Harman Kardon PM655 hum in both channel

[zlatko13]

Hi Guys,
I have a frustrating problem with the PM665 amplifier - a 50 Hz hum at the output that doesn't change with increasing the volume, and is equally audible on both channels. The level is low but audible in silence at a distance of 1-1.5 m.
Taking into account the above, it means that the problem appears after the Volume potentiometer and that its source is something that is common to both channels (eg power supply).
PM655 has the ability to separate the input preamplifier and the output power amplifier, which enables easier diagnostics, so when the input and output circuits are separated, the result is as follows:
the hum before the separation is now divided so that now the hum of the output part is very low - barely audible even by placing the ear directly on the speaker, while the hum of the input part is slightly higher level and of a wider spectrum, but much lower than before the separation.
From this it could be concluded that the hum is injected in both the input and the output part, of course the injection interference in the input part comes to the input of the power amplifier, and is amplified through it and as such added to the independently injected interference in the power amplifier appears at the output.
Checked more or less everything and changed a lot, I couldn't find any problem in the power supply branches so I had to exclude that possibility.
Equally, I 'had' to capitulate and rule out the possibility of a broken element, because what is the probability that an element breaks equally in both channels? Of course, before that I checked almost all the transistors, diodes, capacitors, resistors countless times and resoldered the PCBs.
I feel helpless...
From the beginning, the transformer is getting on my nerves - it's noisy, vibrates and buzzes, and its sound can actually be heard in the speakers - can any circuit or transistor receive this disturbance... ?
That is, does anyone have any experience, idea, suggestion...?

View attachment 2888184

 

[beekeo]

to make sure I understand what you're describing, if you use the pre-out jacks to send the signal to a separate power amp, the hum is audible, but reduced?
or are you referring to the "pre-out/main-in" push button with the speakers still attached to the 655?


I'm hoping that dlucy (or one of the many people with more knowledge and experience than me) reads and comments, but in the meantime...

you mentioned that you've resoldered the PCBs, but here's some basic questions and common things that you have probably already tried/considered:
- deoxit on all of the switches and tone controls (I have an hk 460i, I'm familiar with this hassle)
- does the tone defeat button change anything?
- is the hum audible with headphones?
- re: the transformer issues, from the photos I see online, there are two, is one noisier than the other? is it tightly secured to the chassis, could the vibration be the culprit?

 

[zlatko13]

to make sure I understand what you're describing, if you use the pre-out jacks to send the signal to a separate power amp, the hum is audible, but reduced?

yes it is true

or are you referring to the "pre-out/main-in" push button with the speakers still attached to the 655?

on the PM655 the speakers are connected in one and the other combination and to the external amplifier in separate mode



I'm hoping that dlucy (or one of the many people with more knowledge and experience than me) reads and comments, but in the meantime...

I am very grateful for your answer

you mentioned that you've resoldered the PCBs, but here's some basic questions and common things that you have probably already tried/considered:
- deoxit on all of the switches and tone controls (I have an hk 460i, I'm familiar with this hassle)

True, reading on the forums, HK amplifiers from that time have big problems, especially with small switches (in this case half of them were problematic) and in general with the contacts on the PCB - I opened and cleaned all the switches and resoldered the PCBs

- does the tone defeat button change anything?

tone defeat button was one of those problematic ones and had to be turned on all the time, after disassembly and cleaning it was like new but that didn't change anything

- is the hum audible with headphones?

I didn't have headphones at hand, but if the hum is heard on the external amplifier in separate mode, then ...


- re: the transformer issues, from the photos I see online, there are two, is one noisier than the other? is it tightly secured to the chassis, could the vibration be the culprit?

the PM655 model has one transformer (PM665 has two) but it has two rectifier rails ... it is well attached and connected but noisy and vibrating ... and that is also my question, can this initiate this problem

 

[petes'picks]

If its at the speakers you hear it then its likely capacitor in Pwr Supply and not necessarily the large caps.

Look at this video if its the transformer humming

 

[beekeo]

the PM655 model has one transformer (PM665 has two) but it has two rectifier rails ... it is well attached and connected but noisy and vibrating ... and that is also my question, can this initiate this problem

ah dang, google image search betrayed me! (and I missed that model number discrepancy)

 

[zlatko13]

If its at the speakers you hear it then its likely capacitor in Pwr Supply and not necessarily the large caps.

I was guided by this knowledge and checked all the elements in the rectifier circuit, I didn't find anything bad, but I replaced all the electrolytic capacitors anyway - without effect.
Then I disconnected the power supply to the preamplifier and the control diodes, so only the rail to the power amplifier remained - the hum remained the same as in the case of the separate operating mode.
The main rectifier part is simple and although I replaced the large electrolytic capacitors, I added 1 micro Polyprop in parallel to each - the result was the same. Otherwise, this hum that can be heard from the output stage itself is very quiet and practically inaudible, except in the case of putting your ear directly next to the speaker membrane. It wouldn't be audible by itself and I could ignore it, but the fact remains that it exists.
On the other hand, the hum coming from the preamplifier is probably of similar intensity, however it enters the output stage and is amplified and heard.
Can you infer anything from all this?

 

[zlatko13]

Look at this video if its the transformer humming

thanks
About my case it is not so noisy like this one but it is very desirable to silence it. The noise produced by the transformer is one problem, and my bigger problem is the noise that appears at the output of the amplifier - is it possible that the former causes the latter?

 

[zlatko13]

ah dang, google image search betrayed me! (and I missed that model number discrepancy)

 

[zlatko13]

...that disturbance is of a low level - a few mV... I don't have a professional oscilloscope, only a cheap ALiExpress declared for a sensitivity of 5 mv, but something can be seen in the attached pictures. When I look only at the output in the mode separated pre and out :



And when I look in integrated mode:


Since the gain of the output stage is about 30, I assume that the level of incoming interference is about half a mV ... at the output it is about 10 mV and it is audible. Everything somehow indicates that the source of interference in the power supply ... the preamplifier has its own supply rails and the output stage has its own for each channel separately ... each rail has its own rectifier filters that would have to lead such interference to ground ... in addition the electrolytes were tested and showed no malfunction - I replaced them anyway...am I overlooking something...does anyone have an idea what could be the source of this interference?