Anyone know anything about transistors?

[Liam]

Yes, you're right, the output from U7-A is up at 14.7 V (R90-R91 juntion is at 9.33 V today, must be the weather or something), so it's driving the offset

Pin 2 of U7 is at 0 V, while pin 3 is at 1.339 V, so it's doing just what I'd expect an op amp to do, and is nailed to positive. Can't say I fully understand the U7-A circuit, why is so much DC coupled in this amp?

The gate of Q22 is at -12 V, the source is at 0V, so either it's grounding the inverting input of U7-A, or vice versa.

I'm kind of assuming that U7 is still the suspect device. The 4560 seems to pretty commonly available here in the UK. Maybe I should just order one? Can't seem to find the J111 FET quite so easily though.

I think you might get away with no output devices in this one, but most SS amps need them as part of the feedback loop. Left out, the input stage of the power amp tries to "correct" for the missing output reference.

If I had the output transistors in circuit, and the only load was the dummy for the line out (about 1000 ohms I think), would the big DC offset into the bases not cost me another 3 TIP147s? (I've lasted this long without a Variac ;-)) I'm kind of hoping to get the base voltages so I'm happy with them before I put the transistors back in.

Thanks again, I'm learning a lot here!

Liam

[Enzo]

In a lot of amps, the junctions of the output devices are part of the circuit - the circuit relies on their being there. Looking at this one, I tend to think you can get away with the method you are using.

If outputs are shorted, all hell breaks loose, but if the outputs are OK, but the drive circuit swings them all over to one rail, it won't hurt them without a load. Remember it is the load which draws the current. DC on the output into a load and much current flows. Remove the load, and all you have is a voltage at the output terminals.

But continue without the outputs if it is working for you. Just don't expect that to work in all SS amps.

4560, 4580, 4558, most any op amp will work there.

As to the J111, just lift it out. If it makes the amp OK, then it was bad. If removing it has no effect, then it was OK. it was just a potential source of DC into the op amp input side. I'd try a new 4560. It sounds like its output is stuck.

The JFET is a mute. The nature of JFETs is that they are ON until turned OFF at the gate. That -12v is turning it OFF so it does not shunt the signal to ground. A leak from gate to the rest of it COULD put some DC on the signal line. The JFETs used this way can be most any JFET, as long as you keep P channel and N channel straight.

SS amps are almost always totally direct coupled - why throw a boatload of caps in there?

[Liam]

Thanks Enzo, I lifted Q22, and pin 2 of the op amp stayed at 0V. However, weirdly enough the output and pin 3 went negative (output stuck close to negative supply voltage this time). I've got a feeling it may have done this before, but I thought I just had my meter leads the wrong way around.

I'm still suspecting the op amp, so have ordered some, along with some n channel J fets. I'll let you know.

Liam

[Enzo]

In teh shop here, parts are cheaper than labor, so when I suspect a 30 cent op amp, I just change it. If it fixes it, great, if not, well at least we know is isn't the op amp then.

[Liam]

It's not the op amp. Changed it and just the same.

I think it might be the lack of output transistors though - I can't figure out exactly what the feedback loop on U7-A is supposed to do, but it's pretty closely defined by speaker output, so maybe it's supposed to null the DC? (Although it looks like positive feedback to me.) Going to pop some darlingtons in and check.

[Liam]

Yep, it appears the output transistors have to be in-circuit, at which point the output from U7 biases nicely at plus and minus 1.06V, and there is no DC output to the speakers.

Unfortunately there's not much AC output either...and what there is doesn't sound too good. Tomorrow I will be injecting a signal and chasing it through the power amp. But at least the power transistors and their biasing circuit seem stable.

[Enzo]

Oh well, we live and learn, don't we?

Oh, leave that JFET out and see what happens to your levels. Otherwise, do indeed trace the signal through.

[Liam]

Q22 doesn't appear to make any difference, so I've left it out for the moment.

The signal gets through nicely as far as U6-B, but unfortunately the DC bias voltage at this point is only 0.3 V. It goes up to 1.68 V if I unplug the DSP unit. There is very little signal apparently getting back out of the DSP unit, so I'm guessing the 1.68 V bias is its power supply. Does this mean the DSP unit has died? I hope not because it's all surface mount and looks quite difficult to fault find. Can I lift Q9 out, in case it's shorting the DSP output?

