Newby needs helpl

[jhall299]

Hi everyone,

I'm new to the forum and looking for a little guidance. I'm a ham radio guy and do some restoration work on tube radios. I'm far from an expert and even farther with solid state devices.

I went to the dump the other day and when I was emptying my truck noticed a big ass amplifier sitting on the side. I couldn't resist so once I had the truck emptied I loaded up the amp. When I got it home I discovered it was a Kasino 200U which is a 200watt amplifier with two 15" speakers. My original thought was to salvage parts from it but after looking it over it seemed to be intact.I should mention that I have zero musical ability and no guitar or bass. After a visual examination of the amplifier and finding only minor evidence of previous work I decided it may be worth fixing.

I started checking with the power supply and found it working but the primary rails which are suppose to be +- 39.5v were both about 2v low and the 8v rails which are +-8v were about .4. high. These values didn't seem way out of line so I went on to power up the amp. Long story short the amp didn't work. I went on to troubleshoot and determined that the pre-amp board and all of the controls seem to work correctly. I determined this by putting a signal tracer on the pre-amp output and I could hear a signal and it responded to adjustment of the controls. I was using audio from my computer into the input of the amp for a signal. When I hooked up a speaker to the output I got no output at lower volume but as I increase volume I started to get bursts of audio much like a dirty pot would give and at higher volume toe burst get more frequent. I'm attaching a copy of the schematic for the amp board.

Here are the things I have done so far to try and determine the problem.

1. Removed and tested the final transistors(RCA 36892) using a DMM on the diode setting. They all test good.
2. Removed and tested Q9(RCA 38737) which is the driver for Q22. Again tested with a DMM and good.
3. I tested all of the diodes on the board and they test good with my DMM.
4. Tested all four 1 ohm 5w resistors on the drivers and found them good.
5. I tested all of the resistor on the board in circuit and found none open or shorted. Values all seemed reasonable.

I tried to trace the audio through the board but wasn't very successful. I'm hoping someone can give me a push in the right direction to identify the problem. I have no real need for the amp but if I can get it working, I will likely donate it to one of the local schools.

Thanks in advance for any help you can provide and I will try to be responsive to any suggesting you might have.

Jon

[Jon Snell]

With no speaker load, check the DC voltage on the blue loudspeaker connector. If it is 0volts, with respect to ground, then the amplifier is probably OK.
Connect your loudspeaker if 0volts and touch the inpyt section; R5/C2 you should hear noise.
(The input signal is amplified by Q1 and inverted to drive Q3 whilst Q2 holds the output voltage (blue wire) at around 0volts).

[jhall299]

Hi Jon,

Thanks for getting back to me! With no load the output shows -9.9vdc. Should I remove Q2 and test it out of circuit?

Jon

[Jon Snell]

I would remove the capacitor C ? can't read it. It couples the ground to the feedback resistor.
It is unlikely the transistors have only failed a bit. They will go either open or short.
Removing C? will remove any DC offset from the DC control and if you then get 0volts, that C? was leaky.
Circled in red.

[jhall299]

Hi Jon,

I replaced C8 which was listed as 6.8uf/35v and appeared to be a tantalum cap with a 10uf/35v which was as close as I had. The old cap showed a short. The good news is that it now plays continuously and amplification is working. The voltage on the output was .001v. Unfortunately, it is still distorted. I'll upload a short mp3 so you can hear what I'm getting. Also, since I'm just using audio from a computer for input could there be an impedance mismatch? Audio.mp3

Jon

[jhall299]

Proving the old adage that "Even a blind squirrel finds a nut once in a while" I got the amp working. I wound up replacing one more suspect cap C7 and more importantly I replaced diode CR3. The diode had a bad solder joint(lead fell out) but when I went to replace it the lead fell off the diode. The diode was a 1n3754 and after researching replacements I just replaced it with a 1n4007. My only concern is the original diode was a small metal can and was clipped onto a heat sink. I have the 1n4007 just mounted on the board. Is that going to be okay? If not what would you do?

Thanks again to Jon who got me on track for this.

Jon

[Enzo]

I would note that CR1, CR2 were different from this one and this one was on the heat sink for thermal tracking. Is the part right next to the heat sink, so its own lead wires will reach? At the very least your 1N4007 ought to be against the heat sink and have heat grease around it.