Home » VintageKustom.com » Repairing Kustom Amps » K150-6 pumps out DC voltage when powered on.
Re: K150-6 pumps out DC voltage when powered on. [message #27708 is a reply to message #23707] Fri, 06 March 2020 18:59 Go to previous messageGo to next message
corybrown is currently offline  corybrown
Messages: 51
Registered: August 2015
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Well the new Q4 came in today. Replaced it and sadly it did not fix the problem. I now have -20V instead of -27V so I guess we are going in the right direction.

The only other component that had funny measurements was Q12 but I didn't order any of those. I did unsolder it and remove it from the board and tested it with DMM and it tests good.

So where to next?
Re: K150-6 pumps out DC voltage when powered on. [message #27709 is a reply to message #23707] Sat, 07 March 2020 06:18 Go to previous messageGo to next message
stevem is currently offline  stevem
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I am not sure, I will go back and review the voltage measurements you posted.
Re: K150-6 pumps out DC voltage when powered on. [message #27710 is a reply to message #23707] Sat, 07 March 2020 21:25 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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Ok. I went through and took a measurement at every point on the board. It's probably overkill but I've got to find this thing.

Component In Out
R1 47K 169mV .6V
R2 47K 193mV .6V
R3 82.5K 1% .6V 5.7V
R4 221K 1% .6V -8.5V
R5 2200 5.7V 8.5V
R6 4700 5.7V 5.7V
R7 33K 42mV 0V
R8 39K 42.85V 20.67V
R9 39K 20.68V .6V
R10 4700 -28V -34V
R11 3900 -34V -34V
R12 1820 1% -1.9V -2.2V
R13 33K -1.9V -20V
R14 2200 42V 14V
R15 2700 14V -19V
R16 97.6K 1% -2.3V -20.9V
R17 4700 -21V -33V
R18 470 -28V -21V
R19 470 -33V -34V
R20 470 -34V -43V
R21 1K -20V -21V
R22 100 -20V -21V
R23 1K -20V -21V
R24 100 -42V -42V
R25 820 5% -21V -21V
R26 .5 5W -21V -21V
R27 267 1% -21V -21V
R28 .5 5W -21V -21V
R29 267 1% -21V -21V
R30 820 5% -21V -21V
R31 100K -21V 109mV
R32 18K 43V 9.7V
R33 1200 7.6V 8V
R34 4700 8.5V 7.6V
R35 10K 7.6V 0V
R36 1K 9.1V 8.6V
R37 4.7 8.6V 8.5V
R38 10K 1% 8.5V -.6V
R39 8.25K 1% -8.5V -.6V
R40 18K -42V -9V
R41 1K -42V -42V
R42 4.7 -8.5V -8.5V
R43 510 109mV 109mV
C1 0.005 5.7V 0V
C2 0.33 5.7V 42mV
C3 1.0 35V 20.6V 0V
C4 33 15V -1.9V 0V
C5 27 35V 14V -21V
C6 27 35V -21V -33V
C7 47 PF -28V -27V
C8 0.005 -34V 0V
C9 47 PF 42V 9.7V
C10 47 PF 9.7V 7.6V
C11 33 15V 7.6V 0V
C12 33 15V 8.5V 0V
C13 47 PF -9.2V -.6V
C14 47 PF -9.2V -42V
C15 10 15V -.6V -8.5V
C16 33 15V -8.5V 0V
CR1 1N3193 -19V -20V
CR2 1N3193 -20V -22V
CR3 1N3754 -22V -21V
CR4 FD111 -19V -113mV
CR5 FD111 -21V -21V
Q1 PET8002 B .6V C5.7V E 0V
Q2 2N4249 B 42mV C-28V E .6V
Q3 2N4249 B -2.3V C -34V E .6V
Q4 38735 B -28V C -28V E -34V
Q5 38736 B-19V C 42V E -20V
Q6 38737 B -21V C -42V -21V
Q7 2N3567 B -21V C -113mV -21V
Q8 2N3638 B -21V C -21V E -21V
Q9 PET8002 B 7.6V C 9.2V E 7.5V
Q10 CR10 SZ51218 7V 0V
Q11 2N3567 B -9.7V C 43V E 9.1V
Q12 36892 B 9.1V C 43V E 8.6V
Q13 PET8002 B 8.6V C 9.7V E 8.5V
Q14 2N3638 B .6V C 0V E -9.2V
Q15 2N3638 B -9.2V C -42V E -8.6V
Q16 36892 B -42V C -8.6V E -42V
Q17 2N3638 B -8.6V C -9.2V E -8.5V
Q18 36892 B -20V C 42V E -20V
Q19 36892 B -21V C -42V E -43V
Re: K150-6 pumps out DC voltage when powered on. [message #27711 is a reply to message #23707] Sun, 08 March 2020 00:09 Go to previous messageGo to next message
chicagobill
Messages: 1952
Registered: April 2003
Senior Member
Q8 voltages are off, have you tested it out of circuit?

