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Old 28th March 2004, 08:41 PM   #241
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I bought a case of the 680 VA toroids for 20 bucks a piece and the variac for 50 bucks...all surplus of course. There was a guy in Penn. that was selling the same toroid...I think he also bought them from the same place here in SLC that I did but his price was ALOT higher than 20 bucks. He advertiised them someplace on this site. None left at the surplus place but there are other surplus places that have usable toroids for this project. Check out #3851 for instance at this site..... Just about perfect!
http://www.elect-spec.com/surplusz.htm

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Old 28th March 2004, 08:47 PM   #242
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Indeed, that is about right, i may just order me one of those 560 w's (the one you mentioned)

thanks agian, i guess i needed to get my eyes away from mouser for this one (and digikey)..
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Old 28th March 2004, 09:03 PM   #243
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Hi koolscooby,

From what I've read so far, I think your power supply may be a part of the problem. You should use a split supply with ground, rather than a single supply. However, you can do some initial tests with a single supply and voltage divider to provide the ground...

If we keep the operating currents low, a two-resistor voltage divider between the rails will give you a close enough approximation of ground. To help reduce ground current, use J1b instead of J1a (I recommend J1b anyway). Next, disconnect R7, R9, R36 and R38 to disable the output transistors. It wouldn't hurt to disconnect R46 and R47 too. Now we can concentrate on making sure your diff pair works properly.

Check the value of R23 and R25 with a multimeter, as they may have been cooked when you had C7-C10 jumpered.

Now, with everything connected as above, you should see 9V across D1, and roughly 5.5-6V across R23 and R25. With some signal input, you should get a nice signal between the drains of Q5 and Q7. If you've made it this far, then the diff pair is working just fine, and we can move on to bringing up the output stages.

By the way, you leave Q12b in... putting a jumper across Q12a simply disables it, leaving R17 to set the current through D1. If R17 is 1.5K, then this should work fine with J1b and with or without Q12 jumpered.
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Old 1st April 2004, 02:45 AM   #244
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Chad, thanks very much for the steps, i'll try and do that stuff tomorrow night, or on the weekend, i was out on a conference this week, and am now back and getting unpacked and the crazy things that deal w/ that

question: what resistor should i place between the -15 and ground and +15 and ground? i'm not aware on how to hook something like that up

Thanks everyone
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Old 1st April 2004, 06:50 AM   #245
hifiZen is offline hifiZen  Canada
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For +/-15V rails, a pair of 1.5K 1/4W resistors should do just fine, but note that the ground split will be sensitive to ground currents. Double-check with a multimeter to see if you're getting reasonably equal voltages across each resistor. You can even go so far as to add a 47uF or larger cap in parallel with each resistor. That'll improve AC stability of the ground voltage a bit.

BTW - that last comment in my previous post should say "...you can leave Q12b in..."
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Old 1st April 2004, 10:01 PM   #246
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Chad,
Right now, i'm doing some measurements

it seams that the banded side (to the left) has about 1.72vdc, and the other side has about 640 mv, aka not what you said it should be

I get a signal at r38, r9 where the components have been removed... but i do not get signal at r7 and r36... normal?

and i only see a few hundred millivolts across r23 and r25, does that mean they are damaged?

Thanks much,

Brad
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Old 3rd April 2004, 01:35 AM   #247
hifiZen is offline hifiZen  Canada
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Quote:
Originally posted by koolscooby
Chad,
Right now, i'm doing some measurements

it seams that the banded side (to the left) has about 1.72vdc, and the other side has about 640 mv, aka not what you said it should be
You must be referring to D1, yes? If so, then it sounds like D1 is either damaged or installed backwards. Try putting a new 1N5239B in there and see if that will give you the 9V. What is your supply voltage at? The 'banded side' of D1 should be at +15V, and you should have 9V across D1. Please double-check your supply voltages.

Quote:
I get a signal at r38, r9 where the components have been removed... but i do not get signal at r7 and r36... normal?
Completely normal. This section of the amp is disabled right now.

Quote:
and i only see a few hundred millivolts across r23 and r25, does that mean they are damaged?

Thanks much,

Brad
This means there is very little current running through the diff pair. From what you've described above, it sounds like D1 is at fault. If D1 does not give the proper reference voltage, then the current source for the diff pair doesn't work properly. With the front end starved for current, there will not be enough voltage across R23 and R25 to turn on the IRFP044s when they're connected.

To check if R23 and R25 are damaged, turn off the power supply, and measure the resistance of these with the multimeter. It sounds like they might be OK though.
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Old 3rd April 2004, 02:33 AM   #248
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ok well ...

first of all, i'm dumb
i had a 10x voltage probe hooked in to my oscope that was set for a 1x, so that screwed my measurements by 10 times

but now thats all fixed

i get +/- 15 v at the PSU

but i only get -8 or from gnd to - plane and about +9 from gnd to + plane, is this normal from the voltage divider?

I have this:

+ -- resistor (1.5k) -- ground plane
- -- resistor (1.5k) -- ground plane

+ -- + plane
- -- - plane

why the heck would it do that? or is that the right setup?, right measurements too?

the d1 is okay, and in correct as per schematic, i put it straight on a 15v ps, and yielded 9.1 drop from 15.1 or so to right around 6.05... so thats fine, put it back in, band to the left

thanks much

brad
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Old 3rd April 2004, 03:02 AM   #249
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Umm... I think maybe that's right. It should look like this:
Attached Images
File Type: gif virtual_gnd1.gif (1.8 KB, 1426 views)
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Old 3rd April 2004, 03:13 AM   #250
hifiZen is offline hifiZen  Canada
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Your rail voltages sound too low though. If you have 30V at the power supply, you should get +/-15V on the amp rails. +9/-8V definitely does not sound correct. Perhaps the power supply you're using is loading down when it's connected to the circuit?

At these low voltages, make sure you're using J1b and not J1a.
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