Universal Guitar Amp with Vibrator and Reverberator

[The Dude]

Just because the fuse HOLDER is rated at 15A does not mean the amp should be fused at 15A. Unless you have evidence someone under-fused the amp (unlikely), please put the original values back in the amp. If fuses were blown, there is likely a short somewhere. I'd start by testing the output transistors for shorts.

[tigerzilly]

Just because the fuse HOLDER is rated at 15A does not mean the amp should be fused at 15A. Unless you have evidence someone under-fused the amp (unlikely), please put the original values back in the amp. If fuses were blown, there is likely a short somewhere. I'd start by testing the output transistors for shorts.

I'll heed your advise and replace with the lower amp fuses. Thank you!

I now have a count of 12 electrolitics that need to be replaced with obvious signs of aging and failure. I'll check the power transistors next.

[J M Fahey]

For starters: power transistors need to be insulated from heatsinks where they are mounted , a chassis lip in this case, by a sheet of mica, smeared on both sides with thermal grease

what bthey call "cool rib" is the thick aluminum chassis working as a heat sink.
your picture shows a transistor bolted straight to chassis with neither mica nor grease: a large red flag.
Hint: even a perfectly good transistor is automatically shorted by that.
Please show both power transistor closeups, wouldn´t be surprised if it´s also uninsulated.

You need to order TO3 micas, thermal grease, probably nylon "nipples" which insulate mounting screws from chassis, screws and bolts as needed and a couple connection terminals.

Hint 2: do NOT connect speaker or any load until amplifier is certified good, specially check you do NOT have DC st speaker out terminals.

In fact, maybe your lamp filament does not fully light , just blinks and then fades down to red or orange IF you do not connect a speaker.

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.

[tigerzilly]

Ok. These are good tips! I've been getting into tube amps so not as much experience with solid state. I'll add the items you've listed to my capacitor order. Tonight I can take a closer look at the power transistors and post some pics. I was able to measure resistance across E & B and E/B & C as shown on your diagram. Both transistors measured about the same, although I don't remember the values. I don't plan on powering the amp up again until I replace the electrolytic caps.

Who would have thought working on an old SS amp would be as much fun as a tube amp. I've been digging the cherry red caps used throughout. Looking forward to hearing what this things got!

[tigerzilly]

Here are pics of the power transistors. I ordered the mica, thermal grease, screw insulators and the caps.

[g1]

One of those RCA appears to have a mica spacer.

[52 Bill]

One of those RCA appears to have a mica spacer.

They both look like they have mica insulators to me. A quick meter test for continuity would show if they are still isolated from the chassis or not.

From that era, I'd guess that they are house marked 2N3055 transistors.

[J M Fahey]

The one on the left, clearly has mica insulator:



The other one has not, seen better in this picture:



The right one looks less rusty than the left one, same applies to mounting screws, so I guess "somebody" tried to repair the amplifier LONG ago, within few years of production, got exact replacement (must have paid a fortune) but forgot to replace the mica, of course it started blowing fuses and was sent to basement or attic storage.
Emitter and Base legs are insulated, good, and I seem to see nylon nipples under transistor mounting screws.

The amp is still very repairable, specially because I see no burnt spots on PCB and parts ... are parts, can be bought and replaced.

Tiger must be very careful with PCB to avoid burning/stripping tracks and pads when de/re/soldering.
A good soldering iron, good solder pump/sucker and some desoldering braid will help .

The main filter cap on left is domed out and punctured, both must be replaced.

Maybe Axials are available ; if not Radials can have legs bent and one extended so they fit in available space, they must be glued to PCB to avoid vibration.
SWhat value is prionted on them?
I see 4000uF so modern ones will be 4700uF , can´t read voltage.

40251 was a lower grade of 2N3055H and may be replaced with a modern MJ*1502* any day of the week.

Mushroom shaped drivers are beyond obsolete but modern ones are available, just watch pinout comparing both datasheets.
I guess the ones with tiny clip on heatsinks are drivers.

[Dave H]

What's with the two unused TO66 hole patterns? Did it once have driver transistors? 2N3054 or something.

[tigerzilly]

The main filter cap on left is domed out and punctured, both must be replaced.

Maybe Axials are available ; if not Radials can have legs bent and one extended so they fit in available space, they must be glued to PCB to avoid vibration.
SWhat value is prionted on them?
I see 4000uF so modern ones will be 4700uF , can´t read voltage.

All the electrolytics are domed out. I ordered replacements for all. I found 4,000uf/25vdc from mouser. Now after reading your post, I see I had more options in the 4,700uf value. Hopefully the ones I ordered are ok. Attached is the data sheet.

[tigerzilly]

What's with the two unused TO66 hole patterns? Did it once have driver transistors? 2N3054 or something.

Nice observation. I guess I was ignoring the extra holes. Do you think its been messed with?

[J M Fahey]

What's with the two unused TO66 hole patterns? Did it once have driver transistors? 2N3054 or something.

Good point.
Those tiny plastic mushroom transistors seen on the PCB would *barely* drive a couple TO3 power transistors; now if we add a couple TO66 case ones to the mix, it sounds way more reasonable.

TO66 is obsolete, but was used in the earliest amplifiers, a couple early Ampeg come to mind.

Almost impossible to find today, or in the $30 each class, they may be replaced by TO220 transistors with a little modding.

Problem is we are going to need a schematic of some sort to go on with it.

That said, I see no "missing wires/empty holes" on the PCB, and TO3 transistor wires roughly go to expected points.

Assuming a classic totem pole configuration, and split supplies (very advanced for a 1966/67 amplifier),

the transistor on the left has Red Collector going to +V ; Grey Emitter going to PCB "near" Violet wire (more on that later) ; Orange Emitter going away and I guess towards speaher "hot" terminal ; Green Base wire going towards left mushroom which "should" be the NPN driver, and on the right TO3 we have Black Emitter going to -V ; Orange Base going towards right mushroom and Violet Collector going near other TO3 Grey Emitter, both together being the Speaker Out node.

But I am stacking guess over guess here, really Tiger should try and hand draw a schematic.

Those phenolic PCBS are often somewhat translucid , a strong light under it might make it easier to see components on top and tracks on bottom at the same time.

The amplifier "should" be quite close to some ´66 RCA datasheet or transistor book example.

[tigerzilly]

But I am stacking guess over guess here, really Tiger should try and hand draw a schematic.

Those phenolic PCBS are often somewhat translucid , a strong light under it might make it easier to see components on top and tracks on bottom at the same time.

Ok. This will be my strategy. When parts arrive I will detach the PCB from the chassis to expose the connections on the bottom side. Hopefully I can get enough access to remove and install the caps. Prior to reattaching the PCB, I'll take pics and draw a schematic from what I see on the underside. I'll remount the power transistors with new micas and insulators. I'll disconnect the speakers, power up the amp with the light bulb limiter and see If I still have excessive current draw. If I'm lucky maybe the amp will just power up and work, but I'm not usually that lucky.

[J M Fahey]

You might get lucky, why not?

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.

[tigerzilly]

The underside of the board revealed.