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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Tue 08, 2019 7:39 am 
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check individual components connected to the pins with the voltage way out. anything over 20 percent variation is a problem now find out what part or parts is the culprit.

i really have to check the schematic. time to put my riders usb key in the computer and look up your tv.

voltages are measured from the tube pin to ground unless otherwise noted.

re your 1077 remember when you inject the signal the positive lead goes to the control grid the negative lead goes to the chassis. positive should be the center wire in the coax and negative would be the shield. this would be the bnc connector on your 1077. the other jacks are used for injecting a signal to other sections of the set not in the

if memory serves if the agc voltage is incorrect the front end tubes could overload on a strong signal and control grid bias would be out of spec. check the control grid bias on the if tubes and if it is out of tolerance you found at least part of your problems. you need to fix the agc circuit.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Tue 08, 2019 7:52 am 
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V7 is both "AGK Keying" and "1/2 Vertical Multiplier" with V13 being the other "1/2 Vertical Multiplier" and also "Vertical Output"

However V6 also contains pins 5-9 as "Video Output" and pins 1-4 as "Sync Phase Inverter" so V6, V7 and V13 are often referenced as vertical and video output issues.

I did post a very high resolution scan of the SAMs version of schematic (click here) that can be saved and zoomed/panned around to review. I'd still be curious to see if anything majorly differs from the Riders version. I know some components are not where they should be according to the SAMs photo including the 680k (and doesn't appear it ever was where SAMs said it would even at the factory) so probably a lot changed over iterations.

Quote:
your 1077 remember when you inject the signal the positive lead goes to the control grid the negative lead goes to the chassis. positive should be the center wire in the coax and negative would be the shield.


Yes the coax one works great for IF testing but the standard (banana plug in my setup) plugs for things like checking sync, DC bias, video don't seem to do anything. Well actually applying the DC bias of any value to the input of V7 simply blacks out the screen and audio entirely.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Tue 08, 2019 8:11 am 
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for right now concentrate on the voltage on the pins that are way out of spec. check all components that connect to the pins that are low.

I know some components are not where they should be according to the SAMs photo including the 680k (and doesn't appear it ever was where SAMs said it would even at the factory) so probably a lot changed over iterations.
this is possible. i assume you are talking about the parts layout diagram and not the schematic here.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Tue 08, 2019 3:48 pm 
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The voltage on the control grid of the AGC Keying tube V7 pin 7 is mostly due to the plate voltage of the Video Output tube. Notice that there is only a 6.8K resistor between them. That means that the Video Output plate has 100 time the effect as the voltage on the other end of the 680K. This also implies that the Video Output plate voltage is too high.
When a tubes grid voltage is low (more negative) by even a small amount it will cause that tubes plate to be much higher usually.
Be vary careful if you start using two different schematics, SAMS identifies parts with different numbers than Riders. V7 on one probably will not be V7 on the other, R45 will not be R45, etc.
EDIT: I want to add that if you somehow increased the RF signal level from your 1077 that it could overload the TV and cause similar "interference" problems. The extra lines appearing across the screen
could be retrace lines.

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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Tue 08, 2019 6:26 pm 
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Quote:
In 1918, Edwin Armstrong used only triodes when he invented the superheterodyne receiver. One triode operated in a conventional oscillator circuit. Another triode acted as a mixer by coupling the oscillator signal into the mixer's cathode and the received signal to the grid. The sum and difference frequencies were then available in the mixer's anode circuit.

(Source: Pentagrid converter, Wikipedia)

Normally you would use the first grid (the control grid) for input. But what really matters is the difference between the cathode and the grid. So you can use the cathode for input. But the grid is normally a high impedance load and is easy to drive. The cathode is normally a lower impedance load and therefore you would have to have a circuit that could supply some current to drive it. But if you need to supply two different inputs to a tube, you may well want to use the cathode for one of them.

