HELP! One of my Monoblock amps died..

I've been using a pair of Adcom model GFA-565 monoblock amps since the mid 80's and they have been very reliable and sound great with my Thiel mod 3.6 spkrs. A few days ago about 10min after powering them up with no music playing, I heard a brief arcing sound, the room lights dimmed and when I looked at my amps, the power light was off on one of them. I didn't see any smoke or smell that dreaded odor of fried electronic components!
Upon further investigation I saw that the powerline fuse had blown and the (2) DC rail fuses were OK. I tried replacing the blown fuse and as soon as I powered the amp up I heard the arcing sound again and immediately shut the power off before the fuse blew again.
Well, I'm now asking my fellow "Hoffmanites" for some assistance. Does this sound like it's going to be a major expense. I keep thinking, perhaps the big power toroidal transformr has gone and it will be expensive to repair.
Does anyone here know of a good reputable repair shop on the east coast(preferably the NY, NJ area)????
Thx to all for any assistance you can provide....I'm BUMMED!!!!
Tim "Back to MONO" W.

 

Check directly with Adcom. They have repair in areas and authroized ones too.

Don't mess around. Sounds like your power supply is blown. That will just burn fuses.

www.adcom.com

Good luck, and let us know! A lot of us have Adcom stuff.

 

Well your initial diagnosis could be right, but it could also be the bridge rectifier or even possible the filter caps. I'd imagine it's in the power supply since the rail fuses did not go. I'd think the rectifier is more likely to go out, but I've seen torroids go too.

If you want to to a bit of troubleshooting, you could possibly disconnect the output of the transformer before the bridge. If the fuse still pops you'll likely know it's the transformer. That would be much more expensive to fix than the bridge or filter caps.

Isn't Adcom still in NJ. They would likely be the best place to take the amp.

 

I have had excellent results with Adcom equipment. Call Adcom, their support is excellent, even on their older products. Give them a call, and you will be able to send it to them for repairs. You might consider sending them both, so you can have preventative maintenance done on the other one.

 

They are owned by some Chinese company now (Klein Technologies), and they are now located in Scottsdale Arizona. Luckily Klein has left Adcom alone, and allowed them to keep on making great stuff. However, Adcom's surround preamps are Klein designs.

I would not poke around inside. I would call Adcom, and send it in for repair. If they see that it has been tampered with, it will cost more to fix, because they will have to put it back.

 

Thx for the excellent advice guys! I have dealt with Adcom for parts and they have been excellent to deal with. I wish they were still in NJ!!! It's not going to be cheap sending the amp to them in Az just to probably find out it will be too expensive to repair. That's why I wish I could pinpoint the problem locally and just order the part from them.
Thx again...I'll keep you posted...
Tim

 

Well I appreciate the advice to not mess with the amp, but under the circumstances a diode bridge is around $10.00. Considering shipping and out of warranty repair charges (assumption) it would likely cost you a few hundred bucks to send it in for repair. And as you say, if it is too much to repair then you're wasted your shipping.

The amp modules are generally tough to work on. The power supply, however, is usually not. I guess it all depends on your skill level, confidence, experience and finances.

Perhaps Adcom can recommend an authorized service center in your area. If it is the PS, then anybody reasonably competent should be able to repair it. You can likely find a facility that could at least give you an estimate for less than the cost of shipping to AZ.

But if you choose to do this yourself, and if you succeed, think how good you will feel. If you go this route, and I'm not necessarily sugesting so, then perhaps you could open the amp and post some pictures of the power supply. Then some of us may be able to help by telling you where to look. It might be easy and it might be hard, but a photo might help determine that.

 

Blue Book value on a 565 is about $400. In other words, if it costs close to that amount to fix (including shipping), you'd have to wonder if it's better to just replace it with another one.

 

So true Doug...when u can fix yourself and save a lot of money, it's a very good feeling. I resolved a previous prob with one of these amps by swapping out a couple of semiconductors from the good amp to the other.(Another advantage of having identicle monoblocks!) Total cost from Adcom parts dept: $8!!! If I can figure out how to post a digital pic I will do so(great idea!) . These amps seem to be of a basic design and have been easy to work on. lots of room inside. Pretty much just a couple of small ckt boards, Big transformer, 2 big caps and lots of output trans on the heatsinks.
Tim

Rudy, Thx for the bluebook info. That certainly will help with just how much $$$ I want to sink into this repair!

 

Cool. Just consider the book value as a guide--no doubt eBay madness would send prices higher online! But as I understand it, the Blue Book's values are based on data gathered from dealers who buy and sell used equipment.

I'm thinking of getting a used oscilloscope soon, to troubleshoot a couple of things I have here. One is a power amp that developed a buzz in one channel. I'm almost certain it's a power supply filtering issue. If I could trace it with the 'scope, and compare the left side of the amp to the right side, I'm sure I could find it. (My electronics maybe a LITTLE rusty, but I can still remember how a 'scope works! )

 

Old friend of mine had an HP scope. Nice!

 

Our electronics teacher actually gave away some of their older scopes when they got newer ones, but I didn't get in line fast enough. Those HP scopes are nice. I can't remember, but our newer scopes at school may have been B&K Precision. No idea what the old scopes were...but they probably had vacuum tube guts.

 

I took a couple of digital photos of the amp that I'd like to post. Can someone tell me how I would do it? They are JPG type pics.

