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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Wed 03, 2019 11:31 pm 
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According to the 6F6 specs, the maximum Plate voltage is 375 in push pull configuration. Screen voltage max is 250 volts.

Per the schematic, the design plate voltages are 365 and the design screen voltages are 250.

So, the output tubes are getting pushed very hard particularly with today's line voltages. John is definitely right on this one--this set must be operated at reduced line voltage for sure.


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Thu 04, 2019 5:23 am 
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This may be grasping at straws, but if the AOT DC resistance was only 70 ohms on one side (maybe less) due to its failure, maybe it upset the bias on the 6F6's allowing them to draw excessive current--enough to burn out the back bias resistor (which was a 140 ohm 10 watt resistor)

No. Pentodes have high plate resistance so anything you do in the plate circuit won't affect the bias. Even if you completely disconnect the plate, the tube will continue to draw pretty much the same current. Doing that will, of course, melt the screen grid of the tube, so don't try it.

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Thu 04, 2019 4:56 pm 
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What happens if you operate 6F6's above their max plate and screen voltages? Based on the 6F6 curve it looks like the plate current goes way up unless it is throttled back with some large negative grid bias--at least -20 volts or so. This set has been operating with 400 volt plate voltage and 290 screen voltage. Would that have wrecked the 6F6 tubes? They still test fine but I am worried about using them after I get the plate voltages down to 350 volts or so.


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Thu 04, 2019 5:29 pm 
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This is a terrific thread!
Lots of useful data and information has been posted.

After reading through it several times my suspicion is on the 6F6 audio output tubes. One them either has an intermittent short, or is "gassy."

Unfortunately the types of "shorts" and "gas" that power output tubes develop won't always show up on a tube tester. It may be necessary to operate the tube at "full power" in order to induce the failure to occur. But most tube testers use rather low voltages/currents. That may not heat up the tube sufficiently to get it to fail on the tube tester.

With four output tubes in this radio it could be a bit difficult to isolate which of them is failing. Are these metal tubes? (6F6 is usually metal.) Unfortunately with metal tubes you can’t see if their plates are turning red, or see any sparks or flashes inside the tube. With metal tubes the best way to estimate their current drain is by continuously monitoring the voltage drop across the back bias resistor. This is the resistor that previously burned up when the fault occurred.

For a case like this I will put a 200W or sometimes even a 300W light bulb into my dim bulb tester. The choice of light bulb should result in it just barely glowing while the radio is running normally. A sudden direct short on B+ should cause the dim bulb to get bright. I won't bypass the dim bulb until I am 100% sure that all faults have been fixed.

I also routinely use a Kill-A-Watt because it will show the actual Watts consumed by the radio in real time. A gradual increase in Watts consumed is a sign of trouble.

Sometimes an audio output tube will gradually “run away” rather than abruptly shorting. The observed symptom will usually be a gradual increase in the voltage across the back bias resistor.

Another important measurement is the control grid voltage on each audio output tube. The schematic posted earlier in this thread shows R20 (220K ohms). That is one of two such grid resistors used in this radio. In a push-pull output stage there is a control grid resistor for each side of the push-pull stage. So checking the voltage drop across each of these 220K resistors may reveal which side of the output stage contains a gassy 6F6. The voltage drop across each 220K resistor should be 0V. But if an output tube is gassy, then the voltage across the control grid resistor will rise as the tube gets hotter and hotter. I reject output tubes where the voltage across the control grid resistor gets higher than 0.5V ( 500mV). Ideally I like to see < 100mV.

I’ll post more about this after I take a closer look at the schematic for this radio.

Again, this is an excellent thread for many reasons:
Rare and valuable radio
Intermittent but catastrophic failure
Complicated circuit
Multiple audio output tubes

So, if we manage to fix this one permanently, then we can fix them all.

Best regards, EB

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Fri 05, 2019 10:26 pm 
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EB, Thanks for the info and encouragement. I have repaired several hundred radios in the past and have never found one so challenging as this one.

For all of you following this experience here is the latest update:

I continued my troubleshooting plan yesterday. Attempted to power up the set with all tubes out except the 5U4's to check out the B+ voltage divider. When I tried this, I had no voltage on the divider even though the lamps were lighting just find. I shut it down and found a pin on the plug connector touching another pin. This is where the power supply cable plugs in. Turns out this "intermittent touching of pins" shorts out the HV B+ coming from the PS chassis. Fortunately I shut everything down before the transformer had a change to get hot. I moved the pin away from the other and gave it a robust "wiggle test" to see if something in the plug connector was intermittent. Once I clearly separated the pins I couldn't repeat the intermittent short. So this could potentially be the cause of the initial catastrophic failure of the main power transformer. I am not sure yet about this because the radio had been playing just fine prior to the big failure. So how would those pins have come into contact after it survived the power up that led to the successful playing of the radio?
After taking care of this connector short, I powered up the set again first at 50 volts and then at 100 volts. I had it on a Kill-o-watt meter and monitored watts and volt amps. Everything came up to reasonable voltages with 100 VAC on it. I kept it on for 10 to 15 minutes and everything was stable. Voltages were unchanging throughout the period it was on. I checked temperatures and the transformer stayed at 75, field coil at 75, 7 ohm resistor at 80, back bias resistor (I have a 100 ohm rather than 133 ohm installed @ 10 watts) at 80 degrees. The 3K resistor is 25 watts and got up to 280 degrees (9.5 watts dissipated) and the 2500 ohm which is also 25 watts got up to 250 (8.4 watts dissipated). I am a bit concerned about the temps on these two resistors but I have no experience on how hot they should get.

