Vintage amplifier with germanium transistors - 100Hz hum problem

Thread Starter

Sonique

Joined Feb 17, 2022
23
Hello everyone!

I need a little help from the experts.

I am repairing a turntable with an amplifier 2 x 6W with germanium transistors, the model is a Philips 22GF560 from the 70s. There was a faulty left channel on the amplifier.

I checked that the AC187K was defective and replaced it with the same germanium transistor model.
I also replaced all the electrolytes on the PCB, cleaned all the potentiometers and adjusted the biasing of the output transistors according to the instructions. After the replacement, the voltages on the output transistors were appropriate and the amplifier played nicely on both channels.

The problem is the 100Hz hum (mains is 50Hz) that is heard on both channels. Hum is present even without a signal at the input.
When the volume knob is at 0 the hum is quite strong. By turning up the volume, the hum decreases, only to increase again when the volume is at its maximum.

I checked all the contacts and ground points on the amplifier and turntable, I could not solve the problem.

Can anyone help me how to solve this problem? I don't have an oscilloscope for any serious analysis, just a multimeter and a component tester.

Thanks for your attention and time.
 

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MrChips

Joined Oct 2, 2009
30,708
The first thing I would suspect is the AC-to-DC power supply.
Replace all four filter capacitors C754-757 if you have not done so already.
Sometimes simply turning around the AC plug might make a difference with AC hum.

Philipd 22G560 phonograph PSU.jpg
 

AlbertHall

Joined Jun 4, 2014
12,345
Check that all four diodes in the bridge rectifier are OK. You will need to disconnect it from the transformer to test it fully.
 

Thread Starter

Sonique

Joined Feb 17, 2022
23
Thanks for the answers.

@MrChips - Filter capacitors were changed, turning around the AC plug did not help.

@AlbertHall , @Dodgydave - I forgot to mention that I replaced the bridge rectifier because I also suspected that there might be a problem, but that did not improve the situation.
 

MrSalts

Joined Apr 2, 2020
2,767
If the magnetic field from the turntable motor or from the transformer are too close to any signal wires from the needle (a magnetic device), those magnetic fields may be transferring signal to your input.

Assemble everything with careful routing of wires. It may work better than trying to run it when partially disassembled.
 

Thread Starter

Sonique

Joined Feb 17, 2022
23
The output transistors are not shorted to chassis. A pair of AD162 has a mica insulator and the collectors are connected to ground and chassis.
 

crutschow

Joined Mar 14, 2008
34,281
When the volume knob is at 0 the hum is quite strong. By turning up the volume, the hum decreases, only to increase again when the volume is at its maximum.
That's very odd behavior.
What happens if you short the inputs to ground and turn the volume control knob.
 

MrSalts

Joined Apr 2, 2020
2,767
Do you have a working lamp across the secondaries or something to draw a minimal load from the transformer?
It could be a ground loop. Do you connect circuit ground to the chassis in multiple places or just one (preferred)?
Is it a split supply (+15/0/-15 v) or single ( 0-12v)?
 

MrChips

Joined Oct 2, 2009
30,708
Can you measure the three output voltages at the power supply?
If there is excessive load somewhere this could cause higher AC ripple than normal.
 

Thread Starter

Sonique

Joined Feb 17, 2022
23
@crutschow - I connected the inputs to ground, no change to hum intensity.

@MrSalts - Yes, there is a bulb (24V, 0.1A) connected to the secondary of the transformer.

If I understand correctly the grounding looks like this:

The chassis (where the potentiometers are) is connected to ground on the PCB board, and ground wires run from the PCB board to the speaker outputs and audio inputs.
A grounding wire leads from the speaker output, which is connected to the transformer base.
The PCB board is located in the wooden plinth of the turntable.

The power supply is single (0-30V).

@MrChips - The output voltage of the transformer secondary is 23.6V.
 

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bassbindevil

Joined Jan 23, 2014
824
A signal-tracing amplifier could be used as an audible oscilloscope to listen to the supply voltage (through a capacitor), and check for hum before the volume control. That's just a basic audio amp with a speaker, like a small guitar practice amp.
 

Thread Starter

Sonique

Joined Feb 17, 2022
23
Yes, all three voltages are present. At those points I measured 30.8V, 16.4V and 13.8V. The voltage on C754 is well above the schematic value. Could this be the cause of some problems?
 

Thread Starter

Sonique

Joined Feb 17, 2022
23
A signal-tracing amplifier could be used as an audible oscilloscope to listen to the supply voltage (through a capacitor), and check for hum before the volume control. That's just a basic audio amp with a speaker, like a small guitar practice amp.
Thanks for the advice. I will try to make a signal tracer this weekend.
 
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