Musicman rehab

[Marc]

I've got a 2275-65 Musicman on my bench that is not putting out the sound it should-like 10 on volume and 10 on master give me maybe sound like a 1 on my deluxe reverb....This is one with the schematics that are just about worthless due to the fuzzy copies that are available on the net.

the OT has been replaced with an off the shelf unit at about 6600 impedance. I'm guessing this is way too high an impedance, does anybody know what the correct impedance should be? Like half that?

the voltages seem to be right on the power tubes in both high (700V) and low (450V) settings. The power tubes bias up at .5v as they should.

This is the model with the 12AX7 phase inverter. The voltages seem low to me: the first triode is 205V on the plate and 1.47 V on the cathode, the PI triode is 275V on the plate, 17.5V at the grid and 41V at the cathode.

I've tried moving the LM307's around and that didn't seem to make a difference.

It the most likely cause the OT? Am I missing something obvious?

thanks!

[Enzo]

The original is not very clear either. Schematic Heaven has a bunch of them, for the preamp side use the 2275-130 drawing, it is clearer.


Ahem, first, are you plugged into the MAIN speaker jack? Not the extension speaker jack?

Otherwise...

I wouldn't be thinking about the voltages. If you dialed the mains down to 90v on a Fender Twin Reverb, I bet it still make plenty noise.


Likewise the OT impedance. The OT could be bad, but if it is OK, then it could be WAAAAYYY off in impedance and still get real loud.

You didn;t mention so I assume both channels are equally low?

Look botom right of the preamp half page - see the two zener supplies? Those are the +/-16v for the op amps and stuff. Are BOTH those supply rails up to voltage and clean? Measure right at the pins on the ICs.

The power amp has a triode input buffer and a single triode PI. What signal levels are going into that triode grid? And at its plate? And should be the same at the MV control? And again at the grid to the PI (with maxed MV)

[Marc]

The original is not very clear either. Schematic Heaven has a bunch of them, for the preamp side use the 2275-130 drawing, it is clearer.


Ahem, first, are you plugged into the MAIN speaker jack? Not the extension speaker jack?

Otherwise...

I wouldn't be thinking about the voltages. If you dialed the mains down to 90v on a Fender Twin Reverb, I bet it still make plenty noise.


Likewise the OT impedance. The OT could be bad, but if it is OK, then it could be WAAAAYYY off in impedance and still get real loud.

You didn;t mention so I assume both channels are equally low?

Look botom right of the preamp half page - see the two zener supplies? Those are the +/-16v for the op amps and stuff. Are BOTH those supply rails up to voltage and clean? Measure right at the pins on the ICs.

The power amp has a triode input buffer and a single triode PI. What signal levels are going into that triode grid? And at its plate? And should be the same at the MV control? And again at the grid to the PI (with maxed MV)

Enzo,
thanks for your detailed response.! I've got the 130 schematic up on the clip board now too..

Yes, I did double check for the correct speaker output jack. I haven't made that oops in a while.

Both channels are equally low in volume.

The preamp voltages (at the can) are +/- 15.8 volts all the way down the circuit card. Is it normal for the can tops to be -15.8? I can't say I've measured one of these 'vintage' opamps before. The ac residual on both sides is about .005v wrt ground.

I have measured for continuity from the MV to the PI input grid but I haven't measured signal voltage yet. No broken or open connections that I've found.

I've resoldered each joint on the PI fiber circuit card.
I've lifted both cards to see if there was any stray crud underneath.
I bought this amp at a Hamfest knowing it would be a project, so I am not surprised it has stumped me so far....

I'll try the signal measure and see if my scope will tell me anything useful.(if it's working today)

Thanks
Marc

[Enzo]

Once you have established power is OK< then start signal tracing.

[Lee_ranaldo]

i have a clearer musicman HD-130 schematic from another forum.

currently i have to restore the same amp, the previous owner did a complete "tuning" removing the power tube stage to make it full solid state with to-3 transistors (2N3055 and others).

Anyway i can guide you yo restore the amp to original specs

please BEWARE, musicmans use separate grid voltage and a voltage doubler circuit. (670-725 volts DC!!! on plates).

Please check diodes in the voltage doubler circuit, rectifier diodes for the pre-amp board and bias voltage.

The OT must be way too high for a pair of EL34 i guess.

What i do always on my amp is to disconnect the wire that goes betweent the 2 diodes of the voltage doubler circuit. it's the same as standby on but safer.

