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Old 22nd March 2005, 05:18 AM   #21
djk is offline djk
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"I have been using my Variac set at about 12V so far"

You're only feeding 12V to the amp?

No wonder it doesn't work.

Use the lightbulb. Let it rip. It shoulk blink, and then settle down to a dull glow. If it stays bright, something is really fried. With a dull glos we want the supply voltages toi be about 2/3 of normal at idle. This will allow normal opperation on music to a couple of watts peak so that you can see it working.

After I fix an amp I let it run for half the day or more on the lightbulb. If the bulb starts flashing bright after several hours you will be glad its there, it just saved the amp from total destruction (again), and will run it enough for troubleshooting.

I gave my variac away many years ago.
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Old 22nd March 2005, 01:57 PM   #22
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Quote:
Originally posted by djk
"I have been using my Variac set at about 12V so far"

You're only feeding 12V to the amp?

No wonder it doesn't work.

Use the lightbulb. Let it rip. It shoulk blink, and then settle down to a dull glow. If it stays bright, something is really fried. With a dull glos we want the supply voltages toi be about 2/3 of normal at idle. This will allow normal opperation on music to a couple of watts peak so that you can see it working.

After I fix an amp I let it run for half the day or more on the lightbulb. If the bulb starts flashing bright after several hours you will be glad its there, it just saved the amp from total destruction (again), and will run it enough for troubleshooting.

I gave my variac away many years ago.


I'm still using the variac, because at about 12-15VAC I'm reading -8VDC on the output of the one channel. I have tried raising it a bit and the DC voltage on the output goes up with it.

I don't have much experience yet, but logic tells me that having several volts where I should be seeing mV is not good for the componants. When I first got the amp I plugged it directly into the wall and measured the voltage on the output. I was reading -64VDC on one channel. I haven't plugged it into the wall again, only through the variac. On the good channel I am reading about .2VDC or less.

I am in the middle of putting together the soundtrack for our Easter program right now so I wasn't able to work on the amp last night. Probably won't get back to it until Thursday. I really appreciate all the help you folks have given. I will try and take better measurements when I get it opened up again.

Blessings, Terry
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Old 22nd March 2005, 02:14 PM   #23
K-amps is offline K-amps  United States
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Terry,
The Egyptians used the lightbulb in 4000bc, then the art of softening the supply was lost for 6000 years... then found again Eureka! One of the nicest and cheapest insrances for amplifiers... I have a Variac... don't use it... bulbs rock!

Disclaimer: You kow I am kidding about the Egyptians right?
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Old 22nd March 2005, 02:24 PM   #24
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Hi K-amps,

Don't get me wrong, I am going to build one of those light bulb circuits. I like the idea very much. My point was that I have high voltage on the output, so in lieu of plugging it into 120V I have been using my variac. It lets me bring things on very slowly and watch both meters while I do that. Maybe the light bulb trick will do that as well. I don't know, I haven't tried it yet.

I've got a question about using the light bulb trick. One amp I had would blow the main fuse instantly when switched on. By using my variac, I was able to bring up the power just enough to find the problem without blowing the fuse. Will the light bulb trick allow the amp to power up without blowing the fuse if there is a big problem like that? That would be very cool if it does.

Blessings, Terry
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Old 22nd March 2005, 03:35 PM   #25
K-amps is offline K-amps  United States
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Yes Terry,

Instead of blowing a fuse, the bulb will light up and "eat-up" all the current going to the amplifier and limit it to less than 1 amp or thereabouts. This will prevent turn on catastrophies.

If you have DC on the output, you can attach a dummy load, the bulb will again light up because the amplifier is trying to drive an 8 or 4 ohm load with 75 volts (or whatever the rails or voltage leak is at the output) and load up the bulb which will light up and prevent any components from burning up.

However if you do not have a dummy load, and there's a fault on the input/ VA stages, you could burn up those components as they cannot source enough current for the bult to light up and starve the amplifier of current, hence when troubleshooting, I tend to keep a dummy load on the outputs while using a bulb.

I just switch the amp on/off everytime I trace and fix a fault till the bulb finally goes dim... then I know I am close or done fixing.
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Old 23rd March 2005, 01:15 AM   #26
djk is offline djk
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Go ahead and ignore 30 years worth of experience.

Many amps will not work when brought up slowly on a variac.
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Old 23rd March 2005, 02:41 AM   #27
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djk,

I have read many of your posts found in various threads here and have come to understand the depth of your experience. At this juncture in this thread's discussion it would be helpful to me, and perhaps to others, if you would tell us more about the use of a variac. Can you provide and example of an amp that will not work if brought up slowly with a variac? And, can you tell us why this is so?

I am thinking it might be an amp with integrated circuits used to sense various operating conditions or provide voltage regulation. Am I warm in my reasoning?

Thanks
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Old 23rd March 2005, 02:58 AM   #28
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I have repaired at least a dozen of these. they are VERY stable amps. each one i have brough up on a variac without problems. I have even had these amps running on 20V dc rails!

If you have DC offset, you most likely have bad driver transistors. these go all the time. if those test good, then start yanking mosfets as these do fail!!!

The mosfets are attached directly to the heatsinks! the heatsinks are isolated from everything. the case of the fets and the heatsinks form the main output line. this is speaker level before the output inductor.

Also, check to see that the power supply rails are near the same voltage.

You can pull out all the mosfets and tie a 1K 1/2 watt resistors on both sides of the 180 ohm resistor between the drivers to the output line. this will allow the amp to run without the output devices to a healthy level. those driver transistors are rated for 1 watt!

Without the mosfets you should be able to get a good clean trace with minimal offset.

Start checking mosfets. none will be shorted. but the gate resistance will be all over the place when compared to the others. some will be in the M ohm range those are typically good, others will be in the 100 ohm range and those are typically bad.

Also the inputs to the fets have zeners and 1n4004 diode to limit gate voltage in gross overdrive situations. make sure none of those have shorted. also the fets have 1n4004 diodes across them. make sure those havent shorted.

These are fairly simple, and very robust amps. however the solder traces leave a lot to be desired. they burn off easily.

Email me if you still cant figure it out.

tetech2@doitnow.com


Zc
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Old 23rd March 2005, 03:06 AM   #29
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Quote:
Originally posted by karma


i was told the fan has 2 speeds is this true?
Yes fan is 2 speed, runs at slow speed under normal conditions. when really kicked in the PCR circuit steps into high gear and the fan kicks on full! then calms back down after the input or the temp back down.

I love these little amps! they seriously kick butt!!!! and they did the driver section RIGHT! just my .02c but they realyl do sound good. a little lacking on the ultra low end however. have been wanting to play with the input cap to see if it made any difference. etc...


Zc
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Old 23rd March 2005, 03:14 AM   #30
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I don't quite follow your comment here. The 180 ohm resistors are the gate resistors to dampen oscillation. Do you mean to tie the output end of these resistors to the speaker output? This added resistor would be in place of the removed MOSFET. Is this correct?

Quote:
You can pull out all the mosfets and tie a 1K 1/2 watt resistors on both sides of the 180 ohm resistor between the drivers to the output line. this will allow the amp to run without the output devices to a healthy level. those driver transistors are rated for 1 watt!
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