Can anyone help me with two difficulties I am having with this set?

The major one is related to hum. The symptoms are that when switching on there is a very pronounced hum that rapidly dies away to a very low level. After a short time (less than a minute) it gradually increases to a very loud hum that is unaffected by the volume controls. The radio works on all bands and the residual hum is overwhelmed by the music or speech. The programmes could still be heard behind the loud hum The oddity is that occasionally The loud hum does not develop and the performance is quite acceptable. Switching off and then back on usually produced the fault.

I first assumed the problem was smoothing capacitors so I replaced the 3000mF and the 400mF capacitors but this did not cure the fault. 

The set came from a house clearance sale and had obviously been stored for some time, after work of some sort being done. One obvious thing was a red wire trapped in the hinge at the rear. Another problem was unreliability of the mains switch. I cleaned this and found that the red wire had now fallen off.  The loud hum had gone but it was dead also.

This brings me to second difficulty. The red wire is one of four connecting the power supply and the upper circuit board. There is a connector at one end the other end goes into a hole in the upper circuit board. This is where the break is. I can't find it on the circuit diagram but I assume it provides power to the upper board. To get at the board to re-solder the lead looks like a major dismantling job. It might however be possible to get at the other side of the board from the top. if the perforated cover at the rear of the top could be removed. Looking at the service brochure makes it look as if it should be easy but I can't find the screw about half way along it.

I took a risk buying the set because it looks elegant  and would make a good display item but I am so frustrated by the fact that it seems as if it should be possible to have it working as well.

If anyone can tell me how to remove this grille I might see the answer to the hum problem as well .

Many thanks in hope

John

The humming could be caused by too high idle currents so the power supply is truggling to keep up.
Is it getting hot underneath ?

The aluminum panel with the four sliders lifts right off, it will ease access to the tonecontrol board.

Martin

The humming could be caused by too high idle currents so the power supply is struggling to keep up.
Is it getting hot underneath ?

The aluminum panel with the four sliders lifts right off, it will ease access to the tonecontrol board.

Martin

Hello Martin, I followed your guidance to get at the pcb. I had not expected the slider plate to be clipped as it was a very strong spring. Once off all was easy and I replaced the connecting lead.

On checking there is still the high level of hum the radio can be heard over it. I left it running for about 30 minutes and then checked the under side for heat The power transistors were slightly warm, the mains transformer was cold and the only region of warmth was below the output capacitors. these were warm but not hot. Should I change them? I have never felt these components before so I don't know whether this is normal.  So far I have ignored the -22v line because I thought it worth getting an opinion on the capacitors first.

Regards  John 

Well done !

Did you replace the large cap at the end of the amplifier board ?
I like to mount a 6800uF 63V here.

Martin

Yes I did replace it but perhaps I will try a larger value. you are suggesting one twice the capacity of the original. 

John

Could also be a missing ground connection somewhere.
There's a short black wire behind the amplifier board.
If that doesn't work, you will have no measure around a bit. Start with the idle currents.

Martin

I have tested again and found with only one speaker connected all is fine. This is for each channel separately so it looks as if I need to investigate the idle currents.

Unfortunately this will have to wait because I have serious unreliability with the mains switch and wiring I will resolve this first. The wiring measures O.K but it is old and in places has been trapped and compressed. It seems wise to replace this at the same time as resolving the switch.

I think I can now wind this thread up.

The switch was a sticking plunger on the OFF actuator. Cleaning the dirt and dried grease and a bit more lubricant has solved this. The hum was loading of the power supply but with the new smoothing capacitor this is solved. The only oddity remaining is a very loud noise occasionally when switching on to MW but this might be coincidental because the mains can be very noisy at times. The performance is quite pleasant and certainly worth the effort to, get a working item rather than just a display only model.

if anyone else needs to work on this model things are fairly straight forward but be prepared for some fiddly reassembly. The slider panel lifted off easily but putting it back was rather different. It left me wondering how it was done during manufacture. X-ray vision would have helped me.

Many thanks for all the advice

Regards

John