dual 15 volt power supply positive rail is no longer adjustable

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bryce Goggin

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My compressor blows fuses if its not ramped up slowly. The +15 rail of the supply is up to 20vdc and the 2k trimmer r29 has no effect. I tried another potentiometer with no luck. It does change the voltage feeding back to IC 3 but not the output. All of the other voltages are in range and adjustable.
When I first opened the lid R16, R20, R21,R22, R23 looked like they suffered from some heat. I replaced them, recapped it, and replaced Q5, Q6, Q7, Q8. I tested the rectifiers and they all seem to be good. I disconnected the ribbon connectors to the rest of the circuit and stll blew fuses. Replaced D11, D12 and IC3-5 with no luck.
This all occurred before the purchase of variac. After ramping up the AC and disovering the problem with the +15 volt rail I found a bad diode (D16) I replaced it along with Q11 with no improvement. Anybody have any clues?What progam do you use to write text on pdfs or jpegs?
 

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First, you would use adobe writer for .pdf files.
You can import your photocopy into Word. It's better to take a digital photo of what you have.
Is this your design/build or did you purchase it?
How much current does your compressor draw?

What is the output voltage of your bridge? I suspect it's around 25V since D11 and D12 are rated for 20V.
Do you use the -15V supply for anything. Since you didn't mention it, I am assuming this works OK. The reason I ask is, they both share the same reference at TP A. Your ref. is probably OK if the -15V supply adjusts. You replaced the transistors on the -15V supply. Why if it was working?
What is the voltage meas. at point A? Is it normal? It should meas. 6.9Vdc.
You say you can see the feedback voltage adjust, this should balance around 6.9Vdc. You may see fluctuations initially, but it should settle back at 6.9V. This is under normal circumstances. Since you see the voltage adjust, you have a problem with the op-amp.
You can monitor the output at IC3 as you adjust R29 and it should fluctuate up or down.
IC3 should be outputting around 20.7V in it's current state.

So to recap:
What is the voltage meas. at TP A (It's odd that your output voltage is the same as the zener diodes)? Verify you have 6.9V at pin 3 of IC3. I suspect that IC3 is cutoff at ~20.7V and your TP A ref. might be off.
Is the DC in (output of bridge) the same as the +15V output?
Ohm out the case of IC3 to the +15V ground terminal. It should be 0 ohms.

It looks like, based on the parts list, that some of the schematic is missing (I don't see the relays or switches.).

I hope this help.
 
Can you measure point A? Also is +5v working?

umm never mind, rezer already covered this ...
 
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The output of the bridge is 23.8 v and is the same as pin 6 IC3. The 15 volt output is at 21.6 volts. Test point A (pin 3 o IC3) remains at 6.9v. When talking about the the feedback voltage I was referring to the voltage at pin 2 of ic3 which will fluctuate from 9.2 v to 10.3v when I adjust r29. The voltage at pin 6 does not change when I adjust r29. Pin 4 is attached to ground. Is that what you mean by the case of IC3?

This is the power supply inside a sontec drc 202 compressor. The parts list contains components from other parts of the unit as well as the power supply. I found the docs on a random audio tech thread I apologize for the poor quality.
 

Yes, the case is grounded. The problem I have with your symptoms is, obviously, the pin 6 output should be moving with R29. Likewise the inverting input at pin 2 should be moving, but slightly until the op-amp balances the inputs. Regardless of R29s setting, pin 2 should be at 6.9V.

Judging by the difference between DC in and DC out, you have the correct transistor at Q5 (TIP122, correct?). I forgot this is a Darlington amplifier and will drop 1.5-2V. So that looks OK.

What value do you measure for R26?
 
bryce Goggin said:
The output of the bridge is 23.8 v and is the same as pin 6 IC3.
Click to expand...
If this is the case, then Q5 is shorted from base to collector. Pin6 of IC3 should not be able to rise above pin7 which is the supply voltage pin of the OpAmp. You may have also damaged IC3 and the zener connected to pin7 of IC3 (I can't make out the id of the zener from the photo).
 

It appears that is right. IC3 Vee is at 20Vdc and should not be putting out 23.8V. I missed that in your last post. But I just may have over-looked it since you have already replaced it.

Bryce,
Measure the resistance from IC3.6 to DC in for 0 ohms. It would make perfect sense if this is what the cause is. But you may have another problem if Q5 keeps dying.
 
compressor blows fuses if ac not ramped up slowly



It appears that I hooked up Q5 incorrectly. I connected the output of the bridge to base instead of the collector. I remedied this and the +15 rail is now adjustable. The next time I turned the unit on I blew a fuse. After ramping it up with the variac my multitester showed that the -15 volt rail was down. I found that q8 was damaged and replaced it. Now all the power supply voltages are happening and adjustable. I ran audio through the compressor and everything seems to be working.
The only problem now is that if I turn it on without a variac it blows fuses. Could this due to bad caps?
 
mneary said:
I don't see the primary circuit. Do you use a slow-blow fuse?
Click to expand...
I'm using a 1/4 amp fast blo fuse. I'm pretty sure thats what was in it. There is no indication on the schematic or on the unit what type fuse is supposed to be used. Maybe I should try a slo blo one. I just don't want to damage the unit. The fuse is wired before the transformer.
 

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1/4A is grossly undersized, slow blow or not.

Just charging the input pair of 1000uF capacitors (C7, 8) needs more than a joule to get to 23.8V. If those other circuits are on the same transformer, add another joule. At 120V this is 2A for a half cycle.

Also I judge from R7, R8, R24, and R25 at 1E, that this is a +/- 15V, +/- 28V, 1A supply so it would need at least 150W for continuous operation at full rating. This is more than 1A at 120V.
 

Tried a 2 amp fuse. When I turn the unit on without a variac the -15 volt rail doesn't show up. It doesn't seem to like the spike.
 
Do you have a correction somewhere about the wiring of R19? It should connect to U4 pin 2 and not to the reference node (point A). I'm still trying to figure out how it works at all, even with slow ramp-up of the variac.
 
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mneary said:
Do you have a correction somewhere about the wiring of R19? It should connect to U4 pin 2 and not to the reference node (point A). I'm still trying to figure out how it works at all, even with slow ramp-up of the variac.
Click to expand...

Yes you are correct R18 and 19 meet at u4 pin 2. R19 is not connected to point A. This schematic is funny. C9, c10, c14, c15,c23,and c24 don't actually exist.
 
If Q8 was damaged, IC4 may be also. Especially since the +15 was +20 for a while which would have applied 40V power to IC4. Be sure to use the correct op amp, as many of the pin-compatible alternates exhibit latch-up which the CA3140 supposedly does not.

Are there any other differences between the schematic and the unit that we should know about?
 

Yes there seem to be more differences. R15 and the unnamed resistor attached to point A are not there. There is a 13k resistor between the + pole of c7 and pin3 of ic3. Pin 7 of IC3 is being fed by the +28 volt supply via an 820 ohm resistor which brings the voltage down to 19 volts. Pin 3 is attached to d13 and pin 7 is attached to d11. I see no connection between the 2 pins.
I'm using nte 7144's as opamps.
 
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