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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Tue 18, 2019 4:19 am 
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Only R26 in the candohm was in the circuit. After the failure it still measures 7 ohms.
Joe


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Tue 18, 2019 5:44 am 
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Main computer has failed, this one is like tired snail. The resistor that cooked is, I think the one that takes "B-" to chassis? That would oped circuit the connexion to ground and as noted cause a massive voltage surge. As pointed out I believe it is a shorted electrolytic.

Marc


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Tue 18, 2019 6:17 am 
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One of the common failure modes of candohm is that thin piece of paper deteriorated where the inner resistive wire is no longer insulated from the outer metal case; this could be either a direct short or intermittent. Even if some of the terminals on the candom is disconnected, any fault would affect all sections (because they are all still internally connected).

Only way to know for sure the root cause is to physically find it. I think most of the more likely causes have already been mentioned.


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Tue 18, 2019 6:02 pm 
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Marc,
The most logical root cause is, as you say, R28 (the B- return) failed open (because I mistakenly installed an under-rated resistor) leading to a voltage transient across C42, which caused C42 to short, providing a short circuit path across the HV winding. BUT, if that happened then excessive current would have flowed through C42 which I assume would have severely damaged it. But, C42 looks physically fine. Given the damage to the transformer this was not a transient event as there was excessive heat and smoke coming from the transformer for several minutes before I found it. So I am not convinced yet that this is the root cause. I guess I will need to do a post-mortem on C42 too.
Joe


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Wed 19, 2019 9:40 am 
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I would not call it a transient. It is actually the same sort of surge as I would expect from a startup only worse. The Candohm is a load and between it and the tubes providing virtually no load the 450V caps are going to cop around 600V. (see diagram attached)

As before I do not believe they can handle that and both caps still with supply are in peril. It to me, is quite logical that the cap C42, if shorted was the ultimate demise of the transformer.

The only way to prove that the cap is good, is to check it.

Marc


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Thu 20, 2019 6:59 pm 
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Here are the results thus far:

1. I tested all the electrolytic caps with my NRI capacitor tester. No leakage on any of them at 450 volts. Capacity ratings measured within specs. No visible physical damage to any of them either. I went ahead and replaced the first filter cap C42.
2. I tested all the tubes and all very strong with no shorts except for one of the 5U4's. Filament was open and there was loose debris inside.

This was a catastrophic failure of the power transformer. See picture. Maybe the 5U4 filament shorted (before it opened up) somehow causing the HF transformer to be shorted every other cycle?? OR maybe just an internal fault in the transformer???

I plan to replace all the candohm resistors with power resistors of the correct wattage. But still concerned that I don't have a definite root cause.

Thoughts?
Joe


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Fri 21, 2019 12:59 am 
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Good, we have eliminated one possible cause. I have never seen comment on 5U4 but 5Y3 has only one way it can be run side ways. With it because of the design & type of the filament. it can sag and hit, or flash over to the plate: Similar if the filament broke in the 5U4.

It may be too late to check the filament wires of that tube, but I have had an instance of a filament winding being cut through and immediately flashing over as soon as DC was developed. The one I had recently & mentioned was a chain reaction caused by a 6GW8.

It may be difficult to prove which winding went first. Unless there is earth leakage or an overload, primary side GFCI,s; Circuit Breakers & Fuses will be of no use. Even that it may not help here, like some lighting circuits here, where there is only a low current draw, the breakers are down rated. Eg. 3A rather than 10A. As the bench PSU can be placed in peril it has fuses on the HT and thermal circuit breakers on the LV. The rectifier is a slow warm up Russian 5Y3, so there is no massive startup surge from the caps as that will take out the fuse, as it is run close to the limit of the transformer.

Marc


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Fri 28, 2019 11:55 pm 
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Well, I am back in business, but not without a lot of effort. First, Craig was kind enough to supply a spare transformer. I rebuilt the chassis cable due to missing insulation on a number of conductors. Wanted to make sure that the previous problem was not due to a shorted wire in the cable.
I rebuilt the candohm completely with power resistors. All resistances are now well within specs and the power ratings of each are at least twice what they see in operation. I then replaced the first filter capacitor and checked the other three with my capacitor tester at rated voltage. All tested sat.

I powered it up on the variac and in a few short minutes the 100 uf cap blew. It had a low voltage rating and yes it was connected correctly (+ to ground). I guess it was stressed from the first event. I then replaced all three electrolytic caps even though they had all tested okay previously.

I powered it up again but after a few minutes the audio faded away. Sounded to me like a tube problem. This set has four 6F6's on the output. I pulled a NOS one and began substituting and then powering up to see if the audio faded. On the third 6F6 replacement the problem went away. So I am back in business--but still a bit concerned over not finding a positive root cause. I will be playing the set several hours before I declare victory.

