Checking R57 with an infrared thermometer didn't show much temperature increase after leaving the amp on for ~10 seconds. Definitely smell something getting hot though. Scanning around with the IR thermometer didn't reveal any obvious hotspots. Wish I had a temperature camera!
Anything else I should check before replacing the eBay Mj15022s with Mouser 15024s?
Anything else I should check before replacing the eBay Mj15022s with Mouser 15024s?
So how much voltage are you getting on the emitter resistors, with the bias stack shorted? Shouldn’t get *any*. If you’re smelling something getting hot, there’s current flowing somewhere it shouldn’t.
First emitter resistor I checked (R50) showed a large negative DC voltage (~50V!) on the speaker jack side with the bias stack shorted. Should I check both sides of all emitter resistors? I worry I'll cause serious damage if I keep powering it up in this state.
Confirmed that the +85V wire from the power supply board is connected to the collector of Q19 and the -85V wire from the power supply board is connected to the collector of Q23. I did note that GK's labeling on page 18 of their service manual is wrong. Red is +85v and brown is -85v. I removed the red and brown wires from the output board and confirmed that red is +85V and brown is -85V (a bit less due to the DBT). With the red and brown wires removed from the output board, the DBT remained dim and I didn't smell anything getting hot.
Finally had some time to replace the eBay MJ15022s with Mouser MJ15024s. DBT now stays dim with everything connected.
Next step is wg_ski's recommendation here. I bought a 100 ohm 15W resistor for this test; how long should I run it into this load to make sure there are no issues with breakdown voltage and that it won’t oscillate or latch up? 10 seconds? Should I leave this load connected when exercising the bias circuit (this means just rotating the trimpot while the amp is on, right)?
Next step is wg_ski's recommendation here. I bought a 100 ohm 15W resistor for this test; how long should I run it into this load to make sure there are no issues with breakdown voltage and that it won’t oscillate or latch up? 10 seconds? Should I leave this load connected when exercising the bias circuit (this means just rotating the trimpot while the amp is on, right)?
You can run it on the 100 ohm resistor as long as you like. You’ll know pretty much instantly if there is a problem. The amp will ether work or it won’t.
You should always do the final bias adjust with no load. Remember, you’re measuring a voltage across the emitter resistor. You don’t want any load current mixed in there.
You should always do the final bias adjust with no load. Remember, you’re measuring a voltage across the emitter resistor. You don’t want any load current mixed in there.
Ok, but then I shouldn't have the DBT in the power chain when setting the bias, right? Or should I check the bias first with the DBT?
Check bias function with the DBT first. Then set to zero bias with the DBT. Then set it for real without the DBT, once all issues are cleared.
Ok got it. Forgive my ignorance, but what should I measure across R48, R49,& R50 at "zero bias"? Turn the trimpot until the DC value is at its lowest?
A “zero” bias setting is turned up to just before you start getting current in the emitter resistors. When you exercise the pot, you will see current (voltage on the emitter resistors). You turn it down just past where it goes to zero before you take off the DBT. That will ensure that it’s not set too high when you take it off the dim bulb and you get full voltages. You could start with it all the way down (for minimum voltage between the bases of Q17 and Q21), but then you don’t really know when it’s going to “start” when you go to adjust the bias for real.
Bulb stayed medium-dim with the 100 ohm load attached. Next steps are to set zero bias (no load, on DBT), then play some music through a speaker on DBT. If that all checks out, try music again through a speaker with no DBT.
A bit of a setback: I removed the load and set my multimeter leads across r49. I powered up and initially read 0.0mv. However, a few seconds later I heard a faint click (like a really quiet relay) and the DBT glowed bright. I shut off the amp and could smell hot electronics. The multimeter couldn't have caused this behavior, right? I'm 100% certain I didn't short anything with the alligator leads.
Really stumped on what could have happened...
Really stumped on what could have happened...
> set my multimeter leads across r49. I powered up ...a few seconds later ....the DBT glowed bright. ...The multimeter couldn't have caused this behavior, right? ...
What mode was the meter? Volts? Amps? Ohms? HFE? uFd? Temperature?
What mode was the meter? Volts? Amps? Ohms? HFE? uFd? Temperature?