Gibson SG Systems G100 (Problem with built in 'Wave Form' Fuzz)

[garytoosweet]

Got one of these, original 8417 tubes had failed and took out a few components with it.

I converted the amp to run EL34's, and have everything running great except the built in 'Wave Form' fuzz.

When I turn the Wave Form knob up, it has almost no effect, with the output going slightly down in volume when the knob is at 10.

Here my troubleshooting is falling apart likely due to lack of experience with transistors.

There is some funky voltages around Q6.

Looking at Q6 on the scope I can see the sine coming in at the Gate and a greatly amplified sine at the Drain. (So it's working?)

Looking at Q7 on the scope the sine at the Drain has slightly smaller amplitude than the sine at the Gate. (Should I suspect Q7?)

This part of the circuit is ever so slightly different from the schematic, but should be fine for troubleshooting. Oddly the schematic is dated Oct. 24 1973 and this amp has a production date of Oct. 13, 1973.

[Helmholtz]

1) The fuzz is produced by D1/D2. Check if the relais is working.
2) As Q6 and Q7 have identical circuits, voltages should be the same. Particularly gate voltages should be zero. Check C13 for leakage. Just disconnect one end and see if Q6 voltages change.
3) Check capacitance of C15.
4) What is the voltage at Z5? A 4V zener in this place makes no sense.
5) I don't trust the schematic.

[vintagekiki]

https://ampgarage.com/forum/viewtopic.php?t=8564
SG System & 8417s
_{You must log in or sign up}

http://forums.musicplayer.com/ubbthreads.php/topics/2443246/info_on_SG_Systems_amps
info on SG Systems amps

http://www.guitarsite.com/hotlicks/about4696.html
Please Help!! SG systems schematics

http://img.photobucket.com/albums/v602/myfoot/SG100G1of3.jpg
http://img.photobucket.com/albums/v602/myfoot/SG100g2of3.jpg
http://img.photobucket.com/albums/v602/myfoot/SG100G3of3.jpg

1)
Probably a drawing error
Q7 Drain voltage is 2.65V (not 26.5V)

[vintagekiki]

Looking at Q6 on the scope I can see the sine coming in at the Gate and a greatly amplified sine at the Drain. (So it's working?)

Q6 (TIS58) is correct.

Looking at Q7 on the scope the sine at the Drain has slightly smaller amplitude than the sine at the Gate. (Should I suspect Q8?)

Check (replace) Q7 (TIS58)

Tip
R28 = R33; R30 = R34; Check Ud and Us at Q6 and Q7. Should be almost the same.
If Z5 (4V) is faulty, replacement with any zener diode for 3.9V

[Helmholtz]

Ok, assuming that the 4V zener is correct, Q6 and Q7 would be constituting an overdrive circuit. Low supply voltage facilitates overdrive but wouldn't make sense for a clean gain stage.
Both drain voltages should be about 2.5V.

Are the pencilled voltages in the schematic detail your measurements?

[garytoosweet]

Apologies I misidentified the pair of transistors before, I was reading a different transistors. I have attached an updated schematic detail with penciled in voltages.

Relay checks ok.

Voltage at Z5 is 4.337V. Diode is labelled 749A

Q7 has been replaced with transistor labelled "GE FET2 TI828". 'Singapore" written on top. Searching for a datasheet, I wasn't able to find one.

Capacitance of C15 reads 45uf on my Fluke 179. It's a new Nichicon capacitor. I can pull it out and test in an LCR if there's a reason to suspect it.

Q6 has signal entering and leaving, Q7 has no signal. I pulled C14, tested fine, replaced with a new cap anyways.

Resistance Q7 Gate>Drain was reading 30 ohms with the amp powered on, 124ohms with the amp off.

I'm thinking Q7 is faulty. Ordering some 2N5457 which seem to be a suitable replacement for TIS58.

[vintagekiki]

Cross reference TIS58

https://english.electronica-pt.com/components-cross-reference?ref=TIS58

1)
In order to quickly establish the correctness of TIS58, replace Q7 with Q1 (Channel 1).

[Helmholtz]

Components (resistors, caps, Fets) generally should be tested out of circuit.

As both Q6 and Q7 are supplied be the zener voltage of 4.3V, all Fet voltages must be lower than that. So 27V at Q6 drain is not possible. Check the voltage at the other end of R28.

As said before, both gate voltages must be zero. If not, the coupling cap is leaky. To test C14, use your Ohmmeter. Resistance should be infinite, often displayed as O.L.