So here is what I found:
1. When I replaced R28 I didn't do my homework on wattage so it burn open after playing the set about an hour.
2. When R28 failed it may have created a voltage spike due to the interruption of the transformer output which in turn stressed the filter caps. Particularly the 100uf which later blew as noted above.
3. None of the other filter caps failed due to the voltage spike.
4. I did find one of the 5U4s with an open filament. Perhaps the metal in the filament shorted out the tube causing the transformer meltdown.
5. Perhaps the one bad 6F6 could have also contributed.

Your comments are welcome on what you think happened.
Joe


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Sat 29, 2019 1:43 am 
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It has gone in rather a predictable way & similar to the one here with the 6GW8. A power tube going short can draw excessive current:That is irrefutable and can indeed start a chain reaction & smoke. You can only use a slow blow fuse in series with the CT with a fast heating rectifier tube, or silicon diode etc as the surge into the caps will take a fast blow one out.

One of the reasons I do not up-rate back bias resistors is that I prefer to see them burn. If the capacitors are rated to peak voltage attained, that means that they should hold & the transformer not burn. A lot of transformers here have thermal fuses built in. Naturally cunning says use non reset types. It is an idea to monitor voltage on start-up; The rise & fall is predictable. If it rises & then suddenly falls back, to perhaps near zero. One hits the kill switch.

Electrolytics can be deceiving. I have had them fail before rated voltage and it is important to watch their current. The old Lafayette tester here quotes 10mA as an end point? Modern ones do not consume anywhere near that {0.01CV) is I believe the formula, where "C" is micro farad and V applied voltage. A series 1K resistor gives 1mA for every volt (measured across it). I have actually watched one as it was stepped up, get to a certain voltage and then suddenly start conduction at a hyperbolic rate, so test voltage is important. This is reason as stated before, where I am suspicious of a HV fault in "B": I use the reformer to step up the voltage as it is designed to handle a failure.

I believe this set was an education, because it caused you to have to analyze various scenarios, to find if there was a "smoking gun" that caused this chain reaction, rather than speculate that it was just the transformer alone and end up frying another.

Experience gained: Result positive.

Marc


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Sat 29, 2019 6:35 pm 
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this may be slightly off topic of the original problem, but here goes.

I strongly recommend using 500 volt capacitors in ANY radio or amplifier that uses Direct Heated Rectifier.

I have witnessed a B+ voltage as low as 325 volts surge near and sometimes over 500 volts as the radio powers up. radios/amps that have a B+ of 350-375 or more are guaranteed to surge to 500 or more volts.

new capacitors are not like the old Sprague Twist-Lock Lytics that have a working voltage and a surge voltage rating on them such as 450 working / 525 surge.

I do not like to see any new electrolytic capacitor see more than 75% of its working voltage, even the brands I use (nichicon, Panasonic, Illinois, cornell), much less some no name brand capacitor.

I will most likely get beat for the above statements ; that is fine.

here is a case where a 500 volt capacitor was still not enough and the use of a CL-90 was imperative.

https://www.antiqueradios.com/forums/vi ... 6&t=259314

https://www.antiqueradios.com/forums/vi ... 6&t=282134

steve

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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Sun 30, 2019 12:30 am 
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+1. I do mention (and have in this post: So comment above is relevant) add nauseum that modern caps are not like the old.

Agreed: It is the surge voltage on startup with non-sleeved filament tube rectifiers & rectifier diodes, feeding heater tubes, that is liable to do the most damage: There is as, I noted with my PSU, an initial surge of current into the electrolytic caps, followed by an open circuit, so basically a period of no significant current flow, causing pretty much an EMF near double the sets working voltage and all B capacitors cop it until such time as the heaters become effective.

Many forget that for a 250V rail feeding tubes, there is often 300V plus on the cathode, which is why the first cap on a "choke input filter" is the most likely to fail, especially if under rated.

I too will not use lower than 500V with a 5Y3 / *80 with a 250V rail feeding heater tubes even if it has a voltage divider across "B". I do note (Think its Mouser?) some new Ecaps do, like the old have a SV / PV rating on them.

Heater rectifier's & sleeved types act as current limiters as they slowly develop voltage at a rate pretty much in syntony with the heaters; so there is loading avoiding a surge.

Marc


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 Post subject: Re: I blew up my RCA K-130 Blew a cap again!
PostPosted: Jun Sun 30, 2019 1:10 am 
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Yesterday, I powered it up on my bench in the basement. Played great and I checked all voltages and they were all well within specs. Today, I took the chasses apart (the power supply with 2 5u4s has its own chassis and is connected via 8 pin cable.) I took it to the garage for reassembly into the cabinet.

