Balder lightning guitar amp schematic please!

[nickb]

I did wonder if the trimmer might be a problem. It would certainly explain a thing or two. I should have tested it earlier - sorry.

Let's back up and re-install the 100 ohm then twiddle the trimmer slowly and see how that R36 voltage behaves. If it's not a steady ramp as you turn the trimmer now that you've cleaned it would be best to replace it. I've never had much luck with cleaning pots. Any improvement seems to last a day or too.

PS: Mark has been very busy looking at the feedback resistors incl R23. We'll circle back to that later. 22k should be conservative so leave that in for now.

[bigdrums925]

Mark:
Yes I have a signal generator and o-scope(s)
In the bias circuit R26 measures 38.5k but is marked orange blue orange gold. Perhaps I need to lift one end to measure properly.
It may be time for me to buy a Fluke VOM. Can you recommend which model Fluke I should be looking at?
R25 is 33k.
I'm not certain which is the feedback circuit, consisting of R23, R24? And these 2 are mounted on the board vertically on each side of trimmer RV2 in the picture?
R24 is 220 Ohms, R23 is 22k.

Currently Q11 & Q12 are installed and have been for a while. There are insulators under each of them. There is only 1 screw holding each down at this time.
Q10 is also installed, insulated from the heat sink, and has 1 screw per post 96.
Q13 is not installed at this point.


So should I change R26 from 39k to 10k now? if so what measurements on what terminals should I monitor? Across R36 in mV?
Note R38 is not installed, Nick & I have been adjusting R36 downward, currently 10 ohms.

Please be as simple and straightforward as possible as I just am able to follow what we are doing.

Awaiting your reply, and Thank you!

[bigdrums925]

Installed 100 ohm in R36. Trimmer is a little shaky, 15mV to about 30mV across R36 But it isn't smooth and has a dead spot I think where the voltages jump to 100mV briefly.
What can we do while I order and install the part?

[MarkusBass]

Bigdrums, it's clear that you have to replace the trimmer. You cannot do much if it's not working correctly. You can test it without the output transistors. Just by measuring the voltage between the slider and one of the other pins (in mV range). You shouldn't get any dead spots while doing this.

Mark: Yes I have a signal generator and o-scope(s)

So why haven't you use them? You can test the amp without output transistors just by providing signal to the input and watching the output on oscilloscope. Is it clean and undistorted?

In the bias circuit R26 measures 38.5k but is marked orange blue orange gold. Perhaps I need to lift one end to measure properly.

It looks like it is 36k. Don't change it now. I'm surprised with the high current you get. It is with 100 Ohms resistor, or 1.3 Ohms?

It may be time for me to buy a Fluke VOM. Can you recommend which model Fluke I should be looking at?

DMMs are very cheap. You can buy them even for $10. What you need is to measure voltages in the range of 0-200mV. What are you using now?

R25 is 33k.

It's OK - leave it as it is.

I'm not certain which is the feedback circuit, consisting of R23, R24? And these 2 are mounted on the board vertically on each side of trimmer RV2 in the picture? R24 is 220 Ohms, R23 is 22k.

Yes, I'm taking about R23 and R24. They may be changed in the future but for the moment leave them as they are.

Currently Q11 & Q12 are installed and have been for a while. There are insulators under each of them. There is only 1 screw holding each down at this time.
Q10 is also installed, insulated from the heat sink, and has 1 screw per post 96.
Q13 is not installed at this point.

One screw per transistor is a very bad idea. When the current starts ramping, the transistors will not be cooled properly. Also mounting two transistors from one side and one transistor for the other is a very bad idea. You should use 1+1 or 2+2.

So should I change R26 from 39k to 10k now? if so what measurements on what terminals should I monitor? Across R36 in mV?

No, don't change it now since you get voltages on R36 in mV. Yes, you should monitor voltage on R36 and it should be in mV range (I think that less than 10mV).

Note R38 is not installed, Nick & I have been adjusting R36 downward, currently 10 ohms.

I'm not sure what Nick advised you to do but I think that the output circuit has to be symmetric, meaning: just two output transistors Q10 and Q11, R38 and R39 removed, 10 Ohms resistors installed in place of R36 and R37.

Mark

[bigdrums925]

I removed the trim pot and cleaned it, seems to be working now.
Put it back in with 100 ohm in R36, climbs to about 17mV across R36, variac full on.

Now this is in the configuration Nick and I had it to solve the bias problem: Q11 & Q12 in place while only Q10 in place this side. No Q13

I was using the O-scope when I posted originally, that's how I knew I had oscillation.

My VOM is a Sun DMM-1230

Yes I was a little concerned about testing asymmetrically but hoping we would be able to put Q13 back in soon. But I can't afford to take chances at this point
so I'll wait to hear back with baby steps till we have this in the bag.

So what next?

[MarkusBass]

I removed the trim pot and cleaned it, seems to be working now.

Hmm, "it seems" is not enough, you have to be 100% (or even more) sure that it will work now and in the nearest future. Otherwise, better order some new output transistors .

Put it back in with 100 ohm in R36, climbs to about 17mV across R36, variac full on.
Now this is in the configuration Nick and I had it to solve the bias problem: Q11 & Q12 in place while only Q10 in place this side. No Q13

Bigdrums, I think that you are ignoring some little details and as a result you are not testing what Nick asked you to do. And these little details are the most important in your case. I don't think that Nick told you to leave Q10, Q11 and Q12 in the circuit and change only R36. What about R37, R39, R40 and R42? Are they still in circuit with 1.3 Ohms value?

I was using the O-scope when I posted originally, that's how I knew I had oscillation.

My question is still valid: why don't you use it now when it seems that the amp has a big chance to be working correctly?