+/- 15 volts are ok since the schematic shows +/-16 volts.

if any voltage are way too off the spec, you should replace the 680 ohm resistors and zener diodes in the pre-amp board. I did that with oversized diodes and resistors just in case.

[Marc]

Enzo,
I'm not sure what I am looking for at this point...It sure would be nice if there was more info on the schematics that was legible.....

I set my sig generator at 0.5V at the input and with the volume and mV at 10 the buffer input triode is was 1.34V.

The top of the MV it was 38.4V

The PI input was 38.4V
The PI anode 26.4V
the PI cathode 25.5V

With no power tubes in the sockets the voltage was 26.4V/25.6v at the input grids

I measured on the AC setting on my DVM

What do I do now?
Thanks,
Marc

[Amp Kat]

From the voltages your getting it sounds like your amplfying and I don't have my log calculator but I'm thinking around 35 dB of it so that looks good. What frequency are you coming in at ? I like to set my generator to a small sign wave amplitude and send about 300 to 400 hz signal to the input and check it with the scope at the tip of the jack so you know your signal is there and your jack is good. After the first op-amp that signal should spread out over the spectrum of the scope up into the top area where you have to crange the voltage selector of the scope down. Check it again at the PI grids and the PI plates to verify it's still there. Since you have the model with the non transistors check the power tube grids at pin 5 for that same signal and at pin 3 for it to increase like it did on the first stage but be careful at pin 3 and you may want to keep it in low power mode for these checks. Try wiggling the power tubes gently or the preamp PI tube as you may have a bad connection there which would give you that symptom.

[Marc]

Thanks AmpKat,
I should have mentioned that I'm testing with the amp in low pwer and at 400hz.

I'll try again this afternoon with power tubes in. The tubes are new tube doctor EL34's.

The OT I'm using at the moment was from an organ and it was powered by a quad of EL84's. I figured that would be in the ball park for the impedance and it's nice and beefy. It's foot print is as large as the original OT mounting holes.

thanks,

Marc

[Gtr_tech]

I hope you don't plan to keep that OT in there....it won't hang.

[Lee_ranaldo]

Here it's
a clearer schematic of a HD-130 Musicman.

the HD65 is very similar to this version.

[Marc]

GTR tech,
Yeah I know it's a bit under spec'd but I needed something better than what was there.....

Lee,
thanks for the real clean schematic!

I brought this amp to a local tech who used to work at Musicman back in the day. He has found one bad lm307, so far. Initially we were looking at the tremolo section thinking maybe it was pulling the signal down, but pulling the transistors out didn't make a change. The output section seems to be OK, there simply isn't enough drive.
I'll post more once I have it....

Thanks,
Marc

[Marc]

Well, it's back.
All of the .1 caps on the preamp board have been replaced. They were either open or reading value of 0.01 or less. Now it's better but still not right.
I measured the rest of the preamp caps and they are close to the marked value.

What could have made all these caps fail? Until now I thought this type of can was self healing......
Anybody run across this type of problem before?

I've even considered pulling the board completely and putting in a preamp I built in the early 90's.....heavy sigh....

Marc

[jbltwin1]

musicman rehab

THe LM307's are kind of a weak spot in that amp. There is a lot of chatter about them going bad on the MM board. I personally have not had to replace any but usually end up with tube troubles on most of the one's that Ihave seen. Mostly from guys biasing it like it's a fender amp and they can't figure the IPD. Also, I've found that if you bias them a little on the cold side(about 22-23ma PER TUBE) you get the best longevity on the tubes. If it's a 12ax7 PI amp, It's really important. If it's the SS driver, you can run them hotter but why? I;m really not a fan of biasing those amps from the cathode resistor since it's a ten ohm resistor and drains TWO tubes. You NEED to check the tubes separately, espcially with todays tubes.

[Marc]

update....after setting this MM aside for a while and building a smokin' amp with 6GM5 power tubes and a champ clone, it's time to get this MM back on the bench again.
I've ordered the browndog adapters and new opamps. I'm starting with NE5534's and going from there.... My reasoning is that is if the caps were bad there's a good chance the opamps are too.
So for about $30 I'll find out in a week or so.....and report in.

Marc

[Marc]

Well,
I now have a functioning amp-except the tremolo.
The brown dog adapters with sockets attached and 5344's installed brought the amp to life.
The tremolo probably needs a new transistor and may need the .22 caps replaced......

Marc