Yes, the your new Q1 measurements look nominal. There's some chance the opamp is the entire fault mechanism. Given that the amp output is near the positive rail, the opamp should be attempting a more positive voltage to drive the discrete stage toward 0V--- but it's stuck at about -12V. Sure looks like a failed opamp.
Installed opamp socket, put the 741 opamp in there. New voltage readings:
1) -14.7v
2) -06mv
3) -07mv
4) -14.7v
5) -14.7v
6) -5.9v
7) 14.6v
8) null
I hooked up the meter from on each side of the 33ohm resistors (tried both) on DC mA and I'm still seeing zero there (am I doing this right?) for bias.
Didn't try a speaker load.
1) -14.7v
2) -06mv
3) -07mv
4) -14.7v
5) -14.7v
6) -5.9v
7) 14.6v
8) null
I hooked up the meter from on each side of the 33ohm resistors (tried both) on DC mA and I'm still seeing zero there (am I doing this right?) for bias.
Didn't try a speaker load.
Pin 6 opamp voltage looks encouraging. What is PA amp output voltage? Bias across 0.33 ohm maybe ok, just need adjustment.
You need to measure volts across the 0.33 ohm--- it will be only a few mV.
You need to measure volts across the 0.33 ohm--- it will be only a few mV.
Ah okay, I was looking for milliamps, like biasing a tube amp.
I am seeing zero volts, DC, when I set it to AC I see 0.003 volts. The new, multiturn bias pot seems to do nothing.
No appreciable voltage on the speaker output. That's nice! Should I just plug it into a speaker and see what's going on? I'm putting off the moment of truth.
We're getting very close to the finish line with this one. I plugged it in and it works, for the first time in years!
The sound is distorted, in a way that sounds like underbiasing to me, but it could also be that "wrong" opamp? There is a slight whine at idle. I don't know how wrong to expect that opamp to perform.
I haven't been able to get a reading on the bias. The 0.33 resistors don't seem wrong, but I could just replace them for thoroughness. Measuring the resistance that the bias pot is doing, it is set around 35% of the way up through the 1000 ohm sweep.
I tested it through the light-bulb limiter, and loud notes make the light bulb shine bright; the bulb is off at idle. I won't push it with testing.
Will order a replacement opamp and also another LF351n, which has to come from China.
The sound is distorted, in a way that sounds like underbiasing to me, but it could also be that "wrong" opamp? There is a slight whine at idle. I don't know how wrong to expect that opamp to perform.
I haven't been able to get a reading on the bias. The 0.33 resistors don't seem wrong, but I could just replace them for thoroughness. Measuring the resistance that the bias pot is doing, it is set around 35% of the way up through the 1000 ohm sweep.
I tested it through the light-bulb limiter, and loud notes make the light bulb shine bright; the bulb is off at idle. I won't push it with testing.
Will order a replacement opamp and also another LF351n, which has to come from China.
Distortion might be improved by idle current adjustment. Not sure what to make of whine. A scope would be useful. Opamp replacement might help, but probably a long shot.
Good luck! Keep me posted.
Good luck! Keep me posted.
I was able to get it biased correctly (it was just further up the dial than expected), resolving that distortion, and it sounds great. I'm honestly stunned that it works, considering all the molestation I subjected it to.
It's just about Miller Time. Only thing remaining is a 60hz hum from the preamp section, which is weird because I recently used the preamp section with a separate power supply and there was no hum. Goes away when I unplug the preamp cable.
Many thanks, @BSST and all other punters, for your advice. 🍻🙏🍻🙏🍻🙏🍻🙏🍻🙏🍻
Was well worth the effort to restore this "vintage" amp, and learn how to blow up amplifiers along the way.
It's just about Miller Time. Only thing remaining is a 60hz hum from the preamp section, which is weird because I recently used the preamp section with a separate power supply and there was no hum. Goes away when I unplug the preamp cable.
Many thanks, @BSST and all other punters, for your advice. 🍻🙏🍻🙏🍻🙏🍻🙏🍻🙏🍻
Was well worth the effort to restore this "vintage" amp, and learn how to blow up amplifiers along the way.
Good to hear it finally works. For readers coming to this thread in need of repair,
pls point out what the initial fault was. I also do not see exactly what failed after
that and for what reason.
pls point out what the initial fault was. I also do not see exactly what failed after
that and for what reason.
@as_audio Hard to say what the initial fault/cause was, because this was in my closet 'blown' for years, before I pulled it out and tried to work on it. Earlier on, there was an output transistor replaced, and I overbiased it trying to set the bias, which burned up one of the 22ohm resistors. Replaced that and bias pot. Maybe that blew the U1 opamp. Halfway through this thread, while trying to check live voltages, I shorted one of the TO-220, which caused a reciprocal failure of Q9 and D5, twice, which set me back several weeks of ordering parts and figuring that out. But on the positive side, I didn't pay a dollar to a tech. 🤣 I do hope future travelers will benefit from my mistakes.
It's still not fully solved, as I have a 60hz hum now; it goes away when I unplug the preamp board, but if I attach another audio input signal (off my phone) to the power amp at the plug, I still get that hum. With a signal tracer probe, I can hear it on the poweramp board in many places. There is no fault per se with the preamp section unless it's a matter of grounding or picking up something from the transformer. I don't want to impose more on the goodwill of the troublehooters here, so I'm trying to work it out.
