Denon Amplifier Repairs

Denon TFS-120U Stereo Receiver

This worked okay but the chassis got very hot. The output section looked to be the culprit, using germanium transistors 2SB415. Suspect they were leaky and passing more current than they should or perhaps the biasing was wrong (it was from the early 60s), replaced the output with TDA2030s, sounded quite good afterwards.

Denon PMA-710

This would make a very unusual put-put sound, hum or a buzzing noise depending on how it was feeling and also varied with the volume control. After going over a myriad (how many is a myriad?) of suspect looking dry joints and switch cleaning the plug in connectors on the main board it eventually disappeared. Sounded really good when it was working correctly.

Denon PMA-830

This one worked but was distorting the left channel. I found the direct switch had bad contacts then noticed the left channel didn't go to class A temperature. The small green relay wasn't working, I have not seen these relays before although similar ones are usually found in cassette decks.

Denon PMA-850

This was not energizing the relay. The transistor driver was faulty. I changed the rectifier diodes as the forward voltage varied quite a bit between them. The transistors in the preamp were replaced as the sound varied and distorted.