Q12 is part of the low voltage power supply and has nothing to do with the power amp problem.
Re: K150-6 pumps out DC voltage when powered on. [message #27714 is a reply to message #23707] Sun, 08 March 2020 16:28 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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I took Q8 out of circuit and it tests fine. I also read that Q8 was compatible with the NTE 129 that I just bought. I had an extra so I swapped it in and got the same measurements as with the original Q8 in place.
Re: K150-6 pumps out DC voltage when powered on. [message #27715 is a reply to message #23707] Sun, 08 March 2020 19:43 Go to previous messageGo to next message
chicagobill
Messages: 1952
Registered: April 2003
Senior Member
I thought that you replaced Q4 with an NTE128.

I'd try using the NTE129 to replace Q6 and see what happens.
Re: K150-6 pumps out DC voltage when powered on. [message #27716 is a reply to message #23707] Sun, 08 March 2020 21:41 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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Well that's a good call. I've been out there working on it this evening and realized that Q4 was an NPN and I had replaced it with a PNP. That's on me. Stevem recommended nte128 and like an idiot I bought nte129. My old Q4 tested fine now that I have it out of the board so I put it back in and checked all of my measurements again.

Measurements that make me scratch my head are the collector of Q4. The schematic shows -3.7 at Q4s collector but I'm getting -33V. There is a node at that point between Q4 collector, C7 and R18. Should it be -33V on both sides of C7 or could it be a typo on the schematic?

The other one is CR4 where it goes into the collector of Q7. All of the other diodes have the same voltage on both sides but CR4 has -25V on one side and -256mV on the other side and that -256mV is going into the collector of Q7. Should it be like that?

I'll try the Q6 swap tomorrow evening.

