Frustrated about my 500c!

mr_wetland

Well-Known Member
About 3 years ago I picked up a 500c off CL for $50. Naturally I was very excited but being a newbie on a tight budget I left it alone for a while. Little by little I learned more, picked up some coupling caps and a rectifier, then four new EH 7591's. About a year ago I put in the caps and new resistors as suggested here on AK, then I let it sit because I didn't have a variac and didn't want to blow anything up.

Fast forward a year, I've been rebuilding an old guitar amp so I finally made myself a dim bulb tester. Once I got that going I took the Fisher out and slowly brought that up. I started with a 25w bulb, then went 50, 75, 100, 150 and 200. No smoke, no sparks, no burning smell, no magic smoke. All this over a three day period. So I thought I was ready to plug it in. Hooked up a cd player to aux, and turned her on. After 30 seconds three of the four 7591's were red plating. Please, where do I start looking? I'm new to hardcore diagnosis. I am really disappointed...
 
Have you checked to see if your bias supply is rebuilt? If not, whether it works or not must be rebuilt for reliability. When that's done, you need to check how much current in each of the four power tubes are conducting.

I'm curious, did you connect the speakers and making sure your speaker selector is set right (ex: Connect speakers to Speaker A L and R and set speaker selector to "A") BEFORE you power up?
 
Thanks for the quick responses, gents.

Bias voltage too low - how do I check that? Is that a resistor change, or does the 500 have a pot for that?

Bias supply rebuilt - how do I tell? I changed a number of resistors and caps, but since I'm still learning the jargon, dont know where in the circuit that is. Where is the bias supply located?

I do know that a tube amp should never be on without speakers attached and the switch in the right spot, so yes I did that first.

I know there are a number of AK'ers out there that know the 500c, and I know I can count on you guys to get me through this.
 
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500c is a great receiver. I used to own one as a teenager 25 years ago and bought one six years ago and am listening to it right now with the dreaded JJ power tubes(I know I know but I like it). :D I put alot of work in it. Not completely done because the tuner needs some work, otherwise works.

Let's start here: You mentioned >3< of your 4 output tubes were red plating. Did you connect your speakers BEFORE you powered it up?
 
Yes from the time I put the first 25 watt bulb in and turned it on there were speakers attached. I never test things without speakers.

The fourth tube was likely to turn red too if I left it on. But I didn't want to kill my expensive tubes so I shut her right down.
 
Have you changed the selenium rectifier to a bridge rectifier?

If the selenium rectifier has not been changed, it must be. It's bolted to the sidewall under the chassis near where the AC cord comes in. An original will probably say Siemens on it. If it's a small black square one, it's been changed.

If it has been changed, check the wiring. There are four leads. The two DC ones must be correct. Without looking at the schematic, I believe the plus goes to a 15 ohm bar resistor to the chassis and the - goes to pin 6 of the output tubes. The two AC ones just have to be connected to the two AC wires, which is which doesn't matter.

To check the bias supply, go back to a dim bulb that's enough to get it running. Check the voltage at pin 6 of any of the four output tubes. It should be negative. Ideal at full voltage is -17Vdc to -22Vdc. The closer to 0, the harder you're driving the output tubes. So, -17 drives the tubes harder than -21.
 
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Don't quote me but when I was doing research on restoring my 800C (same as 500C but with AM), there was a resistor change specific to running new production 7591's.
 
Maybe you should post some decent pictures of under the hood and describe which caps you changed and anything else you did. That might give someone here some idea of how to procede.
 
Sam; That resistor change(330K to 220 or 200K) was in conjunction with changing the coupling caps from .047 to .1uf to keep the circuit timing more correct than it would be without the resistor change. (Dave Gillespie wrote a good explanation on it in one of the 800C threads recently). In the Bias Circuit from the Selenium rectifier, there was 2 or 3 different resistor setup's, (all accomplishing the same thing), which could be replaced with a 10K pot to allow some adjustability. Remove C-86, R-131; Replace R-132 with a 10K pot. Further adjustability can be made with an IBAM (Individual BIAS Adjustment Modification) as outlined in more than a few 800-C and 400 overhauls.

Larry
 
Sam; That resistor change(330K to 220 or 200K) was in conjunction with changing the coupling caps from .047 to .1uf to keep the circuit timing more correct than it would be without the resistor change. (Dave Gillespie wrote a good explanation on it in one of the 800C threads recently). In the Bias Circuit from the Selenium rectifier, there was 2 or 3 different resistor setup's, (all accomplishing the same thing), which could be replaced with a 10K pot to allow some adjustability. Remove C-86, R-131; Replace R-132 with a 10K pot. Further adjustability can be made with an IBAM (Individual BIAS Adjustment Modification) as outlined in more than a few 800-C and 400 overhauls.

Larry

Larry, thanks for the tip. I pasted your post in my ticker file for when I get to my pair of 800C's.:yes:

Apologies to the OP for slight detour.
 
No problem at all about the detour, all stuff I should know. It seems like those of us who are going to own vintage stuff and use it on a regular basis had better learn to keep them running on our own. Good help is getting harder and harder to find.

I need to double check, and I will before I turn it on again, but I think what I did was the .1 instead of .047 and 220k instead of 330k. I also changed the selenium rectifier to a rat shack bridge rectifier that was recommended in the Stereophile article. Can not however guarantee that I did that right, although I thought I was careful. I will also double check that and see what is connected to where.