I reconnected the power supply chassis to the tuner chassis and powered it up again to test before putting back into the cabinet. It played fine for 10 minutes or so. I shut it off and we took a day trip antiquing. When we got home this afternoon, I decided to turn it on and was going to let it play an hour or so to make sure all was good.

As soon as I turned it on, I thought I heard a slight "sizzle" so I turned it off. I then thought I was being paranoid from my past experiences with this set and turned it back on. Within 5 seconds it popped like a fire cracker and smoke rolled from under the tuner chassis. Inspection revealed that capacitor C43 (100uf @ 40 volts) had catastrophically failed. Obviously, the cap is experiencing some high voltage from somewhere. I am beginning to suspect that the connector, lug or wiring from the power supply is intermittently applying full B+ to the cap, but that would short out the power transformer.

I will once again do some troubleshooting tomorrow to see if I can find a flash over or short in the cable, plug or wiring. Perhaps it could be a failure of the first filter cap for the reasons you have stated earlier on inadequate voltage ratings of these newer caps. Meanwhile, what do you guys think?

PS I have a very interested buyer in this set, but if I can't conclusively find the problem I will part the set out rather than sell to him.


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jun Sun 30, 2019 2:41 am 
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C43 is on the the back bias:

!. It should be positive to chassis negative to CT. That has been mentioned before.

2. The back bias went open. So what fried the resistor, or why did it come unstuck.

So is there a shorted tube? Possibly one that draws a lot of current like an OP tube. Is there a solder blob, other debris, or bad wire somewhere.

Don't discount a heater cathode short.

Marc


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jun Sun 30, 2019 3:32 pm 
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Dutch Rabbit wrote:
this may be slightly off topic of the original problem, but here goes.

steve


I totally agree with Steve's comments. I would like to include my own RANT about this situation with power transformers and 80 to 90 year old radios. These comments I am about to make come from my own experience of 40+ years during which I have restored in excess of 3000 of these old radios.

My first BEEF. The "TODAY" line voltage to most all of our homes is in excess of 120 volts. Next part these are 80 to 90 year old power transformers that in many cases have already been stressed and it doesn't take much ADDED voltage and current draw to SMOKE THEM. This old RCA radio will run JUST FINE on 105 volts. Yes it might not put out quite as many WATTS of audio as it would with 120-125v. It might have a couple dB less sensitivity with less line voltage. But it danged sure will last another 50 years or so with just a little bit of thought to PRESERVING the radio.
I have done at least a dozen RCA 816K and 813K radios. ALL of them the transformers ran TOO HOT on TODAY line voltage. On 816K radios none ever left my bench without a built in Bucking transformer being installed under the chassis. The last several of that model I have done I also installed a 220 volt Muffin Fan under the power transformers. A 220 volt muffin fan running on 120 volts moves enough air to assist in keeping the transformer cool but not enough noise that most anyone would ever notice it had a fan in it unless they looked under the chassis. This K-130 chassis would have for sure had a bucking transformer ahead of this power transformer. The OP's first posting said B+ voltage was +40 volts above SPEC but that should be OK. Well sorry it wasn't OK. Maybe that transformer had already been stressed to the point that it was ready to let out the magic smoke? Another thing that can be done with this chassis is to get rid of FOUR 6F6 tubes. They draw 0.7 amps per tube of filament current on those 90 year old power transformers. Yes a little bias adjusting might be needed to run 6V6 tubes but think about the reduced filament current. 4 x .7 = 2.8A vs 4 x .45 = 1.8A ... you saved an AMP of current draw on the 6.3v winding equal LESS HEAT in the transformer. Heat is the killer of these power transformers. So like Steve go ahead and blast me if you want to do so but the above is my opinion and my experience.

A good investment for all of us. $20 gets you a handy dandy infrared temp meter. https://www.harborfreight.com/infrared-thermometer-93984.html

John k9uwa

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jun Sun 30, 2019 5:01 pm 
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+1 to what John said.......


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jun Sun 30, 2019 8:22 pm 
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Thanks guys for all your comments. I now have a replacement transformer. I replaced all the caps and the three main filter caps (C42 40uf, C41 30uf, and C40 20 uf) were all rated at 450 Volts which is what was in it before. I really do accept the fact that the first cap C42 should have a higher voltage rating given this set's vulnerability to surge over 450V. I have a new 40 uf @ 600V that I will be installing once I can determine a solid root cause for the latest problem that occurred.