Here are my measurements after putting Q4 back

Component In Out
R1 47K 169mV .6V
R2 47K 193mV .6V
R3 82.5K 1% .6V 5.7V
R4 221K 1% .6V -8.5V
R5 2200 5.7V 8.5V
R6 4700 5.7V 5.6V
R7 33K 38mV 0V
R8 39K 42.85V 20.67V
R9 39K 20.68V .6V
R10 4700 -33V -33V
R11 3900 -34V -34V
R12 1820 1% -1.9V -2.2V
R13 33K -1.9V -26V
R14 2200 42V 11.4V
R15 2700 11.4V -19V
R16 97.6K 1% -2.3V -26V
R17 4700 -26V -33V
R18 470 -33V -27V
R19 470 -33V -33V
R20 470 -34V -42V
R21 1K -26V -26V
R22 100 -25V -26V
R23 1K -25V -26V
R24 100 -42V -42V
R25 820 5% -26V -26V
R26 .5 5W -26V -26V
R27 267 1% -26V -26V
R28 .5 5W -26V -26V
R29 267 1% -26V -26V
R30 820 5% -26V -26V
R31 100K -26V 109mV
R32 18K 43V 9.7V
R33 1200 7.6V 8V
R34 4700 8.5V 7.6V
R35 10K 7.6V 0V
R36 1K 9.1V 8.6V
R37 4.7 8.6V 8.5V
R38 10K 1% 8.5V -.6V
R39 8.25K 1% -8.5V -.6V
R40 18K -42V -9V
R41 1K -42V -42V
R42 4.7 -8.5V -8.5V
R43 510 109mV 109mV
C1 0.005 5.7V 0V
C2 0.33 5.7V 38mV
C3 1.0 35V 20.6V 0V
C4 33 15V -1.9V 0V
C5 27 35V 11.4V -26V
C6 27 35V -26V -33V
C7 47 PF -33V -33V
C8 0.005 -33V 0V
C9 47 PF 42V 9.7V
C10 47 PF 9.7V 7V
C11 33 15V 7V 0V
C12 33 15V 8.5V 0V
C13 47 PF -9.2V -.6V
C14 47 PF -9.2V -42V
C15 10 15V -.6V -8.5V
C16 33 15V -8.5V 0V
CR1 1N3193 -25V -25V
CR2 1N3193 -25V -26V
CR3 1N3754 -26V -26V
CR4 FD111 -25V -256mV
CR5 FD111 -26V -27V
Q1 PET8002 B .6V C5.7V E 0V
Q2 2N4249 B 38mV C-33V E .6V
Q3 2N4249 B -2.3V C -34V E .6V
Q4 38735 B -33V C -33V E -34V
Q5 38736 B-25V C 42V E -25V
Q6 38737 B -26V C -42V -26V
Q7 2N3567 B -26V C -256mV -26V
Q8 2N3638 B -26V C -26V E -26V
Q9 PET8002 B 7.6V C 9.2V E 7.5V
Q10 CR10 SZ51218 7V 0V
Q11 2N3567 B -9.7V C 43V E 9.1V
Q12 36892 B 9.1V C 43V E 8.6V
Q13 PET8002 B 8.6V C 9.7V E 8.5V
Q14 2N3638 B .6V C 0V E -9.2V
Q15 2N3638 B -9.2V C -42V E -8.6V
Q16 36892 B -42V C -8.6V E -42V
Q17 2N3638 B -8.6V C -9.2V E -8.5V
Q18 36892 B -20V C 42V E -20V
Q19 36892 B -21V C -42V E -43V
Re: K150-6 pumps out DC voltage when powered on. [message #27717 is a reply to message #23707] Sun, 08 March 2020 21:56 Go to previous messageGo to next message
corybrown is currently offline  corybrown
Messages: 51
Registered: August 2015
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Went ahead and pulled Q6 from the board. It tested fine. I didn't try swapping with the nte129 that I have because it doesn't have the heat sink.

Thanks
Re: K150-6 pumps out DC voltage when powered on. [message #27718 is a reply to message #23707] Mon, 09 March 2020 05:55 Go to previous messageGo to next message
stevem is currently offline  stevem
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How are you testing these Transistors ?
If you are using just a ohm meter test then that does not rule out that they maybe braking down when powered up.
I would try replacing CR5 with a common 1N1007 or even a 1N1004 will cut it.
Re: K150-6 pumps out DC voltage when powered on. [message #27720 is a reply to message #23707] Mon, 09 March 2020 08:38 Go to previous messageGo to next message
corybrown is currently offline  corybrown
Messages: 51
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Testing the transistors with the DMM. Diode setting and testing B to E, B to C, E to B, C to B and C to E.

But I see what you are saying by breaking down once powered up.

Can Q5 and Q6 be removed from the heatsink and the heatsink then be re-used?

[Updated on: Mon, 09 March 2020 09:04]

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Re: K150-6 pumps out DC voltage when powered on. [message #27721 is a reply to message #23707] Mon, 09 March 2020 11:59 Go to previous messageGo to next message
corybrown is currently offline  corybrown
Messages: 51
Registered: August 2015
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Just ordered some more parts.

I have NTE128 to replace Q5.
NTE129 to replace Q6
1N1007 to replace CR5.

I'll let you know how it goes.

Thanks
Re: K150-6 pumps out DC voltage when powered on. [message #27722 is a reply to message #23707] Mon, 09 March 2020 12:54 Go to previous messageGo to next message
chicagobill
Messages: 1952
Registered: April 2003
Senior Member
You may have figured the problem out already. The voltages around Q4 are off, especially at the collector. The collector should have about -3 volts there and not the -33 volts that you have.

That would lead me to believe that the transistor is breaking down when hit with the full voltage of the circuit or that C7 is shorted. As you don't have the correct replacement for Q4, remove C7 and see what happens.

As for the CR4 question, that diode is part of the limiter circuit. That -25 volts is being blocked by the diode and with a negative voltage being applied to Q7 base and emitter, I would expect the transistor to be turned fully off, preventing any voltage from showing up at the collector.

The power amp does not need the limiter circuit there to operate, so you could remove both CR4 and CR5 from the circuit for testing.
Re: K150-6 pumps out DC voltage when powered on. [message #27730 is a reply to message #23707] Sat, 14 March 2020 17:40 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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Well got the new parts in. I replaced CR4 and CR5 and still the same behavior.

