GAS Thaedra

[d3imlay]

OK, operator error on my part. Relay doesn't pick up if line voltage under 110 vac. Found an open(!) pass transistor in a power supply. Was stamped GAS117. From What I could figure out it was a TIP117. Replaced it with a TIP126 (I think) and PS is good.

New problem is on the tone plugin board. Input OK, ouput distorted on one channel. Tried replacing the opamp with a 4558 since it appeared to be a dual. No luck. Two of the fets on the channel have been previously replaced by someone else. Don't know what they are as they're privately labeled.

Any ideas? Could sure use a schematic.

[Original Burnedfingers]

. Was stamped GAS117. From What I could figure out it was a TIP117. Replaced it with a TIP126 (I think) and PS is good.

Its not a Tip117

[MikeBettinger]

Originally posted by d3imlay
OK, operator error on my part. Relay doesn't pick up if line voltage under 110 vac. Found an open(!) pass transistor in a power supply. Was stamped GAS117. From What I could figure out it was a TIP117. Replaced it with a TIP126 (I think) and PS is good.

New problem is on the tone plugin board. Input OK, ouput distorted on one channel. Tried replacing the opamp with a 4558 since it appeared to be a dual. No luck. Two of the fets on the channel have been previously replaced by someone else. Don't know what they are as they're privately labeled.

Any ideas? Could sure use a schematic.

You probably want to put something closer to the original transistor in the supply. It's dropping approximately 20 volts at a hefty (over 200ma) current. I'll post the generic numbers for the original transistors when I get home.

As for the line amp, any board id numbers? What do the FETs look like? Metal cans, small plastic bead case, T0-92 molded plastic? I can supply you with a set if needed and, more importantly, the pinouts.

As for the problem, check the idle current (heh, heh) I believe it's a 100ma = to 3 volts acrossed each of the 30 ohm power resistors on the board. The offset on the output (which is taken from the collectors of the output transistors, both collectors tied together) should be below 25mv. The dual op-amp (MC1458) is for the offset servos for both channels.

My guess is, since you did not say you got a nasty pop when the relay closed and the fact it is making noise (although distorted), most likely an open output transistor, which the current and offset checks should point to. If the power transistors have an F printed on them I'd start betting money... For a while they used Fairchild TO220 devices that were notorious for opening up and doing exactly what you are seeing. This is most likely the problem with the supply transistor as well.

And then I could be wrong.

I can post a schematic of the line amp but I thought there was one out on the SAE_Talk site already. It's actually a fairly simple circuit. Let me know and I'll fight with my scanner and the file size limitations if there isn't one elsewhere.

Have fun, Mike.

[d3imlay]

The schematic for the preamp online is just a signal flowchart. It treats the boards as a box.

Would really appreciate your effort scanning the schematic. The output of the tone board is clipped on one half . I'm thinking the servo is working as the output shows some DC offset when driven further into clipping but then returns to near zero.

[MikeBettinger]

Originally posted by d3imlay
The schematic for the preamp online is just a signal flowchart. It treats the boards as a box.

Would really appreciate your effort scanning the schematic. The output of the tone board is clipped on one half . I'm thinking the servo is working as the output shows some DC offset when driven further into clipping but then returns to near zero.

I'll give it a try in the morning. Think of the the tone controls as part of the feedback loop; From the line stage output- through the control network- back into the negative side of the differentials. The whole network can be bypassed with a 15K/1K to ground feedback network (bypassing the controls was popular back in the day ).

Mike.

[MikeBettinger]

I've tried to attach it and even though it's a JPEG and only 90K it says it's to large. Nothing I can do in the simple editer I have seems to allow me to post it. I can e-mail it and the setup instructions.

Mike.

[Original Burnedfingers]

New problem is on the tone plugin board. Input OK, ouput distorted on one channel. Tried replacing the opamp with a 4558 since it appeared to be a dual.

The 4558 is overkill and you are probably much better using the 1458 in the servo.

To be honest ...if you have a scope your problem is at best a 15 minute repair.

Given the age of the piece and its history the smart thing to do would be to replace all semi's and caps on the board. Use good caps and bypass them and you will be surprised at the performance.

[d3imlay]

Sure, I've got a scope. I pulled the board and used a power supply for the V+/-. I ran a signal into each channel one was good the other clipped. Respectfully, I'm reluctant to do wholesale transistor changes. I'd prefer to positivly ID failed components. I probably will do the cap replacement. The board has been damaged by previouse replacement of two of the fets so I'd rather not pull those two if I don't have to.

[Original Burnedfingers]

Respectfully, I'm reluctant to do wholesale transistor changes.

I can see where you are coming from. It took a long time for me to get into the habbit of changing transistors in the older equipment. When one knows what to use it is very easy and certainly gets rid of the down time associated with changing one transistor at a time when failure occurs and they do fail when they get this old.

Sure, I've got a scope. I pulled the board and used a power supply for the V+/-. I ran a signal into each channel one was good the other clipped.

And at what point in the circuit did it clip?

[MikeBettinger]

Try this attachment.

One thing. If the board was fired up on the bench it had no feedback; it would be clipped on the good channel; the background behind my comments on the tone controls.