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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Mon 01, 2019 2:58 am 
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i'm going to add one more that might just get hit out of the park for a grand-slam home run.

i will NEVER, NO, NOT, NADA, EVER use no-name chump brand capacitors in any of my radios or high end vintage SS equipment.

did i say never, ever ?

if any of my nichions, panasonics, illinois, cornell, or sprague capacitors fail (which none have), at least i know it was a name brand capacitor and the variable of a POS cheap junk capacitor is out of the equation.

out of thousands of capacitors in hundreds of radios in nearly 40 years, i have only had two 100pf silver micas somewhat fail causing static and hash.

once when i ordered from mouser, i somehow received a few Xicon (mouser house crap brand) capacitors. it was their mistake or mine, but i knew no better in my 20s and used them anyway.
a few years later, with barely 50 hours on them, they leaked some brown goo out of the holes where the leads come out.

these were in a restored vintage tube oscilloscope and i'm SO glad i pulled the chassis before powering it up after sitting idle in climate controlled storage.

that was enough for me.

so, batter up and swing for the fence 8) .

steve

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Mon 01, 2019 3:20 am 
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But guys take a look at the circuit. I don't see how a voltage surge or even a failed first filter cap could over voltage the back bias resistor or capacitor. Can anyone explain how that could happen? I think something would have had to shorted out either the 3000 or 2500 ohm resistors, but there is no short after the event.
Joe


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Mon 01, 2019 4:38 am 
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It must be appreciated that as a back bias resistor in most cases carries all of the cathode current, it can be compromised. If a heater shorts via a cathode there may be enough current to fry the back bias resistor.

If the back bias is open (bonding fail: bad earth) there is normally enough of a path for the voltage to go to surge and the back bias cap will not handle it.

I would actually pay serious attention to the wiring of the rectifiers and the tubes themselves and lead dress in general. If the caps are getting AC: That will not be appreciated.

As suggested before: I totally agree on a 100% inspection of every wire & joint.

Marc


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Mon 01, 2019 4:14 pm 
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Yup, today and tomorrow it will get a 100% wire check and visual inspection.

I just thought of something--I remember cleaning off the chassis with SuperClean. I always try to keep any cleaning materials away from tube sockets. But what if something got into one or more of the sockets while cleaning, leaving a film that might arc over intermittently when B+ is applied to the tube? I suspect I should see some evidence of arcing, but the arcing could possibly be between the materials in the socket. Is there a sure fire way to clean tube sockets? Contact cleaner, etc. and blowing with air?

Is this a crazy idea?

Joe


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Mon 01, 2019 4:34 pm 
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Not familiar with the cleaner you used.

Generally, I don't worry too much about using cleaning solvents that dry with out leaving a residue, although in theory I guess you could flush some native contaminant into a bad place with any solvent.

Other cleaners do leave a residue when they dry. I would steer clear of those in places subject to high voltage like tube sockets and band switches.

So while what you are describing is certainly possible, I would put the likelihood down at the lower end of the list of possibilities.

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Mon 01, 2019 5:41 pm 
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I've been following this tread with some interest so just to be clear, and setting aside whatever caused the transformer to give up the ghost, the working theory here is that something is pulling too much B+ which is returned through R26 and R28 which is causing R28 to burn open. With the ground reference now open this is causing the voltage to increase at C43 causing it to self destruct?


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Mon 01, 2019 7:42 pm 
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jamr005 wrote:
Yup, today and tomorrow it will get a 100% wire check and visual inspection.

I just thought of something--I remember cleaning off the chassis with SuperClean. I always try to keep any cleaning materials away from tube sockets. But what if something got into one or more of the sockets while cleaning, leaving a film that might arc over intermittently when B+ is applied to the tube? I suspect I should see some evidence of arcing, but the arcing could possibly be between the materials in the socket. Is there a sure fire way to clean tube sockets? Contact cleaner, etc. and blowing with air?

Is this a crazy idea?

Joe

Not at all a crazy idea. Unless you had the room dim and eyeballl on that exact spot, you wouldn't see the arcing. Not all arcing or shorting events leave behind dark residue.

Since the radio had been worked on since the transformer failure, there may be more than one fault (an earlier one and a different one now).

We can keep guessing, speculating, and form new hypothesis, but the only way to find the fault at this point is probably to locate it physically; which is 100% verification and inspection.


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Mon 01, 2019 10:06 pm 
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Well, was just getting started on the wire check and visual inspection and stopped to run a few numbers.

According to the technical info, the total designed cathode current is 170 ma. (21.6 ma each for the four 6F6s). All of this current flows through the back bias resistor R28. The voltage divider also contributes current through R28. Per the design numbers the voltage at "the top" of the resistor string beginning with the speaker field coil is 365 volts. Total resistance across this voltage is 6940 ohms which produces a current of 52.6 ma. Adding this to the B+ current one gets a total of 222.6 ma.

The back bias resistor I had installed was 140 ohm at 10 watts. If the total current generated by the set is 222.6 ma then a 140 ohm resistor will dissipate 7.15 watts. I have no idea what wattage the candohm was for R28. (Can anybody help me on this?) Seems to me this is not a very good design, particularly today with higher line voltages (leading to higher B+ voltages and voltage divider voltages) and resistance drift of candohm resistors.

