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Spectro Acoustics

Started by onanparts, February 07, 2012, 07:59:46 PM

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onanparts

Quote from: Robin on February 22, 2012, 02:24:33 AM
It was 1977 I think. We were in the middle of a major cosmetic redesign. I broke my wrist so it was difficult to do the drafting. Remember this was before computers. Spectro was a pretty young company and were all pretty close knot. We would go out behind the shop at every break and play volleyball. One Sunday we all met at a park in Richland to play volleyball. Back then I was young enough that I could still drink, smoke and chase women. I remember diving for the ball after it had already bounced out of bounds. My wrist suffered, so boB drove me to the hospital to get it all betterized. The next three weeks were a @&%@ trying to do the drawings for this major redesign. I had boB's girlfriend do most of the actual drawing since I couldn't draw left handed. Part of this cosmetic redesign was to change the silkscreen color. We tried silver and gold. The gold was pure class. I vividly remember changing the white stripe on our EQ slider knobs to gold. This was a real pain. I think we used a thread dipped in Testers airplane paint. Spectro was the worlds largest manufacturer of graphic equalizers at that time and we had 50,000 white knobs to paint. Boy, those were the good old days.


Only 50,000 knobs to repaint? OK Robin, you opened the door......:)

I got the deluxe kit, it had a solar cell and a meter.

Midnite B17-10. 50kW Cont. 150kW Surge... Me/Myself/And/I

Robin

Looks like there are still more of those darned knobs that need painting.
Robin Gudgel

wanders

Looking for advice on repair of a Spectro Acoustics 202. Just took this amp out of two year storage and the left channel output is about half of the right. I'm not sure of the function of the two trim pots, but both, especially the left, have deteriorated badly. The blue insulation of the left pot literally disintegrated as I was attempting to adjust it. Right was not much better. Are these actually gain controls? Or something else? Probably I could replace them if appropriate.

Opinions sought and appreciated.

Robin

Ut oh, those are bias adjustments. We adjust the bias current for lowest distortion. Put them back to where they were if you can remember.
Maybe Bob can answer the question about lower output?
Did you swap the input cables to make sure it is the amplifier and not the preamp?
By the way, I just bought a Spectro 200SR on E-Bay. We will be modifying it to use on development of our micro inverters. We need a good 120VAC source that can also sink current back into it. Don't know if we can just use a transformer to step up voltage or actually modify the amp for higher voltage?
I think we use3d a Crown amp at Outback for this same testing.
Robin Gudgel

boB

Quote from: wanders on November 23, 2012, 12:26:49 PM
Looking for advice on repair of a Spectro Acoustics 202. Just took this amp out of two year storage and the left channel output is about half of the right. I'm not sure of the function of the two trim pots, but both, especially the left, have deteriorated badly. The blue insulation of the left pot literally disintegrated as I was attempting to adjust it. Right was not much better. Are these actually gain controls? Or something else? Probably I could replace them if appropriate.

Opinions sought and appreciated.

Yeah, that would be a bias trimmer.  If that trim-pot goes open, that channel can get excessively hot.  In that case,
until another trimmer replaces it, I would short it out.  That will bring up the crossover distortion some but you may
not even notice it.  Those trim-pots are 100 Ohms.  R35 and is paralleled by another resistor to help out in case
it does go open, but still may get kind of hot if it does disintegrate. 

I'm not sure about the volume level of the one channel being lower.  What happens when you swap the left and
right channel inputs ?  Does that same channel stay lower in volume ?

Or maybe that one channel is actually clipping earlier than the good channel ??

boB

PS, here is a schematic of the basic Spectro 202 amplifier module...
R35 is kind of in the middle and to the left a little bit.

http://k7iq.midnitesolar.com/Spectro-202-scamp-BW.gif


K7IQ 🌛  He/She/Me

wanders

Thanks for the advice & especially the schematic. Issue is the amp, not the pre, etc. Right trimpot shows a range of 0 to 24 ohms; left channel pot is faulty. Found a temporary 25 ohm pot at r/s, but, of course, that didn't affect the basic issue; if I can find the problem I'll get a proper replacement for the bias pots. I'll work with the schematic and see what I can do. I'll post results down the line.

