Uploaded August 2026 | Updated September 2026, 2 weeks ago
Now this was a pain in the ass to fix, but the juice was worth the squeeze. And as always heres the story:
I picked this up at recently at an estate sale. Seemed to work fine, I did my usual clean out and refurbishment. And after breaking it in a few hours, it cut out.
But it did some strange things - I got some DC in the left speaker. And the unit just seemed to go haywire, and the right channel went completely out.
Thankfully, I was sitting there when it happened. This unit does not have a relay protection circuit, but it has fuses. And a dreaded STK chip. I came to find out it has a muting circuit which detects DC and that didn’t seem to work right.
I had concerns from the beginning that this was an STK chip amp with fused speaker outputs - what the hell happens if you get a shorted chip with DC output and I’m away for five minutes?
Fast-forward after all sorts of schematic, looking, printing, and component testing. Replaced the STK (with a confirmed NOS authentic) and six transistors in the amplifier circuit. Recapped the amplifier circuit - many of the smaller value caps were showing 10 ohm resistance and some were 25% out of spec. I do have strong reason to believe that two transistors were not working right, one of which was in this muting circuit, which cuts off when DC is present. And then another in the right channel bias circuit.
After about 12 hours of total time working on this, I still had the same issue where the right channel was cutting out - but no DC in the left. This thing was just so intermittent the minute I would get excited and put it back together after biasing it would go bad again and again.
I actually pulled up another one on eBay and was considering purchasing one working since I really liked this unit.
Took a Hail Mary finale and looked closely at every solder connection on the amp board and I found the right channel had a bad ground in a conspicuous spot - a bad solder post joint.
Soldered that up and reflowed a whole bunch of other stuff (as insurance!) and it’s working perfect now. I could not find a single solder flaw elsewhere, other than that one spot!
In general, the build quality is very good and I don’t think this has a lot of hours. I don’t think the original STK chip was bad so I’m putting it in my parts collection.
Now this was a pain in the ass to fix, but the juice was worth the squeeze. And as always heres the story:
I picked this up at recently at an estate sale. Seemed to work fine, I did my usual clean out and refurbishment. And after breaking it in a few hours, it cut out.
But it did some strange things - I got some DC in the left speaker. And the unit just seemed to go haywire, and the right channel went completely out.
Thankfully, I was sitting there when it happened. This unit does not have a relay protection circuit, but it has fuses. And a dreaded STK chip. I came to find out it has a muting circuit which detects DC and that didn’t seem to work right.
I had concerns from the beginning that this was an STK chip amp with fused speaker outputs - what the hell happens if you get a shorted chip with DC output and I’m away for five minutes?
Fast-forward after all sorts of schematic, looking, printing, and component testing. Replaced the STK (with a confirmed NOS authentic) and six transistors in the amplifier circuit. Recapped the amplifier circuit - many of the smaller value caps were showing 10 ohm resistance and some were 25% out of spec. I do have strong reason to believe that two transistors were not working right, one of which was in this muting circuit, which cuts off when DC is present. And then another in the right channel bias circuit.
After about 12 hours of total time working on this, I still had the same issue where the right channel was cutting out - but no DC in the left. This thing was just so intermittent the minute I would get excited and put it back together after biasing it would go bad again and again.
I actually pulled up another one on eBay and was considering purchasing one working since I really liked this unit.
Took a Hail Mary finale and looked closely at every solder connection on the amp board and I found the right channel had a bad ground in a conspicuous spot - a bad solder post joint.
Soldered that up and reflowed a whole bunch of other stuff (as insurance!) and it’s working perfect now. I could not find a single solder flaw elsewhere, other than that one spot!
In general, the build quality is very good and I don’t think this has a lot of hours. I don’t think the original STK chip was bad so I’m putting it in my parts collection.
