Kustom amp.

dogooder

dogooder

Well-known member
I can't win. I plug the am in, it plays okay but all the pots are really bad. I take the amp out, spray up all the pots with contact cleaner, put it
back together and nothing. Won't turn on. The fun begins.
 

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I have all the right voltages coming off the power supply. +40 volts off one filter cap and -40 off the other.The only schematic I can find is half schematic and half block diagram. It shows the power supply but the rest of it is just block diagrams. There is a thermocoupler shown in the power supply, the fuse is good. I don't know what the thermocoupler looks like or where it is on the boards. I think I know which component it is? I measure no voltage on it and when unplugged there is no continuity across it. I show -40 volts at the on indicator lamp which does not light. It show -40 on one output transistor leg and 0 where the +40 should be on the other output transistor. I just pulled the transistor and am about to check that. Attached is the only schematic I could find. If anyone knows where I can find a complete one let me know.

Thanx
 

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I found them. They are not downloadable but I think I can take a screen shot and print them out.
There is a "thermostat" in the power supply circuit. Can anyone tell me what they are supposed to look like?
There is a component sitting between the two output transistors. Two wires from the power transformer running
to each side of it and both reading 120 Volts AC. I don't know what it is and what it is for? I am not
used to working with transistors and don't think I ever ran into a "thermostat" before?
 
dwr.jpg

Woooo! Yeah, I get that too - also with my wi-fi router's access site. I just keep clicking through the warnings.
 
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The are probably over 100 variants on the transistor power amp circuit Dogs' but the principles are pretty common so I might be able to help. Let's dispose of that "thermostat" first of all! I am sure that will be a re-settable* Thermal cut out. They usually look like small control knob with two 6mm spades on it. Usually attached to the main heat sink but might be bolted to the power transformer. You have 2X 40V so that we can ignore.

There will be two power transistors, might be metal cased or plastic tab. Somewhere close to them will be the emitter resistors, usually quite big 5W types. Inspect them for signs of distress then measure them...COLD! They are likely 0.22 Ohms. If they are 'blown' then so are the transistors and probably most of the other devices on the power PCB. Fixing transistor power amps is a PITA. But, since you have not reported 'smoke or bang' I suspect you could just have a disturbed wire or bad joint.
Can you attach some photos and then we talk again?

*They usually reset once 10 or so dgrs below 'trip' temp' but some need a blast of freezing spray.

Dave.
 
ecc83 said:
The are probably over 100 variants on the transistor power amp circuit Dogs' but the principles are pretty common so I might be able to help. Let's dispose of that "thermostat" first of all! I am sure that will be a re-settable* Thermal cut out. They usually look like small control knob with two 6mm spades on it. Usually attached to the main heat sink but might be bolted to the power transformer. You have 2X 40V so that we can ignore.

There will be two power transistors, might be metal cased or plastic tab. Somewhere close to them will be the emitter resistors, usually quite big 5W types. Inspect them for signs of distress then measure them...COLD! They are likely 0.22 Ohms. If they are 'blown' then so are the transistors and probably most of the other devices on the power PCB. Fixing transistor power amps is a PITA. But, since you have not reported 'smoke or bang' I suspect you could just have a disturbed wire or bad joint.
Can you attach some photos and then we talk again?

*They usually reset once 10 or so dgrs below 'trip' temp' but some need a blast of freezing spray.

Dave.
Click to expand...
I am bringing it to my buddy. It will take him fifteen minutes to find the problem. I will post the results when I get them. Thanx. The component I don't know what it is sits on the heat sink, which is actually part of the amp chassis, right between the two output transistors with two wires coming from the transformer with 120V on each. I pulled the output transistor that is not getting any voltage and checked it, it checks good. One output transistor shows voltage applied and the other no voltage.. The output transistors are 36892s or substitute 2N3055
 
Brought it to my buddy. Went through everything, all voltages normal ect. We get done and hook it to a speaker and voila? Still needs some work, channel two
is barely there. I suspect the volume pot on that channel and will replace it.
 
I have never come across this before. The amp will not turn on without the speakers being plugged in. I haven't bothered to look at the schematic
to see why yet? I still have the problem with channel 2. I have to buy some more contact cleaner and really clean the pots. If that doesn't work I have to take it out of the chassis to get to the back of the board to really test it.
 
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