Yet another Anniversary Series amp question

[amh]

I'm going through the process of dialing in all my amps and ran into a problem with the 9444Bs. I have the service manual but it calls for a scope and frequency generator to set the DC offsets and bias. I don't have either piece of equipment.

The 94442As and 9444As both can be set with just a DVM. Put the test leads across the channel output and set the DC offset pot for 0 mV ( +/- 10 mV ). Then set bias by putting test leads across the appropriate resistor and set the bias pot for 1.1 V. Job done.

The 9444Bs are a different beast. There are two pots for current limit, one positive and one negative. They are set with a scope and a 4ohm load on the channel. Is there a way to do this with a DVM?

Bias is set with the distortion analyzer, and no resistor is mentioned to check a voltage across. It looks to me like R10 and R11 ( both 22ohm 5W per channel) in the schematic would be the ones to check the bias voltage across. Should they both have the same 1.1V across them as in the 9444A amp.

There is also a LF cancel trimpot. It looks like you definitely need a scope to set that one.

Any ideas?

bfish, you seem good with these amps, any clue?

[bfish]

Howdy, AMH!

Sorry, but I've never had the pleasure of tinkering with a "B"....and after reading your post, that's looking more like a good thing....

You've obviously got a detailed setup guide, but the procedure is beyond resources available to most tinkerers, heck, I haven't even had a (working) distortion analyzer in years, and my scope's largely been replaced with a software scope/RTA via my soundcard. There ARE some really cool new handheld DMMs WITH a scope and more, I've been afraid of looking at them too closely...

Seems like the B was either directly of EV origin, or at least heavily influenced by them, and was more of a replacement for the A instead of a refinement.....another one bites the dust kind of thing.

I don't think you'll be able to apply anything useful from the A literature to your B. Wouldn't even try.

If the amp has an issue I'd say this would be a good one for IraTech, who likely knows it inside out, and can quote you the procedure without cracking a book. Last I heard, he'd do a diagnosis and estimate for $50, the choice to proceed is up to you. If however, you're just wanting to do the setup so you'll know it's right, as I would myself, I'd blow the dust out, clean the pots and fan, ensure proper fusing, look for near-zero DC offset with my DMM, hook it up, and call it good.

[amh]

Thanks bfish,

The amp does work well enough to get somewhat of a baseline from. One channel drifts slowly into protect mode ( over an hour or so of use ). But the other channel is fine. I'll just start comparing readings from good side to bad. I can always check the "out of circuit" value of key components too. Much work though.

Yep, some off those new DMMs are cool, my wife would just love me getting another $400+ toy.

Iratech is an option of last resort. $50 + shipping + service is more than I paid for the amp. Plus, like you, I like to know enough to keep them tuned in and running smoothly.

Thanks again!

[bfish]

A problem that slowly drifts to a limit condition is likely a thermal issue caused by a resistor, thermistor, or zener changing value as the amp warms up. I just looked over the B schematic, and thermistors R25 and 125 would be my first suspect, as they're parallelled with the bias trimpots to provide thermal bias compensation, and could easily cause an over-current protection fault if out of tolerance. If you could get hold of a can of "Cold-Shot" (freon aerosol) like we used to use in the days before DuPont ran the EPA, you could save a lot of time finding the faulty component by rapidly cooling the suspects one at a time. I've seen a few leftover cans on RatShack shelves, might be a good idea to check your local store(s). Having a good channel next door for comparison is a big help.

Understand the shipping/cost issue and preference to DIY. There's great satisfaction and confidence to be gained by knowing what's going on, as opposed to assuming and hoping everything's right.

[amh]

bfish,

Good catch on R25 - 125. I'll check those first. Neat trick with the cool shot too.

I'll keep you posted on my progress.

Thanks again, you've been very helpful.