Kustom PC 5065

[jmaroto]

Hi,

I'm rebuilding a Kustom 250 head, it had a lot of burned parts and some of them were missing. Placed a whole set of new transistors the drivers and the power. In the begining I was reading -39.5V DC on the output jack, then replaced 2 of the 5 diodes of the board and now I have +39.5V DC on the output jack. It's supposed that the 2N3055 power transistors goes half in + and half in -, but all I'm getting are positive readings across the board, the only negative are from collector in Q1 and Q2. I already changed all the transistors at least 3 times to make sure.

On Vintage Kustom forums advise me that I should use 1amp fuse to prevent burned parts, sometimes it turn on with this fuses and other times it just blow.

Does anybody have any idea?
Thank You.

[52 Bill]

Build yourself a light bulb limiter and save your fuses.

Investigate why the negative supply voltage is missing from the board. It should be on the drivers, as well as the outputs.

You say that you have replaced all of the transistors, which ones were replaced and what were they replaced with?

[jmaroto]

Thank You 52Bill .
I have Q1 Q2 NTE159 / Q3 NTE190. Q4 & Q8 are NTE128, Q6 2N4401, Q7 Q5 Q9 are 2N4033, and Q10 thru Q13 are 2N3055. Also replaced CR3 and CR4 with 1N5402, CR1 / CR2 / CR5 are the originals but test good.

[52 Bill]

I just did a quick look up of the NTE numbers and the others that you substituted, and they all seem to be reasonable equivalents. If any of the replacement transistors have different style cases, be sure that you have the correct lead placements.

Using a 1 amp fuse will not protect anything better than the original 3 amp one. Either the one amp will blow from the power on surge or it will blow when one of the new transistors short out. Use the light bulb limiter or simply temporarily wire a 60-100 watt incandescent bulb across the blown fuse.

Have you traced out the negative supply line yet? It should be on the emitters and bases of both Q12 and Q13 as well as on the collectors of Q5 and Q9, also on the emitter of Q3.

[jmaroto]

Well Bill, negative voltages are correct in the Qs as you stated, still +39.5 DC on the output jack.

Do you think it will be a good idea to replace the 0.51ohm 5W resistors? I changed them the first time but I don't know if something could had affected this resistors. As far as I was taught this resistors are kind of fuses inside, but I don't know if they are too delicate.

[52 Bill]

I don't know why you've replaced all of the transistors on the board and some of the resistors, but just swapping parts is not going to fix the amp, unless you get lucky and happen to swap the bad part that is causing all of your problems.

The emitter resistors will normally take a lot of abuse before they bad. If you've already replaced them just check them with your ohmmeter and move on.

Have you compared the voltages on the board with those on the schematic. The bases of all of the transistors must be turned on or they will not conduct.

I've already asked about double checking the transistor orientations, but what about the diodes that you have replaced? Wrong value resistor installed?

[Enzo]

And it may not be a bad part, it could also be a broken connection or broken copper trace on the board.

[jmaroto]

Bill, thanks for your inputs, this is a very old amp I bought it for about $50, Q9 Q5 and Q7 were missing, Q1 and Q2 were melt and the rest of transistor you couldn't see the models, so I "did'nt wanted to take risks" using this old parts. The ceramic resistors were all cracked. The board had a lot of burning marks, that's why all this work.

I know that you would say that it's cheaper to buy another amp, but I'm in Costa Rica and there's no "too much" vintages around.

I have the 5-71 schematic version with the voltages, that's my goal to achieve. Don't really understand what you said about the bases should be turned on.

Hi Enzo, as a matter of fack I was considering make a new board from scrach, maybe it's some trace I'm overlooking, as I mentioned the board was in very bad condition.

[km6xz]

You might be introducing more variables than you think when replacing all these parts unless proven defective.
That will make troubleshooting harder. Don't change any more parts but get a piece of paper and write down the values of each lead of the transistors, diodes and those emitter resistors. That will be better evidence than appearances of any particular part.
Unless you do this everyday, the chances of replacing so many parts and getting all of them in correctly is slim. The worst issue is replacing parts on only pc boards damages the traces and tears pads unless specialized desoldering techniques and equipment is used. Now it is likely that there are broken traces that did not exist before, regardless of what the original problem was that someone gave up on.

[52 Bill]

Some good points were brought up here. Please do check the board for bad traces or component pads.

I regularly fix old outdated equipment for a variety of reasons. Kustom amps were sort of like Peavey amps in their day. Decent, reliable, well made amps. You should be able to get this thing working.

My point about the voltages was, look at your schematic. There are voltages marked on all of the transistors. Bases, Collectors and Emitters. How do your voltages compare with those on the schematic? If you find a spot where the voltage is either missing or way off from what is expected, you have a place to start looking for a problem. Maybe a bad resistor or maybe a bad pc board trace.

[jmaroto]

Good news,

After teaking with the diodes CR1 and CR2, the PC5065 is working fine.

The amp has two boards more a PC5067 with selectone and a PC5066. Right now the PC5066 is working fine with high and low input and good modulation from Bass /Trebble /Middle. But I have no sound at all from PC5067, not even a "hiss" and pots have no effect.

Tested the mollex connections between the boards and they are good, even tested the voltages on the IC and all reads normal.

Anybody has experienced something similar?