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Old 26th June 2002, 09:59 AM   #1
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Default What to do with old broken amplifier

I have a Quatre DG-250c two channel amplifier. It had 125w rms/ch. As far as I know Quatre was a division of a company in California called QMI. The amp was built between 1978 and 1982 when the company went out of buisness. If you have any information pertaining to this amplifier, PLEASE let me know.

OK, I bought the amplifier with only one channel working. I was told the other channel was untested because it had a tendency to blow out tweeters. Well, I blew out the other channel playing it too loud with a 4ohm sub hooked upto it.

The amplifier is mounted in a 19" 3U cabinet which contains a giant 65vdc unregulated power supply. It uses a pretty hefty transformer along with two 18,000uf and two 10,000uf soda can sized (rather old) caps. Lights in the room dim when you turn it on.

Now granted this amplifier sounded very good when working, should I use the ps and make my own amp or tey to fix the one that is currently in there?

Thanks David
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Old 26th June 2002, 02:01 PM   #2
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Default Helpfull Advice

Hello PhaseShift,
I see that the mandatory profile fields are working, and welcome to the forum.
By the looks of your bio you are a youngster with happy ambition.

.....In answer to your questions, from an early 60's model who has been fixing electronics longer than you've been around, the best, quickest and cheapest soloution is to fix what you have, especially if you liked it previously !.
If you don't like the result you can sell a perfectly reliable amplifier.

From long experience, the guaranteed way to bring this now 20+ years old machine (and by your description, quite decent enough) back to usefullness is to check all semiconductors and replace all blown or doubtful semiconductors.

Step one is use a decentish DMM with diode test - a Fluke DMM if possible.
Watch out for any MPSA 06/56 or 42/92 transistors going leaky (they are cheap, so just replace them), and transistors with blackened legs, and be prepared to replace all drivers if any outputs have gone short.
Check all resistors and replace as required.
Replace all signal sized electrolytic caps with modern low ESR SMPS types, for they are by now (20+ yrs) and very doubtfull.
Check all other caps for leakage - disconnect one end.
Treat all pots, switches and connectors with a good contact solvent/oil spray.

Then blanket resolder every solder joint in the whole machine, in one session, and then clean flux away with isopropyl alcohol, a white bristle art brush, and tissue.
I find Multicore Savbit (60Lead, 38Tin, 2Copper) to be excellent tinning, sonically quite fine, and cheap.

Run the amp up 1 channel at a time if possible, with the mains fuse removed and a 60W lamp across the mains fuseholder.
If the lamp goes bright you have a problem, but you usually have plenty of time to track down the fault with a voltmeter, without things cooking - check component temperatures regularly though.

Refit the fuse and set bias currents, and check the outputs terminals for DC offset and adjust to near zero volts if there is a trimpot.
If all seems ok, let it idle for 10 minutes, and check and adjust bias currents and offsets accordingly.
Then connect an audio signal at low level and momentarily connect a speaker to test if all seems ok.
If seems ok, run both channels loaded with a speaker at very low volume for 10 mins.
If it still seems ok both channels, now run it up and slowly and carefully on clean, nice, chordant type sounding music.

If you listen REALLY carefully as you SLOWLY ramp up the volume, you will hear the music get gradually louder but constrictedish (constipated), and then getting towards clip it will let go and become friendlier sounding.
Wind the volume down to zero, and then slowly wind the volume back up into clip, and the sound characteristic will change again.
Do this carefully, and the amp sort of takes on a nice set that stays, but you only get one chance after a blanket resolder.

If you master this, you can now restore any amplifier.
If you now like the sound, that's it !.
If you don't like the sound you can tweak it, or fob it off to your friends with the guarantee that it will run for another 20 years.

If you need more info, by all means feel free to ask.

Regards, Eric.

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Old 26th June 2002, 03:23 PM   #3
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I have a schematic.. some where in my stacks. Will make a note and try to find it.
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Old 26th June 2002, 05:07 PM   #4
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Scotty is the man. He has them all.

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Old 27th June 2002, 03:13 AM   #5
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Scott, I would be willing to pay you for that schematic. I have been looking for one for the last two years.

BTW, the main power output transistors are nolonger being made. I have not tried to look for a substitute. Many of the other transistors in there may not be replaceable.

I have done a trace on the board with a scope (the ch. that never worked). The only thing that I found wrong is a diode was causing some unusual distortion. I only had about an hour to work on it though.

Let me know if you need anymore info on the amp.

Thanks David
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Old 27th June 2002, 04:11 AM   #6
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Thumbs up Found it

I have a nice big clear scan I can send you, this is just a tease post of a tiny one.
Attached Images
File Type: gif smallgc250.gif (9.3 KB, 1601 views)
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Old 9th April 2004, 01:36 PM   #7
ghost is offline ghost  United States
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Default Schematic

Could I have a copy of your schematic? Id be willing to pay for it. I have this amp too and would love to bring it up to spec.
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Old 10th April 2004, 06:04 PM   #8
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I have this amp and the schematic too, but like yours my Quattre was broken when I bought it. I have been trying to restore it, but it continues to break. I am about to conclude it is not stable enough to be worth the effort, but the chassis and power supply would be a good base for something else. If you have better luck, I'd love to hear about it.
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Old 10th April 2004, 06:39 PM   #9
ghost is offline ghost  United States
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Default Quatre

By the way Im a different David,
David Egan
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Old 10th April 2004, 10:13 PM   #10
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Say Scott, old buddy. Would you mind emailing me a schematic on the Quatre as well?

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