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.
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.
d3imlay said:
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.
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.
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.
d3imlay said:
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.
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.
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.
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.
And at what point in the circuit did it clip?
MikeBettinger said:
Many thanks for the schematics. If there is a cost I'm more than willing to pay.
There is a feedback resistor on the schematic that is paralleled with a 10pf cap. Upon further inspection, maybe that's not the feedback loop you're referring to. Are you referring to the non inverting input to the opamp? Anyhow the good channel was OK on the bench.
I'm going to look for an open output transistor. Only half of the waveform is clipped on the bad channel. I've seen this many times when working with power amps. Did these transistors fail on a regular basis? It's odd for a transistor to open up.
Finally:
Is there a cross reference for the transistors?
What about setting for the pots?
Again,
Many thanks for all of the help.
d3imlay said:
The feedback resistor on the board is 10Meg. It's an insurance policy against the external loop having issues, not much FB though, with no path to ground.
The Fairchild transistors failed while in warranty, by opening up; It's a common enough failure. The voltage drop across the 30 ohm resistors tell the tale.
I used the 2SC2344 and 2SA1011 if my memory is intact. The main thing is a Vce of at least 30 volts at a minimum .25A. The stage is basically a class A power amp.
The other postings were the original GAS setup instructions and test point locations (from the vault).
Good luck.
burnedfingers said:
Hi Joe,
In the 80's I had more of a comfort feeling with the Japanese transistors. So I looked in my GAS transistor drawer for the number off the ones I use to use. I assumed it would be used as a starting point, spec-wise. I was going to suggest the MJE15030 and 15031 but I didn't want to take the time to verify the ratings. I've been embarrassed enough time by recommending something based on my memory.
After the Fairchild's, GAS started shipping them with Japanese transistors, 2SD525/ 2SB595's (I believe), which is a low frequency pair.
I guess, as long as the specs are in the comfort zone most name brand transistors will work fine. Hopefully the fix for the problem is this straight forward.
One other thing. The original FETs were quite good, especially the ones that look like little black and white beads. I wouldn't swap them out if they are working. If it has the metal TO-46 cases, same thing. They are in sockets, which I recommend you remove and carefully solder them to the board. The sockets arrangement has a history of causing noise problems and offset drift. Although a bit of deoxit into the pin hole and a few gentle wiggles has the same effect for a while.
All obvious stuff I guess.
Mike.
FWIW, the schematic shows several zeners, and there are none on the board. I have the board on the bench hooked up to a power supply. I understand about the feedback but the output amplifier should work regardless. There are two TO92 xistors on each channel that are not shown on the print.
The ongoing problem is in B channel. It clips off the positive half cycle at zero. There's no drop across the emitter resistors. The base of the NPN driver xistor (GAS 114) that drives the output PNP (GAS 117) is the same voltage as the A channel, which is about 7.0 VDC per the schematic. I didn't think to check the collector. This seems to indicate the fets are OK.
The base and emitter of 117 are both at the same voltage with respect to ground. I couldn't convince myself that 117 was bad but pulled it anyway. I put it on a curve tracer and it looked fine. The NPN output on the negative side did have a Vbe drop so I figure it's O.K. I got tired and quit for the night.
The ongoing problem is in B channel. It clips off the positive half cycle at zero. There's no drop across the emitter resistors. The base of the NPN driver xistor (GAS 114) that drives the output PNP (GAS 117) is the same voltage as the A channel, which is about 7.0 VDC per the schematic. I didn't think to check the collector. This seems to indicate the fets are OK.
The base and emitter of 117 are both at the same voltage with respect to ground. I couldn't convince myself that 117 was bad but pulled it anyway. I put it on a curve tracer and it looked fine. The NPN output on the negative side did have a Vbe drop so I figure it's O.K. I got tired and quit for the night.
d3imlay said:
I keep getting the feeling you have a really early version of this preamp/board. Are there any board #'s 223, 323, etc. to reference? I found an early hand drawn schematic of a simple relay circuit that makes me think that this is a possibility here.
Before I go any further let me put a bit of effort into looking at what I have in my files based on what you've posted here and see if I can't get a bit better focused.
I'm a bit to tired as well tonight, too much so to try ands make sense of this now. It is possible though, hang in there.
Mike.
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