There are tubes made specifically for use as a mixer (a Pentagrid converter). They have a specially constructed second grid that works as a second control grid.

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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Tue 08, 2019 7:17 pm 
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thomas13202 wrote:
he had his 1077 hooked up to a different tv and posted a pic of it. the image looked good. not the distorted mess on the tv he is servicing im pretty sure the source of the problem is in the tv not the b&k. cant hurt to check it though.

could you post a link to the article you referenced about the first superhetrodyne i would like to read it.

my reference to the pict-o-guide was i had a vague recolection of a trouble where edges would look like a pie crust and i couldnt remember the name of it or the cause so i suggested looking for this symptom in a pict-o-guide. i am no engineer just a repair man.



Image

Image

The name of a trouble where edges would look like a pie crust is called piecrust (or geartooth) effect. It is a ringing in the horizontal oscillator that doesn't look at all like the problem at hand.

in the case of the working different TV, it was not stated that the audio tone was on or off. If it were on, the working TV would pass the audio to its internal audio circuits, and the video to its internal video circuits, so I would expect all to be well. In his non-working set, a presence of an audio tone (of "around" the 1000Hz tone frequency), if it were leaking into his vertical sweep, might much more approximate the distorted image. So the source of the interference would have been in the B&K, but the problem would be in the TV.

Obviously there are bad voltages on V7A as posted since. Re-do the resistance checks around that area, and the culprit should become clear.

The Armstrong reading material is here:

https://books.google.com/books?id=REISA ... &q&f=false

Figure 3 on page 9 showcases the single-triode mixer.

Tom Schulz wrote:
There are tubes made specifically for use as a mixer (a Pentagrid converter). They have a specially constructed second grid that works as a second control grid.

Yes, but not in 1918. Armstrong only had so much to work with.

A better example, A lot of early TVs sent video output into the cathode of the CRT instead of G1. Same deal.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Tue 08, 2019 7:56 pm 
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WeekendHacker wrote:
thomas13202 wrote:
he had his 1077 hooked up to a different tv and posted a pic of it. the image looked good. not the distorted mess on the tv he is servicing im pretty sure the source of the problem is in the tv not the b&k. cant hurt to check it though.

could you post a link to the article you referenced about the first superhetrodyne i would like to read it.

my reference to the pict-o-guide was i had a vague recolection of a trouble where edges would look like a pie crust and i couldnt remember the name of it or the cause so i suggested looking for this symptom in a pict-o-guide. i am no engineer just a repair man.



Image

Image

The name of a trouble where edges would look like a pie crust is called piecrust (or geartooth) effect. It is a ringing in the horizontal oscillator that doesn't look at all like the problem at hand.

in the case of the working different TV, it was not stated that the audio tone was on or off. If it were on, the working TV would pass the audio to its internal audio circuits, and the video to its internal video circuits, so I would expect all to be well. In his non-working set, a presence of an audio tone (of "around" the 1000Hz tone frequency), if it were leaking into his vertical sweep, might much more approximate the distorted image. So the source of the interference would have been in the B&K, but the problem would be in the TV.

Obviously there are bad voltages on V7A as posted since. Re-do the resistance checks around that area, and the culprit should become clear.

The Armstrong reading material is here:

https://books.google.com/books?id=REISA ... &q&f=false

Figure 3 on page 9 showcases the single-triode mixer.

Tom Schulz wrote:
There are tubes made specifically for use as a mixer (a Pentagrid converter). They have a specially constructed second grid that works as a second control grid.

Yes, but not in 1918. Armstrong only had so much to work with.

A better example, A lot of early TVs sent video output into the cathode of the CRT instead of G1. Same deal.

