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

[Bob-I]

A friend of mine had this old Kustom SS amp in his garage for years asked me to look at it. It's called Kustom Lead III SC, 130 watt output 2 channel with reverb and trem.

I powered it up and the output is weak and distorted. The line out sounds fine when run into another amp so the problem must be in the power amp section. There is a board and some power Tx's in the output section. On the board there are 4 components labeled RCA 6508 5 9 7646. I get no hits on google so I'm assuming these are drivers. The power Tx's are labeled RCA 7645, again no hits.

There's also an interconnect I can't figure out. It looks like 2 1/2 fuse holders with some wires coming off, between the power amp board and power Tx's.

I took a few pics hoping someone can help me get this amp going. I don't really know how to troubleshoot blown Tx's or houw to source out the parts. The schem is no where to be found so I'm shooting in the dark.

Any assistance is appreciated.

[selloutrr]

just a thought buit call mouser or digikey and ask a tech to cross reference the # or try a local electronic store or library for a component cross reference book.

I personally do very little solid state repair but I'm GUESSING you would trouble shoot this like any other piece of equipment and start at the output and work your way to the front fixing / replacing parts as you go. If the preamp works but the output doesn't your already 1/2 way done.

you may also want to google how to test transistor or any other part you you have in question. If you have access to a tone generator, volt/ohm meter, and scope you'll be able to preform most tests.

I just googled Kustom Lead 3 and found the schematic / service manual for sale from several archive companies.

[Bob-I]

just a thought buit call mouser or digikey and ask a tech to cross reference the # or try a local electronic store or library for a component cross reference book.

Great suggestion. There's an old time electronics shop a few miles away, no tube stuff but lots of old reference manuals.

I personally do very little solid state repair but I'm GUESSING you would trouble shoot this like any other piece of equipment and start at the output and work your way to the front fixing / replacing parts as you go. If the preamp works but the output doesn't your already 1/2 way done.

Yep

you may also want to google how to test transistor or any other part you you have in question. If you have access to a tone generator, volt/ohm meter, and scope you'll be able to preform most tests.

I just googled Kustom Lead 3 and found the schematic / service manual for sale from several archive companies.

LOL, I googled Kustom Lead III and got nuffin. Dummy me

Thx, a few bucks for a schematic will be worth it and my friend is willing to pay.

[selloutrr]

Glad to be of help

just a thought... If you find that one transistor is weak in the group and the replacement isn't super $$$ or near impossible to find. Plan on replacing all of them as the rest may be on the edge of failing.

keep us updated!

[xk49w]

There's also an interconnect I can't figure out. It looks like 2 1/2 fuse holders with some wires coming off, between the power amp board and power Tx's.

That thing between the transistors looks like a thermal cutoff switch. Heatsink gets too hot it opens up.

[Bob-I]

There's also an interconnect I can't figure out. It looks like 2 1/2 fuse holders with some wires coming off, between the power amp board and power Tx's.

That thing between the transistors looks like a thermal cutoff switch. Heatsink gets too hot it opens up.

Yes, that's what I thought, but I'm looking at the interconnects behind the cutoff.

[xk49w]

Yes, that's what I thought, but I'm looking at the interconnects behind the cutoff.

In the 70s I built a couple of SWTP Tiger amps. They had output transistor bias diodes attached to the heatsinks that looked like those. They have two leads each (can't see)?

[iknowjohnny]

http://nte01.nteinc.com/nte/NTExRefSemi ... f/$$Search

[Bob-I]

Yes, that's what I thought, but I'm looking at the interconnects behind the cutoff.

In the 70s I built a couple of SWTP Tiger amps. They had output transistor bias diodes attached to the heatsinks that looked like those. They have two leads each (can't see)?

Yes, 2 leads each. They're held in with fuse like clips and I was wonder if that was an electrical connection or just to hold it. Since you believe it's a diode and there are 2 wires, I think that it's just a hold down.

[Bob-I]

http://nte01.nteinc.com/nte/NTExRefSemi ... f/$$Search

Excellent, but alas no help. The power Tx # came up as a diode, the drivers came up as high speed switching.

There's got to be a way. I'll have to get a schematic and see what these guys are.

[Aurora]

The strange thingies behind that thermal fuse is most probably temp compensation diodes, as said. Used some like those in the 70s...Motorola 1N57xx something IIRC. Try to loosen them an look for a number.

The transistor numbers 7645 and -46 looks more like the typical production codes used back then... week 45-46 of 1976....but I may be wrong here. Are there no other numbers on those outputs? on the sides ??
Larges manufacturers often had parts with proprietary marking..........
That interconnect looks like an early Molex, but there was/are alot of similars, I'm afraid.....

