hifi engine

harmon kardon 930 twin power receiver -- heated up

lwong2's picture

Dear friends in HIfi Engine,

I have a problem with the above receiver and seek your advise on the likely cause and what shd be done to rectify this problem.

proble area : The left channel out put heat sink is very hot like an electric iron. So is the transistor.
I checked the voltage at the speaker out put it was Rt channel about 5 mv and left channel 78 mv.
This problem comes erractically -- sometimes it is normal and some times after i on it for about 20 mins , the left side got heated up very fast. The other channel is ok. It seems to be something is not stable and drifting.

My question : Can this issue fry my speaker ?
Can this be cause by the biasing ? if so how can i adjust the biasing or other areas i should look at.
This receiver has been more than 30 years old, should i replace the two pots that control the speaker out put voltage.

Hope to receiver your advice and guidance on how to tackle this problem.

Thank you and best regards,

Wong



Re: harmon kardon 930 twin power receiver -- heated up

It might be just dirty bias controls; these are on the amplifier board. Memorize where they are set, then move them several times--if you have control cleaner, use it on them.
Could be poor connections. I would not use the amp with this condition; could cause damage.

lwong2's picture

Re: harmon kardon 930 twin power receiver -- heated up

Hi Johnnysan , i have tried spraying on the control cleaner on both the pots. Its still the same . Perhaps it concerns the bias adjustment.

Thks.

Wong

lwong2's picture

Re: harmon kardon 930 twin power receiver -- heated up

Thank you Mr Johnnysan.

Kindly advise me which pot shd i lubricate. Looking at the manual for audio driver adjustment VR601 is for left channel. When i look at the schematic VR 601 is located at the right channel, while VR 621 is at the left.

So VR601 shd be correct for the left channel ?

Thks

Best regards,

Wong

Re: harmon kardon 930 twin power receiver -- heated up

On the schematic the left channel is R601 and the right is R621; this may be reversed.
You may have failing bias diodes (D 601, 602--if the schematic is reversed then 621, 622). Capacitor C604 may be bad (47uf 50 volt). Transistors TR 603, 604 and/or 605 may also be bad. I would carefully check all solder connections.

lwong2's picture

Re: harmon kardon 930 twin power receiver -- heated up

thank you . I think i should send it to a competent technician for repair.

Best regards,

Wong