When I got to the 200 watt bulb on my tester, the output tubes were glowing with that pretty blue glow that they have, but I was no where near red-plating. That didn't happen until full voltage. Is that in any way an indicator that the rectifier is in right but something else is wrong? Is there a way of measuring how much voltage was getting through with the 200w bulb, and what the bias voltage should be at that point? I'm assuming that if you are not at full voltage to teh unit, you are not going to get the full "-17v to -21v" at pin 6. I'm checking pin 6 for this, correct? One probe goes to that and the other to ground on the chassis? I'm glad you suggested that I go back to the tester and not try getting the bias at full voltage, because "she who must be obeyed" has a very strict budget these days for hobbies, and I can not afford a new quad of EH 7591's at this time.

I will also post some pics as soon as I get a few that look helpful.
 
"I changed a number of resistors and caps." doesn't tell us what resistors and caps, and that's critically important before we can help. We need to know what you did and didn't do yet.

If you're not comfortable saying "I changed R-xxx", or "I changed the bias supply cap", then you'll need to post some well-focused pics of the underside of the chassis for us.
 
I thought I was a little more specific in my last response, but maybe not. I changed the coupling caps and the resistors that go with them (that's the .1 caps and the 220k resistors). I also changed four more caps that you can see to the right of the orange drops in the one photo, and three caps over by the rectifier (second photo). I also changed out the two electrolytics that used to be in that cardboard can. And I removed the selenium rectifier and replaced that. I admit to not knowing the technical names for these caps, would be happy to be educated in that regard.
 

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That power resistor (green) in the middle of the photo looks like it has gotten pretty darn hot...are you sure your voltages are correct?

-D
 
I am not at all sure my voltages are right, that is why I keep asking about the best way to check them. I posted some pictures and can post some more if they would be helpful. That resistor actually looks okay in real life, but this discussion started because I have red plating output tubes and don't know how best to diagnose that. So I'm sure that my voltages are not correct.
 
The power resistor does normally get hot. If that resistor blows, it'll kill all the voltages AFTER that. I don't think it's the result of redplating.

When you replaced the rectifier bridge, did you make sure the rectifier is wired +(POS) to the 15 ohm resistor connected to ground? Wiring -(NEG) to 15ohm resistor connected to ground will guarantee redplating. Also in your 2nd picture, there's a brown can. That is part of the bias circuit. I would suggest to replace that with two 1000uf 35v caps.
 
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The tubes are redplating because they're passing too much current. There are ways to measure this.

The best way, a direct measurement, is to install 10 ohm resistors in place of the short piece of wire that connects pin 5, the cathode, to ground. If you have not done this you might consider it.

The second best way is to measure the bias voltage being applied to the grid of each output tube. It's an indirect measurement, but you can easily do this right now. Be careful, since you will need to measure this voltage with the unit turned on and the bottom cover off. If you touch the wrong spot with a finger you could die. Literally.

With the receiver on and warmed up, (with speakers connected), connect the black, negative lead of your DMM to a spot at chassis ground. Touch the red, positive lead of the DMM to pin 6 of the first output tube. Since this voltage is lower than ground potential, it will read as a negative. It should be between approximately -18 volts DC and -20 volts DC. Anything less negative than -17, such as -16, -15 or worse, is bad. This will allow too much current to flow and the output tubes can fail. Check the voltage at pin 6 of each output tube.

When you replaced the cardboard capacitor with two new electrolytics, (the pic still shows the cardboard cap, is this a "before" pic?), the resistor that controls this -18-20 volt bias voltage is also usually replaced. This is R132, and the stock value is 5.6 k. By adjusting this value you can adjust the bias voltage.
 
The tubes are redplating because they're passing too much current. There are ways to measure this.

The best way, a direct measurement, is to install 10 ohm resistors in place of the short piece of wire that connects pin 5, the cathode, to ground. If you have not done this you might consider it.

The second best way is to measure the bias voltage being applied to the grid of each output tube. It's an indirect measurement, but you can easily do this right now. Be careful, since you will need to measure this voltage with the unit turned on and the bottom cover off. If you touch the wrong spot with a finger you could die. Literally.

With the receiver on and warmed up, (with speakers connected), connect the black, negative lead of your DMM to a spot at chassis ground. Touch the red, positive lead of the DMM to pin 6 of the first output tube. Since this voltage is lower than ground potential, it will read as a negative. It should be between approximately -18 volts DC and -20 volts DC. Anything less negative than -17, such as -16, -15 or worse, is bad. This will allow too much current to flow and the output tubes can fail. Check the voltage at pin 6 of each output tube.

When you replaced the cardboard capacitor with two new electrolytics, (the pic still shows the cardboard cap, is this a "before" pic?), the resistor that controls this -18-20 volt bias voltage is also usually replaced. This is R132, and the stock value is 5.6 k. By adjusting this value you can adjust the bias voltage.

Fisherdude: Would you suggest removing the power tube to test if there's bias voltage present? I hesitated to suggest that because of possible unloaded overvoltage concern. As in the beginning of this thread, OP stated he had it on 30sec and 3 of 4 tubes started redplating.
 
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As a general statement, unless you're really an expert, and I'm not, turning on tube equipment with some tubes removed is high risk. You can't make broad statements as to what is acceptable, since different circuits behave differently with tubes removed.

A quick measurement of the bias voltage to one of the redplating tubes will tell us a lot.

Or, assuming the bias caps have already been replaced, even though the pic doesn't show them, before he turns it on again the op could replace R132 with a smaller value and give that a try.
 
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