Gibson GA-95 RVT

[Travst]

Phil, R50 is probably R30. I have a hard time reading the schematic in places. It's the 220k resistor with a 451v note next to it next to T1.

What I marked as R47 is the 4k 5w resistor to the left of L1.

I'll have a look at the rest after my son's bball game.

[Phil_S]

I owe you a bit of an apology. I am using the Epiphone EA-300RVT schematic, which appears to be identical, but has different part number references. You may find it easier to read and there are a number of useful voltage notations. Look here: http://www.webphix.com/schematic%20heav ... mbassy.pdf

You may find some minor difference, but these amps are supposed to be identical twins.

Also, I managed to post before I was done. You may have read it while I was editing. Please re-read and check the voltage chart again. Sorry about this.

Enjoy the game.

[Travst]

I'll work from the Epi schematic, as it's much easier to read.

R47 on the Gibson schematic is R55 on the Epi.

R50 = R56

C29B = C30B

C29A - C30A

C30 = C31

C31 = C32

The voltage at the 100k resistors off V1 is 23.7

I've attached 3 labeled pics of the guts and a revised chart.

[Phil_S]

It's impossible for the voltage at the junction of the 100K resistors to be 24V. Plate voltage is 150-ish on V1. It has to be higher than 150V. I am going to guess it is in the range of 250-300V. I posted one of your pictures with a circle around the pair of plate load resistors for V1. The junction is at cup #2? (obstructed), about 3 or 4 cups from the left edge. Red on that junction and black on the chassis, what is the voltage?

See if you can find the pair of 100K for V4 configured exactly like the first pair I circled. Follow wires from V4, pins 1 and 6 to the board. At the end where the two 100K are joined, there should be a wire connecting the bottom end of the "V" made by each pair. So the voltage reading at both points should be the same. Follow the wire from the bottom end of the "V" back to C29A, and that's where you'll also find R43 (4.7K).

I posted a jpg of the schematic with the junctions of the 100K's circled. The circle on the left corresponds to the circle in the picture.

You speculate the blue cap at cup #50(?) is C29B. I don't think so because C29B connects to the two white sandblock resistors on the terminal strip. On the one you guess is C29B, there are two resistors for which I can't read the stripes. What values are those? There is also a wire headed straight down from the board -- where does that wire go?

R55 (4K 10W) and R44 (4K 5W) appear on the terminal strips, bottom of the picture, below the two blue caps.

The smaller one (R44) goes to V10, C29B, R55, and R43 (4.7k). You'll find C29A on the other side of R43.

The larger one (R55) goes to C30B and the choke (L1).

Above R44 and R55, is a small light brown carbon film resistor. It seems the 3 of these are connected at the left side of the picture. That suggests the small resistor is R43 (4.7K), but I can't read the stripes. Please confirm the value. The right side of what I'm guessing is R43 will connect to C29A. There is a piece of purple-ish cloth wire there that I think connects to cup #50, where you have guessed is C29B. I suspect that is C29A, and the two resistors there that form a "V" must be the 100K plate load resistors for V4. See if you can trace back to V4.

I hope I was clear and that I didn't contradict.

See if you are able to trace where these go and get DC voltage readings, black probe to ground.

Still no reading on V10. Can you find anything there?

You can easily identify C31 and C32 for the bias circuit. Because you need negative voltage there, the + side of those caps will be grounded. I'm not able to see the grounds., but I suspect you got those right. You have -44 and -55 at tje - side of those caps? The + side has the dimple, and the - side (away from the power transformer) is plain. Counting from the top of the picture, it looks like lugs 4 and 6 on the terminal strip. I see a lone resistor from lugs 5 to 7, what is it's value, as I'd like to try to identify it?

Finding this problem will require lots of patience. These amps are very difficult to work on. The board is a mess. It would be easier to work on a bowl of spaghetti. Stick with it.

[Travst]

The measure of both 100k junctions is 263v. I clipped the resistor wires and got the wrong reading for some odd reason. Some of these points are dirty and/or corroded, so it's giving me some trouble.

I'll have a go at the rest tomorrow. I feel I'm getting close. How would using the pop test at each solder cup on the board work as a plan after I get all the caps right?

[Phil_S]

Actually, 263V is a good number. It compares appropriately to the schematic value of 224V. If you are seeing 263V at the junction of the two plate load resistors for V4, then the problem is likely on the other side of those resistors. The first thing I'd do is swap V4 for for a known good tube. If you don't have a spare, swap with V5, where you are getting good high readings. See if the problem follows the tube or stays with the socket.

Since your plate voltage readings are high, I'm suspecting maybe the plate load resistors or the cathode resistors. The cathode bypass caps may be leaky causing a partial short of the cathode(s). Pull V4 and meter the resistors R12, R13, R28, R29 to see they match the schematic values. Do a live circuit test to see if voltage is flowing across C15 and if there is a cap across R12, same test.

Do this. Hold off on the other stuff.

[Travst]

Will do. I have swapped a couple of known good tubes into V4. The voltages vary slightly, but it's not significant. This thing came full of NOS tubes and they're all good. I was fortunate.

