Universal Guitar Amp with Vibrator and Reverberator

[J M Fahey]

Cool.

[tigerzilly]

Underside cleaned.

[tigerzilly]

As said before, I suspect a botched repair , and even a "perfectly healthy" amplifier will blow fuses if somebody forgot his micas on the table.

I pulled the suspect power transistors. Mica and screw insulators are all there. I'll still replace the micas.

New caps are all in with some slight damage to the corner of the board. I'm still kicking myself for that.

[tigerzilly]

Astro Amp Lives!

I reinstalled the power transistors with new mica and thermal grease. I replaced the two fuses with 3amp fuses to match what it came with. Powered it up with speaker unattached and had no sign of light from the lightbulb limiter. I connected an audio generator and scope and voila! Astro Amp Lives!



I connected the speakers. The tone came through well, but I could hear static. More so on channel 2 and even when volume was down. I also wasn't seeing the vibrator or reverberator working.

I began working on a schematic. This will be a challenge.

[tigerzilly]

Follow wiring to find *where* are those fuses connected to: one "should" be a Mains fuse; the other "might" be a speaker fuse, in series with speaker out wire, but please check and you tell us.

Correct. I looked at the fuses. One is a main fuse. The other is on the speaker extension jack.

[J M Fahey]

Ok, the main point is that they work

You were very lucky

Now you basically need to clean (and eventually replace) "mechanicals" , such as switches, jacks, pots, and connectors, but that stuff is visible/understood without schematics.

As of reverb and vibrato, move their controls and check, also might need some footswitch to enabvle them ... is there a footswitch jack anywhere?

Again, best News is that main ampbasically works.

GTest it both with generator and a real Guitar.

Curious about power supply rail voltages .

[tigerzilly]

Ok, the main point is that they work


Curious about power supply rail voltages .

I'll check voltages tonight. The footswitch jack is in pieces. I have a new one to install. The reverb cable grounds have pulled from the board. I'll reattach them and connect the reverb pan. I'll re-clean the pots and jacks and see if any of the crackly noises go away.

It does seem I may be out of the worst of the woods. Thanks for all your support and advice Juan!

[tigerzilly]

Back at it. Had my son play his guitar through the amp and I thought it sounds pretty good, but he's not convinced. Still some low level noise that needs to be resolved.

I reattached the reverberator cables to the board and attached the reverberator tank. Reverberator still not working. I have signal to the tank, but not out of it.

I didn't install the new footswitch jack to maintain access to the underside of the board until I get a handle on creating a schematic. I tried different combinations connecting the wires to the jack to see if I could get it to work. I did see some variation in signal when I change the speed of the vibrator, but nothing from the intensity pot.

I measured voltages off the transformer and got 21.6vac on each leg of the secondary and 43.9vac across the secondaries. I measured 28.6vdc at first filter cap and -29vdc at second filter cap. I had 23.2 and -24.8vdc at the other two filter caps.

Attached is a pic of the reverberation tank.

[J M Fahey]

Thanks.
I expected around +/-25V rails since 40251 is rated 50V.

Transistors typically stand 5, sometimes even 10 extra volts, but not beyond.

[tigerzilly]

Getting the amp to power up and pass signal appears to be the easy part. The amp is noisy. Noise is unaffected by volume. Still no reverberator or vibrator. Reverb tank seems bad so I'll test with another.

I drafted a layout of the pcb. It shows component values and measured resistor values. Next I'll shine a light from under the board and try to draw a schematic.

[tigerzilly]

I'm having trouble staying focused with this one. I started the schematic, but have a long way to go. So I started desoldering pots to test and clean. The resistive paint on a number of them has flaked off in spots. One pot labled 100k only measured 10k. Looks like I need to find replacements.

[trobbins]

Were you able to check how close your speaker line is to 0Vdc - that will be a simple indicator that your output stage is biasing up ok.

Was a signal on your scope looking fairly clean for mid-tone controls, and could you up the volume (perhaps with a resistor load instead of speaker) to show symmetric clipping?

[tigerzilly]

Were you able to check how close your speaker line is to 0Vdc - that will be a simple indicator that your output stage is biasing up ok.

Was a signal on your scope looking fairly clean for mid-tone controls, and could you up the volume (perhaps with a resistor load instead of speaker) to show symmetric clipping?

When I get the pots back in, I'll power back up and check!

[The Dude]

......One pot labled 100k only measured 10k......

Are you sure? I've not seen resistors or pots lower in value.

[J M Fahey]

Now I can believe itīs actually 10k, you measured 10k, and maybe you misread it as 100k.

Suggest you do not pull parts ar random, and even less having no schematic.

An accident waiting bto happen.

Absolute worst case, leave it aside for now.

Better unrepaired but original than *destroyed*.