Denon POA6600 Help

[Bigred]

Does anybody have the service manual for the Denon POA6600 and NOT the 6600A version? Or at the very least know what the proceedure for setting up the idle current and neutral point voltage is?

[Bigred]

I have a full POA6600A manual with the idle current setup procedure but since the 6600 is a dual super non-nfb curcuit and the 6600A is optical class A.... I don't think the 6600A procedure would be any benefit Has anyone got any ideas? Is there a tech whizz that could give me a rough idea from looking at the schematic?

[Roar Malmin]

Drop me a mail at
roar(at)musical-innovation.com
I have the schematic.

[Bigred]

Thanks for the response. I have a full service manual for the 6600A and JUST the schematic diagram for the 6600 but require the rest of the service manual (specifically the adjustment procedures).

[anatech]

Hi Bigred,
I would think your DC offset adjust control would go for minimum DC voltage from the audio output to ground. Less than 10 mV since it is adjustable.

Bias current is another issue. Most products tend to sit from 10 mA to 30 mA per device.

If you have the schematic in electronic format, could you email it to me please? bhome at sympatico dot ca.

-Chris

[Bigred]

Thx Chris.... schematic and short novel sent your way

[anatech]

Hi Kevin,
The procedure for setting bias simply warms the heat sink up to what would be close to a known temperature. Adcom and a few other people do things in a similar way. Just follow the same procedure and check the value and placement of the test points so you can correct for any differences.

Your DC offset should be very close to zero. If this channel has been repaired before, it's possible the J-Fets were damaged. Also, check your +/- 15 VDC supplies for the front end.

To match J-Fets, you first measure a bunch for IDSS and separate them into piles having similar values. From each pile you can run them through a matching test jig I made. You can also use the jig for testing NPN pairs or even individual beta.

Note that I reversed the current select scale listed. It goes from right to left instead of the other way around. Measure between TP 1a and TP 1b for the lowest reading to get a match. I normally run the drain voltage around 10 VDC.

-Chris

Edit: Brain Fart! It's the POA-6600A that I do not have. Can anyone help me out here?

[Bigred]

The one 6600 mono block was in protection and had the 2sa1492 and 2sc3856 complimentary pairs done along with a 15v zener in front end, some smoked resistors (can't remember exact ones) and spkr relay 701 (contacts MELTED!! lol). I dont have the means to have handfulls of transistors for matching purposes unfortunately. It is back pounding but runs a little warmer than the other and has the -14mv dc-offset (cooler amp +1mv). So the search on how to setup idle current continues. One other note: After amp was back running, found the amp would go into protection with loud pop thru spkrs if you switched it from normal to balanced mode. Since normal mode was only ever used and amp functioned just fine until output prob, it is not known how long this circuit has been faulty(cause, coincidence, result who knows). In any case I traced it back to opamp NJM2068(IC401) and replaced. I dont have a balanced capable preamp to fully test but no more protection and pop. Even though its not used, I wouldn't feel good giving back until I fully check so am gonna borrow my buds preamp with balanced outputs after I am confident with the idle current and neutral point voltage setup. Looks like I will have to jar loose and grab the SM from servicemanuals.net since its not available from Denon anymore.

[anatech]

Hi Bigred,
With damage like that, it is very possible to damage the front end. Check the feedback resistor in case it got overheated as well. The case is simply that those fets have been damaged and should be replaced. They should not be expensive and if you pick up 20, you should get a few good pairs out of that. One point is that this channel may have higher noise and distortion (almost guaranteed!). It may also fail later on.

Understand that enough current went through the fets to wipe out a zener. That should be reason enough right there.

-Chris

[Bigred]

Well I have confirmed all resistor values are within tolerance and all expected voltages are pretty well spot on. There MIGHT be some fatigued components but I do not think this amp is in any dire need of more repair. Yes the offset should be ZERO but I didn't think 14mv is worth the trouble of building a jig and matching handfuls of fets etc. I may be wrong with this thinking and am open to being convinced otherwise.

I have since learned there are 3 adjustment procedures needed to be performed with the 6600
1: Idle current 2: Neutral Point voltage 3: Distortion factor. The 6600A has no mention of distortion factor. So I will be more at ease when I get the 6600 manual and confirm these adjustments. The only thing confusing at the moment is the fact for the 6600A version all it says for the adjustment of neutral point voltage is to hook meter up to terminals and confirm the voltage indicates within +/-100mv value. THATS IT!! If I'm understanding correctly... any reading +/- of upto 100mv is acceptable here. So if I got 99mv reading or neg.99mv its good. I happen to be gettin -14mv so I assume its well within "spec". It puzzles me that the manual is calling it "adjustment of neutral point voltage" but no actual adjustment procedure just to confirm the +/-100mv value. What if I got 150mv for example??