74 Musicman HD130 From Hell (HELL I TELL YOU!)

[fdesalvo]

I took in a basket case HD130 (12ax7 PI) with some serious baggage:

1. 2/4 OT mounting bolts are wood screws
2. V1 socket mowing any type of retainer. Held in by stock fasteners aided by those metal finishing eyelets you’d find on the rear panels of any given Fender.
3 V2 socket missing some material with exposed female ends of pins.
4. V1 socket missing material on one side
5. Chassis spray painted black.

I couldn’t play test it, as the caps had puked all over themselves. Hell, I didn’t even dare turn it on, thinking of that 700v lurking under the hood. I replaced all electrolytics, cleaned/lubed pots, Replaced PI socket, and installed some known good tubes.

With no tubes in, the PT makes a very loud buzzing sound with strong vibration. The mains fuse then blows within 3 seconds. This amp was obviously dropped. I’m just not sure of why those sockets had missing material. Overheated? Does it look like the PT’s lacquer coating melted and then was painted over? Was the chassis painted to hide damage? Many questions.

[xtian]

Good photos!

A light-bulb limiter would be helpful at this point, for preventing further damage while you figure out what's wrong. First step might be to disconnect PT secondaries from the circuit, and see if PT still makes noise or draws a lot of current. For reference, a healthy PT in 100 watt amps will draw 10-15 watts all by itself, due to losses in mechanical (magnetic) vibration and heat.

[fdesalvo]

Thank you. I’ve never heard or experienced vibrations this severe prior. Made me think there was some internal arcing. I’ll be able to run tests on the trannies tonight.

Cheers!

[Helmholtz]

No-load losses of a good PT are caused by the magnetizing/exciting current and the primary DCR as well as the magnetic (hysteresis and Eddy current) losses of the core.
No noteworthy mechanical/vibrational losses.

I have some professional experience in transformer loss analysis.

[olddawg]

Honestly, if it’s a beater those damaged sockets may or may not cause a problem. If they don’t arc and the pins connect... who cares? I would float the secondaries on that PT and see if it’s still drawing current before I do anything. I’ve seen old and abused PTs hum like that for years. If you get it going just make sure it’s properly fused and blast it on a dummy load for a while if you have one.

[fdesalvo]

Alrighty. Ive disconnected all PT secondaries and have added them back one at a time. No buzzing PT/Fuse issue until the HV wires were reconnected.

Does this suggest the OT?

[Enzo]

To me it suggests whatever those HT wires connect to. Disconnect the OT CT and that will narrow it down.

SChematic would be nice.

Do the output plates have flyback diodes? They fail more often than the transformers certainly. Turn out the room lights and power up - easier to see tiny blue arc sparks. See any? Especially on power tube sockets.

With CT disconnected, measure resistance to ground from plate pins of power tube sockets. That should tell us a short to frame or to secondary.

[fdesalvo]

Thank you, Enzo (hope all’s well). I’ll have. A look at the ot and will report back. I can’t seem to find a suitable schematic, so I’m at a loss there. Would make life easier. So would a better understanding of how output transformers work. Always learning ha.

[g1]

Schematic is pg.3 of this set:

[fdesalvo]

G1: thank you!

Enzo: ot ct disconnected. Plate leads do run to a diode on the tube sockets. From the plate lead to ground getting around 25M resistance. After the diodes, getting around .5 Ohm. The diodes are only conducting in one direction. Meter reads OL reverse biased. Forward biased reads around 2V. I applied power with the ot ct removed and fuse blew immediately.

[Enzo]

The CT of the OT is its connection to the power supply, so I'd be looking at the power supply, maybe a shorted rectifier or cap.

[fdesalvo]

The power/bias supply has all new caps. I will test the diodes. Thanks

[g1]

Sorry, pg.3 of those schematics. (edited post above)
Make sure of the polarity on those 100uF in totem pole arrangement. Also try lifting the choke at the C44/C45 end to see if the fault is in the 725 or 360V line.

[fdesalvo]

All good, G1. Thanks again.

The noise and issue are present on both sides of the power selector (high and low). Noise is louder and fuse blows quicker on high setting as you’d imagine).

[g1]

You might want to build a lamp limiter to save on fuse costs.