[FallenAngel]

Hey guys,
 
Our good friend charliex has asked me to help fix his SAC KH1000 amplifier. I've opened it up and looked around but can't seem to see any obvious issues. None of the resistors or diodes look burned up, none of the transistors get warm and the only sign of problem is that the "ON" light keeps blinking instead of turning off when it's ready.
 
Looking over the board, I noticed the TA7317P circuit protection chip that I would like to replace first and maybe the MC1455P1 timer. As the symptom is a flashing light, I'm thinking that the amp is designed with a warm-up circuit but it doesn't turn off. This could either be because of the timer or because of the protection circuit acting up. Unfortunately, these are out of production and not easy to get (without paying $30 for shipping or waiting a month for delivery).
 
Does anyone have experience with this amp or have any ideas? Anything would be most appreciated.
 
Thanks!
 
 

 

[Avro_Arrow]

Has the amp developed a DC offset? That will trip this circuit.
You will have to check before the mute relay.

 

[FallenAngel]

Yep, I'm seeing about 180mV on each channel. I decided to replace the protection chip first in case that's gone. After that it's the timer chip though I think it's OK and all 4 opamps. If that didn't fix it, I'd really hate to have to replace to92 components as they are out of production. Output BJT are not impossible to get though. The rest are lm317/337 and diodes and resistors which I don't figure would fail.

 

[Avro_Arrow]

You should be able to see the 555 time change state after a period of time even if the protection chip does not respond to it.
I tried to find a data sheet for the protection chip but information was limited.

 

[Avro_Arrow]

Pin 8 on the protection chip should be the mute timer input.
It should go from low (muted) to high (unmuted).
You could even drive it high manually.

 

[FallenAngel]

I haven't seen a 555 timer, there's a 1455 timer. I don't hear a relay click so obviously it's not activated.

Overall the amp looks like a higher voltage ckk2.

I can try looking closer at the time chip but I may be a little over my head in debugging it. I'm more of a brute force approach guy that swaps out all silicon that could fail until it works. The parts are cheap so I don't mind. I just want to make it work for my friend before the next Toronto meet.

 

[FallenAngel]

Further thinking, I see 180mV offset so DC protection circuit is obviously not working!

 

[Avro_Arrow]

I don't have a schematic or the amp in front of me so I have to speculate.
Sorry, the 1455 is a dual 555.
180mV is pretty high for a headphone amp.
With the protection circuit engaged, you should see nothing at the headphone connector.
The relay could be implemented as either open circuit or crowbar.
Open circuit, as the name implies, keeps the headphone jack disconnected until the
protection says it is safe to connect it.
Crowbar shorts out the output until the protection says it is safe to un-short it.
Obviously, the first is more common.
 
You should be able to jumper 5(?) volts from the 1455 timer to the relay to force
it to engage. This should give you a chance to get a true reading of the offset
voltage and (with a expendable) pair of headphones, hear what the amp is doing.

 

[FallenAngel]

I've just hit a serious issue. This is an old PCB and was difficult to desolder, so while removing the opamps, I've damaged the board.
 
I don't see a way to fix the board at this time. Old copper lifts and gets destroyed easily and I hate it! All I can think is drilling out the PCB and wiring connections. Not something I'm interested in doing but just in spite, I just might!

 

[Avro_Arrow]

Bummer...