There is also cathode introduced negative feedback in aduio amps and the long tail pair phase inverter/ balanced amplifier. The long tail pair is a pair of triodes that share a cathode resistor and the plates of which should always pass the signal 180 degrees out of phase of each other. When used as a phase inverter the long tail pair ties one grid to a fixed bias with no signal (such as ground) and the other grid is tied to signal. The tube tied directly to signal at it's grid changes cathode current and thus cathode voltage across the shared cathode resistor varying the grid-cathode bias of the grounded grid triode by driving it's cathode. If an additional stage of gain is desired between the phase inverter and the output a second long tail pair can be used with both grids being fed separate phases from the phase splitter and the plates driving separate halves of the push pull output. IIRC the long tail pair was also used in early tube computers as a sort of tube based OP-amp.

Some of this is interest and and intentional research some of this is having checked completeness of over 500 Sam's photofact folders over the past week and a half...and I'm still going. (I was given a huge hoard with lots of duplicates and need to get it down to nearly 1/3 before winter steals the temporary storage they are occupying.

I'm still convinced this is a horizontal disturbance. There is no bright line of retrace and if you look at the OPs pic carefully you'll see the vertical is rock steady and the video of 1-3 line(s) gets shifted horizontally every 1-4 dozzen lines. The picto guide shows a nice sinusoidal disturbance affecting all lines, but the OP has a pulse (or just the clipped peaks of a sinusoid) only effecting the sync timing of a select few lines.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Wed 09, 2019 4:15 am 
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I didn't make any good progress with this yet, but to clarify a few points.

1. I did not have the 1077 attached when the issue was first observed, instead I was testing the game console that had previously worked well other than the vertical roll. It occurs on any input device and even if the tuner is fully disconnected.

2. The 1077 outputs 1000K audio signal on both the working TV and the Admiral speakers, but the Admiral one has a lot of static ever since this glitch began.

The only thing I recall altering on the Admiral before and after the issue occurred was that ancient 680K resistor fed to pins 7-8 of V7 which matches the schematic. However I have tried two new resistors (checked) in same spot and tonight cleaned up the solder around that whole area and reapplied but there is no difference in the problem. So clearly something else did go astray. Didn't have a chance to test all resistors but the others I checked on that circuit (both leads still soldered) were within the ballpark of what they should be. Removing resistors from this set is a real pain due to how they are bend around and clamped back to the PCB with solder applied all around so I try to do as little of that as possible. I do not see any areas where the trace may had broken either along the circuit.

Is there a general guideline on how far back one has to go in tracing connected elements just to troubleshoot this one voltage issue? Many end up branching off and interconnecting a lot throughout the entire chassis. There are dotted lines around various blocks in the schematic, are these representative of a full circuit? Or can the exhibitted problem still be caused from some other region. (Full resolution available at this link).


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Wed 09, 2019 5:40 am 
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What is the voltage reading across the 680k resistor? Measure as close to the resistor body as possible.

MattPilz wrote:
... These values are the same with or without the 680k resistor...
Highly suspicious.

Also, vary the "Noise Gate Control" from end to end and report voltage readings across the resistor, at both limits.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Wed 09, 2019 2:49 pm 
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Will try this tonight. I tested the resistance (not voltage) of the 680k in circuit and it started at 25K and slowly climbed to 100K and did same regardless of noise gate position.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Wed 09, 2019 6:40 pm 
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MattPilz wrote:
Will try this tonight. I tested the resistance (not voltage) of the 680k in circuit and it started at 25K and slowly climbed to 100K and did same regardless of noise gate position.


Testing the resistance across the 680K, in circuit would yield meaningless info. The reading rose over time due to your meter current very slowly charging (or possibly discharging) C1B and C1A, or C1C. Or both circuits.

I meant check the voltage across it, we need to know why:
Quote:
... These values are the same with or without the 680k resistor...


It would be impossible if the circuit is wired the same as the schematic.

R45 and R43 are a voltage divider that feeds V7 pin 7 with 36.4% of whatever voltage is across the pair. The left side of R45 is at your main B+ 265V rail, the right side of V43 varies from ground to +150V depending on the Noise Gate Control setting. With the noise gate control set all the way clockwise (at ground) the voltage across the pair is 265V, 36.4% of which is 96V. Fully counterclockwise, the voltage across the pair is 265-150, or 115V, 36.4% of which is 42V. So pin 7 should vary between 42V and 96V with the noise gate control.

