help with a Kustom 200B

[Enzo]

Well, this is just a linear power supply, and not a ton of current at that.

The large transistors are 2N3055 or similar.

I don't see 2N4249 in the circuit, but those are general purpose audio transistors, so a 2N5086 maybe or an MPS4249

2N3567 is like a 2N3391 or something. They don't need to be low noise, but MPS8097 or MPSA18 should work, or something common like MPSA06

2N3638 is the same thing but PNP. so 2N5086, MPSA56, MPS8599

[toytunone]

Kustom 200B

Enzo is right on with troubleshooting this thing, he is better at this than I. However, I had a chance to work on a Kustom 200B owned by my nephew a few months ago so I thought I would tell you what I found. Refer to schematic PC-703.

The problem was a fairly loud 60 cycle hum on the output which actually measured 1.5 V rms, also loud hiss on output measuring about .7 V rms, also low volume from the mixer front panels making a 1 V RMS 1 Khz input produce less than 1 watt on the output. I thought it should be full power ~125 watts.

One 8 V Power leg was also off, I think it was about 3.5 volts. I unsoldered the wire to the main board and the power was 8 V again and the hiss and hum were reduced. Q 700 was shorted b-c. R&R and same problem, but the power supply was correct at 8 V.

Since the input > output was so low, obviously the problem was a gain stage, also since the problem was common to all four front panel mixer boards, it was obviously on the main board. Like you, I verified the output transistors were not the problem, (at first, I suspected a short here), by measuring the voltages on them and they were symmetrical. At this point I decided to use a .01 uf Capacitor in series with 1K ohm resistor test jig and bypass some transistors in the preamp and drivers, (this can blow transistors if you screw up where you put it, but it is a quick and dirty way to troubleshoot stages). When I bypassed Q 707, a 2N3638 PNP General Purpose driver, I heard the .1V 1Khz signal I had fed in earlier with a coupling capacitor at the input of R701 get much louder. It sounded like about 5 watts. I removed Q707 and checked it and it was b-e open, so I had several I had pulled years ago and I picked the one with the highest gain and lowest leakage and replaced it.

The hum went down to .15V RMS on the output, the hiss became .015 V RMS and I couldn't feed more than about .1 VRMS 1Khz to the preamp for fear of blowing my test speaker.

Here I rewired the 120 VAC plug for 3-wire and the hum went down to about .05 V RMS.

I also found an open coupling cap, I've forgotten which one, but it was in the driver circuit. That cap reduced the hum to less than .01 V RMS on the Speaker output. I worked on the amp about 2 more hours, but couldn't find anything else wrong, so I disconnected the power to the reverb board and Bingo, the hum disappeared completely and the hiss went to about .01 V RMS, acceptable, but not silent. After testing the reverb board a few hours, I concluded it was the spring itself that was causing the residual hum since all the impedances were low and wouldn't cause a hum problem. Also when I shorted the input after the spring with 200 ohms, the amp grew deathly quiet. The front mixer boards are also low impedances and do not contribute measurable hum in spite of a lack of shielding.

So I sprayed all the mixer pots with Caig ProGold G5 and pronounced it well. BTW, Creedence great John Fogerty always used these amps for his sound, maybe he still does.

Sorry for the length of this post, also your mileage on voltages may vary, but I hope it helps you troubleshoot this amp.

[Valvehead]

thank you sir