It would also appear that the amp has had a "new" input socket recently, and the P20 breakaway looks like it has been out. I'm trying to imagine any scenario in which the "power amp in" socket could have been swapped for a bad input socket, and then might have caused all the problems I've found so far.

All good fun, advice most gratefully received as ever....

Liam

[Liam]

Done a bit more, the DSP board is pulling the whole of the 5V supply down. If I plug it in the 5V supply pulls down to less than 2V with no load and no signal. Tried changing U5 and leaving out Q9, just in case they were shorting the output from the DSP, but makes no difference.

The "power amp in" socket has not been swapped, so it looks like a DSP circuit problem. Unless anyone (and I guess that means Enzo!) has got any better ideas. Going to try to get my head around the digital circuit tomorrow. Not sure how a power transistor problem could have caused all this, but I guess it will all make sense eventually.

Liam

[Enzo]

The Stage 160DSP has a 5v rail on the main board, but on the DFX board it feeds a 3v regulator. If it loads down the +5, I'd check for a shorted 10uf cap before or after that regulator if not the regulator itself. On the DFX - C23, C18, U5. The 1.8v Vreg could be bad, but I don't think it would load the +5 down. The only other place +5 goes there is the reset generator U11.

I cannot imagine how the DSP would affect the power amp unless loading down the +5 is so severe it is loading down the AC that makes +/-16v, which would make offsets in the op amp U7 in the PA.

Q9 is a mute, if it shorts - or is not turned off by the SHUTDOWN signal - it will silence the DSP out, but won't affect the +5.

If teh DFX loads the +5 down, none of the other stuff matters, there will be no output to mute. When on, Q9 shunts the output of the DFX but it does it with 10k of resistance.

If you want to continue without the DFX, look at the drawing. The balanced input to the DFX hits pins 1 and 3 of P19, and the outputs on pins 2 and 4. You MIGHT get away with two jumpers 1-2 and 3-4 across P19. The signal will be very hot, so keep the volume minimal but it ought to work - or at least not hurt anything. And might allow continued troubleshooting.

[Liam]

Enzo, I am a total idiot, I'd got the plug the wrong way round for the DSP board. (Surprised they don't have a dummy pin to prevent this but I guess they shouldn't have to!)

Got a bit more output now, but it's a bit distorted, so still trouble shooting. Hoping I might not have damaged the FX board...

Thanks for your continued help, I'm starting to feel like I'm close!

Liam

[Liam]

Tried the amp with the DSP unti plugged in the right way around. Strange! Effects work, getting signal through, but at low signal levels output is rather distorted and faint, but when I put in a big signal I get plenty of signal out. Used a guitar to check this. Stabbed chords ring loud and clear if I hit them hard, but single string is distorted. The quieter I play it, worse it gets.

Have spent the day worrying about whether I'd killed the DSP unit, and the evening chasing signals through the amp with a meter. Pretty sure the DSP is OK, AC levels look OK, will scope it tomorrow and check where the distortion occurs.

AC and DC volatges all look OK, until I get to TP30, which is a little on the high side for AC. TP33 and TP34 are very low, about half the level on the schematic at TP33 (1V), next to nothing at TP34.

What do I do next? I'll try taking Q22 back out of circuit, but what does Q20 do? Would it cause this sort of problem if it was leaky?

And on a more general note, 2 questions for Enzo:

What is the U7-B circuit for? Some sort of negative feedback for U7-A?

How can I ever thank you enough for guiding me this far?



Liam

[Liam]

Hi,

Done a bit more. The preamp/FX is definitely OK. Took the preamp out to another guitar amp and it sounds fine.

Tracing the signal, it all looks fine as far as U7A (which I have changed for a fresh op amp of the same type), and the bases of the Darlingtons seem to be nicely biased and to have good signal. But the crossover distortion at low signal levels is absolutely terrible. The output transistors all check out OK with a meter, and seem similar to one another in output on a scope, but I'm still confused as to what the U7B circuit is supposed to do.

