Help with Hitachi HMA 8500 MKII

[cedus]

After trying a lot of things : DIY Chipamp, DIY Tubeamp, Marantz 8B etc. I settled 1 year ago on a wonderful Hitachi HMA 8500 MK2, dual mono with 2SK134/2SJ49 as output devices for 100W of pure pleasure. This amp is just amazing and looks also gorgeous !
But in the last 3 months I have had a very tiny distortion in the mid-range, which is becoming more and more annoying.

I plan on changing the PSU electrolytics in the forecoming month but I am not sure it is coming from there. Is there anything obvious I should check first ?
What is the methodology to track where the problem is coming from ? I had a look with the search function but no explanation seem obvious.
I have at home a proper dual trace scope and a laptop with USB soundcard for tone generator ?

The first diagnosis indicates that nothing is burned or melt down. It is just aging of the beast. Should I go for a complete change of the transistors (except the output pair which is the unobtainium field) ?

I am not in a rush but this great amp deserve to be back in shape !

I attach the shematic for the one who knows.

Many thanks for your help.

[poetry2me]

This is really a great amp!

The most likely cause of the distortion you describe is the power relay that protects the speakers. There are two relays in this amp, one for each speaker terminal. To verify, you may want to test connecting your speakers to speaker terminal B instead of A.

The relay's contacts corrode over time and need to be cleaned. There may be burned contacts from too many switch events under load or there may be oxidation. Both can normally be cleaned out using special electronic anti oxidation spray (like Kontakt 60) applied to a small sheet of paper which is then pulled throught the closed relay contacts. In very bad cases you may need to replace the relay. I have seen it on ebay, specifically offered for this amp.

Hope this helps
Poetry2me