Ashdown MAG300 EVO2 problem

[Mick Bailey]

I don't think this is a failed capacitor. High ESR would mean excess ripple, but you'd still read a voltage. If the cap goes open you'll just have the rectified supply. A shorted cap will blow fuses.

It's a total mystery how the +65v rail can drop and the output still centre on 0v. Can't wait to find out how this one works out.

[m1989jmp]

I don't think this is a failed capacitor. High ESR would mean excess ripple, but you'd still read a voltage. If the cap goes open you'll just have the rectified supply. A shorted cap will blow fuses.

It's a total mystery how the +65v rail can drop and the output still centre on 0v. Can't wait to find out how this one works out.

Well, it was an operator error. I measured V+ and V- in respect to safety ground because it was impossible to get under the board.

This time I ripped the whole board out and measured to filter cap junction ground; voltages are -69 VDC and +69 VDC, that seems right. Sorry for throwing you guys off there.

Voltage at output still at 0 VDC. V+ and V- voltages are present at output transistor collectors.

How do I proceed from here, it seems rail voltages are fine?

[g1]

So you still have loud hum but 0VDC on the output?
And the line out/preamp out has the same hum or not?

[m1989jmp]

So you still have loud hum but 0VDC on the output?
And the line out/preamp out has the same hum or not?

Line out has no hum and EQ responds along with compressor etc.

Tomorrow I'll try hooking up power amp again to see if it has hum, but I won't hope it sorted itself out

Also, I'll make voltage measurements of all transistor junctions to see if something's off.

Any other voltage measurements I should make that could point to something?

Thanks

[J M Fahey]

Unplug power amp signal in PL1 , any change?

Short across R3 or C3 ... does hum disappear?

With amp OFF measure R4-13-20-23 Are they "almost a short" or open?

Measure output rail voltage at the pcb (the junction of these resistors) and at the output jack hot terminal. Do you get the same value?

Measuring Vbe and Vce at every transistor is fine and I often recommend it, but slipping a probe tip can easily short 2 very close pins and cause a DISASTER so be very careful.

Sharpen tips like needles (yes, on a grinding stone) so they "bite" where they touch and do not slip, preferrably touch solder pads because solder is soft, transistor legs are hard and slippery.

Or much better, get a set of these:

you thumb push and release them so they firmly catch component leg and stay there

[The Dude]

FWIW: These probe extensions work well, too. Great for getting into tight spots. I have a pair and wouldn't be without them. I'm not suggesting you buy them here- they just had better pictures than most.

https://jet.com/product/detail/d5ce6...9-62a575d2ec05

[Mick Bailey]

I find a good check (once I've ruled out shorted/open power transistors) to measure the voltage across the emitter resistors. This tells you whether the transistors are getting their bias voltages on their bases. In an amp that was previously working OK you should get a reading. Only millivolts, though. No voltage could indicate the bias circuit onwards is damaged.

[m1989jmp]

I made some measurements on the circuit.

I've attached the schematic with corresponding voltages; keep in mind that I started from input transistors towards output transistors, and supply voltage dropped from 70/-70 to 69,5/-69,5 during the measurement.

All relative voltages for each transistor are good though, since the voltage couldn't change that much while making measurement on a single transistor.

I noticed the TR7, TR8, TR12 and TR13 got a bit warm by the time I finished measuring, about 15 minutes.

Also, I've included measurements on the 4 diodes in the circuit.

[m1989jmp]

Unplug power amp signal in PL1 , any change?

Short across R3 or C3 ... does hum disappear?

With amp OFF measure R4-13-20-23 Are they "almost a short" or open?

Measure output rail voltage at the pcb (the junction of these resistors) and at the output jack hot terminal. Do you get the same value?

Measuring Vbe and Vce at every transistor is fine and I often recommend it, but slipping a probe tip can easily short 2 very close pins and cause a DISASTER so be very careful.