Thx

Robin

You may need to get a hold of an oscilloscope and audio generator. IT is pretty easy to follow the schematic and trace the signal on both channels to see where one drops off. Getting the equipment is another matter. You might even luck out and find some cheap stuff on E-Bay?
Robin Gudgel

wanders

Quote from: Robin on November 24, 2012, 12:58:05 PM
You may need to get a hold of an oscilloscope and audio generator. IT is pretty easy to follow the schematic and trace the signal on both channels to see where one drops off. Getting the equipment is another matter. You might even luck out and find some cheap stuff on E-Bay?

I have access to an oscilloscope and been interested in getting in to that. Thanks for the advice. If I can get this thing fixed, I'll post the results.

boB


If the main problem is low gain, it ~may~ be that the input op-amp is not working.  It gives the amp a gain
of about 3 or so.  If the zener diodes that supply that op-amp are shorted, then maybe it is just bypassing
the op amp or maybe the op amp itself is dead.  I think it is just a single 741 type pin-out.

The earlier amp modules did not have that op amp.  Instead, the feedback of the amp module
was just set higher...  In fact, as I remember, R15 was 27K Ohms instead of 8.2 K Ohms in
the schematic.  If the problem IS the op amp circuit, you could bypass the op-amp gain
circuit and just change R15 back to 27K and get that gain back.

boB
K7IQ 🌛  He/She/Me

wanders

#54
boB,

Thanks for that suggestion. My amp does not contain an op amp and, as you suggest, R-25 is 27K. The information, however, caused me to look more closely in that area and I found that C-18 (little hard to read the schematic there) is spec-ed at 10 uf. On the good pcb, the component reads 12 uf (out of the circuit), however, on the low output board, the cap reads 33 nf. In your view, would that be sufficient to cause the output loss?

Thanks,

boB

#55
Quote from: wanders on November 25, 2012, 01:23:40 PM
boB,

Thanks for that suggestion. My amp does not contain an op amp and, as you suggest, R-25 is 27K. The information, however, caused me to look more closely in that area and I found that C-18 (little hard to read the schematic there) is spec-ed at 10 uf. On the good pcb, the component reads 12 uf (out of the circuit), however, on the low output board, the cap reads 33 nf. In your view, would that be sufficient to cause the output loss?

Thanks,


Yes, the low capacitor will definitely cause that problem.  Great detective work!
K7IQ 🌛  He/She/Me

wanders

Thanks, I'll replace that part and post the results.

wanders

boB, Robin,

The replacement cap fixed the problem. For good measure, I replaced the rest of the electrolytic caps on both boards. I'm listening now and, even with all new caps, the amp sounds better than I remember. As noted, I benched this amp a few years ago in favor of a better sounding Hafler P3000. Think I'll give the Spectro another shot at the starting rotation.

Thanks for the help.

Warren

fitz43

I would like to know what the connection was between Impulse Audio and Spectro Acoustics. When I was considering purchasing a Spectro Acoustics P-101 in 1975, I was given marketing material that had Impulse Audio written on it. I was told it was the same people. The items appear very similar. I do have the material scanned as a pdf file, but don't know how to upload it here.
Was the Canadian dealer who said he could sell the pre-amps Jan of Jan Stereo West from Vancouver, BC? I purchased my Spectro Acoustics P-101 from one of the franchise stores. It is a silver face with a SN of 74008. Thanks for your time. Doug

Robin

Jan Rybar was a mover and shaker in the HiFi industry in Vancouver. He was one of the first to sell the Spectro line. Before we built Spectro, we built the Impuls Audio preamps.
Here is the real story. Ken Cox and I (future president of Trace Engineering), built 23 stereo preamplifiers while working at Phase Linear. Some of the emplyees and us, wanted a preamp to go with our Phase Linear power amps. Bob Carver was taking his time in messing with the Phase 2000 preamp. We just got tired of waiting for him and decided to make a few much more simple preamps for the interum. Bob Carver even bought one.
A Canadian salesman came down to Phase for a visit one day to see how the progress was coming on the Phase preamp. The Phase preamp wasn't ready, but he saw one of our little preamps. I think it may have been called the Rob-Ken pre-amp? He said he could sell a bunch of them. That statement launched Spectro Acoustics. Ken and I decided to build preamplifiers, but something a bit better than our 23 simple designs. The Impulse Audio preamp was born. I doubt that we ever made more than a few hundred of them before switching to Brian Morreson's designs of the Spectro Acoustics gear. The Impulse Audio was my design, and I am not much of an electrical designer. They didn't sound bad though.
Robin Gudgel