I am working on restoring a vintage amp (a Heathkit, relevant schematic shown below). I believe I shorted Q707L collector to ground (silicon heatsink pads = bad, lesson learned, stick with mica). Q707L of course was bad, R741 literally burned up, as did Q709L so I replaced those components. I also replaced Q706L as it seemed to not be testing good. All of the surrounding components seem to be testing good, including the diodes, resistors, and other transistors like Q705 and Q703, as well as Q704 and Q708. I get sound at a decent volume from the channel but it is a bit distorted and phases in and out a little bit. Putting the other channel in place sounds good so I do not think it is the preamp section or anything upstream. I've exceeded my ability to interpret the circuit to diagnose the symptoms the channel is exhibiting - any thoughts on what could be suspect that would cause the amp to distort but not be dead? I appreciate any thoughts!
Through the decades, a tech needing to replace a bad output transistor ALWAYS replaces both outputs preferably in a matched set.
And along with that for good measure, both resistors coupling them together, in this case the .33 ohms.
There should be NO voltage to ground at idle, maybe a few millivolts only.
Not having the amp on my bench, it's got to be troubleshot, comparing idle voltages from the other side.
And/or an o'scope used to check the sine waves from out to input.
And along with that for good measure, both resistors coupling them together, in this case the .33 ohms.
There should be NO voltage to ground at idle, maybe a few millivolts only.
Not having the amp on my bench, it's got to be troubleshot, comparing idle voltages from the other side.
And/or an o'scope used to check the sine waves from out to input.
Thanks guys. I will recheck the .33 ohm resistors and start doing some voltage checks tomorrow. My fear was to not leave it powered on for too long if something was still wrong but hopefully I’ve passed the smoke test at this point.
Silicon washers aren't bad at all. I've been using them for 16 years, as I prefer them to mica plus that silicon grease muck, but they are more subject to being punctured by tiny bits of metal. You have to be scrupulously clean when fitting them.
I took the coupling resistors out of the circuit to measure and one is indeed bad. I’ll have to get that on order then can reevaluate.