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Old 19th February 2011, 02:26 AM   #21
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I'll give it a try again tomorrow and see what happens. Too tired to start again tonight. Enough frustration for one day. Thanks Bob. I'll let you know how I make out.
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Old 19th February 2011, 06:00 PM   #22
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ok...I'm back at it again today. Here is where I'm at. The best I can do across R7/7 if .35V without hitting the wall and sending the offset to where I lose all adjustment. My concern is the bias ability around the output stage. With the frontend biased for this level I'm only reading 2.5V to ground from +DRV and about 350mv from -DRV. Adjusting P3 and monitoring the voltage on +/-DRV I can't get any more. That tells me I won't have any bias adjustment around the output. Time to try the other side and see I get better results.
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Old 20th February 2011, 03:31 PM   #23
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I think I've figured this out. Back to my original suspicion I think it's a problem with the zeners Z3,4 which protect the output stage. The voltage drop across them is more than I can adjust for. I don't have any spares in the box. Before I order replacements thought I'd ask what Mfgr part number you're using. The 2nd question is could I substitue them with say a 12V. I'm running the IRFP140/9140 for the output. It's been too many years and I've forgotten how to intrepret these data sheets.

Thanks
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Old 21st February 2011, 03:20 AM   #24
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Something's AFU with your measurements if you are seeing Anything other than the same voltage across Z3/4 and +/ drv - that is the same point. Again, ignore the voltage across +/-drv until you have your R7/8 voltages close.

I bought a bunch of 1N5239 ages ago. I think the ones I have are Vishay. you sould substitute a 12V part there, but you shouldn't be seeing 9V unless something is totally messed up.
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Old 21st February 2011, 10:26 PM   #25
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I should've paid attention back in highschool during Electronic 101. Consquently I know just enough to be dangerous. It's alot harder diagnosing problems than building. I took the zeners out and tested them and they seemed to be ok so back in they went. Next I starting pulling transistors and testing again. Discovered Q3 was bad. VGS was ok but when I measured the R between Gate and Source/Drain got a really low reading. Replaced and started testing again. Better but still not great and all over the place (+/-) despite taking my time and being patient. Finally after several attempts and the frustration meter maxed out I turned off the power. This is where things got interesting. Still had the meters hooked up and was still getting about .4V at the output. Grounded the inputs and outputs to drain off whatever charge may have been in the circuit. No change. Measure the Reg supply +/-V at the frontend board and what do you know....1.5V. Unplug the AC power cord..and it drops to 0V. Now it looks like I've got AC issues. Lucky there aren't many parts ahead of the transformer. I'm thinking either the Triac or C2 is bad. Maybe once I get this fixed the adjustments will be easier. Know of any good audio repairs shops?
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Old 21st February 2011, 11:15 PM   #26
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Go to your bathroom and look in the mirror. That's the best repairman for this job. If you find a commercial guy to touch your home built amp, the bill will likely be greater than the cost of building it anew.

Yes, it seems likely that C2 or the Triac are leaking. Was the Triac mounted to the case to dissipate the heat generated?

If you used the proper cap there is little chance that C2 is conducting. X and Y rated caps are supposed to fail open. To be sure, just remove it. Plug in, check for voltage on your PSU. If you still have something on your PSU, disconnect the Triac. Repeat the test. You now know what is leaking.

If you want, you can replace the Triac with a switch. Leave C2-R1 across the switch, but pull R2 and disconnect the current switch. The switch should be rated for 250VAC and 10A or better.
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Old 21st February 2011, 11:53 PM   #27
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Yes. The Triac is mounted to the rear wall of the chassis which is a not so heavy duty aluminum sheet. Maybe I should remount it to the bottom of the chassis or, better yet, to the heatsink. Does it really generate that much heat? All the parts I used were what was specified for the project. I don't have another Triac in the box. No wonder these projects take up so much time. Thanks for the suggestions on a workaround. I'll do the tests you suggested this week. Tommorow it's back to work so it may be the weekend before I get to it.

Thanks for all the help Bob. Keep it up and I'll have to put you on my X-Mas list. I'll keep you posted.
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Old 22nd February 2011, 02:43 AM   #28
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If you look at the triac datasheet, http://www.littelfuse.com/data/en/Da...or_Qxx40xx.pdf you've got around 2-3W dissipation, based on 3A draw. Not a lot but you need to give it some heat sink. You're probably OK where you are, but if it failed moving it to the case bottom isn't a bad idea.
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Old 27th February 2011, 11:06 PM   #29
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Ok. Back at it again this weekend. Power Supply puzzle solved. C2 when in the circuit will always conduct. Not much but enough to give you .3-.5V DC unless the power cord is unplugged. For now it's out. So it's back to the front end. No matter what I've tried I can't get this thing dialed in. Even upped C9,10 to 330pf thinking this might add some stability. this thing is so sensitive that even moving my hand over the circuit will drive the offset crazy. At this point I cant get more than .3V across R7,8. I'm thinking about upping the gate resisitors on everything from 100 to 220 or so and give that a try. Anyone have any advice?

Last edited by Dmatuszak; 27th February 2011 at 11:21 PM.
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Old 27th February 2011, 11:28 PM   #30
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Do you have a scope to check for oscillation? Oscillation stability isn't the same as thermal stability. If you don't have oscillation increasing the gate stoppers won't do anything to make the offset more stable. Bring C9 and C10 back to normal values or your top end will suffer. Yes, the sensitivity to air currents is amazing. I ended up just sliding the top open far enough to make an adjustment and then shutting it quickly then allowing 15 minutes or more for everything to settle in before trying again. I think it was Peter Daniel who drilled holes in the enclosure so he could adjust without opening. It takes a while, just let things settle between adjustments
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