[Phil_S]

After sleeping on this, I'm thinking the problem is at the cathode(s). Those plate resistors (If I'm right about them connecting to #50) look new. As you raise the value of the R on the cathode, plate voltage should drop. Leaky caps or out of spec (low reading) resistors are a good place to start.

The bias on a preamp tube is sort of like doing plumbing. Think of the cathode as the drain. As you raise the R value, you are reducing the diameter of the drain pipe. This forces pressure backwards onto the plate, resulting in higher voltage.

While you are in there, please get a voltage for V10. I attached one of your pictures. Look for the black circle at the end of R44 (4K 5W). That's where you should be able to get a voltage for V10, on that lug.

[Travst]

The voltage for V10 is 151v. You're correct, the solder cup is marked 50 on the board.

If you're referring to the 2 100k resistors for V4, they are new. I replaced them after testing the old ones that were out of spec.

[Phil_S]

I should have realized and asked earlier....the other end of R44 (the end not circled; 4k 5W) is connected to C29B. What is the voltage there? You had reported that it might be 476V, but I think not, especially if you have 151V at V10. I believe this is part of the puzzle, or I wouldn't bother to ask.

Another thing, to save for later, is that the tremolo voltage on V5 is off. You can tweak the cathode voltage with the trimpot on the board. Adjust it until you have something close to 1.65V on V5, pin 8. See what this does to plate voltage (pin 6). It should rise closer to the spec 176v(?). I say save for later because I don't think it will have any meaningful impact on the main problem, which is lack of output.

[Travst]

I'm reading 290v at the other end of the resistor. The voltage at pin8 of V5 ocillates. I turned it down so it was hitting closer to the 1.65v and measured 151v on pin 6 of V5.

I'll check the V4 stuff tomorrow. I was tied up all day today.

[Phil_S]

IMO, 290 is a good number for C29B. It drops to 263 at C29A. This is across the 4.7k (R43). The schematic shows a drop from 251v to 224v. Both are a 27v drop. So, I think you power supply rail is operating properly.

Also, the 27v drop across a 4.7K resistor means that the combination of all the tubes on the C29A node (V1, v2, V4 if I read the schematic correctly) are drawing about 6mA. The average, 1mA per triode is, IMO, a little on the low side. Nevertheless, it is what the schematic says is right.

This gives me further cause to think that the problem is very localized and I'm hoping we will soon isolate it. No problem with taking your time. I stop by regularly and I'll see your post whenever you get around to it.

To tweak the V5 tremolo, turn the tremolo off. That should stop the oscillation. Also, keep it turned off until you finish all the diagnostic work. There are two good reasons for this. One is that the oscillation may have an impact on the circuit operation, making it harder to get stable voltage readings. The other reason, the one that's really important, is that the optoisolator ("roach") that runs your tremolo is an obsolete part that can't be had for love or money. If you should blow the bulb in it, it can't be replaced. I have a whole pile of notes on possible solutions, but, no matter what you do, it is going to feel like a crisis if you blow the bulb. Be real careful in that area.

[Travst]

To turn off the tremolo, I'll need to jumper the DIN plug for the footswitch. Any thoughts on which pins should be jumpered together?

And, it seems like modifying the plug would allow a regular 2-button footswitch to be used... your thoughts? It looks like only 3 pins of the 5 are actually used, so couldn't a stereo 1/4" plug work for this?

[Phil_S]

To turn off the tremolo, I'll need to jumper the DIN plug for the footswitch. Any thoughts on which pins should be jumpered together?

And, it seems like modifying the plug would allow a regular 2-button footswitch to be used... your thoughts? It looks like only 3 pins of the 5 are actually used, so couldn't a stereo 1/4" plug work for this?

I am easily confused when it comes to switches. I assume the f/s and cable are gone?

I think I'd see if I could get a standard DIN plug of similar size to replace what's there. you'll need 4 terminals to manage the two switches, which are simple on/off type.

The dashed lines on the schematic indicate shielded cable for the reverb. So, you will need cable with three conductors and a shield. Ground the shield and one of the inner conductors. One conductor goes to the reverb (3), and the other goes to the tremolo (5). If you have a metal DIN plug designed to use the shell of the plug as the shield ground, then 3 connectors is good, but don't know if such a thing exists or not. I have no meaningful experience with DIN plugs. You can probably buy what you need at Radio Shack.

If there is nothing plugged in and the tremolo is on, well, then we know that we need to short 4 and 5 to turn it off. A short wire with two clips should do the trick, or, if you have a bit of solid bare copper of suitable size, you might be able to plug it into the receptacle from the outside. I think I might try a bit of 16AWG household wire...I'm not sure how fat the original plug is. I can pull mine later and measure it with the caliper if you like.

[Travst]

I've looked around for a suitable plug, but it seems it'd be a lot easier to set it up in the "modern" fashion with a 1'4" stereo plug to use a conventional 2-button switch that I already have.

I plan on doing the V4 tests you outlined above tonight, as well as jumpering the trem as a temporary measure.