Yamaha PD2500

[jkuetemann]

I recently acquired an ill Yamaha PD2500 power amplifier, gratis. The unit was indicating that the protect circuit was active (the reason it was a freebie). Upon my initial checks I discovered that lifting the front left corer would clear the protected status. I found two bad caps and a burned resistor on an auxiliary power supply and a whole lot of cold / dry solder joints everywhere, especially in the PSU.

I have replaced the bad components with new and have totally re-soldered this unit. Installed all new Sil-Pads on the outputs and re-compounded the handful of other items that were disturbed during the operation. Yamaha Canada's service department was even so kind as to send me a scan of the service manual, and this is where I would like to solicit some opinions. No other faults found and it does work now.

The manuals sent appear to be complete in so far as the actual contents match the printed table of contents, but there is no schematic. A service manual without a schematic? Nor is there any reference to calibration / bias settings. Seems odd.

The bias settings are quite different from channel A to channel B. The test points read across both the 'upper' and 'lower' output emitter resistors as is typical. I get ~32mV on channel A and only ~11mV on channel B. It looks like channel B may have been mucked about with since the value seems quite low. So, my question...

Without seeing a schematic and without a reference to what the engineer who designed this unit specified for bias setting what to adjust it to then? My initial thought was to go with 26mV across each emitter resistor (52mV at the test points) but that was substantially higher than the potentially 'good' side.

So, some quick thoughts. This unit operates on +/- 80V rails and has 7 pairs of outputs per channel and 0R47 emitter resistors. If it were set for 26mV across a single emitter resistor then each pair of outputs has a quiescent current of ~55mA, and therefore each channel ~385mA. This would mean each channel would be dissipating ~62W at idle. Would one expect something like this to dissipate ~120W at idle? It is actively cooled but that seems somewhat high. Would it have been somewhat under biased to limit quiescent dissipation?

Any advice on what this units bias should be set to? Is it even worth the effort? I haven't given it a serious listen as yet, but it is an old professionals amplifier. My guess is that ruggedness was the primary concern and sonics may be disappointing in a domestic setting. Would it be good for something like a subwoofer?

[jkuetemann]

This was on hiatus for a while but it turns out to be a nice sounding amplifier. I never did get any feedback regarding the biasing and so just went with the channel that seemed to be untouched and set them both to 32mV on the test points. Seems to be thermally stable and with the cooling fan and really large heat sink in this thing it stays cool.

Any ideas for replacing the existing centrifugal 'cross flow' fan with something that is as close to silent as possible? The heat sink won't work via convection due to its design and position and will need to remain actively cooled. I'd like to bring this into my current system but it would be nice to have a quieter running fan if possible.

[sgrossklass]

You can try posting pictures of the whole affair, maybe someone has a bright idea then. Sounds like a radial fan is used.