Question on how to adjust DC offset on the output of a Dayton SA1000 SW Amp

[tronboy_rf1]

I have two of these amplifiers.  The first failed (stopped enabling in response to the trigger input) just a couple of days before a championship football game several years ago - so I ordered a second and put the first on the side for repair.

Recently, I've gone through the first unit.  When I opened it, I immediately saw a couple of bulged caps, so I made the decision to replace all of the elec. caps on all 4 internal boards - however I ended up not replacing the low-leakage caps as I tested several of them (they were all good for value/esr/leakage at rated WV).    The first unit does work now (there were many bad caps that had drifted in value or were really leaky - I measured them all and recorded results for each).  I've now run 150W sustained into a 4 Ohm dummy load for 10 minutes or so and looked around for other problems.   As a final check, I looked for any DC offset on the speaker outputs, and I'm getting 0.25V.    I also checked the second unit that is currently in service and still has all of its factory parts, and the second unit is showing 0.17V of offset (maybe this is why folks complain of speaker pop with this amp when it enables).

I'm not aware of any public schematics or service manuals on this model, and I've also searched for the same on the Snell SPA750 (appears to be the same SW amp).    I've reversed some of the circuitry in this unit, but I'm hoping someone has figured out a thing or two on the amplifier board so I don't have to pull it back out.  There are two pots on the amplifier board, and I also see a quad jfet op-amp package (so I'm asking myself is this DC servo'd or not).   I'm also reading in other forums that this model has a tracking downconverter on it, which I'm thinking is a fancy linear regulator that varies the rail voltages based on audio level.  If so, it seems that this would muddy the waters on the output DC offset at any operating point other than having the output enabled with no audio.  This may be OK as long as the DC offset doesn't mover around too much I guess.

Hopefully someone has gone down this path before and has figured out how to adjust the DC offset on this unit.  I contacted Dayton, and so far I haven't received any reply.

Thanks for any information that you can share.

[coromonadalix]

No schematics are available  it seems, searched lots of forums  as you may did

some  did reversed schematics

post pictures of the amp board  .... 

[jonpaul]

Dayton/Parts Express = junk from China, just replace

j

[coromonadalix]

Could  be   but with picture(s), you may find some trimmers on the amp board

Measure the values between the pins and very slowly turn them to check the dc offset on the outputs if it change or not ??

[CaptDon]

I believe your dayton is one of their 'Digital' amplifiers, basically a class D switched? I have some of those amps that were given to me and I never bothered to track down the offset. Those amps get such poor reviews (the truthful reviews) that I figured it wasn't worth my time to polish a turd. The other amps (class AB) that came in the same pile were so pathetic!! I was going to trash them. I viewed an online video where the guy showed the burned resistors and shorted zeners. Sure enough that was the problem with this one. The more I dug into it the more I hated it!!! Measuring the voltage drop across a few 'darkened' resistors showed .5 watt resistors running at .65 watts and 1 watt resistors running at about 1.5 watts!! What a piece of garbage. I upgraded the parts and actually the 250 watt class AB unit sounds very good and may actually be reliable for who ever I donate it to. Let us know what you find. Cheers!!

[tronboy_rf1]

Hello again.

A couple of folks wanted a picture of the amplifier board, so I've attached one here.

This won't be rocket science for most on this board, but when you look at the picture the top section takes the power feed from the the main power board.  The mains transformer and power board create approx. +/-140V quasi-DC which runs through these filters and through the MOSFETS at the top (IRF640).   This looks like power cleanup and regulation for the output stage.

The devices on the heat sink at the bottom are the output stage (two 2SA1943's and two 2SC5200's).  This is a single channel amp so it makes sense that these are in parallel.

At the very bottom where the larger caps are with red marks on them are a couple of linear regulators (+/-15V) with a quad op-amp package to the left of the regulators. 

I pulled back the wire bundle to show two available adjustment pots.    I was speculating that one might be for bias adjustment and the other for DC offset.    The crossover distortion looked pretty good so I didn't really want to play with the bias (whichever one that is).

Comment away.  Any suggestions are appreciated.

Thanks.

[coromonadalix]

well you have 2 trimpots  i would take all the 3 pins values between the tap / wiper and extremities and slowly check if the dc offset change, if not put them back to their original values

if not,  you dont have many choices, accept the way it is,  or reverse engineer it

[tronboy_rf1]

OK.   I was hoping someone here had seen this and would have an aaahaaa moment.... 

I did get a response from my note to Dayton Audio.  They did not completely blow me off on my question about an adjustment procedure, but they didn't promise that I'll get an answer either.  They only said they would forward the request into the company and see what comes back.

I'll find a window in the next couple of days to figure it out.

Thanks for the feedback.