Without the 680k resistor at all, the voltage at pin 7 would be whatever voltage drop from 150V you get thru the noise gate control and R43 (or R70) into the remaining circuits. Perhaps even the measured 112.8V you measured. So in theory the 265V rail isn't feeding your R45 (but it IS feeding your V7 pin 8 (measured at 277.2V), so the failure should be very isolated.

Again, measuring the voltage across the 680K resistor, at both extremes of R7, would tell us all we need to know.

Your 277.2V reading is probably high due to your line voltage (usually around 123V these days) being higher than the nominal 117V demanded by note 4, bottom left box of your schematic. Can usually be ignored, although for some early sets it matters enough to build in a bucking transformer or some other means of returning the line voltage to exactly 117.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Wed 09, 2019 7:14 pm 
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The resistance measurement changing while you look at it is probably because a capacitor more or less is in parallel with the resistor. The cap is charging up and throwing off the measurement.
The dotted lines around some of the circuits on the schematic indicate the boundaries of the different circuit boards.
I think although the 680K resistor area was obviously a good early suspect the resistor has been eliminated at this point.
It is not out of the realm of possibility that a tube or something else has failed.
When you want to understand a circuit it helps to determine where the voltage (and current) is being sourced from.
On a tube the plate, cathode and sometimes the screen can carry the main current, the grid is a very high resistance (many megohms) so there is practically no current ( unless the tube is shorted). The grid voltage (more exactly the difference of the cathode and grid voltage) controls the cathode and plate currents. So for instance the grid of V7 (pin 7), the grid itself can be ignored. That leaves three resistors connected together, one of them is 68K, much lower than the other two, so it can influence the voltage there more than the others. If you trace back on the other side of the 68K there are two coils (usually coils do not have very high resistance) and a 47K 2W resistor and some capacitors connected. The capacitors do not conduct DC, so they are out of the matter. On the other side of the coils is the plate of the Video Output which is shown with 90V which is the same voltage the grid of V7, so right there it looks like both voltages are probably off. At the other end of the 47K 2W is a 220 ohm then a line labeled 150V. A higher wattage resistor is a clue that there is a lot of current flowing and it might lead to where the power is coming from. When an entire wire is labeled with a voltage it very well could be a power supply line. If the voltage is off in a circuit the supply voltage is something to check (150V in this case). A little more use of the same detective work leads to the cathode of the Audio Output tube V11. With a little more TV experience you can recognize the audio circuit as being what is called a split supply source. It became fairly common. The voltage that appears at the Audio Output cathode is used to power other circuits in the TV.

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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Thu 10, 2019 3:24 am 
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New Voltage Tests of R45 and R43:

R45 680K End-to-End: 180.5V
R45 680K (Connected at Pin 7): 90-94V (depending on Noise Gate position)
R45 680K (Connected at Pin 8 ): 272V

R43 390K End-to-End: 60-90V (depending on Noise Gate position)
R43 390K Right Side: 0-153V (depending on Noise Gate position)
R43 390K Left Side: 89-93V (depending on Noise Gate position)

Note that Pin 7 contact only varies between 90V and 94V depending on which end the noise gate is at (not 42-96V).

Interesting, when I try to get another reading of V7 Pin 9 (including direct through socket or underneath with tube plugged in and heated) the multimeter cannot get a good or sustained reading and keeps glitching. Perhaps pin 10 has poor solder connection or some short? is it possible the tube socket itself may be bad?


