Boogie - cuts out

[acorkos]

Alright fellas, you've convinced me. (I really know why I love this forum) I'm gonna measure the voltages first thing tomorrow morning and get right back to you.

good man

And yes, Bob, my DMM has a diode checker - I'm quite embarrassed that I just don't know what to use it for.

i don't completely understand transistors....but i do know that my DMM beeps when there's a short, and when a transistor is open, there's no voltage where there should be voltage.

[txstrat]

Actually I've thought about buying a new (better) one frequently. Maybe it's the right time now.

[txstrat]

OK here we go. All negative voltages of the EQ except one are in spec of +/- 0.3 volts.
For the connection between Q2 and Q4 the schematic shows 12.4 (positive) and is actually -10.6 volts.
Even if I assume the schematic is missing the minus sign there's still a difference of two volts. Don't know if that's OK.
Also when I touch these points with my meter the amp starts to hum badly and I receive a radio station that can clearly be heard over the amps speaker. Not too bad either, huh?

The Transistors are:
Q1 = [MPS A63]
Q2+Q3 = [MPS A70]
Q4 = [MPS A20]

Looking at another Boogie schem (of the dual caliber) theres the same Transistor configuration and similar voltages but the connection between Q2 and Q4 shows a negative voltage. I assume this should be the same in the amp on my desk too.

What to do now?

[acorkos]

hmmmm

don't know about that positive voltage being negative actually....maybe someone else with more experience with these things will chime in with an explanation. is that +7.5V between Q2/Q3?? or is that negative too? i might consider replacing Q4 for the heck of it. also, check the caps at the input/output of the EQ circuit for DC leakage...the .01uF and the 1uF. and check the 10uF polarized electrolytic to make sure it's not shorted or showing signs of leaking

[acorkos]

what about general gunk in the faders themselves? it wouldn't hurt to slide each one up/down a hundred times following a teeny spray of deoxit.

[txstrat]

The faders seem to be OK - no noise while moving them. And each changes it's frequency band smooth and even. So far for a short check.
The voltages ought to be negative in the whole EQ section, ragarding the Dual caliber schematic, which uses the same Transistors and close by voltages. (see pic)
I don't mind to change some parts and I'll check the mentioned caps next.
Why do you assume Q4 might be bad? I have no Idea where the signal goes in this circuit. Especially when the EQ is off.

[Robert M. Martinelli]

The faders seem to be OK - no noise while moving them. And each changes it's frequency band smooth and even. So far for a short check.
The voltages ought to be negative in the whole EQ section, ragarding the Dual caliber schematic, which uses the same Transistors and close by voltages. (see pic)
I don't mind to change some parts and I'll check the mentioned caps next.
Why do you assume Q4 might be bad? I have no Idea where the signal goes in this circuit. Especially when the EQ is off.

Hi again Matt!

I wouldn't trust the last schematics you posted, as some of the voltages shown are clearly wrong (e.g. Q1's collector is tied directly to the -15V rail but the schematic calls for -11V).

There's no way any voltage can be positive with reference to GND, because the EQ is supplied with a negative-to-ground voltage, so at least some signs in the previous schematics are wrong too.

I'd definitely pull the four BJTs off the circuit and test them with your DMM.
(those 10V out of Q2 are suspect )

Hope this helps

Best regards

Bob

[txstrat]

I've desoldered all transistors and tested them with my multimeters diode tester. All readings were OK except the forward voltage of the A63s collector showed some weird behavior. When I connect the DMMs com to the base and the DMMs + to the collector it shows a reading for half a second which then disappears and the DMM shows infinite (open).
I borrowed my neighbors DMM (which has a transistor tester) and the A63 showed 000 while all others showed 004. Which is, I assume, the forward voltage.
Does this mean the A63 is bad?

[acorkos]

possibly. for $5, you could replace them all. and then you would know with certainty that they are ok. and if the problem persists, you can rule them out as a possible source.

[txstrat]

possibly. for $5, you could replace them all. and then you would know with certainty that they are ok. and if the problem persists, you can rule them out as a possible source.

Yeah, that's what I thought.
I'm gonna buy these little rascals tomorrow and see what happens.
Hopefully my electronics supplier has them in stock.

