Kustom 400, burned 100 ohm resistor

[52 Bill]

Not to muddy the waters but I'm having trouble matching my board marked PC704A and its almost mirror image PC704B with the schematic for PC704 posted here. There are 3 more circuit boards in the amp, PC 303, PC 403 and PC 504. I'm wondering if the PC704 schematic is a combination of PC 704A (or B) and PC 504.

That could be, as I remember it the power amp board you are working on does not have any of the voltage regulator circuitry on it. There are a few differences at the input section for the stereo/mono switching as well.

Did you ever verify the condition of the driver transistors that we were discussing earlier?

Try turning on the amp and see what the light bulb does. Most amps will draw a lot of current at start up and then settle down to idle level, so see if the bulb stays bright or dims down.

Besides the power amp board that you are working on, there are other things that could be wrong with the amp that you haven't looked for or found yet, like the other power amp and the power supply. If the light bulb stays bright, try disconnecting the red and green power supply wires going to the board that you are working on and retry turning the amp on. If the light dims then you know that there is still a problem with the board. If the light stays bright then you have additional problems with the amp that will need to be searched out.

[davohilts]

The driver transistors tested similar in circuit to the driver transistors on the other board, so my thought was that they were good. I tried turning on the power switch and leaving it on and the light bulb did dim to almost off, however a 100 ohm resistor on the other side of the board started to smoke so I shut off the amp. I will replace it with a flame proof resistor for good measure. When I get a minute I'll pull the red and green supplies to the board and see what happens.
I'm tempted to just order replacements for all the transistors on the board including the driver transistors but will hold off until I can see what happens with the other board when I isolate it by disconnecting the board that I'm working from power.

[52 Bill]

If there is a resistor burning, something is drawing too much current. You should check all of the transistors and diodes again to see if you missed something or if something new has died.

One point that I'll warn you about is the diode that is clipped to the power transistor heat sink. It is part of the bias string and is supposed to be thermally tied to the output devices to compensate for temperature changes. The leads break off very easily from flexing so try and treat it gently. I will usually pull it out of the clip and leave it floating until repairs are done. You should also make sure that it is well soldered to the board.

Powering up with the repair board out of circuit will let you know if there are any other problems that will need to be addressed. The rest of the amp should work normally without the bad board.

[davohilts]

I have replaced the diode that is clipped to the board as it had broken a the base as you have warned. I took the board out of the circuit and the light bulb glows for a very short time and then goes out so I'm going to assume that all is well with the other board (thank goodness). I agree that there must be an issue with another (or more) transistor so I'm going to go ahead and pull them one at a time from the circuit and test them. Stay tuned, but it will probably take me a while. I'm getting more confortable doing work on this board so I'm not going to be shy about pulling transistors out of the circuit.

[davohilts]

I have replaced all the driver transistors and gone through the board and replaced several out of tolerance resistors all to no effect. I am still missing something major because I still get a bright light on my current limiter test but when I disconnect the board (704A) I can get the amp up and running without issues. Right now I'm out of options, I must be missing something but can't find it. I did manage to obtain the schematics for the 704A and B power amp boards and will attach them here. The are mirror images of eachother with wires that run to the stereo/mono switch that connect to the 704A board.

[52 Bill]

Now that you have a schematic, can you tell us all of the parts that you have replaced based on the part numbers on the 704A drawing? Like which was the first 100 ohm resistor that burned?

Did you clean up the carbonized sections of the pc board? Is the board correctly grounded to the chassis? I think it uses the mounting screws for the ground connection.

[davohilts]

Yes, I did cleanup as best I could the carbonized sections of the board by scraping off the build up and the board is grounded to the chassis using the machine screws and grounding lugs.
Here is what I initially did:
R733 is the one that burned up, it damaged R736 also and I replaced R737 as it was way off. I replaced it with a 10 watt 1 ohm ceramic type resistor. Testing showed that Q2 and Q706 were bad.
After replacing these components I reinstalled the board and tested it again with the current limiter. The bulb still glowed brightly but then dimmed, I then noticed that R724 was burning up and shut off the amp.
Testing then showed that CR702 which was clipped to the board had a broken lead and that Q3 and Q703 were bad.
I decided to go ahead and order replacement parts and elected to also replace Q704,5,8 and 9 as well.
Q700,701,702,707 tested good so I left them in place.
I replaced Q1-4 with matched pairs of NTE 130 power transistors.
I replaced Q703 (38735) with a NTE 128
I replaced Q704 and 8 (38736) with NTE 128
I replaced Q705 and 9 (38737) with NTE 129
I replaced Q706 (2N3567) with NTE 123
I replaced 1N3754 with NTE5801
I replaced CR703 (FD111) with a 1N4748A

