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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Tue 20, 2020 11:01 pm 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
Some voltages are shown on the schematic, not the wiring diagram.
A good place to start is with the voltages from the power supply shown on the schematic just below the CRT drawing. 225V, 150V, -3.5V, -14V, -85V and -17.5V a little further down. Another to check is the 265V at the junction of the field coil (on the speaker) at the yellow wire, the centering control and the positive terminal of C127A or C132A. Wherever is easiest to get to.
I would get the horizontal problem fixed first. The horizontal section creates voltages that are used elsewhere.
Do be extra careful if you are not well, you could get yourself zapped, (or blow something up).
You are using service data from Rider I believe. The SAMS info that was linked to is not the same set.

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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Wed 21, 2020 12:17 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 1149
Location: Belrose, NSW, Australia
Quote:
I'm currently dealing with vertigo. Trying to fix a tv with vertigo is a very interesting combination.

OT, but my wife had very bad vertigo until just over a week ago. Now completely cured!

You might be very surprised what causes it, I know I was. Your Otoliths have come loose!
Even more surprising is in 80% of cases it can easily be fixed. You can even do it yourself, although it's easier with a helper.

Have a look at this link:

https://dizziness-and-balance.com/disor ... /bppv.html

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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Wed 21, 2020 12:40 am 
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Joined: Aug Sun 23, 2015 12:44 am
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Hi irob2345,


Thanks much for the link, I'll check it out!

Steve


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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Wed 21, 2020 12:45 am 
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Joined: Aug Sun 23, 2015 12:44 am
Posts: 39
Hi Tim,

Thanks much for the info. I noticed that the SAMS was "similar to chassis". For my money, should have been exact! I'll let you know what I find.

Steve


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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Thu 22, 2020 2:48 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 1149
Location: Belrose, NSW, Australia
Steve, you have a PM.

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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sat 24, 2020 10:36 pm 
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Joined: Aug Sun 23, 2015 12:44 am
Posts: 39
Hi Tim,

I found that voltage on 5V4 plates way too high. My DVM went off scale! The HV filter cap (500PF) is going to the plates instead of chassis ground. I suspect this cap is either leaky or shorted. Caps block DC. Possibly carrying second anode voltage to 5V4 plates. Your thoughts?


Steve


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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sat 24, 2020 11:09 pm 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
Be very careful when measuring things connected to anything connected to the flyback transformer (T105).
The HV rectifier V110 should never be measured without a special high voltage probes and even then only
the cathode side (going to the CRT). The plate of the horizontal output tube is never measured unless you want to buy a new meter and probably pick yourself off the floor. There are very high voltage spikes that can jump through most insulation. Many times the damper V111 is also off limits.
The damper tube V111 rectifies pulses from the flyback and creates what is know as the Boost voltage.
The regular power supply circuit is only supplying +280V, the damper diode is used to create a higher voltage
in this case about +340V boost voltage. The boost voltage is used to bias several things in this set.
If the 500pf cap is C142, (it is better to use the part identifier because there could be other 500pf caps) then it should be connected to the plates of the damper. Check the schematic. Don't re-engineer things at this stage.
What did the other voltage read from the power supply?

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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sat 24, 2020 11:34 pm 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
Whatever meter you used it was probably confused by the large AC signal on the plates of the damper.
This signal is used by the horizontal section of the yoke to deflect the electron beam in the CRT.

Here is some light reading that may help you understand TV circuits.
https://worldradiohistory.com/BOOKSHELF ... h-1955.pdf

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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sun 25, 2020 12:02 am 
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Joined: Aug Sun 23, 2015 12:44 am
Posts: 39
Hi Tim,

My error. Yes, it IS c142. I'm used to seeing this cap go to ground. Its shown on schematic that this plate connection to 5V4 is only on SOME CHASSIS. Im familiar with "Do Not Measure". I only measured where DC voltages were indicated and shown on voltage chart. I avoided 5V4 Cathode. Voltages on 6BG6 were nearly spot on ( except the plate, avoided that). Supply voltages appeared very reasonable. Not perfect due to i'm sure, differences in test equipment then, to today. Our test equipment today is more accurate than back then. Thanks for the link. I downloaded the book to a thumb drive. Good reading for sure. It's been many years since i studied and read about this. I admit, am a little rusty. Memory not as good as before.

Steve


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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sun 25, 2020 12:06 am 
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Joined: Aug Sun 23, 2015 12:44 am
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Tim,

Question, Why are voltages shown for places to avoid? "Do Not Measure" should be indicated??

Steve


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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sun 25, 2020 1:13 am 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 2642
Location: Dallas, TX
SJK wrote:
Tim,

Question, Why are voltages shown for places to avoid? "Do Not Measure" should be indicated??

Steve

That is a fair question. I wish I had a good answer, some meter types may do a better job than others.
They didn't have digital meters back then and they certainly didn't have auto-ranging ones either which
might be confused by the AC or pulses. Digital meters might show more decimals but either type could not
be calibrated well.

So, do I understand that the problem is that you have a small length horizontal line on the screen and nothing else on the screen? It is odd that you have good enough high voltage to see the line but not enough horizontal length. The HV is derived from the horizontal sweep.
The vertical sweep is another circuit.
Is the yoke all the way to the front of the CRT, up against the "funnel"? Did you try another horizontal output tube V109 (6BG6), or test the one in the set? Does the width control have any effect?