Conclusions thus far:
1. The short in the connector plug could be root cause of the catastrophic transformer failure.
2. The failure of the back bias 10 watt resistor appears to be tube related as I did not see any problem when I powered up the set without the tubes.

Next steps:
1. Reinstall all tubes except the 4 6F6's, power up to 100 Volts and monitor voltages and temps to see if the problem recurs. If not, that will suggest the problem is in the 6F6 circuits. I think I will do this using dim bulb tester at first.
2. If I don't see the problem (as evidenced by a rising voltage on the back bias resistor), I will power up with the 6F6's installed and monitor the back bias resistor. Will try this at 100 VAC first and if okay raise the voltage to what ever level needed to keep 6F6 plate voltages around 350 V.

Joe


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Sat 06, 2019 2:58 am 
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I would replace the back bias resistor with the correct value. Using a significantly lower value like you have will cause the output tubes to draw too much current.

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Sat 06, 2019 12:33 pm 
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Jim Mueller wrote:
I would replace the back bias resistor with the correct value. Using a significantly lower value like you have will cause the output tubes to draw too much current.


This is a very good idea. Keeping the current down is important in these old sets. A bucking transformer is a must when this one is finally fixed.

It sounds like you had two things wrong with this radio. The physical short in the plug you found is probably the major cause of the transformer failure. Then you add the B+ voltage up high and caps blow up.

One suggestion is to never power up a radio with rectifiers in and no load on the power supply, all other tubes out. This is a bad practice and can cause blown caps etc. it’s best to test for shorts with an ohmmeter.

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People may not remember how fast you did a job, but they will remember how well you did it.


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Sat 06, 2019 1:02 pm 
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I have not followed the details of this, but I want to echo / reinforce a major point:
Don't attempt to trouble-shoot something that has the wrong value parts installed!!
A common corollary is: If there is evidence of past rework, check everything against the schematic.

One recent comment caught my eye: Checking for a "runaway" tube by measuring the grid voltage. That strikes me as doing thing the hard way. If you suspect any kind of issue with a tube, monitor either the plate current or the screen current---(or both, by monitoring the voltage across a cathode resistor)
The common cause of a "runaway" tube is grid emission. This will appear as a continuing rise in plate current after the initial warmup.
My only other comment is my current sig.....

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"Even if you don't understand Ohm's Law, you are still required to obey it."


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Tue 09, 2019 6:44 pm 
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pixellany wrote:
One recent comment caught my eye: Checking for a "runaway" tube by measuring the grid voltage. That strikes me as doing thing the hard way. If you suspect any kind of issue with a tube, monitor either the plate current or the screen current---(or both, by monitoring the voltage across a cathode resistor)
The common cause of a "runaway" tube is grid emission. This will appear as a continuing rise in plate current after the initial warm up.
My only other comment is my current sig.....

I agree that "grid emission" is a common failure for audio output tubes. Unfortunately a tube with this fault may test "good" on a tube tester. Failure occurs only when the tube operates "in circuit," where the voltage, current, and plate dissipation are much higher than what a tube tester applies to the tube.

Also, I agree that each of us may have a preferred method to identify rogue power output tubes. That's fine.

Personally I will stick with measuring the voltage across the control grid resistor. For me this always works. If it is > 0.5V then I check (or replace) the coupling capacitor and substitute a different output tube.

I also check the resistance value of the control grid resistor(s). For example, nearly every AA5 radio has a 470K grid resistor. Sometimes an old resistor will increase greatly in value (I've measured up to 1meg). The data sheet for every audio power output tube specifies the "maximum control grid resistance." For example, the 50C5 datasheet specifies 500K as the maximum grid resistance for cathode-biased operation. So it isn't good if the existing resistor has drifted way up to a much larger value.

Note: "Fixed bias" operation (example: many guitar amps and hifi/stereo amps) requires a considerable smaller grid resistance (typically 47K to 270K) vs. "cathode biased" operation. Technically speaking, this RCA K-130 is more like "fixed bias." Therefore the factory grid resistors are only 220K. I don't think any of us brought this up yet, but both of these grid resistors in the RCA K-130 should be checked. If they are out of tolerance, then they should be replaced. Also the two coupling capacitors should be tested for DC leakage, or (better) replaced with modern capacitors.

