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
 
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.



:)
 
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
 
Found it

I have a nice big clear scan I can send you, this is just a tease post of a tiny one.
 

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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.
 

opera2000

Member
2006-10-09 11:25 pm
I have three of these amps. two of them are dg250 and one gc500 by QMI. the QMI amp was great till I turned it up.
I have two of them apart now. and after replacing the outout transitors I noticed that the heatsinks dont fit up with the chassis.
The two outboard heatsinks bow out in the middle. All three are like that. being fixed with only four screws this is why they blow up. I am going to tap a few more holes in it to see if I can make them fit better. What I would like to know is does any one still have a schematic for this? I need to know what to set the bias at.
and the gc500 has three pots. one for bias, One says DC gain cell
(Which I assume to be DC offset) and another that says AC gain cell (I think that means gain)
If anybody can help me with the bias adjustment or a schematic I will be greatfull.
thanks
 
GC 500

opera2000 said:
I have three of these amps. two of them are dg250 and one gc500 by QMI. the QMI amp was great till I turned it up.
I have two of them apart now. and after replacing the outout transitors I noticed that the heatsinks dont fit up with the chassis.
The two outboard heatsinks bow out in the middle. All three are like that. being fixed with only four screws this is why they blow up. I am going to tap a few more holes in it to see if I can make them fit better. What I would like to know is does any one still have a schematic for this? I need to know what to set the bias at.
and the gc500 has three pots. one for bias, One says DC gain cell
(Which I assume to be DC offset) and another that says AC gain cell (I think that means gain)
If anybody can help me with the bias adjustment or a schematic I will be greatfull.
thanks

I have been in contact with the designer. He indicated that the QMI version of the 500 was simply a 300 with a bigger power supply - they did not scale the design and they did this on their own. If I have any any success getting a 300 schematic, will let you know.
 

opera2000

Member
2006-10-09 11:25 pm
GC500

I would think the designer would keep a schematic in his portfolio.
Are you saying that the amp will fail because of the heatsinks being warped? or is there another issue to deal with.
I now have four of these amps and every one of them had warped heatsinks. I have spent many hours sanding heatsinks drilling and tapping more holes for screws and I replaced the output transitors. and got it working. I got the dc offset down to almost zero. and let it run at low volume. every few hours I checked the dc and it was stable and cool I let it run all night. the next morning I checked it and it had fried the left speaker. I would never trust one of these amps with good speakers.
you also mention they had a 300 model. from what little I have found out about these are they supposedly offered to upgrade dg250's to the 500 for a while. the only difference I can tell between the 250 and the 500 is six output transistors of MJ15004 and Mj15003 vs four RCA410 so they changed from all NPN to NPN and PNP which looks like a push pull arangement.
what I would like to know is what do they mean by "Gain Cell"
If you know more about these amps please post it.
thanks DOT.SYSTEM for keeping this thread alive for six and a half years.
And Scott Nixon I know your here. you seem to be the only person in the world with a schematic. Would you please post it I was so deperate I tried enhancing and enlarging that teaser you posted. Be kind
 
Quatre/QMI basically bought or licensed designs (not sure about the speakers). The later owners produced variations on the designs. The only amps I am believe Dave Gore designed were the 250s and the 300. They also listed a GC200 in their lterature - never heard of anyone actually having one. The same for the GC-20 JFET gain cell preamp (3 chassis - single faceplate) but a prototype of this was built - not clear if any were sold. There was also a GC-2 head amp that was sold - the marketing literature copied that of a head amp competitor Marcof.

I think the designer is saying he did not design the 500 and what QMI did to create the GC500 was not adequate and that the protection circuitry plays a role in the failure."The GC500 is better not to own - when the 300 PS was raised other parts were not changed to suit and it was unreliable -they made these changes themselves...The GC-500 will continue to blow up as long as the protection circuit is not readjusted or removed. "

At this link, the gain cell is described as an log/antilog transconductance multiplier.

http://www.freepatentsonline.com/4155047.html

I had a 250c schematic but I gave it to somebody at Audio Ayslum when my old 300 blew up and I junked it. "The GC-300 schematic is close to the 250 but the outputs are complementary. I don't appear to have one...You can fix it without a schematic. However the TO-3 style output transistors are obsolete. So you have to buy replacement parts that are way too much money."

"You will lose one or more outputs usually on one side and at least one driver and predriver transistor. Sometimes a resistor burns - replace that also."

Other quotes:

"The heat sinks warp in the extrusion process - they always looked that way. They used lots of white grease to fill the gap. The right way is to have the extrusions flattened by the shop that drills the holes."

"One change QMI made that was retrograde was replacing the dual transistor in the load circuit (TO-78 can) of the input stage with cheap descretes. Not all boards have this. This controls the gain cell function. Then they put a pot in the circuit and just turned the function off. QMI did not believe it worked.
In my last meeting with QMI- I applied heat to these descretes selectively - to force them to match and showed on the THD meter how the distortion dropped to nil. They need to match - so the dual was superior. "

"Putting a cap across the rails on each board - cuts crossover distortion and ground noise. This was the major problem with the power supply wiring. This mod solves the problem. Use a 50uFd polypropylene or stack of smaller values that will fit - from -60V to +60V - not ground - right on the board - foil side is good - in the power transistor area. The grounded bypass caps on the board for stability actually induce ground noise if increased."

The cap goes across the rails - no ground connection that is tapped - for this bypass. Power is drawn sequentially by the output transistors - like telegraph keys - in a sense. The noise and dirt produced by this process (class A self cancels and thus does not make this noise) has to be cancelled where it happens - on the board preferably at the power transistor pins. Each phase draws through a separate wire. The remote power supply in the amps required 12 to 24" of wire per run - this is death - the situation is hopeless. There is a spaghetti of wiring - the self-inductance of this wiring presents a high impedance at HF."

YMMV - I am comfortable with things like installing PS bypasses caps, new RCAs, binding posts, etc. and probably will not be trying circuit board mods.

Hope this helps.

Bob
 
I popped the hood on a used GC300 I bought. This one says QMI on the faceplate and has a cheaper chassis than the earlier gain cell amps I have seen. There is a gap where the cover meets the faceplate.

Anyway, there are only 2 large electrolytics caps inside. I seem to recall more on my junked DG250/300. How many cans are in your units?

I think my 250/300 had also some sort of divider panel inside. Here is an Ebay listing with pictures of the inside of a 250:

http://cgi.ebay.com/QUATRE-GAIN-CEL...photoQQcmdZViewItemQQ_trksidZp1742.m153.l1262

Bob