When I reinstall the board and connect it up I get the bright light on the current limiter and shut it off quickly. It dims slightly but stays light. If I disconnect one or the other of the 39.5V supplies the limiter bulb dims to a very low level but both 39.5 supplies connected result in bright limiter glow.

thanks for any help.

[davohilts]

I was unable to get the amp working using some replacement parts that I got to replace the drive transistors Q704,705, 708 and 709 and the diode replacement for CR702 that clips to the heatsink is not metal and may be a problem with the bias control. When I powered up the amp with the replacement parts R724 began to smoke. I pulled out the board again and decided it was time to do some reading on solid state circuits and to educate myself so I can move ahead and fix this sucker.
After a month or so of studying up on solid state circuits I have dove back into the Kustom 400 and am trying to get it up and running. The amp was in really bad shape and here is a report on where I am. I removed the components around the area of the board that originally had burned up prior to my getting this nightmare to work on. I had cleaned the carbon off with solvent as best I could before but this time I pulled out the resistors around the area and took a dremel tool with a wire wheel and removed all the carbon around the area. I then tested all the transistors in place and got conflicking results so decided to pull the components and test them out of circuit.
I have removed the PC704A board and pulled all the transistors and tested them. I have removed the original rca output transistors and have ordered replacements, new NTE130 matched pairs, I have managed to obtain NOS rca transistors for Q704, 705, 708 and 709 complete with heatsinks (not the modified ones I tried to use, I have replaced the blown CR703 (FD111) with a NTE177, the bad Q703 (38735) with NTE128, the bad Q707 (2N3638) with NTE129, a bad cap C707 (47pf) with 2-22pf mica caps in parallel, a burned up R730 (95.3 ohm) with a 97 ohm new resistor, a fried R724 (100 ohm) with a 99.4 ohm new resistor, and have found a source and ordered a NOS replacement for the broken lead diode CR702 (1N3754) my initial replacement was an NTE5801 that was not in a metal enclosesure so I was worried that it might not provide bias control as specified when clipped to the heatsink on the board. I also replaced an out of tollerance R737 (1 ohm 5 watt) with a new one.
I have also gone through the back side of the circuit board and cleaned around the connections to make sure there were no traces that were shorted together.

I have a question concerning the output transistors Q1, 2 and Q3, 4. Am I correct in that these pairs also correspond to the drive transistors in the heat sinks and that Q1&2 should be associated with Q704 and Q708, and Q3,4 with Q705 and Q709?

Another question I have is once I get all my replacement parts installed, what is the best way to power up without the danger of burning up my new parts if something else is wrong?

I have learned a lot so far and appreciate all the advice and experience you guys have shared. I also noticed how in the beginning I was only understanding about half of the suggestions and now look back with embarassment that I didn't follow or understand some key advice.
Stay tuned.
Dave

[Jazz P Bass]

Quote:[I have a question concerning the output transistors Q1, 2 and Q3, 4. Am I correct in that these pairs also correspond to the drive transistors in the heat sinks and that Q1&2 should be associated with Q704 and Q708, and Q3,4 with Q705 and Q709?]
You are correct.

Quote:[Another question I have is once I get all my replacement parts installed, what is the best way to power up without the danger of burning up my new parts if something else is wrong?]
Build yourself a 'Lamp Limiter'.
The schematic has all of the neccessary design voltages marked. (rare indeed).
Your testing goal is to make sure that the voltages are as they should be.
Keep in mind all voltages will be low with the limiter.
If something is still wrong with the amp, it will show up as a full bright lamp limiter & or voltages that are incorrect.

