Peavey Encore 65 Misc Parts Question

[Enzo]

Difference in DC resistance of winding.

Read the thing I posted about the Vactrols. They are equivalent. I don't know which numbers are which, but the document describes the diffrences in types. Should be able to reason the applications.

[TomCarlos]

Unfortunately, finding a datasheet for the 21L628 or the 21L565 has been next to impossible. I will see what kind of parts Peavey ships.

Per the measurements from the actual circuit, there is 9ma flowing through the LED (in the LDR). The center point of the internal resistors is not connected to the circuit. So the circuit is using the outer pins only (both resistors in series).

In the meantime, I have a question on tube V3A (12AT7) of the reverb circuit. As you can see in the photo, V2B (12AX7) and V3B (other side of the 12AT7) appear to be biasing as expected. The issue is with V3A. It seems to have twice the current flowing through the tube as expected.

Blue Lead voltage at top of Reverb Transformer, 434v
Plate Voltage = 427v
Grid Voltage = .05v
Cathode Voltage = 4.62v

I changed tubes (3 different 12AT7 tubes), same results.
I checked R31 and it measures 475 ohms.
I replaced C20 with a new 1uf electrolytic cap, same results.
In fact, changing the cap from the .47 caused a slight increase in the voltage.

So it's not clear to me why I am getting close to 10ma when I would be expecting to see around 5ma.

Any ideas on this?

[Enzo]

Doesn't need to be linear. It is used as a switch. LED is ON or OFF, not slowly dimmed down.

I doubt the reverb tone is from the transformer, rather the circuit around it.

70240102 is the Peavey part number. The part itself is a 21L628. The 21L628 is the data sheet to look for.

I don't offhand know the difference, but one thing that comes up is speed. Light them up the resistance goes down. Turn the light off, and some revert to high resistance faster than others. it may be difficult for you to test for that. 200 milliseconds versus say 20 milliseconds.

Here:
http://denethor.wlu.ca/pc300/optoiso...troduction.pdf

[TomCarlos]

Ok... new reverb driver / transformer is in and the reverb circuit is working. I got the Fender Replacement Transformer (SKU P-TF22921) from Antique Radio Supplies. Perfect fit. Unfortunately, even though this is a "Fender Replacement", don't expect the deep lush sound that you'd get from a real Fender Amp!! But for a Peavey, hey, can't complain.

In a day or so, I will come back to this thread. Enzo got me thinking about the LDRs. Mine are working. But I want to learn more and see if I can figure out the differences in the 40101 and 40102. I bought myself one of each from Peavey and they should arrive next week. In the meantime, I setup a test circuit to evaluate an Xvive VTL5C3/2. No, you cannot measure the "Off Resistance" of 10 Meg. Your meter will show an "Open." But I was able to push 40ma through the LED and was able to determine that the resistance across the series resistors was close to 1.5K (close enough to the 2K stated value). And sure enough, if you decrease the current through the LED, the resistance on the LDR outputs increases. However, the LED current to Resistance Out is NOT linear. If you look at the datasheet's "Output Resistance vs Input Current" curve, that shows how the LDR behaves. Anyway, when the Peavey LDRs arrive, I will report back.

[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.

[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]

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.

[Jazz P Bass]

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

[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.

[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....

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]

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]

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]

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]

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.