Yes I was a little concerned about testing asymmetrically but hoping we would be able to put Q13 back in soon. But I can't afford to take chances at this point
so I'll wait to hear back with baby steps till we have this in the bag.

It's fine that you are concerned about it but why you are still doing it (testing the amp in incorrect way)?

Next steps depend on your answer how Q11 and Q12 are connected to the circuit. Also remember (I remind you for the second time) that assembling the power transistors with 1 screw only it extremely risky. I hope you understand it.

Mark

[bigdrums925]

I noticed while trying to clean the circuit board numerous times that it does not clean like others I done. I can dissolve the coating (flux and the like) but it does not clean up. I have a working microscope, the kind with some distance under the first optics for soldering and the like. I could see there was also this difficult coating on the trim pots electrical contact surface. Because this trim is an "open" type, I was able to clean it effectively. I then tested it with the meter and was satisfied it would perform reliably. Reinstalled, and was able to adjust smoothly from about 4mV to 17mV.

I'm quite sure said nothing about Q11, Q12. At the time there were only the MJ11014's installed and the 8ea 1.3 ohm resistors in place. He told me to put in Q10 and remove R38, and change R36 to either 100 or 330 ohms. Since the others were not mentioned, I did nothing with them. I am trying to follow the directions verbatim. I understand that we should be balancing both sides but there has been no sign of trouble on the MJ11014 side. However I mentioned I could measure no voltage drop across the resistors on that side either. Of course I am concerned that removing and reinstalling these parts is taking it's toll on the traces. I gather from you that I should balance both sides, either 2 or 4 MJ1101x's in place and remove the other resistors as well for further testing. I never know who will be responding next, and sometimes the directives from different tutors here don't necessarily agree, making it difficult to know exactly what I should be doing and not doing. R37, R39, R40 and R42 are still in the circuit as the 1.3 ohm values.

I have yet to have the amp working with a signal applied and a load since I first posted. I have already removed and reinstalled the MJ's a few times so I was only using the 1 screw to hold it in place before soldering. Haven't had a heat issue since posting.

So I haven't heard from Nick for a bit so what you want me to do as I understand it is to remove Q12 and the resistors for Q11 replacing them with 1ea 100 ohm resistor to make both sides even. Then put the 2ea screws in for Q10, Q11. Is that correct? If so, what shall I test for at that point?

[MarkusBass]

I thought that initially all four power transistors were removed (because you had a short circuit in the output section of the amp). Wasn't there a photo with removed transistors? Now, it seems to me that it was not the case. Let's wait for Nick. I don't want to interfere with his ideas how to proceed. I'm sure he will be here tomorrow in the morning.

Mark

[nickb]

I'm back! Sunday is my day off so we went out to enjoy a rare nice October day.

Our immediate aim here is to get the DC conditions right. I can see the merit in using the scope to check its not oscillating but only after we are sure that we are safe DC wise. Running asymmetrically is beneficial for now. First if it all goes horribly wrong you will only loose the one PNP transistor Q10, the second is as least twice the voltage is developed across the R36 emitter resistor making it a more sensitive measurement. It will do no harm. On the other hand you would not want to drive a load with too much power like this.

If you are 100% sure that the trimmer is now good and go back to where we were, i.e Q10, Q11, Q12, R37, R39, R40 and R42 fitted. Q13 out and R36 = 100 ohms. Set the trimmer to the R25 end again, monitor the R36 voltage and do the careful variac thing again. We should be able to get to 100% voltage without the current zooming off. Now check the operation of the trimmer. Report what range of voltages you get across R36 with the trimmer at extremes. If it gets over 1 volt something is not right.

BTW the 39K resistor will have a tolerance, probably 5% meaning about 37k minimum. Now add in you meter accuracy, often about 3% on the resistance range and you can see a measurement of 36K is easily possible.

[MarkusBass]

Great. This explains the procedure. If you manage to fire up the amp without the fire , I would also measure voltages on R37 and R40 (and report here what values you got).

Mark

[bigdrums925]

25-45 mV across R36 sweeping the trim pot, pot is linear as I sweep.

0.0 mV across both R37 & R40

What should be the next step?

[nickb]

25-45 mV across R36 sweeping the trim pot, pot is linear as I sweep.

0.0 mV across both R37 & R40

What should be the next step?

The range is OK, perhaps a tad on the high side. I expect it will come down a little once we have a idle current flowing. Q6 needs to be in good thermal contact with the heatsink and all transistor screws fitted. Install Q13 and all the 1.3 ohm resistors again, set the trimmer to the R25 end and run up the variac. Set the trimmer for 25mV across R36 (it should be about the same on any of the 1.3 ohms). Let it warm for a few minutes and recheck.

Next check the DC voltage on the output and adj R1 to get close to zero. With no load, hook the scope to the output and look for any sign of oscillation. Now apply a 1Kz 200mVpp signal to the input and check the output you should be able to get about 82Vpp without clipping.

With no input, Recheck the 25mV idle and the output offset voltage. If all is OK you can connect the load and give it a try. Play for a bit to warm it up and check the idle once more.

[bigdrums925]

Well it is working hooray. The idle current is only about 0.6mV however
Tried it with a speaker load and a guitar plugged in and that works.
0.0 offset, nothing is getting hot and no oscillation.
1kZ 200mVP-P renders a max of 53 volt output
What do you think about the idle current being so low?

[nickb]

I have a strong suspicion that the trimmer was the problem all along. With only 0.6mV I think you will be able see crossover distortion on the output. Now, please....don't hit me... change the R26 39k back to 33k and try once more.

200mVpp gives 53V(pp?) under what settings of the controls?

[bigdrums925]

Changed the R26 to 33k but now only get a 0.0 reading across R36-R43
Haven't tried all the controls yet but both gain and master volumes all the way up.