I recommend to check if the Fets are actually wired as shown in the schematic.

[garytoosweet]

As both Q6 and Q7 are supplied be the zener voltage of 4.3V, all Fet voltages must be lower than that. So 27V at Q6 drain is not possible. Check the voltage at the other end of R28.

Double/triple checked 27V at Q6 drain. Tracing back, at junction of R28 and R32, I have 30VDC. The junction of R28 and R32 is actually connected directly to the 30V rail, on the opposite side of R32 that is shown on the schematic. I have edited the schematic detail and added my voltages which were the same as last time. Looks like you were correct to not trust the schematic.

As said before, both gate voltages must be zero. If not, the coupling cap is leaky. To test C14, use your Ohmmeter. Resistance should be infinite, often displayed as O.L.

Mentioned in last post, I pulled C14 and checked in LCR meter, tested ok. Replaced with new Panasonic film cap, no change in amp.

I pulled Q7 from board and there is now 0VDC at the point. (With no transistor installed in Q7's place)

I recommend to check if the Fets are actually wired as shown in the schematic.

We have found 1 error so far. Also I noticed there are 2 33UF caps, Q6 and Q7, source to ground on the schematic, but only 1 in the amp. Q7 Source has 8.2k and 33uf to ground, Q6 Source has just a 4.7k to ground. So there are some differences here. Hoping I don't have to trace it out.

Finally, I pulled Q7 from the board. Attaching meter leads to Gate and Drain I got a jumpy reading of around 100ohms. By lightly taping the TO-92 case the resistance is jumping around wildy from only a few ohms up to 1M ohms. I am thinking Q7 is faulty, the voltage I was getting at Q7 gate was leaking from the voltage at Q7 drain. Does this sound reasonable? I will swap the transistor when the 2N5457's arrive tomorrow.

Thank you for help so far, very much appreciated!

[Helmholtz]

We have found 1 error so far. Also I noticed there are 2 33UF caps, Q6 and Q7, source to ground on the schematic, but only 1 in the amp. Q7 Source has 8.2k and 33uf to ground, Q6 Source has just a 4.7k to ground. So there are some differences here. Hoping I don't have to trace it out.

Does the wiring look original?

Yes, seems that Q7 is leaky.


https://www.allaboutcircuits.com/tex...ansistor-jfet/

[garytoosweet]

New 2N5457 in Q7 and all is well.

All wiring is original in the amp. It's possible on the schematic the drain voltages for Q6 and Q7 are swapped. Q6 being connected to the 30V rail explains why there is 26.5VDC at the drain of one of the transistors in the schematic.

It also appears the schematic available for this amp is for a later version, with the one I'm currently working on being a slightly earlier version.

I had a model from 1974 a few months ago, it had one continuous PCB for the channel 1 and 2 preamp sections and mixer section. It also seems to have the 2 33uF capacitors on Q6 and Q7 source as in the schematic. In this earlier version these sections were separated to two PCB's.

Thanks again!

[teemuk]

Is this diagram a better match?
http://www.prowessamplifiers.com/sch...ages/sg100.jpg

[garytoosweet]

Yes that looks to be a much better match for this version. Thank you teemuk, I didn't see that in my search. This one is 'Rev A' while the one I posted is 'Rev B'. I sent Gibson an email to see if there is a higher res copy available and will post it here if I get it.

On final testing I realized the Reverb circuit is not working. Appears to be an issue with the Reverb send IC.

The IC in the amp is labelled "GE-IC263 I85 7716". "KOREA JGET" written on its side. TO-99 case with 8 pins. I wasn't able to pull up any info on this. It's likely it's not original.

However the circuit is exactly as in the schematic, which calls for MC1709CL.

I'm a bit uncertain as to how to determine a substitute. I want to make sure the circuit will still work with the impedance of the tank, as the tank appears good.

It would be great if it works with the DIP14 socket on the board without too much re-fussing of the circuit. I'm looking for options now. There are also NOS IC's available on Ebay.

[garytoosweet]

I think I can rewire the send circuit to use a more common DIP8 op-amp like a TL072 or similar. I can use the existing socket, may need to cut a trace or two and swap a few components. This seems like the best solution to me.

Is there a simple circuit that is recommended to drive a 200-ohm input reverb tank? I was looking at the Fender Hot Rod circuit and Peavey C30, both very simple, should be doable.

[g1]

There is a 14pin socket on the board? That is loaded with an 8pin can?