I've had this amp for awhile. I won't bore everyone with the long story.
First what I have done.
I bought a new input board from Chris Hoppe and built it. I checked it out and everything is good. Before I installed the board in the amp the amp worked. The only thing wrong was a high THD in one channel. So the output transistors were all working fine.
After I installed the new board the right channel is perfect! I can't wait to listen to it. The left channel not so good.
Below is the output of the left channel;
Everything on the heat sink is good. I have not only checked all the transistors and resistors, most have been replaced with new ones. I only have 1 pair of output transistors installed right now.
I have done a visual inspection of the new board. Everything looks good. Comparisons to the right channel looks good, components are the same.
I checked Q107(good) and checked Q108(good). I replaced Q108 just in case it was bad and wasn't showing up on the transistor checker. No change in the output.
When I turn the bias pot R119 it doesn't change anything. I checked the resistance of R117, R118, & R119 and they are good.
It looks like the transistors are turning on too late on both sides. So I'm guessing the problem is in the bias circuit, but I'm stumped, I don't know where to look next.
Any help would be greatly appreciated!
Gary
First what I have done.
I bought a new input board from Chris Hoppe and built it. I checked it out and everything is good. Before I installed the board in the amp the amp worked. The only thing wrong was a high THD in one channel. So the output transistors were all working fine.
After I installed the new board the right channel is perfect! I can't wait to listen to it. The left channel not so good.
Below is the output of the left channel;
Everything on the heat sink is good. I have not only checked all the transistors and resistors, most have been replaced with new ones. I only have 1 pair of output transistors installed right now.
I have done a visual inspection of the new board. Everything looks good. Comparisons to the right channel looks good, components are the same.
I checked Q107(good) and checked Q108(good). I replaced Q108 just in case it was bad and wasn't showing up on the transistor checker. No change in the output.
When I turn the bias pot R119 it doesn't change anything. I checked the resistance of R117, R118, & R119 and they are good.
It looks like the transistors are turning on too late on both sides. So I'm guessing the problem is in the bias circuit, but I'm stumped, I don't know where to look next.
Any help would be greatly appreciated!
Gary
No bias. I suggest comparing the DC bias voltages between the two channels, either from NPN gates to PNP Gates on outputs or across the bias transistor. Might be a bad bias transistor or an open pot.
WOW, I can't believe Nelson Pass replied! Thank you very much.
I got part of the measurements done tonight but I will finish tomorrow. From the measurements I made it looks like the bias voltages are way too positive.
I'll post them all tomorrow night.
When you say bad bias transistor are you referring to Q307 on the heat sink?
Thanks again
Gary
Hi Gary!
Yes, on your drive lines, points 5 and 6 after the 100 ohm resistors, you should be seeing 1.1V to 1.9V and -1.1 to -1.9V, varying with the position of the bias pot. These voltages should be opposite polarity and pretty close in value.
Yes, Q307—mounted remotely by cable on the heatsink—is the bias transistor, part of the bias spreader circuit. The bias voltage across the positive and negative drive signals, around 3V, is maintained from collector to emitter of Q307.
I've seen problems with corrosion on the JST board headers and connectors causing the bias spreader to go all out-of-whack. Since April 2021, I've been including new cables and board headers with every board. Are you using them? Or did you buy yours before I started including those cables? If that's your problem, I can send you a set.
Yes, on your drive lines, points 5 and 6 after the 100 ohm resistors, you should be seeing 1.1V to 1.9V and -1.1 to -1.9V, varying with the position of the bias pot. These voltages should be opposite polarity and pretty close in value.
Yes, Q307—mounted remotely by cable on the heatsink—is the bias transistor, part of the bias spreader circuit. The bias voltage across the positive and negative drive signals, around 3V, is maintained from collector to emitter of Q307.
I've seen problems with corrosion on the JST board headers and connectors causing the bias spreader to go all out-of-whack. Since April 2021, I've been including new cables and board headers with every board. Are you using them? Or did you buy yours before I started including those cables? If that's your problem, I can send you a set.
Hi Chris,
I bought my board from you about a month ago, so it came with the connectors. I didn't install them but I will now. Thanks for the insight. I'll post my findings in a bit.
Gary
Below are my measurements;
Q106
Base 85.2V
Collector 84.9V
Q107
Emitter 85.7V
Base 85.0V
Collector 11.0V
R118
L-drv+ 14.8V
R119 Side 12.9V
R117
L-Drv- 12.5V
R119 side 12.7V
Q307
C-E 2.7V
All measurements are to ground, except for the Q307 number.
