Ronco that Adcom!

ben62670

Well I got an Adcom GSA-700 for $50. It was not working and I figured I just wanted the case anyway. Well I got to poking, unscrewing, unsoldering, and did some cutting with a 4 1/2" angle grinder with a cut off wheel...

Face

Not just an amp anymore eh?

ben62670

It was a a Dolby pro logic decoder... I spent a lot of time trying to save the remote capabilities, but after figuring out the chips, and ribbon cable I found out the 6 channel pot was bad:( It appears that it sent full signal to the right channel and the relay for the speaker protection died in the off position for the right rear channel. I fixed that. Now I have a 3 channel amp. I cleaned out the lame electrolytic input caps, and now I have a clean 3 channel. The Schematics were miss labeled, but I figure where the inputs went, and ran the line levels straight to the amp section. I have my laptop driving it now. All channels good. No DC offset. The toroid has lots of outputs so I can keep the digital, and analog all on its own supplies. I am going to drop my DIY DAC board in there, and my DIY phono pre. I'll end up with a decent integrated/ DAC for $50:) I am going to replace a couple regs with 15v+- units instead of the Adcom 12v units for the pre, and DAC. For now I am going to slip my Alps motorized pot in there. It has a balanced option so I may be able to add a couple channels till I can afford the 6 channel unit.

Systems

Thats a beefy tranny and output stages for a pro logic processor/amp, alot better than the chip amp stuff usually found in them.Am I correct in that the center ch amp has four output devices and the left and right surround sections each have a pair?

The analog section of the DAC will work just as well with +-12volts.

ben62670

Thanks GV for the tip on the DAC, and yes of coarse you nailed it. 4 on the center, and 2 each on the surrounds. It appears to be NPN/PNP outs and not Mosfits according to the diagrams. There were a lot of crappy electronics in the circuit for sub filters, and processing. I have it hooked up to some full range 4" Tang Bands for now. I think I am going to bypass the speaker protection circuitry. At first the right channel was a little wishy washy(sorry for the technical jargon) but it smoothed out. The trannies by themselves would have cost more than the amp I repaired:) When I get a few $ I am thinking about jumping into the DAC board where the second set of outputs are to use it as the gain stage for the Pre. I need to figure out how to calculate gain for OpAmps. There are jumpers on the DAC, and I think it would be pretty simple to jump in there? Do you think the NE chips are capable of driving the active XO? I am not sure which OpAmps I want to go with yet on the DAC. I may just go with the BB OPA134's because they are so cheap, and have decent performance. Also I may drop in some Dayton .47uf caps in the inputs on the amps just because I have them and would like a little protection against DC. The amp is rated at 50 rears, and 100 center at 8. It would be handy if I could drive the Vifa's with the RR/RL amps, but I may be better off going with the gain clone? I wish there was more room and heat sink available to mount the amps in the same case, but it isn't going to happen:( What do you think of my cave man approach to shrinking the board:eek:

ben62670

I found a cool link:)
http://www.bcae1.com/opamp.htm
There is a mistake on the inverting OpAmp diagram. They are feeding the V+, and not the V-.

Systems

ben62670 wrote: »

When I get a few $ I am thinking about jumping into the DAC board where the second set of outputs are to use it as the gain stage for the Pre.

If you wanted to add some gain I would utilize the 5534 in the unused filter/output buffer section.Some minor mods and it would work.

There are jumpers on the DAC, and I think it would be pretty simple to jump in there?

Do you mean the jumpers used to select the the 2volt or 4.5 volt filter/buffer section?I would not add gain ahead of the filter , you don't want to amplify all that high frequency noise.

Do you think the NE chips are capable of driving the active XO?

No problem,they can actually drive a 600 ohm load.

I am not sure which OpAmps I want to go with yet on the DAC. I may just go with the BB OPA134's because they are so cheap, and have decent performance.

Yeah give them a try.If you do you might want to disconnect the little orange 22pf caps that are near the IC sockets.The 5534 needs them for stability but the 134 does not so it may not like them.

