Kustom SS amp help, ala, confession of an SS idiot

[Bob-I]

Update.

I took out one of the power Tx's to get a better look. Put it back and fired it up get get voltage measurements. The damn thing's working.

There's a little fizz on the output, but plenty of power. My guess is now I have dirty sockets for the power Tx's and dirty pots.

Gonna do some general clean up work and see if she'll clean up.

[Aurora]

Clean all interconnects and switches, pots etc, and I guess you're good to go Had my suspicions... SS amps usually blow all the way, at lest in the output section.

BTW- as it's friday evening in my corner of the world, I was fooling around the net - these sites claim to sell the manuals:

http://www.broadwaymusicco.com/Kustom10.htm
http://www.musicparts.com/products.asp?Company=Kustom

[jaysg]

Look for signs of heat damage to resistors. My limited experience is that American solid state amps from that era usually show signs of misunderstandings by the designers, as to how Joe Jr. Rockstar was going to interact with his shiny amp.

[Structo]

Of course you probably know this but be sure to clean the heat sink compound off and replace that when you re-install the power transistors.

[BobW]

Bob,

If the problem is more than dirty connections, usually you can replace the T0-3 type transistors with anything that can handle the max rail voltage and current rating. NTE /ECG makes a lot of crossover parts. For the NPN's a 2N3055 would work but, for best results you are better off buying the complement NPN PNP pairs.

Older Kustoms and Peaveys had the thermal cutoff switches mounted directly to the heatsink. Also the output spade connectors and Molex types oxidized over a period of time, and have found about 25% of the repairs only required cleaning and new heat sink grease as Aurora and Structo suggested.

Measure the Collector to ground rail voltage on all transistors , then you'll know what voltage rating the transistors must exceed.

[Aurora]

2N3055s will probably not survive in this amp, as Vcc+Vdd is probably 100volts +.
In any PP, the outputs should have a higher Vce than the total supply , - Vcc+Vdd, as on a full negative output swing, the NPN device will have to sustain almost the full positive to negative rail voltage. PNPs vice versa.
Always shoot for output devices with a Vce larger than 2xVcc.

[Bob-I]

I don't have any heat sink compound so the final cleanup will have to wait. I checked the thermal cutoff as BobW suggested. When I pulled off one of the connectors it came off in 3 pieces in my fingers. I soldered it on.

The amp is acting intermittant now so I'm quite sure that the heat sink compound and general cleanup will take care of it.

I agree that the designers had no idea how Jr/ RockStar would push these amps. They're all pretty underdesigned but this amp is 30 years old and most likely can be easily fixed, plus even the reverb and trem work. That's pretty impressive.

The Vcc is 50V not all the high. I'm impressed with the physical size of the filter caps, monsters but no numbers on them so I have no idea what they are.

I contacted the broadway music but I got an auto reply that they're out of the country. No hurry and I bet I won't need the schem after all.

Thx everyone, this has so far been a real education.

[M Fowler]

Most amps that I used to work on of this type back in the day such as the Carvin B2000 were using RCA 2N3055 or RCA 40411 or RCA 2N6254. For the rest of the circuit the 2N3391A.

My notes say that Delco DTG110A power transistors were changed to the RCA 2N3055 or 40411.

Not sure if any of this helps.

Mark

[Aurora]

Not to be nit pickin', but 40411 was the one spec'ed in RCAs original design notes. It has a Vce of 90V, 2N6254 is 100V, while DTG110A was 70V and 2N3055 only 60V.
Nevertheless - 2N3055 was indeed the workhorse of many SS designs well into the 80s.

[M Fowler]

Aurora,

Yes you are correct on that. I didn't read the solid state service bulletin for power transister replacement very well, too fast.

Mark

[Structo]

Did these power transistors have the mica insulators between them and the heatsink?

[M Fowler]

Yes mica/dope or paste on large heat sinks bolted to the chassis.

Mark

[Bob-I]

Did these power transistors have the mica insulators between them and the heatsink?

Yes, and they look like the heat sink compount is all dried up. I think I'll get replacement mica and heat sink compound and try that before replacing anything.