In addition to the new caps mentioned above, the other initial conditions prior to the latest failure are as follows:

1. The candohm was replaced with power resistors R24 (3K) and R25 (2.5K) are 25 Watt, R26 (7 ohm) and R28 (133 ohm) are both 10 Watt)
2. The back bias capacitor C43 (100 uf) was replaced with a 160V capacitor. It was connected correctly, i.e. + to ground and - to the transformer center tap.
3. Speaker FC resistance was per spec.

I brought the set up on a variac slowly and checked voltages along the way. Raised the variac up to 120 VAC. All voltages in the set were in spec, the radio played just fine, and the transformer was actually cool to the touch. (I was doing this in the basement where it is cool). I tested the motorized tuning to verify the 24v winding worked in the new transformer and the tuning worked just fine.

After a few minutes I noticed that the audio started to fade. Suspecting a 6F6 output tube, I began substituting each 6F6 with a NOS metal one. On the third substitution the fading stopped and I was satisfied that I had fixed the audio fading problem. The set played for probably 15 minutes. The transformer stayed cool. The temps on the power resistors were high of course per my temp detector (200-250 degrees).

Feeling I was in good shape, I took the radio apart (separated the speaker, power chassis and tuner) and moved everything upstairs to my bench in the garage to get ready to reinstall in the cabinet. Plugged it in after re-cabling it to make sure it was still okay and it played fine. Played maybe 5 or 10 minutes. The wife and I went out for dinner and when I came back I tried the radio again. When I first turned it on, I immediately heard a sizzle (I thought) but then thought I was just being paranoid so I turned it back on. In 5 seconds or less one of the caps blew like a fire cracker and the smoke rolled. This morning I checked out to see what failed and found:
1. Back bias resistor R28 (133 ohm) is now open. I couldn't see any physical damage to it though.
2. Back bias capacitor C43 (100uf @ 160 volts) catastrophically failed.

I am looking for a high confidence root cause. I don't see how a failure of the first filter cap would cause these back bias resistors to be over-voltaged leading to their failure. I do see how a shorted tube plate to ground could do it, but not sure that would be the root cause.

So I am looking for someone who can describe a logical root cause for this outcome. Thoughts?

Thanks,
Joe


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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jun Sun 30, 2019 10:53 pm 
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When I encounter a problem like this, I look for something I may have inadvertently done during the initial restoration and rule that out first.
Nothing exotic, just look for things like wiring errors, a solder splatter that is shorting out a point, or a terminal somewhere that is grounding out on something. If it is intermittent, that makes it a little harder to nail down.

Unfortunately, it usually means you have to visually review just about everything.

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Last edited by processhead on Jul Mon 01, 2019 1:54 pm, edited 1 time in total.

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jun Sun 30, 2019 11:53 pm 
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The point I would make is that it is important whether it is new or old is do a post mortem on the basis is of did it just fail, or did something cause it to fail?

Marc


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 Post subject: Re: I blew up my RCA K-130
PostPosted: Jul Mon 01, 2019 2:44 am 
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k9uwa wrote:
Dutch Rabbit wrote:
this may be slightly off topic of the original problem, but here goes.

steve
. So like Steve go ahead and blast me if you want to do so but the above is my opinion and my experience. John k9uwa


amen, brother

that's why my signature says what it says...

steve

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 Post subject: Re: I blew up my RCA K-130 Blew another cap!
PostPosted: Jul Mon 01, 2019 2:44 am 
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A few comments that haven't been touched on yet.

The rectifiers in this set have the two diodes in each connected in parallel. If one tube failed with an open filament, it would cause the transformer to be operating into a half-wave rectifier. There would be DC passing through the secondary which would tend to saturate the core. This would cause the transformer to run hotter than normal. Whether this would be enough to cause the old transformer to fail is undecided.

The data sheet for the 5U4, http://frank.yueksel.org/sheets/137/5/5U4G.pdf, says that they are sensitive to horizontal operation. Operating one in the wrong orientation could cause a filament to plate short.

The voltage surge that occurs at turn-on isn't just applied to the input filter capacitor but goes through the whole set to every capacitor in the B+ circuit. They all have to be rated to handle it.

This set has a capacitor input filter, not a choke input filter. But it doesn't make any difference in regards to the turn-on surge.

Starting the set on a variac may not reveal the true voltages that occur during start-up. Nor will measuring the voltages while it is operating. You have to measure the voltage while it is warming up at full line voltage.

Testing the electrolytic capacitors may not find anything. They sometimes self-heal when voltage is removed and will test good even though they are compromised.

I have, on a few occasions, seen output tubes with screen to control grid shorts. This will cause them to draw WAY more than their normal current. Sometimes this condition is intermittent. If this happened while the set was operating, it would cause a loud pop in the speaker.

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