If OTOH all the outputs are of the same type, this is a quasi complementary output stage, but I tend to think not - if the amp is made in the late 70s or later...? Any clue about the prod. year?

If the amp gives output at all, it is not likely that the outputs are blown....

I have a fairly exstensive parts data library at work, and may be able to help, but somehow those numbers looks kinda suspicious for being RCA part numbers....???

[Bob-I]

The strange thingies behind that thermal fuse is most probably temp compensation diodes, as said. Used some like those in the 70s...Motorola 1N57xx something IIRC. Try to loosen them an look for a number.

Ok, but unless there's reason to suspect these are the problem I don't want to start ripping things apart.

The transistor numbers 7645 and -46 looks more like the typical production codes used back then... week 45-46 of 1976....but I may be wrong here. Are there no other numbers on those outputs? on the sides ??
Larges manufacturers often had parts with proprietary marking..........
That interconnect looks like an early Molex, but there was/are alot of similars, I'm afraid.....

That makes complete since, especally since this is most likely a late 70's amp. I've pulled one of the power Tx's out and these are the only numbers 7645 and 3 7 0000. My guess is RCA was hiding the part number for Kustom.

If OTOH all the outputs are of the same type, this is a quasi complementary output stage, but I tend to think not - if the amp is made in the late 70s or later...? Any clue about the prod. year?

If the amp gives output at all, it is not likely that the outputs are blown....

I'll buy that. There IS output but weak and distorted. So what do you think the problem could be then? MY assumption is that the components on the board are drivers of some sorts, Tx's or Darlingtons. Could this be the problem?

How would I best troubleshoot this? With tube amps I typically inject a sine wave input and look at the output of each stage for a change in the wave form. If I were to do that on an SS amp, were would I look?

I have a fairly exstensive parts data library at work, and may be able to help, but somehow those numbers looks kinda suspicious for being RCA part numbers....???

Thx for the offer. I'll hopefully be able to source out a schematic and get more information.

[Structo]

On those weird looking two lead devices, what type of resistance do they show whent the amp is cold?

[Aurora]

On those weird looking two lead devices, what type of resistance do they show whent the amp is cold?

If they are diodes, they would show an ordinary diode junction of .7-.8V when measured. Heat will lower the junction voltage, thus pinching the bias to avoid runaway.

RCA published a few amplifier schematics of good working solid state PAs using their newly developed output devices, that became models for several successful designs in the early 70s, and often copied with other brand devices and minor variations. If these designs may be the basis, there are some questions:

Does the outputs look like being all of the same type , or maybe 2 pairs?
Measuring the polarity of the two outermost drivers should tell if this is a fully complemetary with NPN/PNP devices or quasi complementary with all 4 outputs of same polarity.

By measuring and looking at the wiring, are the output pairs driven in parallell ( maybe with a emitter resistor of around .5 ohms +/- )

What is the supply voltage off those big caps? 130W should command at least +/- 50 Volts supply...

A better picture of the output section only would help, too...
As large in resolution as the rules here will allow...


EDIT:
Actually, supply voltage should be measured first, along with the general check of fuses etc. BTW - is there a fuse in the loudspeaker output line?
Some desings used an R/C network in the output, that were short circuited by a fuse. If the fuse blows, the output will be just a tiny squeek...

[Bob-I]

On those weird looking two lead devices, what type of resistance do they show whent the amp is cold?

If they are diodes, they would show an ordinary diode junction of .7-.8V when measured. Heat will lower the junction voltage, thus pinching the bias to avoid runaway.

Ok, I understand

RCA published a few amplifier schematics of good working solid state PAs using their newly developed output devices, that became models for several successful designs in the early 70s, and often copied with other brand devices and minor variations. If these designs may be the basis, there are some questions:

Does the outputs look like being all of the same type , or maybe 2 pairs?
Measuring the polarity of the two outermost drivers should tell if this is a fully complemetary with NPN/PNP devices or quasi complementary with all 4 outputs of same polarity.

By measuring and looking at the wiring, are the output pairs driven in parallell ( maybe with a emitter resistor of around .5 ohms +/- )

What is the supply voltage off those big caps? 130W should command at least +/- 50 Volts supply...

A better picture of the output section only would help, too...
As large in resolution as the rules here will allow...


EDIT:
Actually, supply voltage should be measured first, along with the general check of fuses etc. BTW - is there a fuse in the loudspeaker output line?
Some desings used an R/C network in the output, that were short circuited by a fuse. If the fuse blows, the output will be just a tiny squeek...

No, there's no fuse on the speaker.