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Thu 10, 2019 4:54 am 
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MattPilz wrote:
New Voltage Tests of R45 and R43:

R45 680K End-to-End: 180.5V
R45 680K (Connected at Pin 7): 90V
R45 680K (Connected at Pin 8 ): 272V

R43 390K End-to-End: 60-90V (depending on Noise Gate position)
R43 390K Right Side: 0-153V (depending on Noise Gate position)
R43 390K Left Side: 89-93V (depending on Noise Gate position)

Interesting, when I try to get another reading of V7 Pin 9 (including direct through socket or underneath with tube plugged in and heated) the multimeter cannot get a good or sustained reading and keeps glitching. Perhaps pin 10 has poor solder connection or some short? is it possible the tube socket itself may be bad?



I'm guessing "pin 10" does not have a problem. It's a 9-pin socket, right?

Pin 7 is now 90? It was suspected bad at 112.8 before. What changed.

The Noise Gate control is apparently working ok. It would vary pin 7 from 42 to 96V as I said before, except I didn't realize it's bucking the 150V line through R64 and R37 and R44 as well in a sort of roundabout way, and now the 3-way voltage divider math is too complicated to do in my head. So we'll say your 89-93V is correct as well. Again, still wondering where the 112.8 went.

So if all this checks out, then the symptoms must have just coincidentally appeared when you changed out R45.

Here is a description of a "Noise Gate":
https://en.wikipedia.org/wiki/Noise_gate

which may be the same thing, or not. It's not something I've seen in my (pre-1952) TV collection. The wiper of this control goes to the control grid of the sync sep (v12) tube, but also to the control grid of the video output tube (V6), so no telling what "noise" is being gated, but probably not audio noise. Is there any documentation on the operation of this control?

It should probably be noted that the 150V line goes straight from the noise gate control to the cathode of the audio output tube (V11), which, if dirty, would likely cause static in the speakers. That line also runs the cathode of V10, and supplies the plate of V8, all in the audio section. Putting a 'scope on the 150V line might yield some interesting clues.

Might be time to try a different 6BH8 in V7, maybe it coincidentally internally failed, or maybe the heat from soldering in the new R45 failed it.

You should re-do the resistance checks for V7 pin 9 and maybe V1 pin 2. If the -63.9V persists. Now that that's not repeatable, I guess all bets are off.

And I assume the video game was playing some sound too, so my original thought that it was audio "noise" isn't really ruled out yet. Maybe a fail somewhere on the audio board is putting noise on the 150V line as well, screwing up the sync thru the noise gate, and screwing up the audio thru V10's cathode. Putting a scope on it would be nice.

Also, if you are using a digital multimeter, you might see glitchy behavior measuring DC on a DC test point that has an AC signal on top of it. In the old days, an analog meter would invisibly "average out" the AC component, just by the inertia of the needle wanting to stay put. A digital meter can read all over the place under these conditions. So maybe your -63.9V was a consequence of that, and it's really an "average" of -1.3V, and wasn't bad at all. And put you on a wild goose chase. If this is the case, you should try to find a vintage VTVM.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Thu 10, 2019 5:17 am 
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Quote:
I'm guessing "pin 10" does not have a problem. It's a 9-pin socket, right?

My mistake again, pin 9 is what I meant. It has troubles reading any values steadily with my multimeter and will attempt another read every 1-2 seconds as if there is no good contact being made. The values that do flash on read -2 typically but is again not steady. I can try reflowing new solder to that connection as I notice it is quite an opening where the pin 9 socket lead attaches to the trace so perhaps something got loose.

Quote:
Pin 7 is now 90? It was suspected bad at 112.8 before. What changed.


Yes - no idea as nothing should had changed. I will try all of this again tomorrow with a different multi-meter, perhaps an analog one.

Quote:
Might be time to try a different 6BH8 in V7


I believe I did already try the original from the set that I had previously replaced. I am going to be investing in a tube tester shortly I think to help eliminate these easy potential causes.

Here is what the Noise Gate description reads for this chassis. Since my feeds will always be direct RF sources and not over-the-air it should always be set fully counterclockwise according to this.

