Denon PMA-700V - problems and failures

[P. S.]

Hi,

First - apologize for a lengthy post. I got this mystery going on with my amplifier Denon PMA-700V. Bought it recently, cleaned deoxited etc. and performed some checks. DC offset in both channels is acceptable - 9.4 and 9.6mV.

Now - what I did was probably dumbest thing ever. I noticed that RIGHT speaker does not play sound so went ahead and tried to reseat the cables. But in the process I forgot to shut off the power first (!) ;-(. Result was spark at the positive speaker wire and smell of burnt circuit from the right speaker terminal. (no blown fuses in the amp though)
Once the smoke cloud and smell resolved - I still attached my test speakers to it.

Now the real mystery begins:
I finally had sound coming out of both speakers ;-) The right speaker plays unsteady though - there are a few times I heard distortion. The issue disappeared once the amplifier warmed up for a few min.

I went on and started adjustment of bias. The procedure is as follows:
Start with cold receiver and adjust at 3 time points: 10sec (0.4mv), 5min (5mV), 15min (8mV).
No problems at all with setting the bias on left channel.

Now on the RIGHT channel:
- no voltage at all for the 1st minute or so - not even when you twist the adjustment pot across the scale
- after a minute or two voltage shows up but keeps fluctuating across 5mV range (e.g. if I set it to 8mV - it will fluctuate anywhere between 11 and 6)
- when I twist the adjustment pot it seems to have no effect at all besides very small range around 1-2PM

Other things I did:
- deoxited the bias pods
- checked voltage between power supply board and the board where bias is adjusted (50v steady for both channels - seems no issue here)

I am not super knowledgeable in electronics and how amps work (that's why I keep reading Audiokarma! ;-).
I could not find anywhere on the forums the description of my case where voltage is unstable on the bias measuring point.

Anybody has an idea what might be the issue here?

 

[mr.muddles]

Did you look for bad solder joints?

Not sure about your original problem with no sound on the right channel but those binding posts break off really eeasily and maybe one of them was not connected properly. Maybe check all the solder joints at the speaker terminals... I don't know of the relay circuit can effect what you are experiencing but might look at that too...

I have the 900v but have not had any of these issues, aside from the binding posts breaking off the back.

 

[P. S.]

That's good hint. I will try to touch up the posts with my solder - that I think will include the posts where I put my clamps for testing the bias (might be they are screwed up?) Will keep you posted about what I find out ;-)

 

[mbz]

Smoke is never a good sign, some damage has been done.
Have a quick look at resistors in the right power amp for signs
of heat stress, eg, R562, 564, 566, 590(?) 568, 570, 570, 572

If you are comfortable taking voltage measurements, measure dc voltage
to ground/chassis at TR508base and TR510b. Don't probe transistor legs, probe
at R552 and R554 which connects to the TRb. Expect about +1.2 and -1.2Vdc
If these voltages are not ok then measure at TR502c (R546) and TR502e (R548)

Adjusting the trimmer should have a small effect on all above voltages.

Monitor voltage measurements from power On.

 

[P. S.]

@mbz thanks a lot for advice.

The values on on R552 and R554 are both between 1.10- 1.15Vdc right from the power on. They do not fluctuate so I assume all is good. No visible damage of the resistors either.

 

[P. S.]

Ok so seems like I may know the reason why my readings were off. The rods where the bias is measured (marked as "T.P" on the board) protrude from the board for around an inch. I measured at the ends and it seems like I was pressing against them them too much. This plus old solder joints is causing them to wiggle a bit. I started measuring right on the board level and my readings are way more solid. Adjusting the bias right now. After that will let the amp seat overnight in room temp and repeat the measurements. Hopefully they are good now!