Mesa Boogie D-180

[CJ]

tried plugging the bass into input #1, sounds like they guy on Dance to the Music/Sly Stone

 

[CJ]

here is a guess at the turns  and wire size for the speaker to 600 ohm balanced line transformer based on a 2" Mean Length Turn of #38 wire on a 25EI core and the measured DCR and inductance,

usually the signal taken off the speaker is pretty dirty, so this transformer output probably does not get used much in the real world, but hey, jus sayin...

looks like some padding going on, and some filtering to get the stage light dimmers off the signal,

 

[CJ]

there is a chunk of copper  surrounding this 600 ohm xfmr,

 

[CJ]

here is a shot of the micro caps used in the preamp, one was bad so we replaced all three as a precaution,

this amp comes with spare parts, notice the current limit diode circled in red, both leads soldered to the ground trace, was wondering why the schemo shows 2 but the pc board had 3,

this is a tight layout, probably trying to keep surface area and thus noise down as this is a high gain amp, to get a smaller footprint you would have to stuff the caps inside the vacuum tubes, reminds me of a Blaupunckt car radio,

 

[kafka]

OK, glad you figured this out.  This explains why everyone hates the MV on the D-180.  This is basically the same circuit as the Limit knob on the Mesa S.O.B., which everyone hates just as much. (I know, because I own one).  The amp sounds just fine when you turn the Limit all the way up (er, down, as it's backwards on the S.O.B.), but is just a fizzy mess at anything lower than 8 (or, higher than 2 on the S.O.B.)

Probably the best thing one can do for the D-180 is get rid of the weird MV and install a more conventional one.

here is the inverter circuit with V-ac test tone voltages in red and DC tube voltages in black,

of interest is the master vol control in the cathode of V4, this concept eliminates a dual pot master and 2 coupling caps which would be normally used to implement a master vol on the inverter, pretty clever, but does not have a linear increase in sound when the knob is turned, perhaps a rev log would work better, we have a linear 1 meg in there now,

not much negative feedback being applied, almost to the 'why bother' level,

 

[Matt C]

I'm working on repairing one of these amps right now and running into an odd problem that seems to be coming from the weird master volume circuit.  The amp will pass signal but only if the master volume is turned almost all the way up.  Through most of its rotation, the amp is completely silent with no change in signal level, but towards the top, the amp suddenly comes alive again.  I've tried a different tube and a different pot with no change, so I can only guess that the TCR508 diode is to blame, but I'm not sure how to go about confirming that, and also not sure where to find a replacement - they seem like a rare component.  Any insight into how to tackle this problem??

 

[PRR]

> .....how to tackle this problem??

DC voltmeter? What goes wonky when it cuts-out?

 

[Matt C]

should have included a couple more details, sorry.  Right now moving the master volume pot changes the positive DC voltage on the cathode of this stage, and from what I understand that varies the level of the signal coming out of the tube.  Right now that cathode bias voltage varies between about 20Vdc (master volume at 0) and 2.4Vdc (master volume at 10).  Through most of the rotation, between 20V and about 6V, there is no output at all.  Then things finally come to life between 6V and 2V.  Also noteworthy is that when the voltage is high and there's no signal, there's also no forward voltage drop across the current regulator diode.  At some point around 5V there is a big sudden jump in volume and suddenly there is a forward voltage drop across the diode.

 

[PRR]

I don't understand the failure mode. Not going to beat my brain about it.

Well, if we had a negative supply, we could do it bullet-proof for 12 cents.

Uh, there's a negative supply over to the side. It is a bias source, variable and soft, but it may come from a solid place.

Where's the rest of this amp's schematics?

The CRDs may as well be dumb resistors. Assuming about -58V is available, the resistor is more than 10X the cathode impedance, very little effect, "infinite impedance, close enuff". With two stages cascaded, balance will be 99% "perfect". And perfect balance is not needed in a guitar amp.

FYI: the CRDs are 2.1mA. There are a ton of other ways to make a 2.1mA CCS. Most need more spare voltage than the FET diode, so again a negative rail. If the only rail is like 50V then standard 40V parts won't do, but we could cobble something. But in this case they "should have used resistors". They just got too clever for the parts they had. (In fairness, they had no concept that these amps might be around in the 21st century.)