[acorkos]

i'm having some thoughts here...maybe someone with a better understanding of this circuit can correct me if i'm not seeing this correctly. it looks like your signal passes from left to right through the EQ circuit (Q1 Q2 Q3) when the EQ is switched 'OFF'. Switching the EQ 'ON' connects ground to the lower part of the EQ drawing (EQ faders, inductors, and associated caps and resistors)...Q2 and Q3 are bypassed when the EQ is 'ON'. The switching Vactrol LDRs in these older Mesa amps are known to go bad....maybe LDR5 in the EQ switching circuit is causing a problem...?

[txstrat]

I've though about them LDRs too, but if one (i.e. LDR5) would go bad I think that wouldn't explain the oscillating when the five EQ faders are all up.
I plugged a guitar cable into the input, switched the EQ on and moved all the faders full up. The signal was oscillating like mad, no matter what the volume was set at.
If you want me to do any testing just let me know. But give me time to solder them new transistors in.

[Robert M. Martinelli]

I've desoldered all transistors and tested them with my multimeters diode tester. All readings were OK except the forward voltage of the A63s collector showed some weird behavior. When I connect the DMMs com to the base and the DMMs + to the collector it shows a reading for half a second which then disappears and the DMM shows infinite (open).
I borrowed my neighbors DMM (which has a transistor tester) and the A63 showed 000 while all others showed 004. Which is, I assume, the forward voltage.
Does this mean the A63 is bad?

Not necessarily,
The MPSA63 is a Darlington BJT, that means you actually have two BJTs in the same envelope, with a total hfe ( gain factor ) which is the product of the hfes of the two Darlington-connected BJTs. When you test transistors with a diode checker you get a reading which is the forward voltage of the junction you're testing ( e.g. B-C or B- E ) when the junction is forward-biased; when testing a transistor using a DMM's "transistor tester" function you get a "gain" reading instead. The DMM applies a small Base current, measures the Colcector current and gives you a gain reading dividing the second one by the first one. The MPSA63's hfe can go as high as 5000, so it's impossible for the transistor tester to state the collector current ( even a 0.1mA Base current would result in a Collector current the DMM simply can't provide ).

Having said that, the "004" reading you get when testing the other BJTs is suspect, if they tested good when testing the junctions using the diode tester, they should be considered good, so, when tested with a transistor tester, they should show a higher gain factor ( 4 is definitely too low for a small-signal audio BJT, with values from 100 to 3-400 being fairly common ).
Could it be they've been inserted with the wrong orientation/polarity into the DMM socket? ).

Hope this helps

Best regards

Bob

[txstrat]

Could it be they've been inserted with the wrong orientation/polarity into the DMM socket? ).

Oh man, yes, could be wrong orientation. I checked the data sheets for orientation before I tested and even though I might mixed up E and C - darn.
The DMMs hfe tester has NPN and PNP sides and each has four (4) holes with the letters E,B,C and E.
I think I recall having put them in like in the attached pic.
Will take another reading when I'm back home and double check the orientation before.

When testing the transistors with the diode tester the display showed a number of around 1000 (actually in between 996 and 1004) for the forward voltage and infinite (open) when I interchanged the DMMs probes.
Does that mean something?

[Robert M. Martinelli]

Oh man, yes, wrong orientation. I checked the data sheets of them to check the orientation. Then I put the pins in the appropriate holes of the transistor tester (hfe). At least I thought so. I mixed up E and C.
The DMMs hfe tester has NPN and PNP sides and each has four (4) holes with the letters E,B,C and E. I put them in like in the attached pic.

When testing the transistors with the diode tester the display showed a number at around 1000 (actually in between 996 and 1004) for the forward voltage and infinite (open) when I interchanged the DMMs probes.
Does that mean something?

Hi Matt,

Those BJTs have the same pinout, EBC with the BJT flat side facing you, so according to the pic it looks the BJT on the right has been tested "wrong" with the E and C plugged backwards....EDIT : oh....now I see you've just realized it and edited your post accordingly....OK, good!.

Also, the forward voltage of a Silicon Junction should normally be in the 500 to 600 mV range, so something in the 1 V range seems not good ( just like the gain factor of four you got out of the BJT tester ).

HTH

Bob