Just for a laugh, I entered your question into an AI chatbot- https://chat.openai.com/chat
Here is the result, kinda badass actually...

I think it might have nailed it with #3?

"There are several possible causes for a hum like the one you are describing. Here are a few things you can try to troubleshoot and potentially fix the issue:

1. Check the AC mains voltage: The hum could be caused by an issue with the AC mains voltage. Make sure that the voltage at the outlet you are using is within a normal range (usually between 220 and 240V). If the voltage is outside of this range, it could cause problems with the amplifier.
2. Check the grounding: Make sure that all of the components in the system (amplifier, turntable, speakers, etc.) are properly grounded. A poor or inadequate grounding connection can cause a hum.
3. Check the power supply: The power supply in the amplifier could be causing the hum. Make sure that all of the capacitors in the power supply are functioning properly and are not leaking.
4. Check the input wiring: Make sure that all of the input wiring is in good condition and that there are no loose connections.
5. Check the transformer: The transformer in the amplifier could be causing the hum. Make sure that it is properly connected and that there are no loose connections or damaged wires.

If you are unable to find the cause of the hum using these methods, it may be necessary to seek the help of a qualified technician who has the tools and expertise to diagnose and fix the issue."

 

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?

I don't think so, that 12V rail is for the preamplifier section, it could be a shielding problem if the capacitors have been replaced on the PSU..

 

Depending on the level of hum, it might just be working as designed. New speakers, earphones or even shorter
distance between the speaker and your ear might be causing a "normal for the past" amount of hum to be objectionable.

I had a GE based AM/FM radio which had a much higher noise in one channel than the other.. After
replacing the bad preamp transistor that channel went quiet but the other channel now was obviously bad too.
With both transistors replaced the noise was gone but there was residual hum. Capacitor replacement didn't help,
running the radio on pure DC from a 3 terminal regulator didn't help. The hum turned out to be coupling between
the power transformer and the audio transformers in the audio section. This hum had probably been there since
the radio was built...

 

I do not believe this level of hum was present when the device was operating as intended. Very interesting story, thanks for sharing.

I would be satisfied if I could reduce the hum to some acceptable level. However, this is not a HiFi device, it has more of a sentimental value.
I made a signal tracer from an old pc speaker so I'll have a go at locating the problem

@Dodgydave - Thanks for the reply. I'm continuing my investigation.

 

C755, C756 and C757 could be replaced with one fat, modern 3300uF cap. These three caps are the most likely culprits. The only weird thing is the variation in hum intensity with volume position.

also, is the hum coming from the transformer or from the speakers? Touch the transformer frame with a plastic stick to see if you feel it vibrate. I don't want to suggest a finger directly because it may be shorted to chassis if it is buzzing loudly.

lastly, a ground loop is possible. Ideally, you should have one path to ground from each location. Try isolating the PNP collectors from the chassis of the unit. Make sure they are connected to wires that run back to the 0V near the filter caps (eg C577).
Unless I misunderstood when you said...

A pair of AD162 has a mica insulator and the collectors are connected to ground and chassis.

 

It would be worth checking for all the solder joints. Sometimes you'll discover connections that were never soldered, but it's more common to find hairline cracks, mostly with heavy components or connectors that are subject to stress.
And check that the pots behave properly using an ohmmeter. If resistance jumps around while the pot is rotated, a squirt of contact cleaner may help.

 

also, is the hum coming from the transformer or from the speakers?

The hum comes from the speakers, the transformer is quiet and does not vibrate.

 

And check that the pots behave properly using an ohmmeter. If resistance jumps around while the pot is rotated, a squirt of contact cleaner may help.

I did not measure the behavior of the potentiometer with an ohmmeter, but I did spray them with contact cleaner. All potentiometers work ok, without crackling.

 

The hum comes from the speakers, the transformer is quiet and does not vibrate.

I don't think that is what he meant.

Of course we understand that the hum emanates from the loudspeaker.
It is very likely that the source of the hum is from the transformer, not mechanically but electrically.

 

I don't think that is what he meant.

Of course we understand that the hum emanates from the loudspeaker.
It is very likely that the source of the hum is from the transformer, not mechanically but electrically.

I did mean the vibration of the transformer - it can buzz quite loudly if there is a strongly leaking filter cap or a reversed filter cap - or any other shorted component. Otherwise, I think it may be a damaged or leaking filter cap on the power supply (one of the three 680uF caps.

 

Today I listened to the hum with my homemade signal tracer. I did the check without connected speakers and without a signal at the input.

