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Old 24th Jan 2023, 3:33 pm   #1
paul1962
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Default 1970's GEC power supply issue - help required

Hello everyone,
I have a situation that is starting to confuse the hell out of me and wondered if any of you may be able to assist.
The scenario is that I have a GEC 2805 radiogram circa 1972 which has been "got at" in it's past. My confusion stems from the power supply.

All checks have been done. Transformer ok, rectifier fine, Mains Capacitor fine and 24v supply to the main board correct.
I can see there has been resoldering from this to the main board, which is confusing me. I checked the power supply side of the board, and had to change C323, which should be 100uF and was actually reading 22uF (open circuit according to my scope)
C336 is the smoothing cap all good. C323 is new, and C305 is good also. I'm all out of capacitor issues at this point.

Cutting to the chase, the service sheet states that the live is wired to chassis and the power circuit is negative. Checking the circuit diagram, this would mean that negative is supplying the positive side of C323 electrolytic ? Currently, the positive supply is wired to the positive side of C323, but the wires have been resoldered and I question at the right polarity.

The fault is bad mains hum and both amplifiers are giving good output.

I attach the service sheet circuit and description. Can anyone make sense of this ?

Kind Regards,
Paul
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Last edited by paul1962; 24th Jan 2023 at 3:38 pm.
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Old 24th Jan 2023, 3:54 pm   #2
Cruisin Marine
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Default Re: 1970's GEC power supply issue - help required

Checking the circuit diagram, this would mean that negative is supplying the positive side of C323 electrolytic ? Currently, the positive supply is wired to the positive side of C323, but the wires have been resoldered and I question at the right polarity.

Looking at the diagram, the common is Positive, so chassis is + volts, so + goes to the positive side of C323, which is what the diagram appears to show, it goes to common, or positive in this case.

Make sure the wiring on the common side is as shown in the diagram.
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Old 24th Jan 2023, 4:07 pm   #3
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Default Re: 1970's GEC power supply issue - help required

Although C336 may be the correct value, at this age it might have a high ESR so
check by substitution. Note that MR1 is a metal rectifier, and nowadays a silicon bridge
would be used. You can measure the dc ripple on an oscilloscope.
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Old 24th Jan 2023, 4:33 pm   #4
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Default Re: 1970's GEC power supply issue - help required

Quote:
Originally Posted by Cruisin Marine View Post
Checking the circuit diagram, this would mean that negative is supplying the positive side of C323 electrolytic ? Currently, the positive supply is wired to the positive side of C323, but the wires have been resoldered and I question at the right polarity.

Looking at the diagram, the common is Positive, so chassis is + volts, so + goes to the positive side of C323, which is what the diagram appears to show, it goes to common, or positive in this case.

Make sure the wiring on the common side is as shown in the diagram.
Many thanks for your input Cruisin Marine

I see your point, and yes, according to the circuit diagram, we are of the same conclusion.
In the real world circuit board, this is what I found. Please see attachment where the positive has been wired to the positive side of C323.

The negative of C323 is not connected in circuit according to the printed side of the board.
I have numbered the things in question. 1) The new c323 cap, 2) Positive from supply and 3) being the negative of the supply.
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Old 24th Jan 2023, 4:37 pm   #5
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Default Re: 1970's GEC power supply issue - help required

That piccy looks right to me.
Check continuity Neg side of C323 makes with the bottom end of R325.
Also, as referenced by Resto, make sure C336 hasn't gone bad.
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Old 25th Jan 2023, 1:38 pm   #6
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Default Re: 1970's GEC power supply issue - help required

Is the hum on both channels?
Does the hum increase with volume?
Is the hum the same level on all input selections?
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Old 25th Jan 2023, 2:07 pm   #7
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Default Re: 1970's GEC power supply issue - help required

Hum can be caused by wiring the positive and negative outputs from the rectifier to the wrong place on the circuit board.

The output from the rectifier is a pulsating DC.
It needs to be 'tamed' by the reservoir capacitor C336. Even then it will have a significant amount of AC ripple on the DC power.

If I was building a new amplifier from your schematic I would wire directly from the rectifier DC outputs to C336.
I would then run separate wires from C336 to the circuit board.
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Old 25th Jan 2023, 4:16 pm   #8
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Default Re: 1970's GEC power supply issue - help required

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Originally Posted by PJL View Post
Is the hum on both channels?
Does the hum increase with volume?
Is the hum the same level on all input selections?
Hi PJL,
The hum is on both channels and is not affected by adjusting the volume. It's the same level from minimum volume as maximum, and also whether radio on all bands or record player is selected.
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Old 25th Jan 2023, 4:19 pm   #9
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Default Re: 1970's GEC power supply issue - help required

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Originally Posted by Silicon View Post
Hum can be caused by wiring the positive and negative outputs from the rectifier to the wrong place on the circuit board.

The output from the rectifier is a pulsating DC.
It needs to be 'tamed' by the reservoir capacitor C336. Even then it will have a significant amount of AC ripple on the DC power.

