Repairing a Denon PMA-350 Amp (distortion)

[neuralnet]

Hey guys,
I'm hoping someone will be able to give me some help repairing a Denon PMA-350 Amp. The output from the amp became progressively distorted in both channels. I've swapped out the amp and the rest of the system plays fine, so I know it's not the speakers or the source at fault. I was hoping it was some caps blown or something simple like that, but it's not the case. I have a multimeter and can identify components, but have no experience diagnosing a fault like this and would really appreciate someone taking the time to give me some pointers. The service manual can be downloaded here; https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Denon-PMA-350-Service-Manual.pdf
Please let me know if you need any other information.
Thanks!

 

[rjenkinsgb]

With it being both channels, a power supply fault seems most likely.

Check the voltages on the main smoothing caps C003 and C004, around centre right on the schematic.
They should each have around 40V - 45V or so on them.

If they are OK, then check the low voltage regulators to the left of those, across C013 and C014. They should each have 15 - 16V across them.

Post back what readings you get?

 

[audioguru]

A sinewave at the inputs and an oscilloscope showing each output will quickly show what kind of distortion there is.

 

[tvtech]

That's a nice amp. But the SE is vastly superior to the normal 350. You sure you posted "your amps" diagram in the link?

**broken link removed**

 

[unclejed613]

describe the distortion, does it get worse as the volume goes up, or worse as it goes down?

 

[unclejed613]

do you know if there's DC offset on the outputs? if there's more than 0.1VDC on the speaker terminals, replace C327 and C328 which are the DC blocking caps for the feedback. as a matter of fact replace them anyways, the amp is old enough that if those caps are dried out, they will suck out all the low frequencies. that effect would be one that progressively gets worse. the remaining audio would only be mids and highs and would sound tinny.

 

[neuralnet]

That's a nice amp. But the SE is vastly superior to the normal 350. You sure you posted "your amps" diagram in the link?
**broken link removed**

Good catch, I thought I'd linked the correct user-manual, and despite the site listing it as PMA-350 & having a PMA-350SE manual listed separately next to it.. both are links to the SE model. I'm struggling to find the 350's service manual at present.. Sorry, should have double checked the manual with the hardware before posting. Does anyone have a link to the basic 350 manual? My google-fu is failing me..

Amp belongs to a friend and I offered to take a look at it because listening to music on his amp was fairly painful.. Distortion in situ sounded like the speakers had blown. It was evident at relatively low levels, not sure if it got worse with volume. I'm currently at my own place and testing the amp with headphones, and it sounds clear, sharp and plenty of bass.. so I'm guessing it's to do with the speaker output section or the power is just too low to drive speakers? Not sure if this is relevant but the unit has been unplugged for a number of weeks inbetween use and testing. I'm going to grab some speakers from the garage and will post back.

 

[neuralnet]

do you know if there's DC offset on the outputs? if there's more than 0.1VDC on the speaker terminals, replace C327 and C328 which are the DC blocking caps for the feedback. as a matter of fact replace them anyways, the amp is old enough that if those caps are dried out, they will suck out all the low frequencies. that effect would be one that progressively gets worse. the remaining audio would only be mids and highs and would sound tinny.

With the amp on and the volume fully down, the speaker outputs are showing around 0.01V.. so those caps should be ok for now at least? Should I post some pics of the board or hold on and see if we can find a PMA-350 manual..

 

[unclejed613]

if they are dried out, they won't have leakage current through them (which is what would cause DC offset), but would severely reduce the amp's gain at low frequencies. measuring the ESR (effective series resistance) is the best way to determine if they are good or bad. a 220uF cap should read less than 5 ohms on an ESR meter. if you don't have an ESR meter, an audio generator and oscope can be used to measure the ESR. connecting the scope input in parallel with the audio generator, and setting the frequency to 50khz, and the output voltage to 1V p-p, you connect the capacitor across the terminals, and you have now created a voltage divider between the 50 ohm source impedance of the generator and the cap. if for instance the voltage goes down to 0.1V thats an ESR of about 5 ohms.

 

[neuralnet]

Thanks unclejed for the extra info..

I'm glad to report that this tale has a happy ending although I owe you all an apology. It turns out that the amp is not at fault. I tested it at my home with a spare pair of speakers I had in the garage and I couldn't get the amp to distort, clean output on both channels. Today I took it back to my friends house and swapped it back in to his system and the distortion is apparent. I popped the mesh covers off his speaker boxes to find that the glue on the dust covers from the speaker cones had dried out and both had fallen off, leaving them wedged between the corner of the cone and mesh cover. It was these that was causing the distortion to the sound, albeit mechanical not electrical. Why he thought that the amp I lent him fixed the problem, I have no idea unless they had moved themselves into a place that obstructed less..

In any event, thanks all for pitching in with your advice.