I came up with the following information:

Power supply:

C754, C755 for sectors B (16.5V) and A (12V) - no buzz
C756, C757 for sector C (30V) - strong buzz

C742, C743 are after the volume potentiometer - no buzz

C744, C745 - after BC147A - no buzz

At all other points in the amplifier sector (section C on the diagram, measured at C746...C753) there is a buzz, but it is much weaker compared to the one that occurs on these two capacitors.

Preamplifier:

C726, C727 no buzz
C728, C729 weak buzz
C730, C731 weak buzz
C736, C737 no buzz
C738, C739 no buzz
C740, C741 no buzz


Based on this I thought that there was a problem with the capacitors C756, C757 which was later transmitted to the entire sector C (30V) of the amplifier, so I replaced them with a new pair. However, the result is the same, a constant buzz on these two capacitors.

Is this method I used even correct?

 

Where on the schematic did you connect the ground lead from your signal tracer ? Do you get a buzz touching the input of your signal tracer DIRECTLY on the negative side of C756 and C757 ?
One thing you could try is connecting 10 nF capacitors in parallel with each of the four diodes that make up the bridge rectifier.

Les.

 

I connected the signal tracer ground to the chassis. The buzz only occurs when the tracer input touches the positive side of these C756 and C757.

 

There will probably be a fair amount of ripple on the positive end of those capacitors as they are directly connected to the output of the bridge rectifier.

Les.

 

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.

That's strange.

How does adjusting balance control effect hum with volume control at low/high/midpoint?

What happens to hum if you short TS430 base to emitter?
This should turn off TS430 leaving the driver/final transistors (a single channel test).

It's possible is more than one path for hum to get into signal. If some of these are opposite phase they might cancel each other out and reducing one might make the hum worse due to the loss of cancellation. It's possible (unlikely?) that part of the construction depends on some cancellation to reduce hum and having the gramophone apart might be reducing one of the sources of hum thus making the result worse. Or as I said previously, the designers might just have accepted more hum then we would today.

manual in pdf format (with an easier to read schematic)
https://elektrotanya.com/philips_22gf560_sch.pdf/download.html

A calculated guess at the ripple (really a sawtooth) at point "C" with both channels at idle.

First the current used by the driver/final amplifier stage:

The bias adjustment guide (page 1) says idle bias is 6 mA (TS434a & TS434b).

The driver transistor TS434 has an emitter current of 1.9V/68ohms -> about 2.8mA

R552 18K has one end on the final output which is likely at 1/2 the supply or 15V and the other at 2V (schematic value).

So 13V/18K -> about .7 mA.

Total for a single channel is thus: 6+2.8+.7 -> 9.5mA, call it 10mA so two channels is 20mA.

The current through R573 1.5K from point "C" to "B" is:

"C" 30 V (measured 30.8V)
"B" 16.5V (measured 16.4V)

(30.8-16.4)/1.5e3 -> 9.6mA about 10 mA.

So the total current from "C" at idle is: 2 * 10mA + 10mA -> about 30mA.

The ripple at point "C" (C756 & C757) for a 100 HZ system should be:

V = I*T/C -> 30e-3*(1/100.)/(2*680e-6) -> 0.220 volts peak to peak

It will greatly increase at higher currents.

 

I'd try replacing the small 680uF caps with a pair of 3300uF caps and the ripple should vanish.

 

I don't currently have a pair of 3300uF capacitors, I have one 6800uF (50V). Would it make sense to connect it in parallel with the C757 for a trial run?

 

@michael8 :

Thanks for this in-depth information and excellent analysis of the ripple at point "C". I have so much to learn and your comment is very helpful.

I haven't tried short-circuiting the TS430 base and emitter. I could do it from the top side of the PCB board (+ side C742→R550 emitter side) but to be honest I'm afraid of doing some damage in the process, I'm not experienced enough and I already have a few short circuits (unintentional).

Based on your calculation, it seems that the device is designed to work in that way as you wrote in post #23.

I would also consider replacing the 2 x 680uF with 2 x 3300uF as MrSalts suggests. When these values are inserted into the equation, the ripple voltage is 0.045V.

Thanks for the schematic link, it's much better than the one I have.

 

Hello,

When the hum is created by the powersupply, connecting your capacitor accross C757 would cut down the hum.

Bertus

 

Thank you for your prompt response.

I connected the 6800uF to the C757 with alligator clips and then I listened to the positive side of the C757 with a signal tracer and the buzz is noticeably lower.

It still exists, but it is certainly 40% lower in intensity.