If I was building a new amplifier from your schematic I would wire directly from the rectifier DC outputs to C336.
I would then run separate wires from C336 to the circuit board.
Many thanks for the suggestion Silicon
I can confirm that this is exactly the way it has been wired.
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Old 25th Jan 2023, 4:24 pm   #10
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Default Re: 1970's GEC power supply issue - help required

Quote:
Originally Posted by Cruisin Marine View Post
That piccy looks right to me.
Check continuity Neg side of C323 makes with the bottom end of R325.
Also, as referenced by Resto, make sure C336 hasn't gone bad.
I have checked continuity Cruisin and can confirm there is continuity.
Based on no difference with hum from minimum volume to maximum, I feel this issue is before the amp or pre amp circuits and I think possibly it's time to swap out C336 Mains Cap.

Now, concerning this, it is currently (no pun intended ) a 2000uF 30v can type of electrolytic. These don't seem to be widely available so do you feel that a 2200uF would be ok in this circuit ?
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Old 25th Jan 2023, 10:02 pm   #11
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Default Re: 1970's GEC power supply issue - help required

It does sound like a PSU problem but we should check it out before swapping parts.

Can you take some voltage measurements (from earth/positive):
1. Voltage on C336
2. Junction of C329 & C331
3. Junction of C330 & C332
4. Voltage across R337
5. Voltage across R338
and what is the value of R337/R338 so we can calculate the current.
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Old 26th Jan 2023, 6:11 pm   #12
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Default Re: 1970's GEC power supply issue - help required

Hi PJL

Oh boy, are you going to love these readings
With negative connected to chassis and positive probe, these are the readings.
Voltage on C336 24v out of circuit and 17.1v in circuit.
Junction of C329 and C331 is -9.5v
Junction of C330 and C332 is -9.3v
Voltage across R337 is -9v
Voltage across R338 is -9v (reading -9.1v one end and -9.7v other end, but both matched identical)
According to the parts list, both R337 and R338 are 1 ohm.

Something is wrong here I feel. Surely the readings should all be positive voltage and not negative ?

Kind Regards,
Paul

Last edited by paul1962; 26th Jan 2023 at 6:16 pm.
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Old 26th Jan 2023, 6:21 pm   #13
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Default Re: 1970's GEC power supply issue - help required

2000uF 30v can type of electrolytic. These don't seem to be widely available so do you feel that a 2200uF would be ok in this circuit
Yes, that will be fine.

With negative connected to chassis and positive probe, these are the readings.

Sorry, that doesn't make any sense when I read it.
You want the RED + POSITIVE of the multimeter to chassis.
Then measure voltages with the BLACK - NEGATIVE lead.
These readings should all be - or negative with respect to chassis.
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Old 26th Jan 2023, 6:34 pm   #14
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Default Re: 1970's GEC power supply issue - help required

You need to look at the chassis as your O volt line, then it will all make sense.
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Old 26th Jan 2023, 7:15 pm   #15
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Default Re: 1970's GEC power supply issue - help required

Sorry Cruising, I'm old and easily confused

I will take readings again, this time, the right way round. Positive to earth never sat well with me

Ok, all readings are the same, but now at positive voltage. (e.g) -9.5v should be +9.5v etc.

Last edited by paul1962; 26th Jan 2023 at 7:23 pm.
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Old 26th Jan 2023, 7:36 pm   #16
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Default Re: 1970's GEC power supply issue - help required

Voltage on C336 24v out of circuit and 17.1v in circuit.
The voltage should be higher when it is in circuit, if it is not there is maybe a faulty capacitor.
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Old 27th Jan 2023, 12:43 am   #17
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Default Re: 1970's GEC power supply issue - help required

What do you mean by 'out of circuit' when you measure the voltage on C336?

This could mean 'completely isolated from the rest of the circuit'. In this case he voltage will decay to zero.

Seeing as the voltage increases when it is 'out of circuit', I assume you mean that it is isolated from the load and the power supply voltage increases as a result.
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Old 27th Jan 2023, 12:46 am   #18
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Default Re: 1970's GEC power supply issue - help required

Also you need to measure the voltage across R337 and R338 by putting the red probe on one end and the black on the other end of the resistor.

Apologies for confusion!
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Old 27th Jan 2023, 3:55 am   #19
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Default Re: 1970's GEC power supply issue - help required

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What do you mean by 'out of circuit' when you measure the voltage on C336?

This could mean 'completely isolated from the rest of the circuit'. In this case he voltage will decay to zero.

Seeing as the voltage increases when it is 'out of circuit', I assume you mean that it is isolated from the load and the power supply voltage increases as a result.
You are correct Silicon. When voltage is taken and the wires are disconnected from the board, it's 24v. When connected drops to 17.1v.
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Old 27th Jan 2023, 4:08 am   #20
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Default Re: 1970's GEC power supply issue - help required

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Originally Posted by PJL View Post
Also you need to measure the voltage across R337 and R338 by putting the red probe on one end and the black on the other end of the resistor.

Apologies for confusion!
My fault entirely PJL, you did say across, I should have read it better.

The voltages across R337 and also R338 is 0.5v

I think I need to be careful here. The transistors on the heatsink is starting to suffer with overheating, I nearly burned my fingers on some of the transistors
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