Sharpen tips like needles (yes, on a grinding stone) so they "bite" where they touch and do not slip, preferrably touch solder pads because solder is soft, transistor legs are hard and slippery.

Or much better, get a set of these:

you thumb push and release them so they firmly catch component leg and stay there

No change with both suggestions.

I've hooked it up to a bulb limiter with 8ohm load and I get both humming sound (100Hz if I'm correct) and the bulb lights up bright.

The output voltage is the same on all places you suggested me to check.

Emitter resistors are withing specs (almost short).

Can I short other transistor bases upstream like you suggested with that differential amp input?

[J M Fahey]

No change with both suggestions.

Just to make it certain: both input shorting suggestions are tested with : amplifier ON, straight into mains, no bulb limiter, speaker connected ... did you test it that way?

I am slightly doubtful because you mention:

I've hooked it up to a bulb limiter with 8ohm load and I get both humming sound (100Hz if I'm correct) and the bulb lights up bright.

and thatīs an *entirely* different problem, and not what I am trying to test: whether the "strong hum" comes from the preamp or is generated within the power amp itself.

So please confirm this, and if you had been testing with bulb limiter ON, speaker connected > turn on please repeat the test without the bulb limiter.

Basic voltages and remitter resistors look good, also power transistors look healthy.

Can I short other transistor bases upstream like you suggested with that differential amp input?

Wonīt help, if shorting the amp input connector or the input resistor differential stage itself didnīt work, then any earlier stage will do even less.

Please confirm my doubt.

[m1989jmp]

Just to make it certain: both input shorting suggestions are tested with : amplifier ON, straight into mains, no bulb limiter, speaker connected ... did you test it that way?

I am slightly doubtful because you mention:


and thatīs an *entirely* different problem, and not what I am trying to test: whether the "strong hum" comes from the preamp or is generated within the power amp itself.

So please confirm this, and if you had been testing with bulb limiter ON, speaker connected > turn on please repeat the test without the bulb limiter.

Basic voltages and remitter resistors look good, also power transistors look healthy.

Wonīt help, if shorting the amp input connector or the input resistor differential stage itself didnīt work, then any earlier stage will do even less.

Please confirm my doubt.

I've repeated the test in the following ways:

1) no bulb limiter, speaker connected, PL1 inserted. RESULT: loud LF hum (100Hz most likely) followed by HF squeal. I shut it down immediately.
2) no bulb limiter, speaker connected, PL1 pulled out. RESULT: no hum at all
3) no bulb limiter, speaker connected, PL1 inserted, R3 shorted. RESULT: no hum at all

This would indicate fault in the preamp, right? But when I plug line out into another power amp it works fine.

Below is the preamp schematic:

[Jazz P Bass]

[QUOTE=m1989jmp;465419]But when I plug line out into another power amp it works fine.

That still leaves IC14B as a suspect.

[m1989jmp]

[QUOTE=Jazz P Bass;465421]

But when I plug line out into another power amp it works fine.

That still leaves IC14B as a suspect.

Yeah, and the only suspect I suppose, but I won't jump to conclusions.

[J M Fahey]

Or the problem might be as stupid as poor or no ground at PL1 or the wire connecting them or open ground wire or worn/stressed connector ground pin or cracked ground pin solder.

Please solder a single wire, from C2/R3 ground end at the power amp to R98/R99 ground end (should be joined by a trace).
Plug/unplug connector PL1 and post what happens.

[m1989jmp]

Or the problem might be as stupid as poor or no ground at PL1 or the wire connecting them or open ground wire or worn/stressed connector ground pin or cracked ground pin solder.

Please solder a single wire, from C2/R3 ground end at the power amp to R98/R99 ground end (should be joined by a trace).
Plug/unplug connector PL1 and post what happens.

So you're saying the problem is not in the power amp section?

Based on the fact that there is no hum when shorting the input transistor base?

I'll certainly do what you suggested tomorrow, thanks a bunch.