I have the replacements for Q5 and Q6 buy I'm curious about the heatsinks. Is there a way to remove the heatsink so it can be re-used?

Thanks
Re: K150-6 pumps out DC voltage when powered on. [message #27732 is a reply to message #23707] Sat, 14 March 2020 21:58 Go to previous messageGo to next message
chicagobill
Messages: 1952
Registered: April 2003
Senior Member
Yes, there is a way to reuse the heatsinks, but it is not a simple job.

For testing, you don't need to have the heatsinks on the replacement transistors. The amp will need the heatsinks if you drive the amp hard and for an extended period of time.

Did you try testing Q4 or removing C7?
Re: K150-6 pumps out DC voltage when powered on. [message #27734 is a reply to message #23707] Sun, 15 March 2020 10:51 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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I have tried so many things I'm losing track.

Q4 has been replaced with a brand new nte128.

I tested with Q7 out of the circuit and experience the same behavior.

I tested with CR4 and CR5 out of the circuit and with replacement parts 1n4d004 and also tried NTE177. Same behavior.

Last night I took out Q5 and Q6 and put in NTE128 and NTE129 and again same behavior.
Re: K150-6 pumps out DC voltage when powered on. [message #27735 is a reply to message #23707] Sun, 15 March 2020 13:55 Go to previous messageGo to next message
chicagobill
Messages: 1952
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Again, did you test or remove C7 the small cap next to Q4?
Re: K150-6 pumps out DC voltage when powered on. [message #27736 is a reply to message #27735] Sun, 15 March 2020 19:05 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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I am so sorry. I misread your post before and I thought you meant Q7. I just now removed C7 and we are back at 0V on the output.

It's not a stable 0V it keeps bouncing between 0V and -.25V. Not sure if that's a problem.

Thanks
Re: K150-6 pumps out DC voltage when powered on. [message #27737 is a reply to message #23707] Mon, 16 March 2020 06:06 Go to previous messageGo to next message
stevem is currently offline  stevem
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That small amount of D.C. Is normal.

Now get some top hat type slip on heat sinks for those Transistors.

[Updated on: Mon, 16 March 2020 06:08]

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Re: K150-6 pumps out DC voltage when powered on. [message #27738 is a reply to message #23707] Mon, 16 March 2020 16:10 Go to previous messageGo to next message
chicagobill
Messages: 1952
Registered: April 2003
Senior Member
So the cap was shorted. Good to know that you found the problem.

So let's see, have you reinstalled all of the original transistors again? Is the amp keeping fairly stable and not going into failure mode? How does the amp sound?

If the amp is sounding good and staying stable, I'd just replace the bad cap and call it good. The cap is a ceramic cap 47 pf (picofarad) and rated at 100 volts or higher.

The fluctuating 1/4 volt on the output is fairly normal and may go away or lessen once C7 is replaced.
Re: K150-6 pumps out DC voltage when powered on. [message #27739 is a reply to message #27737] Mon, 16 March 2020 18:56 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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Can you point me to that part somewhere I can order it. I searched for that description and didn't get anything.

"Now get some top hat type slip on heat sinks for those Transistors."

[Updated on: Mon, 16 March 2020 18:57]

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Re: K150-6 pumps out DC voltage when powered on. [message #27740 is a reply to message #23707] Tue, 17 March 2020 07:46 Go to previous messageGo to next message
stevem is currently offline  stevem
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Mouser electronics part number 532-322505B00.
Once you install these place a dab of silicone sealer down in the hole to keep it held onto the Transistor.
Re: K150-6 pumps out DC voltage when powered on. [message #27742 is a reply to message #27740] Sat, 21 March 2020 11:34 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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Thank you I'll look at getting some of those ordered.
Re: K150-6 pumps out DC voltage when powered on. [message #27743 is a reply to message #23707] Sat, 21 March 2020 11:57 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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Okay well the new part for C7 came in. I got 47p 100V 10% from newark.

I also reinstalled all of the old components and installed this new capacitor to replace c7. We are indeed stable and it is no longer going into fail mode.

However I just noticed in my meddling that I broke the leads off of CR3. I was warned against this and now I've done it.