I am going to continue the wire check because for the 10 watt R28 to have failed open, it would have needed to be seeing north of 267 ma.

No matter what I find, I will be considering:
1. Removing 2 of the 6f6's to reduce B+ cathode current
2. Requiring the use of a bucking transformer to reduce to line to 110 volts or a bit less.
3. Increasing the size of the back bias resistor to 15 watts or more.

Thoughts on this?


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Mon 01, 2019 10:34 pm 
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10W resistor is fine, just don' go lower. Since you're looking for inputs, I'd say don't mess with the design until you've found the existing fault or at least until the radio would operate consistently for multiple days/weeks without problems.

The set was originally designed for up to 125V, so it's unlikely the components were undersized.

If the design is so poor that components would just blow up, they all would've failed and none would've lasted long enough to be in the vintage market.

Since they're constructed from resistive wires, I don't think candohms drift much, but rather prone to failure due to the way the hot resistive wire is (barely) insulated from the metal outer case.

Someone posted this a long time ago. Don't know the original source, it might help :)


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Tue 02, 2019 1:41 am 
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If you have the currents Current squared by the resistance guides you on the wattage that is being applied to the resistor. As I pointed out I do not, like "heavying up" backbias resistors as they can act as a "fusistor" and save the transformer, from a fault, in your case after the field.

This is where I like Some German & European circuits. They give currents & voltages on most, or all of the tube elements. One of the things on the list of oddball faults is an OP transformer secondary or VC going open, causing the coil to "ring" & generate EHT and a massive flash over.

The valve data sheets should give an indication of current draw of the tubes.

I think this has reached the point of examining if you have done the candohm correctly and testing all of the tubes. If the sets voltages are within 20% it should be OK.

On of the things I did recently on tossing a metal rectifier & adding 1N4007's was to add 100 Ohm resistors in series with the diodes AC side. Voltage was spot on.

You can initially add a resistor to the Heater & filament rails to cut their voltage; Pull the rectifiers & other tubes & ensure there are no hidden shorts on that rail and there are no mistakes like cathodes earthing via heaters (should produce hum) & ensure no heater cathode shorts. Some times that can be picked up by an ordinary ohm meter.

I doubt you have the gear but one of the things I do with swine like this, is to use the bench PSU to power the B+ (No tubes no mains power) to see what it draws. Only current should be the total resistance of the candohm and a rough guide 0.01CV for the electrolytic draw.

If that has a field coil I would be tempted to insulation test it.

One step at a time.

Marc


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Tue 02, 2019 2:08 am 
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Adding to what Marc said... I would have the radio hooked up to my metered variac. I would also have several other meters looking at voltages and currents before I started the radio up again. I would be running it first at around 100 volts input and monitor and check lots of voltages. Observe it all over a couple hours of run time. Although yes I read the specs on the radio up to 125vac that radio if my radio would NEVER see 125.

Here is an example of what happens when Line Voltage is increased to an old radio. The example is a Zenith 9s262 chassis. I had one on the bench. It was all done with the usual total overhaul. That day my line voltage was above its normal 125v Line was 130v and the current draw of the radio was 0.76amps. = 99 watts. I lowered the incoming voltage with my variac to 109volts. Current draw dropped to 0.56 amps = 61 watts. A 17% drop in line voltage created a 24% reduction in Current draw and a total of 1/3 reduction in total watts. When you increase the filament voltage to a tube it conducts more than it did at the correct number of 6.3 volts. That explains part of the increased current draw. The rest of it? Heating of the Power Transformer. at 130v you could fry an EGG on top of that transformer if left running for about an hour. No way would it survive that voltage for any length of time.

John k9uwa

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Tue 02, 2019 6:35 am 
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The variac will allow the set to be run on its original voltage which will help in comparing the set to any manufacturers data sheet with quoted voltages.

If you wish to Megger / insulation test. You can insulation test the filaments minus tubes, but to do that to heaters, no tubes & lamps & lift the transformer wires. I can only see a miss wire there. I am still inclined to believe the problem lays wholly in B+.

Is there some way of testing the tubes? That can involve another set/s that takes the same tubes & monitoring the voltage & current (current can be voltage drop across a known resistor) and seeing what happens.

Marc


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Tue 02, 2019 3:14 pm 
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I plan to finish my 100% wire check and visual inspection today.

So far I noticed that the DC resistance of the audio output transformer primary is unbalanced (70 ohms on one side and 120 on the other). It will get a good visual inspection or maybe replaced.

At this point, I think my problem is one (or more) of the following:
1. Bad tube that over conducts when hot.
2. Wiring error (either mine or the guy who was in there last)
3. Audio Output Transformer Primary

Joe


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Tue 02, 2019 3:23 pm 
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Not unusual to have an OT with unequal DC resistance on the primary winding halves, depending on how it was wound.

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Wed 03, 2019 12:11 am 
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There are grounds to insulation test the OP transformer & making sure any wire into it is not able to short ISO types especially.

Get the chassis mounting screws and put them in to ensure that they do not hit anything.

