Peavey Encore 65 Misc Parts Question

[TomCarlos]

MEF members:

I have a Peavey Encore 65 that is in need of some maintenance. I am checking parts and making my shopping list.

I noticed several caps on the schematic that have electrolytic cap symbols. For example, C7 is shown as a .47uf, 50v. On the board, there is a purple-ish .47 16 volt cap. The schematic shows less than 2 volts at the cathode. (There are other tubes that have a similar setup). The yellow cap in the photo is C5, .047uf, 630 volts. That appears to be a poly film.

I checked the other "Encore 65" threads here on MEF and other threads. No one asked about that cap. The cap measures fine but I was wondering, is this really an electrolytic?

Thanks, Tom

[Leo_Gnardo]

The cap measures fine but I was wondering, is this really an electrolytic?

It sure is. But if you want to sub in a film cap, no problem. It's not under a lot of stress as a cathode bypass, what's it going to see maybe a volt or two. More than likely it's OK for the next 20 years at least, I'd leave 'er be.

[g1]

The purple is electrolytic. It is marked for polarity. 0.47uF used to be a fairly large and expensive size in non-electro, so they used electrolytics. Now you could just use a non-electro there.

[Enzo]

It looks like an electrolytic, is made like one. The schematic shows the polarity next to it. WHy should we think it is not one?

[TomCarlos]

Mmmm.... my lack of experience with these.... have not seen this before, I cannot find one like this on the net .... take your pick :-) Thanks for the replies.

[Enzo]

If it isn't bad, don't replace it. Or if you have some 1uf, close enough.

Or:
https://www.mouser.com/ProductDetail...3YSdqOPfP4w%3D

[TomCarlos]

Next Q - Referring to the schematic pic in #1, any reason why a plate resistor needs to be 1 watt? It looks like all the preamp tubes have a 1/2 carbon installed.

[Enzo]

it dissipates about 1/6 watt, but they might have used the larger resistor for noise reasons or some such.

A lot of film resistors are smaller than what we are used to for a given wattage.

[TomCarlos]

Thanks Enzo....

I didn't think the power dissipation would be over 1/2 watt, even when under a full signal.

On to the next item I am brushing up on - Optical Isolators. I get what they do. Most of the examples and tutorials show a 4 legged sample. Peavey uses a 5 leg bug. #2 and #3 are marked as 40101. The original Peavey part number was/is 21L565. #1 is a 40102. That part number was 21L628. I am trying to find specs sheets on these so I can get a better understanding of how they work.

Interestingly enough, #3 is connected somewhat differently than #1 and #2. The center leg is not connected to anything. So that is why I am trying to get a spec sheet on this. Oh, one other thing, #3 appears to have some "crud" on the leads. Maybe that isn't a big deal.

I am recapping this amp, touching up solder joints, etc etc etc. Soon, I will reassemble the board to the chassis. But this is a good time to study up on the circuit design and parts.

[Enzo]

Optos are optos. A light source - in this case an LED, compared to the neon bulb in a Fender - and one or two photocells. Photocell is a light sensitive resistor. Dark the resistor has high resistance, Shine a light on it, the resistance goes down. LDR means light dependent resistor.

Three legs? Just means it has two resistors. One end of each connected together. In the case of #1 and #2, that common end is grounded. The LED has no idea how many resistors are watching. In the case of #3, I think what they did was use the two resistor type, and left the center connection open, so the two resistors are in series now. That just gives twice the resistance when dark, making the reverb more OFF when supposed to be.

Don't worry about the oxidation on the legs.

Look at #3. Normally the LED is lit by current through R37. This shines on the photocell lowering its resistance. So the reverb passes through it from the control to V3b. Reverb is ON. Ground the tip of the footswitch jack, and it shorts across the LED, making it dark. That allows the photocells to go high resistance, effectively turning off the reverb.

[TomCarlos]

Thanks Enzo...

After reading your explanation, I found a white paper on Vactrol Optical Isolators. Vactec introduced the compact Resistive Opto-Isolator branded as Vactrol

I'll see if I can find the Vactrol (or other brand) replacement part numbers (if I ever need them) and distinguish the different between the 40101 and 40102. I am guessing the difference is in the dark resistance spec and temperature coefficient. More to come on this.

[Jazz P Bass]

All you need to know: http://denethor.wlu.ca/pc300/optoiso...troduction.pdf
Enjoy.

[TomCarlos]

I finished recapping all the electrolytic caps and replaced a few resistors that were slightly out of tolerance. So the amp is passing a signal - with a couple issues. I will deal with Issue 2 another time.

Issue 1 is a non working reverb circuit. I will deal with the LDRs later. But for now, I have a burned out Reverb Transformer. I discovered this when measuring 0 Volts at Pin 6 of tube V3A. The Blue Wire of the Transformer has 455 volts. But there is nothing at the pin for the Red lead. I removed the wires and there is an open on the primary windings.

I'll give Peavey a shout tomorrow and hoping this transformer (705-00152) is available. If not, does anyone have a lead on a replacement? I cannot find specs on the Peavey Xformer.

Thanks, Tom

UPDATE: Photo of the transformer and the reverb circuit.

[Enzo]

Specs/schmecs, it is a 12AT7 tube driving a reverb transformer. I'd drop a Fender in there in a heartbeat.

meanwhile, pull the transformer, it needs to come out anyway, right? Open it up and see if possibly it is just a failed connection between the winding wire and the insulated wire. Of a broken winding wire. Might be fixable.

[TomCarlos]

Thanks Enzo.... I found the Fender replacement, wasn't sure if it would be a match. But yes, I will yank it out and look at the leads coming out of the paper to see if they are cut or damaged.