What is the replacement part for that?
Re: K150-6 pumps out DC voltage when powered on. [message #27744 is a reply to message #23707] Sat, 21 March 2020 12:26 Go to previous messageGo to next message
stevem is currently offline  stevem
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It's a 1N3754.
Electronic parts outlet in the Netherlands has some for E7.50 each.

Do not operate the amp without atleast a 1n1004 or 7 in its place .
Re: K150-6 pumps out DC voltage when powered on. [message #27745 is a reply to message #27744] Sat, 21 March 2020 15:10 Go to previous messageGo to next message
chicagobill
Messages: 1952
Registered: April 2003
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stevem wrote on Sat, 21 March 2020 11:26
It's a 1N3754.
Electronic parts outlet in the Netherlands has some for E7.50 each.

Do not operate the amp without atleast a 1n1004 or 7 in its place .
E7.50, Ouch!

A long time ago I started to test other diodes to use as a replacement for these diodes. Generally little glass diodes like 1N914 or 1N4148s seemed to work the best. The problem comes in mounting them in a way to get them to react to the heat of the output transistors.

If you don't want to buy the expensive NOS one, get a 1N914 type and solder two thin wires to the ends. Heat shrink the connections so that there is no chance of shorting out to the heat sink. Then using as thin a bead of silicon glue as you can, glue the diode to the center of the heat sink near the clip that held the original diode. Then run the wires to the pc board making sure that you keep the correct polarity.

This diode string ties the lower half of the power amp to the upper half. If the diode string breaks circuit, the upper half of the circuit turns fully on and will cause the power amp transistors to fail. So don't run the amp without the diode in place.
Re: K150-6 pumps out DC voltage when powered on. [message #27746 is a reply to message #27744] Sat, 21 March 2020 15:26 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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stevem wrote on Sat, 21 March 2020 12:26
It's a 1N3754.
Electronic parts outlet in the Netherlands has some for E7.50 each.

Do not operate the amp without atleast a 1n1004 or 7 in its place .
I tried googling this but I'm having no luck. Do you have a link?


I have some 1n914. I'll play around with those.
Re: K150-6 pumps out DC voltage when powered on. [message #27747 is a reply to message #23707] Sat, 21 March 2020 16:42 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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Okay I took the 1n914 and added some wire and heat shrunk everything so it wouldn't touch the heat sink. Then I placed the 1n914 in the same bracket that the original CR3 was in. I laid it in there with it kind of looped around it as seen in the attached photo.

https://photos.app.goo.gl/jkrZrfFi42oysPVx8

Now when I turn it on I don't get the dc hum any more but I get a ton of static.
Re: K150-6 pumps out DC voltage when powered on. [message #27748 is a reply to message #23707] Sat, 21 March 2020 20:10 Go to previous messageGo to next message
chicagobill
Messages: 1952
Registered: April 2003
Senior Member
The way that you have the diode mounted looks fine. I will assume that the polarity is correct.

As for the static, might be totally unrelated to the diode. Describe what it sounds like. Do the controls have any effect on the sound of the static?
Re: K150-6 pumps out DC voltage when powered on. [message #27749 is a reply to message #23707] Sun, 22 March 2020 12:24 Go to previous messageGo to next message
corybrown is currently offline  corybrown
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It sound like when you have a scratchy pot but it's full blast and continuous. The controls seem to have no effect on it.

Thanks
Re: K150-6 pumps out DC voltage when powered on. [message #27750 is a reply to message #23707] Mon, 23 March 2020 06:06 Go to previous messageGo to next message
stevem is currently offline  stevem
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The 2 blue wires on the left end of the output driver board are the audio inputs from each channel / preamp so if you remove them one at a time you can then narrow down if it's one of the channel preamps , or the output driver board
Re: K150-6 pumps out DC voltage when powered on. [message #27751 is a reply to message #23707] Thu, 26 March 2020 06:59 Go to previous messageGo to next message
stevem is currently offline  stevem
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If you have not located a diode yet let me know as I have found that I have a spare original one.
Re: K150-6 pumps out DC voltage when powered on. [message #27752 is a reply to message #23707] Fri, 27 March 2020 21:12 Go to previous message
corybrown is currently offline  corybrown
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Thank you for the offer. I did find two on ebay for $10. I've paid for them and just waiting on them to ship. I'm hoping to get back to this some this weekend.

Thanks
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