[davohilts]

I have a limiter with a 150 watt lamp, the problem I am experiencing when testing with it is it glows bright, dims to very low and then as I'm taking voltages, the resistor (R724) starts to smoke. I then test the transistors and at least one (Q3) has blown. One thing I did last time was to disconnect the feeds to the output transistors Q1-4 and this allowed me to power up the amp and not have the burning resistor issue. What I want to do is compare the 704b board voltages to the 704a voltages without destroying things. I don't know what actually caused my problem but I suspect it was the driver transistors and the diode which clips to the heatsink which were not new old stock. I took new transistors and soldered them to the old heatsinks and used some heat transfer grease on a silicone diode in the clip.

[Jazz P Bass]

At this point, seeing that you are having difficulties in identifying failed components , let me suggest this:
With the power off & the power supplies drained, use your meter, set it on ohms & measure key points of the schematic & compare each amplifier.
This may help prevent any damage to the working amplifier. (we all know what a slip of the meter leads can do).
Take any & all readings that you want.
This to that. That to this.
Pick any transistor & measure the resistance of the base to emitter, base to collector, collector to emitter.
What you will be looking for is something different from one amplifier to the other one.
This test would have caught Q3 as shorted.

[davohilts]

Thanks! I'll do that prior to firing up the amp with the new parts.

[52 Bill]

I have a limiter with a 150 watt lamp, the problem I am experiencing when testing with it is it glows bright, dims to very low and then as I'm taking voltages, the resistor (R724) starts to smoke. I then test the transistors and at least one (Q3) has blown. One thing I did last time was to disconnect the feeds to the output transistors Q1-4 and this allowed me to power up the amp and not have the burning resistor issue.

I think I had told you before that a resistor burns only because something that it is connected to is drawing too much current. So Q3 shorts and then it starts to burn R724. You have to ask yourself why is Q3 shorting? I can't keep track of all of the parts that you have replaced and frankly I don't want to, because you have taken to the approach of throwing parts at this thing and not finding and fixing the real cause.

At this point I will recommend that you start your thinking from scratch. Look at every part that you have replaced (and those you haven't replaced as well) and test each of them to be certain that you have not damaged them during the testing process. While doing this, be certain that you have installed the correct transistors in the correct locations, PNP where the PNP should be and the NPN where the NPN should be. Also check all of the diode polarities to be certain that they have the cathodes where the cathodes should be, etc. If for example the temperature sensing diode that mounts to the heatsink is wired in backwards it kills all the idle bias voltage.

I don't know what actually caused my problem but I suspect it was the driver transistors and the diode which clips to the heatsink which were not new old stock. I took new transistors and soldered them to the old heatsinks and used some heat transfer grease on a silicone diode in the clip.

At this point do not concern yourself with the details of how the drivers are heatsinked or if the bias diode is mounted to the clip or not. These things do not matter until the amp is running normally and will not cause the destruction of parts that you are experiencing. You should be concerned about how the output transistors are mounted and whether or not the mica insulators are still ok. There cannot be any continuity from the chassis to the transistor cases.

Be certain to not connect any speaker loads to the amp until it is safe to do so, and I personally would try a smaller wattage bulb in the limiter. I use 60 and 100 watt bulbs in mine.

[davohilts]

Thanks for the reply. It is good to know that what I've been obsessing on and worrying about is not the main issue. After the last go round when the output transistor blew and the resistor began to smoke I took a couple of weeks off and read about transistors and testing transistors and have a good grip on how to do so now. I pulled all the transistors and tested them and reinstalled the good ones and have been careful to insure I'm using the correct one in the correct place and that npn goes to npn and pnp to pnp. I will reduce the wattage in my limiter to 60 and start again following the circuit and hopefully by tracing back from Q3 see if I can find what is the root of the problem.
Thanks for all your help.
Dave

[davohilts]

Well, I did start from the beginning and went through the board circuit carefully, everything checks out. Then I checked the power transistors and they all checked out. Then I checked (for the first time!!) continuity from the case to ground and guess what? The mica washer on Q3 was folded back on itself on one end and the transistor was grounding to the chassis! Thanks 52 Bill!! You hit the nail on the head. I don't have matched transistors installed as I await a new matched pair but the amp now powers up without the limiter glowing brightly. I installed a 60 watt bulb as you recommended and now have a stable if not completely working amp.
One side of the stereo preamp works but the side with the most of the effects only hums, and doesn't pass a guitar signal. So now I guess I need to trace the signal from the input to see where I loose it.
Thanks you for helping me solve this mystery with the power board. To say I've learned a lot is a huge understatement.
Dave