Quote:
NOISE GATE ADJUSTMENT

The Noise Gate control is used to improve sync stability in fringe and noisy areas. Set the Noise Gate fully to the left (counterclockwise), Set the Channel Selector for the strongest TV station. (Be sure that the Vertical and Horizontal adjustments are correct.) If the picture is unstable (jitters or rolls), slowly turn the Noise Gate control to the right until the picture just becomes stable, Check adjustment on other TV stations, and if necessary, readjust control. Caution: If the Noise Gate is turned too far clockwise for a strong signal, the picture may roll vertically, tear horizontally or disappear.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Thu 10, 2019 4:11 pm 
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The cathode of the Audio Output V11 is used as the source of the 150V supply. Notice C2 (60 uf) should ground (filter) out any AC (sound mostly) signal there. The DC cathode current gets to ground to a large extent through the other circuits. This split supply circuit is used to provide a lower operating voltage for some of the circuits instead of using high wattage (power wasting) resistors to drop the voltage. It is possible that because of a problem in the audio output circuit that it is oscillating, disrupting the audio and the circuits that get powered by the 150 V supply. By the way some time ago I said something about operating the set without the speaker but with a resistor substitute, I see that was wrong since you would still need the audio output transformer which is mounted on the speaker.

WeekendHacker seems to get confused on what has the most major effect on the voltage at a certain point.
Sometimes he has "the tail wagging the dog".
Be careful about using terms like 'noise gate' when they mean something different in other uses like modern audio than they do in vintage TVs.

Tube sockets certainly can go bad and cause problems.
I've always advocated the use of an oscilloscope to troubleshoot a TV. SAMS is good for this since they almost always show the waveforms at certain points. However if you look at a point where no waveform is shown you have to figure out if what you see is correct yourself. Power supply lines are easy because there should be very little AC signal (there is always some). I posted some links to some of w2aew's videos on youtube about running a scope earlier. There is one complication however, SAMS gives the scope sweep frequency instead of a time, because SAMS did not use a triggered scope (which was rare in a service shop at the time).

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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Thu 10, 2019 6:31 pm 
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Wow, very cool. 6BF5 tubes are a power source, I didn't know that. I'm going to buy a bunch of them and see if I can run my table saw with them.

I always thought the only power source in a TV came from the electric company. Burning fossil fuels and all that. But I get easily confused.

I'll wag the dog briefly, and suggest that the 117VAC coming from the electric company is transformed (via a transformer) into 285VAC. Then it is rectified into DC with a 5U4GB tube, to a sloppy 280VDC. A speaker field coil and an L23 choke, along with a C1A and a C1B electrolytic capacitor, smooth it into a fairly constant 265VDC "source" rail. I think that's called a pi filter, but I'm kinda dumb.

This 265VDC rail is then used to charge two series electrolytic capacitors, C3 and C2B to 265VDC against ground. These two capacitors, in series, with 265VDC across them, are then "center tapped" between them, which, due to the fact that one is 60MFD and the other is 40MFD, don't split the 265VDC exactly down the middle, but at 150VDC (compared to ground) instead.

Then this 150VDC source is used to float the 6BF5 cathode.

But I'm surely confused, and I prefer the idea that 6BF5 tubes are a free power source. I can't wait to start cutting lumber with my tablesaw, for free. That will be fun!

Oh, p.s. it's against forum rules to call out another member by name and criticize him. I don't do that.

I offer my advice in the hopes of getting people thinking. Others may do the same thing. Eventually, hopefully, we all reach a positive goal.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Thu 10, 2019 7:16 pm 
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WeekendHacker wrote:

This 265VDC rail is then used to charge two series electrolytic capacitors, C3 and C2B to 265VDC against ground. These two capacitors, in series, with 265VDC across them, are then "center tapped" between them, which, due to the fact that one is 60MFD and the other is 40MFD, don't split the 265VDC exactly down the middle, but at 150VDC (compared to ground) instead.