Also I may drop in some Dayton .47uf caps in the inputs on the amps just because I have them and would like a little protection against DC.

Do it, accidents happen.

What do you think of my cave man approach to shrinking the board:eek:

Very resourceful.

ben62670

If you wanted to add some gain I would utilize the 5534 in the unused filter/output buffer section.Some minor mods and it would work.

That's a good idea. I'll take a look at the board tonight. Later I need to figure out how to add a couple COAX inputs and move the optical. For now I just need to get it going:) In a couple days I am going to get a few things from digikey. They OPA134's are only 2.10 each. I may just snag 25 for $42 and do my Adcom DAC before I sell it:) Here are some receives that may work. I know I need to keep the cable run very short to the board.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=TORX177PLFT-ND
Maybe I should get some new cans now that I look at it. They are typical 25v 220uf's that I can use in other projects. I'll have to figure out what to get. The Elna pieces that came on the board look pretty good, and have a decent rep.

Systems

ben62670 wrote: »

Later I need to figure out how to add a couple COAX inputs

I removed the coax input on the board and ran some 75ohm cable to a BNC connector on the back panel.If you need 2 coax inputs a simple toggle to switch the center conductor of the coax between 2 inputs should suffice.

The Elna pieces that came on the board look pretty good, and have a decent rep.

Yeah the Elna cerafines are low ESR types equivilent to the Panasonic FC's.IMO I don't think you will gain anything by changing them.

Just in case you don't already have this.http://focus.tij.co.jp/jp/lit/ug/sleu037/sleu037.pdf

ben62670

I read it inside and out:) I'm going to leave the PS caps on the board alone, but I want to upgrade the amps cheesy caps, and have left overs in stock. They will be used in future projects for sure. I have been playing with experiments long enough to have dealt with the frustration of not having what I need on hand:( Shipping kills DIYer's like myself. I think the 220uf 25v's would be handy to have around. Now that I am thinking about it I may go with 50v's to cover any gainclone projects I have. I still have 10 populated boards that I used cheap caps on. I am now learning that the caps near the OpAmps and transistors are more important than having some huge PS caps. Too much is lost do to runs on the board. I am thinking about having a run of 3way XO boards made to my specs. My present idea is a 12db 12db 24db piece. If you want to get in on it we can design it together and split the cost? Seeing I want to make a few for myself I would be happy just to send you a few just for helping. I know enough just to be dangerous:eek:

cnh

I certainly don't have your abilities in repair, reconstruction.

But man, for 50 bucks the toroid alone is worth more than that. Looks like a lot of work but some really nice pieces, at least the handful I can recognize.

Great purchase...wish you luck with that. Would love to hear a review of the sound when you're done.

Hint!

cnh

ben62670

cnh wrote: »

...But man, for 50 bucks the toroid alone is worth more than that...

I'm very pleased. It's less watts than I am used to playing with, but like you said I couldn't beat it even if the amp section was shot. It took some thinking because the schematic was off, but some patients, a multi meter, and careful examination have paid off. I have been looking for some DIY cases for a while, most are light weight junk, or cost a lot of money for a larger case. I'll post a pic later of the back. I needed lots of ins and outs. A few minor mods in the amp section, and this thing is just what the doctor ordered:)
Thanks
Ben

Systems

ben62670 wrote: »

I am thinking about having a run of 3way XO boards made to my specs. My present idea is a 12db 12db 24db piece. If you want to get in on it we can design it together and split the cost?

Well I'm pretty much set as far as actives xovers go having recently purchased a blank board from Linkwitz labs.I will gladly offer assistance and have some ideas but as far as contributing financially it would depend on the actual costs involved and the end product.We can discuss details via PM .;)

evildad

I just picked up a GSA-700 on ebay and I want to play around with it. Your mod looks very interesting. You indicated that you had a schematic for it, where did you get it, that would be really handy.