VintageKustom.com: Repairing Kustom Amps » K100-2- Pre-Rev-3...no Q131...1/2 the power amp

Home » VintageKustom.com » Repairing Kustom Amps » K100-2- Pre-Rev-3...no Q131...1/2 the power amp (One half of my power section doesn't work)

Show: Today's Messages :: Polls :: Message Navigator

K100-2- Pre-Rev-3...no Q131...1/2 the power amp [message #21623]

Fri, 03 October 2014 10:45

Jerrybass1955
Messages: 45
Registered: August 2014
Location: NC

Member

Hi Folks,
I am still working on my K100 which now appears to be a version from prior to Rev. 3 since it has no Q131. I am struggling to get the dead half of my power amp working. I have tried several things including replacement of the output transistors, drivers and most all of the resistors in the neighborhood. I may try replacement of the bias diodes next, but here are a few symptoms/questions for you guys:

* At the base of Q126, collector of Q124 I am reading about 26.6 volts. This looks pretty consistent with the K100-1 schematic (same circuit), but the K100-2 schematics show 32.8 at the same location. Is 32.8 wrong? Seems odd that it would show up as 32.8 on the rev. 3 drawing and not have been corrected by the rev. 4 drawing - especially when it is 26.9 volts on the K100-1 schematics.

* The problem I have is that the channel driven by Q128 and Q2 is not working. If I drive the output relatively hard and look at the signal at the base of Q2, I get approximately square wave of maybe 1/2 volt or so and no real contribution at the speaker terminal. (good news, is that I am getting about 10 watts out of the other channel operating class A until I hit asymmetrical distortion. Earlier stages don't appear to be distorting too soon or asymmetrically.)

* This is the same thing that happened with the original driver transistors. I replaced Q127 with a 2n3440 and have tried the original transistor as well as a 2n25415 and a 2n5416 for Q128. Currently the 2n5416 is in and when it was installed I immediately got a high current draw situation with things getting hot. I shut down the amp and need to address that. But otherwise, I get the base drive signal mentioned above which is the basic problem. Any reason these transistors shouldn't work?

* I disconnected the protection circuit at the diodes to see if that was a contributor but there was no change (except that perhaps not having it in could have contributed to the high current situation when I later tried the 2n5416).

* I replaced the 1 ohm resistors which had drifted with a pair of matched resistors. I also replaced the 95 ohm resistors in the protection circuit with 100 0hm 1% (they were 5% in my circuit - should I try to get closer to 95 ohms?). R205 and R207 were replaced with 1% 2 Watt resistors. Most if not all relevant resistors have been replaced with metal or carbon film resistors of correct value.

* It appears that the Q1 side is working well and putting out about 10 watts into 8 ohms before asymmetrical distortion sets in. No sign of unreasonable asymmetrical distortion in stages prior to the driver.

* It seems to me that Q2 and Q128 must be turned on suggesting a bias problem. Perhaps the diodes are bad. They test good in-circuit, but I know that is not always reliable. I shorted out all the diodes which I think should have likely led to an output from Q2 (albeit with crossover distortion), but saw the same signal at the base of Q2.

* I replaced the power transistors Q1 and Q2 with matched MJ15015s for good measure and nothing seems wrong there. They test the same when I pull the wires as they did out of the circuit and Q1 is obviously working like a champ.

* Some DC readings Q126 collector =-3.6V, Q128 Base =-0.7V, Q124 Base = 47mv, emitter=0.6V, Q125 base=54mV. I forgot to bring other voltage readings with me.

I am at a loss at this point. Any suggestions would be appreciated. I have double checked my replacement resistors to assure myself that I didn't do anything wrong, but if something seems suspicious, I'll triple check.

Thanks,
Jerry

Previous Topic:	New member with a Charger question
Next Topic:	tuff fix