I recently found a Denon PMA-560 Stereo integrated amplifier for sale with the remark that the left channel had stopped working. For a small fee the seller shipped the unit to me and this is the log of diagnosing and fixing the issue.
The Denon PMA-560
First, something about the Denon PMA-560. It’s a Stereo Integrated Amplifier from 1991 and is an analog design. I can really appreciate this design, using no digital IC’s or MCU’s.
As always I was able to find a trove of information on this unit at hifiengine.com, including a scan of (a German version of) the service manual.
I bought this Denon on Marktplaats, a Dutch alternative for Craigslist, with the remark that the left channel had stopped working. This could be due to a lot of things, but it will probably be fixable.
Diagnosing the problem
The first step with any repair job I do, before connecting it to mains and powering it on, is to open up the device and check for any obvious signs of malfunction: blown fuses, leaking or buldging capacitors, fried resistors or other burned looking things, disconnected wires, etc.
A first inspection showed that everything looked the way it should. There was little bit of dust, which was quickly taken care of.
Time to turn it on!
To be safe, I always start with, if possible, disabling the speaker outputs. The PMA-560 has a nice rotatary selector on the front turn the A and B speaker connectors on or off - it turned them all off and connected a pair of headphones.
Next, I hooked up my CD player to the CD input, turned the volume all the way down and powered it on. Nothing exploded. Yay!
Slowly increasing the volume revealed that both the left and right channel on the headphones were working just fine. I also tested the different inputs (omitting the phono input for now) and everything worked beautifully. Also the balance, bass, treble and loudness buttons worked as expected.
Next up: speakers.
First, I connected two 8Ω speakers to the A-channel, and as expect no sound on the left speaker. Connecting the speakers to the B-channel, however, had the same effect as the headphones: great sound on both left and right channels.
These simple tests told me several things:
- The power-amp section is working fine for both the left and right channel.
- The inputs work correctly.
- The adjustment section works ok.
- Headphones and B-channel work fine.
- Only the left speaker for A-channel is not working.
Finding the root problem
The service manual (you can find it on hifiengine.com) contains the full schematic and it was rather easy to trace the single point of failure for this particular set of symptoms.
In pink I’ve indicated the wire connection to the headphone PCB, where the headphones are connected. In blue are the three relays that enable/disable each channel or headphones.
If you follow the lines you see that the left channel goes straight to the A, B and headphone outputs. The same goes for the right channel. The only point of failure between the common left channel output and the A-channel left output is relay, RL602.
It’s not surprising that after about 25 years of turning off and on again, the relay for channel A encountered a (probable) mechanical failure. I tried the speaker selector and the RL602 makes a distinct, heavier sound than RL601 for B-channel.
The fix is still pending. The relays are clearly marked and a Denon part number is specified in the service manual. Finding a replacement part was straightforward and a new relay in currently in the mail. In a few days I’ll replace the relay, which will most likely make A-channel fully functional again.
What I liked most about this repair is that is was very easy to diagnose. I didn’t even touch my multimeter or scope. Stay tuned for an update when the relay has been replaced.
The spare parts have arrived, read the update and replacement log now.