A control grid resistor going open will turn a tube into a diode. OP ones have an an attrition rate, more commonly high.

I only use 1Watt resistors on HT as many lower, no longer have a decent voltage rating & can flash over & fail. If a supplier cannot tel me the voltage rating I will not buy.

When heated, an oversize blob of solder can short, proximity of wires falls into lead dress catagory. If a tube terminal is a bit wobbly, I have sleeved the whole terminal, or adjacent one/s if I see a danger of contact.

Its possibly something obvious. Do consider walking away; sometimes the mind gets in a rut & leaving it & doing some thing alien will clear it and you come back with a different approach.

Marc


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Wed 03, 2019 5:57 am 
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Quote:
No matter what I find, I will be considering:
1. Removing 2 of the 6f6's to reduce B+ cathode current

That's a bad idea. The radio used to work fine with all 4 6F6s; it should be able to work fine again. Get it to work as designed, then if you want to modify it go ahead. But if you change things while it has a problem it's just adding the possibility of a new problem. Don't add complications. Make your task as simple as possible.

A shorted primary in the output transformer won't have any effect on the power supply. It will cause bad audio but it won't increase the load. However, a short from the primary to ground would be bad. The same goes for the speaker field coil. It is possible for either of these to be intermittent.

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Wed 03, 2019 11:58 am 
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+1 That circuit would have been well tested in the factory and meddling with it only adds complication.

One reason that I never power an unknown radio (aside from the fact that if its here it probably died, or is about to) until I have inspected it. Alarm bells should always ring if some one has tampered with it. They can do bizarre things, & some I get are quite dangerous, often from the Monkey that tampered with it.

If it has been tampered with: Check everything.

If you do not have an insulation tester, that set has a divider. You could perhaps risk removing the heater tubes & you should be able to control the voltage with the Variac & see where the current is being drawn: The divider is predictable as its a fixed resistance. A lot of them were 25K which should be 10mA @ 250V. (2.5 watt dissipation)

Marc


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Wed 03, 2019 10:00 pm 
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Progress this afternoon. First the wire check and visual inspection is complete and everything is Sat.
Next on the troubleshooting plan was to inspect the Audio Output Transformer and Field Coil. FC looked Sat, but the AOT has melted sometime in the past. The primary measures 71 ohms on one side and 123 ohms on the other side. I thought that odd but since neither side was open I didn't put priority on inspecting the speaker (AOT is on the speaker, see picture). To melt the AOT primary, either C47 or C48 failed in the past (I have since replaced them both) or one or both of the 6F6's shorted plate to cathode or were greatly over-driven. I don't think the AOT failure is a root cause but rather a symptom. I will be testing all the 6F6's again to see if I can find a short on any of them--I'll leave them on the tester for at least 5 minutes to give them time to really warm up.
Meanwhile I will be searching for an AOT replacement (I hope I have one in my stash). The schematic says the VC has an impedance of 11.5 ohms at 400 Hz. My other info says that 6F6's have an output impedance of 10K ohms when in push pull configuration. I'm not sure how Parallel P-P configuration will impact that output impedance, but my guess is it will reduce it in half to 5K ohms. So I will be looking for an AOT with a turns ratio of either:

1. square root of 10000/11.5 = 30
2. square root of 5000/11.5 = 21

I must say that I have never run across an output tube problem like this potentially is. Seems strange that a bad tube could do all this damage.
Thoughts?
Joe


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Wed 03, 2019 11:02 pm 
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This has been interesting. Definitely in the output cicuit. The easy answer is the pair of plate caps zapped at some point. Not many of these original output transformers are seen on these late '30's RCA consoles. Four output tubes yanking on the primary, what were they thinking? Other companies did that, too. It would be interesting to tack the new one in place and try it all out. Sorry, that transformer l had is long gone (to another collector), so it's not in my stash. Contact me if you want a mongel out of the scrounge box


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Wed 03, 2019 11:21 pm 
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I tested all the tubes again to see if I could make any of them short out after heating for 5 minutes or so. None failed.

After further contemplation, even a failed (shorted) AOT would not have caused my back bias resistor to open and its parallel capacitor to blow since there is no ground connection to the speaker. The only way to short out a section of the B+ (and hence burn the back bias resistor open) would be:

1. for one of those caps (C47 or C48) to short, but I replaced them before the last failure and they are fine now. So they didn't cause the AOT problem or the latest back bias resistor problem.
2. a plate to cathode (ground) short on one of the tubes, but I just tested all the tubes and they are fine (and I beat on them to try and cause a short)
3. a short across one of the tube sockets, but that seems to be a long shot doesn't it? I looked on the topside of all the tube sockets and they all look just fine and dandy. I'll look again on the bottom to see if I can see any arcs but I think I have already done that.

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)

I think I have a universal AOT that will work. The primary DC resistances on it measure 350 ohms on each side compared to the 70 ohm and 120 ohm measurement on the bad one. Need to test it to see which leads give me a 25 to 30 turns ratio.

Anybody else see how B+ could be shorted out at least partially to burn out the back bias resistor?

This one is still a puzzler.

Joe


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