Then this 150VDC source is used to float the 6BF5 cathode.



Ignoring the obvious sarcasm and correct circuit description elsewhere....you do have your conception of the circuit wrong here.

Capacitors can not divide DC voltage and current. The audio output tube is acting as a resistive voltage divider in tandem with the circuits driven by the 150V rail.
If that doesn't make sense think of it this way: a resistance can be defined as anything that has a voltage drop across it when current flows through it. A tube biased into it's conduction region (as a class A audio output normally is) satisfies this definition ... however the audio signal messes with the amount of current flowing (but if designed and operating right never interrupts it). To get rid of those variations we put a cap across the audio stage (265v-150v) to absorb those fluctuations and maintain DC on the 150v line....so what of the other stages on the DC line, surely their current varies with signals too?...well there is the other cap from 150vTo ground to eliminate their fluctuations too.
Now let's say you need a stereo phono preamp and have only 2 identical mono (with capacitive signal input and output coupling) units that have bad power transformers,a separate fixed power supply that puts out heater voltage, and twice the designed B+ of your preamps, but no spare parts and no time to order any. The amps are already driving the tubes hard and double the B+ will kill them fast....the only solution to put the correct B+ into both amps is to treat the B+ load of each preamp as a resistance and stack the B+ loads of the amps to form a voltage divider.
This concept was employed as far back as the Motorola VT71 and was super common in low power battery 2 way radio where wasting power as heat in power supply voltage dividers was a massive battery life disadvantage.

I hope I've made this system clear for everyone who may be confused by it...It certainly is not the most straight forward power supply design out there.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Fri 11, 2019 5:25 am 
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Joined: Sep Thu 20, 2018 8:53 pm
Posts: 103
Turns out the differences between my first readings and second were dependent on whether or not I had an RF signal connected.

With an RF signal through the tuner, the first voltage tests I posted are correct (e.g., R7 Pin 7 = 112-120V, R7 Pin 9 = -63V).

Without an RF signal through the tuner, the last voltage tests I posted are correct (e.g,. R7 Pin 7 = 90V; V7 Pin 9 = Undetectable/Varying).

I tried to check over any other area to find any loose connections and thought I found one at C2-B but even redoing it did not alter the defect.

A reading of C2-B was giving me 280+V instead of 265 but I didn't verify if I was running at 117V or slightly higher through variac; however lowering the voltage input to the extent the TV still functions continues to exhibit the same picture glitches that now plague it.

V7 is the only tube I don't have a spare for. It was a NOS replacement I purchased because the original had gone to air before I got TV.


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 Post subject: Re: 1956 Admiral Console - Some general technical questions
PostPosted: Oct Fri 11, 2019 3:54 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 1792
Location: Dallas, TX
I guess you typed R7 where you meant V7 a couple of places. Also I think you meant C1B where you had C2B.
V7 is sort of an odd ball tube circuit. Normally for a tube to pass current the plate must be more positive than the cathode. If the cathode is more positive that the plate the tube doesn't conduct so it isn't doing anything.
However if you look at the waveform W4 you see that the plate actually has a AC signal that has a peak-to-peak voltage of 600. So V7 conducts only during the positive peaks of the plate signal. Using a meter to look at the plate only shows the average voltage which is a bit negative. This illustrates why a scope is used to get the whole story.
Since the current problem seemed to appear just after you replaced the resistor connected to V7, that area was a good suspect for the location of the problem. That might have been a red herring however. Something else went bad about the same time. You had all the tubes out when you measured some resistance on the sockets, are you sure you put them back in the right place? Maybe a socket went bad. Have you replaced all the paper and electrolytic caps? I think you should go back to square one and check some overall basic things like the 150V line (on C2B) as well as some waveforms.
Whatever is causing the problem on the screen and the sound, it seems like an interfering signal is getting into common circuits. It could be an oscillation or a overload of signal or something else.

_________________
Tim
It's not the Destination, It's the Journey.


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