It's still not fully solved, as I have a 60hz hum now; it goes away when I unplug the preamp board, but if I attach another audio input signal (off my phone) to the power amp at the plug, I still get that hum. With a signal tracer probe, I can hear it on the poweramp board in many places. There is no fault per se with the preamp section unless it's a matter of grounding or picking up something from the transformer. I don't want to impose more on the goodwill of the troublehooters here, so I'm trying to work it out.
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Sometimes dollars to a tech are a good investment, but the problem is
finding a good one. The people who replied here are of this profession,
I assume.
..
The hum problem may indicate a remaining fault. Have a close look.
finding a good one. The people who replied here are of this profession,
I assume.
..
The hum problem may indicate a remaining fault. Have a close look.
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Couldn't agree more. The local amp tech recommended to me refused to even look at it. Said it's easier to make money fixing tube amps, solid state amps are a PITA. I'm DIY or die anyway, I enjoy the agony and the ecstasy of climbing that mountain.
Much appreciate it. Hopefully enjoyable problem solving, because it's genuinely making the world a better place.
I am kind of stuck.
I get good ground continuity from the input jacks and at the speaker output, which is where the power amp board goes to ground.
The filter caps are relatively new.
Not much hasn't been replaced on the power amp board. There are two 1uf and three 10uf electrolytic caps. Tried paralleling newer ones to no effect.
It really sounds like a missing ground.
Looks like this on my quasi-scope. 60hz but a little distorted.
What to test?
As you can probably tell, I enjoy troubleshooting. 🙂
I'm afraid I don't understand the picture. Can you provide some details re tested point and the Y and X axises units/div?
If you ground the input to the power amp at R1, is the output hum and noise acceptable? What is your chief problem and what test best provokes the issue?
I'm afraid I don't understand the picture. Can you provide some details re tested point and the Y and X axises units/div?
If you ground the input to the power amp at R1, is the output hum and noise acceptable? What is your chief problem and what test best provokes the issue?
This question is very helpful. 👆
I tried grounding R1: when I ground it to the front face of the amp chassis, it does not eliminate the hum. However, when I ground R1 to the back of the chassis, it goes dead silent. But when I ground R1 to the front plate and run another alligator clip from the front plate to the back plate, it does not go silent.
So there is maybe a problem in electrical-ground coupling of the front to the back of the chassis, or a ground loop? I think this is a known problem with GK amps- which are built from an aluminum chassis screwed together at the corners. However I tightened those screws, tried running a ground (and R1) to the input jack to back plate with no luck. This puzzles me.
So I'm almost there -- it's a ground thing isn't it? My meter beeps continuity between front and back of chassis with less than 1ohm resistance.
BTW the image is from the line out signal of hum at idle, with an Oscilloscope plugin in my DAW. It's showing the sample is at (roughly) 60hz. Y-axis level is arbitrary -55db.
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Yes, it does sound like a grounding issue. They can be very vexing.
How bad is the hum? Can you quantify how much hum reduction you need? Can you post some pictures to show construction and grounding details? I've no suggestions to offer at present. I can't resolve graphics in the pic--- inputs on the left, speakers out on the right?
How bad is the hum? Can you quantify how much hum reduction you need? Can you post some pictures to show construction and grounding details? I've no suggestions to offer at present. I can't resolve graphics in the pic--- inputs on the left, speakers out on the right?
@BSST Yes grounding problems are a bear - seems like most older amps have them.
The hum is too loud to let it be, I'll put it that way. Something is certainly off. I have another similar GK amp that is dead quiet. It sounds like this from the line out:
I'm inclined to take the chassis apart and sand the corners where they meet to freshly raw metal. I can get R1 to ground on the sides as well, but not the front, although I'm not sure what is actually not grounding right. The 15v to the preamp? The main purple ground wire goes from PA to speaker jack. That seems fine. The chassis is aluminum with machine screws like this:
The hum is too loud to let it be, I'll put it that way. Something is certainly off. I have another similar GK amp that is dead quiet. It sounds like this from the line out:
I'm inclined to take the chassis apart and sand the corners where they meet to freshly raw metal. I can get R1 to ground on the sides as well, but not the front, although I'm not sure what is actually not grounding right. The 15v to the preamp? The main purple ground wire goes from PA to speaker jack. That seems fine. The chassis is aluminum with machine screws like this:
All parts of the chassis have to be conductive re each other, but in rare cases the loop formed
by a cabinet can form a secondary winding of the transformer and couple hum this way, yet I
think this is not the case here.
Conductivity can be a problem when panels with heavy paint or strong anodising are used.
by a cabinet can form a secondary winding of the transformer and couple hum this way, yet I
think this is not the case here.
Conductivity can be a problem when panels with heavy paint or strong anodising are used.
Cleaned up the inside of those chassis points and now the grounding of R1 now works on the front panel, but it did not solve anything with the hum.
At least eliminated one factor, but back to the drawing board. What was R1 going to tell us about in the first place?
At least eliminated one factor, but back to the drawing board. What was R1 going to tell us about in the first place?
The R1 test was to confirm that the output stage was acceptably quiet. But you've progressed well beyond! 🙂
Some grounding exploration: Can you remove both output jacks from the front panel so that they don't touch the chassis? Any change in hum?
Depending upon results, perhaps also remove input jacks so they don't touch chassis either? Assuming all jacks floating re chassis, does an ohmmeter check reveal any bonding of amp circuit common to chassis?
Some grounding exploration: Can you remove both output jacks from the front panel so that they don't touch the chassis? Any change in hum?
Depending upon results, perhaps also remove input jacks so they don't touch chassis either? Assuming all jacks floating re chassis, does an ohmmeter check reveal any bonding of amp circuit common to chassis?