If the tube is gassy (or if the coupling capacitor from the previous stage is leaky, or the grid resistor is out of tolerance) then this voltage will be positive and will often rise slowly over a period of several minutes. If it gets above 0.5V then I turn off the radio and diagnose before allowing the radio to stay powered up for a long time. As mentioned by the above post, faults may cause the plate current to continue rising. Eventually the tube may "red plate" or fail catastrophically.

But when all parts are good (output tube, coupling capacitor, and grid resistor), then the DC voltage drop across the grid resistor remains very close to 0.0V. Including after several hours of operation.

Monitoring the voltage drop across the grid resistor works for radios, guitar amps, and hifi/stereo amps of all sizes, from 1 Watt to 100 Watts.

Please note that in this RCA K-130 radio, the voltage to measure is ACROSS each of the two control grid resistors. Neither of these resistors is connected to circuit ground in the RCA K-130 radio. In contrast, when testing an AA5 radio or any other "cathode biased" amplifier circuit where the grid resistors ARE connected to circuit ground, then this is a very simple test, with one lead of the voltmeter connected to circuit ground.

Best regards, EB

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Tue 09, 2019 6:50 pm 
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jamr005 wrote:
I shut it down and found a pin on the plug connector touching another pin. This is where the power supply cable plugs in. Turns out this "intermittent touching of pins" shorts out the HV B+ coming from the PS chassis. Fortunately I shut everything down before the transformer had a change to get hot. I moved the pin away from the other and gave it a robust "wiggle test" to see if something in the plug connector was intermittent. Once I clearly separated the pins I couldn't repeat the intermittent short. So this could potentially be the cause of the initial catastrophic failure of the main power transformer. I am not sure yet about this because the radio had been playing just fine prior to the big failure. So how would those pins have come into contact after it survived the power up that led to the successful playing of the radio?

Could you post photos of this plug and socket?
With them both "assembled" and "unplugged from each other?"
Maybe something happens when there is a tug on the cable?
Could there be a spot or two along the cable where the insulation is bad?

Good luck!
-EB

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 Post subject: Re: I blew up my RCA K-130 Blew another cap! UPDATE
PostPosted: Jul Wed 10, 2019 1:25 am 
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I think I have found the root cause of my problems. I believe there were two separate and distinct problems, each with its own root cause.

The first problem was the catastrophic failure of the power transformer. I believe that was caused (as discussed previously) by a shorted pin on the chassis connector to which the power amp cable connects. The HV output of the power amp is supplied to the chassis on adjacent pins (2 and 3). The distance between those pins is small and I found them intermittently touching. I straightened them both and have not had any other problems.

The second problem was the burning out of the back bias resistor and the catastrophic failure of its parallel electrolytic capacitor. The resistor was 10 watts and the cap was 100 uf @160 volts. Obviously, a high voltage somehow was impressed across this resistor/capacitor combination. I went through a number of power up configurations to see if I could isolate the culprit. I started with rectifiers in but all other tubes out and everything was fine (note the rectifiers had the voltage divider as a load). I then put in all tubes except the eye and the audio tubes and everything was still fine. My next step was to add only the eye tube. When I went to put the eye tube in, I noticed that the 4 conductor bundle of wires seemed a bit brittle. When I wiggled them I could see that several conductors entering the eye tube socket had bare wires showing. I then rebuilt that entire 4 wire assembly. When I removed that wire assembly I could see other bare wires where the bundle was knotted below the chassis to form a strain relief. I think that these bare wires could easily have shorted to ground. The wire feeding the plate of the eye tube, if grounded would have put high voltage on the back bias resistor and would quickly blow the parallel cap. So I believe the bad cable feeding the eye tube is the root cause of this problem.

After rebuilding the eye tube cable, I installed all the tubes and slowly raised line voltage up until the highest B+ voltage (365 V feeding the 6F6 plates) was achieved and I was pleased to find that all the other B+ voltages are nearly dead on per their specs. I let the radio play for 10 minutes or so. The variac is set at 110 volts, so a line voltage of 121 would increase all the B+ voltages by 10% which is too high in my opinion.

My next steps are to run the set using my bucking transformer unit set at -14 volts (~108 here) and let it play for an hour or so and then for a couple of hours to see if it is okay. If it is I will conclude that my assumed root causes are correct. My last step will be to install a bucking transformer in a metal box so that it is permanent to the set.

I have learned my lesson with doing a good visual inspection on all connecting cables. So if you have radios with multi-chasses with interconnecting cables, I recommend you give them a good look over. Same goes with speaker cables.

Joe


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