Gibson GA25RVT Chassis Shots, Parts Lists?

[stereomonostereo]

Hey guys,

I'm working on a 1965 Gibson GA25RVT that's an absolute nightmare. Typical rat's nest, but with added work by some unknown tech. I should add that this is the 7591 model. There are big differences between what the schematic shows and what i'm looking at. The schematic has labeled capacitors, but doesn't show any values. Does anyone have any clean chassis pics or maybe a parts list that shows capacitor values?

[nickb]

The cap values are all shown in the version I have. The units are all uF.

[Enzo]

And Gibson is notorious for the schematics not matching the amps. I have one model that I have three different published factory schematics for, plus three more I have hand drawn myself from chassis, all of which are different. They might start a run of a model, then half way through change the innards to some other model with the same control complement. Ooops this GA15 is really an EA45 or whatever.

And so many of them have the narrow terminal board with parts on both sides, so gut shots still won;t show half the components.

[stereomonostereo]

This one seems to be close to the schematic, but i feel like something is missing. Originally, the -17vdc bias feed would slowly creep up to +135vdc when a signal was applied. I found the LCR was dead and replaced it with a VTL5C3 using a resistor to knock the voltage down to 150v on the LED +, and read 121 on the other end of the LED. The bias is a steady -17vdc now, but the amp sounds strangled. The lows are buzzy and the notes have a bad decay to them. It really sounds like bad power supply capacitors. That being said, i tested and then replaced C29, C28, C26, C25, and C24. All of the large voltage capacitors. The problem is still there.

I've tested the output transformer, the driver transformer, and all of the voltages. Tubes all test good. Everything looks to be working properly. Still it seems like the amp isn't getting the proper current. Could the replacement LDR possibly have something to do with it? If i'm looking at the schematic properly it really only affects the tremolo. Also on a chassis shot that i came across, I'm counting six large value electrolytics, instead of the five i'm seeing on the schematic.



Two firecrackers with two positive leads, and two single axial leads. That would suggest i'm missing an important cap, or one was added later on.

[g1]

If you have a schematic with a bias circuit and LDR, please post it.

[stereomonostereo]

Here's the schematic:

[nickb]

I wonder if the LDR in that Vactrol is too low a resistance - it's 30K max. You probably need something with a few Megs dark resistance. It will kinda work but it probably be attenuating your signal far too much. I suggest you remove it and substitute with the correct part or leave out for now. What test did you do to check the function of the old optocoupler?

Also it cannnot affect the bias as there is no connection between them, unless it was been miswired of course. Something else is going on - measurement error? The power tubes would be red plating if the bias really got up to 135V.

Also, out of curiosity, what was used originally for the light source?

The distorton you describe sounds like some DC is way off. Do some DCV measurement and compare with the schematic.

[g1]

If you want to completely rule out the LDR, disconnect the resistive side of it.
Did you test the drive transformer with a neon tester, or just with ohm meter?

[stereomonostereo]

With the original optocoupler unit (which was a VTL5C2 that someone put in) i was getting zero voltage out of the LED - side. I replaced it with a VTL5C3. These used a different optocoupler than the old Fender circuits, but i don't have the original from this. I'm going to try disconnecting the resistive end like you suggested. ALl of my DC voltages seem to be correct. I swapped a known good driver transformer with the original.

[stereomonostereo]

Okay, with the resistive side unhooked there's no difference

[nickb]

With the original optocoupler unit (which was a VTL5C2 that someone put in) i was getting zero voltage out of the LED - side. I replaced it with a VTL5C3. These used a different optocoupler than the old Fender circuits, but i don't have the original from this. I'm going to try disconnecting the resistive end like you suggested. ALl of my DC voltages seem to be correct. I swapped a known good driver transformer with the original.

Well the "3" is a MUCH better choice than the "2"

[stereomonostereo]

It's 1 better

[stereomonostereo]

Going over all of the voltages again, i'm reading 314vdc at C24. The schematic is showing 150v going into the optocoupler. R45 tests out good. I'm getting 114vdc at the other end of R44.

[Enzo]

Ohm's Law tells the whole story. You have 300v instead of 150v because too little current is being drawn through R45. 400v down to 150v is a 250v drop and R45 looks like 26k. SO I = 250/26,000 = 9.6ma. Call it 10ma. Some of that came through R44 as a branch circuit. SO if I can read the thing, looks like they expect 90v at the bottom of R44. So 150 to 90 is 60v, and R44 is 150k, so I = 60/150,000 = half a milliamp. So lets call R45 current for the opto only 9ma.

The original opto had an incandescent lamp in it, which will behave different from your LED unit.


Have you tried adjusting R56?

On the other hand, we need to make it work, not make it match the schematic. You need to get the current through the LED to vary so as to cycle the photocell part between high and low resistance.

[nickb]

Intersting info about the incandecent bulb Enzo.

I see it drops 7v at 6mA from the sch. I'm guessing a 60V 20mA type would do the trick - it would be easy enough to make your own replacement opto with one of those and an LDR inside some heatshrink, a bit like the Fender (neon) ones.