AC30 - Freshly repaired, but failed yet again

[Mick Bailey]

Here's one for my Black Museum.

A guy called me to say he had an AC30 that had been repaired several times by a local music shop, but had failed again. He just dropped it off and it has to be one of the most messed-up amps I've seen. This is just one section, the rest is similar;

[Enzo]

They do run hot, so I see extra ventilation...

[Mick Bailey]

There was also a penny on the circuit board. I never knew they did those super-ventilated models as a coin-op.

[xtian]

Oh, please, show more photos! That's a good one. Is there any chewing gum and foil wrappers on it?

Repair shop: "Geez, man. I have no idea why it keeps failing! Works here at the shop...for the 30 seconds I tested it..."

[g1]

I'm sure those vent slots in the top just scream out 'feed me' to kids. I'm surprised we don't hear about this a lot more.

[Justin Thomas]

I'd make a new quasi-PCB with turrets instead, and jumper wires for traces. The pins on a lot of PC-mount components will fit nicely inside a turret...

Justin

[Leo_Gnardo]

There was also a penny on the circuit board. I never knew they did those super-ventilated models as a coin-op.

They ARE coin-op. But they're not cheap, won't work on a penny. You have to deposit gold sovereigns to keep your Vox happy. Nothing less will do.

[drewl]

Nice!
That's an '80's model right?

I was going to post about a GK 200RCB that was hacked up pretty bad by someone attempting to "repair" it.

Blown output drivers, an 8 amp fuse that melted the bridge rectifier trying to feed dc into a dead short and 220k resistors where they were supposed to be 470 ohm.
Guess the "repair guy" read the melted colors wrong.

Not to mention the ripped up traces
Oh well, it's fine now.

[Mick Bailey]

It's a 70s amp. The boards have a 71 before the location reference, so maybe that's when the circuit was drawn up. They're well-known for burning the PCB (and probably one of the worst AC30s you could buy), and it isn't flame retardant. My plan was to rebuild the power amp as a classic 60's style. A replacement chassis panel in sheet metal, with tag strip and the tube sockets mounted on the new panel for heat dissipation. All in the same locations as a '64. Maybe also to move to separate cathode resistors and bypass caps to prevent current-hogging. A dual-section can cap means just a few resistors and caps to wire directly to the sockets. I have the means to scan and re-create the original PCB with FR4 and heavier copper, but it ends up being the same problem for a future generation.

The preamp needs an overhaul, too.

The thing about these amps is the chassis was a re-use of the old one, just with a large rectangle cut out to clear the PCB. In this respect it's fairly straightforward to rebuild it and dispense with any circuit board.

Sadly, the power transformer has been replaced with an incorrect type. The original PT acted as a chassis brace, but the replacement is another bodge and the chassis flexes so needs further stiffening.

More pics to follow.

[Gingertube]

See my post #5 here:
http://music-electronics-forum.com/t42816/
Cheers,
Ian

[Dave H]

Maybe also to move to separate cathode resistors and bypass caps to prevent current-hogging.

I once did some measurements on the same pair of EL84s and found that bias current matching was 4 times better with separate cathode resistors than it was with a common resistor. Fixed bias was about 2 times better than a common resistor. I did fixed bias by replacing the common cathode resistor with a 15V zener.

[Mick Bailey]

Separate cathode resistors it is, then.

[Mick Bailey]

A minor setback with progressing with this job. It's a Stolec Vox, possible around '72 with reverb, and the reverb tray and connecting leads have been removed. There are some redundant holes full of dust in the bottom of the cabinet that I presume are the tray mounting points. The problem I have is the owner insists the reverb was working. So there's that one to resolve.

Does anyone know if these amps were fitted with a piezo reverb? There's no sign of a transformer or any mounting location/holes and I can't locate the specific schematic for this particular amp

[52 Bill]

Does anyone know if these amps were fitted with a piezo reverb? There's no sign of a transformer or any mounting location/holes and I can't locate the specific schematic for this particular amp

Don't know for sure, but I doubt that it still had a ceramic cartridge reverb by then. Could have had a direct tube driven reverb like an Ampeg or a Gibson.

The basic schematic for the AC-30 didn't change that dramatically from year to year, so count the count the preamp tubes. Is there one or two additional ECC83?

[Mick Bailey]

One additional ECC83 compared to a JMI-era top boost model. That's 6x ECC83 + 1x ECC82 total in the preamp.

Here are a few more pics 'as-received'