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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sun 25, 2020 1:49 am 
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Joined: Aug Sun 23, 2015 12:44 am
Posts: 39
Thats what baffles me. High voltage, but only 1 inch of horizontal sweep. i'm using a 90 degree test crt. An 8XP4. Raster wont fill the screen, but enough for test. I notice that upon advancing BRIGHTNESS CONTROL the one inch goes out, as if "Blooming". Perhaps HV not as high as it should be. I tested both 6BG6 and 5V4 both passed. of coarse, thats not definitive. I suppose replacement of those is in order. Yes, crt is up as far as will go towards yoke.

Steve


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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sun 25, 2020 4:46 am 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 2642
Location: Dallas, TX
Oh, I guess I didn't realize you were using a test CRT. I wonder if that has some effect. I'm not too
concerned with the brightness behavior because the gun characteristics are probably different.
What is the width control effect? It could be open or something.
Do you have an oscilloscope?

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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sun 25, 2020 5:32 am 
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Joined: Aug Sun 23, 2015 12:44 am
Posts: 39
Hi Tim,

What you said about the gun makes sense. The width coil was removed from the circuit, not by me. Probably to get more width. I do have a scope. Difficult to get to bc of my setup. Should have pulled it out in the beginning. I was considering buying a 5V4 and 6BG6 to definitely rule out the tubes. Then if need be, the scope. Tubes are, a weak link.

Steve


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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sun 25, 2020 7:08 am 
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Joined: Aug Sun 23, 2015 12:44 am
Posts: 39
With the help of my little Nephew, got out the scope. And, ordered the 5V4 and 6BG6.

Steve


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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sun 25, 2020 4:59 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 2642
Location: Dallas, TX
I've been thinking about a few things related to what you are seeing, sometimes I just can't help myself.
The 8XP4 might not be the best test CRT to use in this case, too bad you don't have a 5AXP4, it matches the deflection angle of a 10BP4 better. That might cause the raster to be a bit smaller on the one you have.
Also the 10BP4 has an outer coating on the CRT bell that forms a capacitor of about 500 pf that helps smooth the CRT anode voltage. The test CRT doesn't have this (no need to discharge it). This particular set might be sensitive to the difference, less of a capacitance to charge up to the peak of the flyback voltage. It might work better if you added a 500pf 10KV cap while you use that test CRT. You would need to discharge it after the set is on though, discharge it with a 1K resistor in the path.
Speaking of HV ceramic caps, you don't know that the 500pf HV cap that is in the set is OK. I think it is held in a clip so if you have a capacitor tester it would be easy to test, although you can't test it for high voltage breakdown.
Here is a restore of a RCA 621 that a fellow did, that is in the same vintage radio club I'm in. That set is one from a year earlier than the 721, but is somewhat similar. On the forth page he found that the HV ceramic cap was bad. He was able to re-stuff it! He is very meticulous (understatement).
viewtopic.php?f=3&t=315410
I'm not sure whether removing the width coil would increase or decrease the horizontal sweep. Anytime someone modifies the circuit it is bad news. Many times they do that to correct something that really is caused by something else. They may have even removed that part so that they can fix another set.
Unless you find another fault that is causing the problem, I would think that the thing to do is replace the width coil.
You could always replace every part in the set to correct a problem, all you would be wasting is money, time and effort (is that all?). Of course you would have to re-adjust and align everything also!
Hopefully the problem will be fixed the smarter way.
Back when this set was only five years old a serviceman could easily just swap out parts like the yoke to find a problem, now the parts are not that easy to come by (or cheap).

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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sun 25, 2020 5:08 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 2642
Location: Dallas, TX
The reason I brought a scope is that the Rider service info shows a signal trace on the horizontal yoke coil.
Be sure your probe and scope can handle the voltage however, 1100V peak-to-peak! Use AC coupling.
Since the horizontal centering control is connected to one end of the yoke it could be bad or dirty.
The voltage on the control would be DC, but the schematic doesn't show what it should be.

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 Post subject: Re: HELP NEEDED FOR RCA KCS-26 CHASSIS
PostPosted: Oct Sun 25, 2020 11:36 pm 
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Joined: Aug Sun 23, 2015 12:44 am
Posts: 39
Hi Tim


Thanks very much for all the use of your brain :)

When my wife was alive, I had a 621TS. Lost it when she passed (long story). I used an 8XP4 to successfully restore it. The only difference is the 621 was electrostatic focus. 721 is magnetic focus. i removed the focus coil from its mounting so as not to interfere with the 8XP4. I will reconnect the width coil which is still there. I dont think my scope will take 1100 volts. the probe either. I'll check the scope's manual to be sure. I dont want to destroy it with that high a voltage. I WOULD like to scope the control grid of the 6BG6, and try replacing both tubes as well. Then move forward from there. It's true, i don't know if the HV cap is good. Don't have one of course. I'm printing your recent comments which I find very valuable. I'm now awaiting my tube order. I'll report back regarding the sawtooth on the 6BG6 grid and go from there.

Steve


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