All of them seem to be way too positive compared to the right channel and based on Chris's input above.
Whatever was going bad may have bitten the dust finally, see the waveform below;
It seems like Q107 is shorted, but I checked it and it was good. I didn't replace it. I haven't checked it since the output changed.
Question, does Q106 help set the bias for Q107?
Anyone got any ideas?
Gary
Q106
Base 85.2V
Collector 84.9V
Q107
Emitter 85.7V
Base 85.0V
Collector 11.0V
R118
L-drv+ 14.8V
R119 Side 12.9V
R117
L-Drv- 12.5V
R119 side 12.7V
Q307
C-E 2.7V
All measurements are to ground, except for the Q307 number.
All of them seem to be way too positive compared to the right channel and based on Chris's input above.
Whatever was going bad may have bitten the dust finally, see the waveform below;
It seems like Q107 is shorted, but I checked it and it was good. I didn't replace it. I haven't checked it since the output changed.
Question, does Q106 help set the bias for Q107?
Anyone got any ideas?
Gary
Yes, indeed the light should come on. Make sure the LED isn't wired backward.
So something is pushing or pulling everything too positive.. The voltage across Q307 is normal, so the bias spreader seems to be working fine, just that everything is shifted positive.
Could this be something like a resistor in the wrong place? Like a decimal place shifted? There are 33.2 ohm resistors in the kit, as well as 332 ohm, for example.
So something is pushing or pulling everything too positive.. The voltage across Q307 is normal, so the bias spreader seems to be working fine, just that everything is shifted positive.
Could this be something like a resistor in the wrong place? Like a decimal place shifted? There are 33.2 ohm resistors in the kit, as well as 332 ohm, for example.
That was the first thing I thought of and I checked everything and it was good.
I'll check again, maybe I missed something.
Thanks
I'll check again, maybe I missed something.
Thanks
Update,
I checked Q101, Q102, Q103 & all are good.
I pulled out the bias pot R119 and it is good.
I also checked every resistor in the left channel and they are all the correct value and good. I didn't find anything in the wrong location.
I pulled out C104, I thought I might have it in backwards or it was shorted, everything was good.
Schematic below if you need it,
The only thing remaining is the DC servo circuit and the diodes.....
Gary
I checked Q101, Q102, Q103 & all are good.
I pulled out the bias pot R119 and it is good.
I also checked every resistor in the left channel and they are all the correct value and good. I didn't find anything in the wrong location.
I pulled out C104, I thought I might have it in backwards or it was shorted, everything was good.
Schematic below if you need it,
The only thing remaining is the DC servo circuit and the diodes.....
Gary
...the output section isn't completely off the hook yet though is it?
It might be helpful to isolate the problem to either the input board or the output section. You can test the input board via the procedure in the documentation, under the heading "Conditions required to operate the board without the output section connected:"
It might be helpful to isolate the problem to either the input board or the output section. You can test the input board via the procedure in the documentation, under the heading "Conditions required to operate the board without the output section connected:"
The voltages on each pin of IC101 & IC151 match within a few mVs.
The DC offset at the outputs are good.
I'm running out of things to check.
The DC offset at the outputs are good.
I'm running out of things to check.
I thought it was, but now I'm beginning to wonder. I checked all the transistors and have replaced them. I only have one pair of output transistors installed right now. I checked all the emitter resistors, they are good.
I checked Q307.
I checked D201 thinking maybe it was shorted and clipping the + waveform. It is good also.
C202 is new
I have not checked R201 & C201. I'm thinking that wouldn't cause what I'm seeing on the output.
I did the check on your board and it checked OK,
One odd thing was when I have it connected to the output section, if I look at the board output with my scope, the output goes away. It's like the scope kills it.
The real odd thing is, this amp worked before I put your board in. The only problem was a barely high THD. It was about .08% in the left channel.
I do still have those odd voltages on the bias circuit though, so it's got to be there somewhere.
I haven't replaced Q107 yet. I did check it and it was good.
I'm using a Atlas DCA75 Pro checker and I know they won't catch everything. That's why I replaced everything on the heat sink.
I haven't replaced Q107 yet. I did check it and it was good.
I'm using a Atlas DCA75 Pro checker and I know they won't catch everything. That's why I replaced everything on the heat sink.