Looking again at Q20, it works fine, and it just matches the output impedance to the load, right? Be surprised if that could cause massive crossover distortion. What could I have missed or done wrong? I can't find anything obvious.

Going to try to figure out if one half of the signal is working better than the other.

TIA

Liam

[Enzo]

This is a grounded emitter amp - I call them flying rail amps, a term Crown uses. Where you normally expect the output to be is grounded, and the speaker draws from th PT CT which is also the main filter common. Speaker between there and ground.

In the ground return of the speaker, they inserted the series resistances R120-123. All 0.22 ohms, they total 0.06 ohms. Speaker current will drop a voltage across this resistance. That voltage signal then proceeds to U7B where it is fed back to the input U7A. When an external speaker is used, then the current increases through the load and hence R120-123. Q20 is just an attenuator switch to alter how much of that signal is fed back. Ity doesn't affect load impedance though.

Right above R120-123, see L1, the speaker output line inductor, an at its left end the CT connection. That is where the speaker current comes from. Flowing to the right, it feeds teh speaker, adn branching to the left is the feedback loop to the U7A. That and the U7B signal constitute your NFB. Put to me directly, I'd have to say U7B was likely more a limiter than anything.

Crossover distortion always sounds worse at lower levels - it becomes a greater percentage of the overall signal. But is it actually xover distortion?

Are the bases of the outputs both at the 1.08 and -1.08 called for? If they are low, you will get xover. You replaced Q10,11 I recall, but do both diodes D19,20 have good drops? They look like Shottky, so the drop will be small.

Otherwise what does the signal look like at those bases? Clean on both sides?

If any one is truncated, try lifting D21,22 to disable those limiters.

Is the signal the same at both ends of C57, 58? In other words, they are indeed acting to bypass the signal around Q10,11 bias setters?

[Liam]

Hi Enzo, apologies in advance, done quite a bit, so I'll try to keep it short as I can.

D19/20 drop 0.272 and 0.269 in the correct directions. The signals at the bases look clean and correctly biased, and C57 and C58 appear to be passing signal perfectly. I'll go through todays testing in slightly more detail.

I started by measurng all DC biases under zero load, no signal conditions.

Darlington bases at +1.083, -1.189. Supplies at +/- 47.2 V. Emitters between 0.002 and 0.005, all in correct direction. U7B at 0V, but U7A floating at + 0.25 V. R90/91 junction is at - 0.060 V.

Applying an 8 ohm load the positive Darlington bases go to 1.074, negatives to -1.175, power rails and emitters stay where they were. U7 outputs and R90/91 junctions stay as they were.

Injecting a 67 mV sine wave into power amp input, U7A puts out 350 mV, but retains the 0.25V positive offset. R90/91 junction looks symmetrical however, with 140 mV AC signal. This also appears at the bases of the Darlingtons with DC offsets of +1.1 V and -1.092 V. The supply rails stay up at +/- 46.3 V.

Going left to right across NPN then PNP Darlingtons the emitter DC voltages are at:

+ 0.008 0.013 0.012
- 0.020 0.009 0.005

AC voltages are at:

0.008 0.012 0.012
0.023 0.009 0.005

(The emitter is the leg going to the 0.47s isn't it?)

Looking at the signals, the +ve transistors do a good job of the +ve half, but the -ve transistors do not seem to achieve quite the same "width" of the bottom half of the sine wave. The signal shows a classic crossover notch just below the 0 V line with my scope on DC.

Speaker output at this point is 1.08 V into 8 ohms. If I remove the load the output leaps up to 12.7 V rms, the crossover distortion disappears and I get a perfect sine wave.

I'm coming to the conclusion that either that U7B feedback circuit does something weird (can't see how it could, inputs to Darlingtons look OK to me), or there is something wrong with the TIP142/147s that I bought, OR I have made some stupid mistake that I can't see, like getting a transistor of the wrong type, or the wrong way around in the bias circuit (but the voltages look OK).

What do you think, should I go and buy some more Darlingtons. What's the white grease on the back of them called? I'm running out of "surplus" left on the heat sink!

Any help MASSIVELY appreciated, I'm not running out of patience yet, but I don't feel like I'm having fun anymore

Liam