Thank you,
Tim

 

You cant attach pics over 120kb. Hit "post reply" at the bottom of the last page, then type your message, scroll down, and hit "manage attachments". A box comes up, click "browse" and find the image on your computer that you want to attach, then click "open" then click "upload". Thats it!

 

I"m on my way out the door, but I'll get you started. If you use the "Post Reply" button here, you will get to the reply screen. Go down below the message window and look for the Additional Options area. (If this area is just a bar, click on the arrow on the right side of the bar to expand it.) From there, you click on the "manage attachments" button, which will pop open a window. What you do is click the "browse..." button to choose the which image you want to post, then be sure to click the "upload" button to send it to us. Repeat this for each photo you have, unless you want one photo per post (in case you want to add a note to each photo), which is OK too. Close the window, and verify that your images are shown in a list in the "Additional options" area. If that's OK, send your reply!

 

Here are the pics...

 

Sorry....I know it didn't work....still trying.
Tim

 

I think this is my problem...My pics are well over 120kb. Guess I'll have to re-shoot at a lower quality???
Tim

 

If you have an "image processing" program that can open and save JPG files you probably can use that instead of reshooting. I use a program called LviewPro, and it allows you to set the image quality/compression ratio of JPG files. Set it to lower quality/higher compression ratio and the resulting file gets smaller.

 

Tim--you have a PM. We'll take care of ya.

 

Rudy, Got your msg and will send you the pics on monday.(They're on my pc at work). I really appreciate your offer to help with this!
I did some more tshooting today and have traced the prob to 1 of the 2 large 100v/35000uf caps. I had them tested at a local electronics store. Looks like these were custom made for Adcom.(Their logo is on them). I'll be calling Monday to see just how much 2 of these babies will cost.(If they still carry them....I hope!)
Quick question...Since I have 2 seperate monoblock amps..if i replace the caps in 1 should I also do the other? Will the 'sound' be affected and sound different if I only do the caps replacement on 1 amp???
Thx again all for your input everyone. This site great!....I miss my tunes BIG TIME!!!!
Tim

 

Well, I can't believe I found the problem and can't get the replacement caps!!! I called Adcom and they don't have them anymore. I've been searching the internet(digi-key.com, Partsexpress.com) and not having any luck.
Does anyone here have any other ideas??? I would really appreciate any recommendations at this point.
Here's what I'm trying to get:
100v, 35000uf capacitors. They're big: 5" x 2.5"diameter with 2 threaded screw holes on top.
Thanks guys.....
Tim

 

Try Mouser. Don't give up.

Hopefully, that's all it is too....caps.

 

You don't have to use Adcom caps. Anything that can handle the maximum power supply voltage and has the approximate capacitance or more will work. The biggest problem will likely be getting the right diameter to fit in the mounting brackets.

My thought is that these filter capacitors don't really have any specific impact on the sound of the amplifier. Their function is to keep the power supply voltage smooth from hum, and stable from voltage fluctuation. IMO, the more capacitance the better. You don't necessarily need 100v capacitors either. Measure your power supply voltage on the other unit. If it is 40v (for example) you just have to make sure you have a good margin above that. In this example, 60 or 75 volt caps might work just fine. Of course, get as high a voltage as you can that has the capacitance. Your power supply voltage should not rise much above the 40v or so, so the caps will not likely blow from over voltage. Of course, I don't know what voltage your power supply has. The more power the amp puts out, generally the higher the power supply has to be. Your amplifier will never be able to put out more voltage than the power supply provides, and will generally put out square waves if you attempt to do so. Clipping is what happens when an amplifier wants more voltage than the power supply has available.

All your power supply needs to do is put out a steady, stable voltage. The best power supply possible would be a battery. Of course, a battery will drop voltage over time, but that is the goal of any power supply, to be as close to a battery as possible. In light of this, as long as the amp puts out clean, strong, dc (and usually plus and minus) than it doesn't seem to matter how it does it. Hence, you should be able to use any power supply that puts out the right voltage and current, and have virtually the same sonics. IMO, you would not have to have the same caps in the power supply sections of your mono blocks. Now if you were asking me about having different capacitors in the signal processing sections of dual mono blocks, I would say that wound not be a good thing, but I think it depends on what the capacitor is doing in the circuit.

I once had a client that had an old Elam 241 microphone without a power supply. He was unable to get a replacement power supply for it. I built him a good solid state power supply and that mic sounded absolutely amazing. Would it have sounded better with the original power supply? I would think no, but I'm guessing that some would disagree. And it's entirely possible that it sounded better with the new supply. I've never heard another mic sound that good. Of course that was years ago, so who knows what's out there now.

Of course this, and anything I post here, is just my opinion.

 

If you can't find exact caps, you'll have to make do with what you find. In a power amp, the big caps would do two things: 1) filter the incoming AC voltage to make it as pure DC as they can; 2) store energy, which gives you dynamic headroom.

On Mouser's site, I found that they do have computer-grade 35000uF capacitors, but only rated for 40 volts. You can go higher on the capacitance, as long as size is not an issue: a 40,000uF capacitor will work as well, if not better. Another way to do it is to put the caps in parallel: two 17,500uF capacitors connected in parallel equals 35,000uF. (It's the opposite of resistors, which are summed when connected in series.) Found some 39,000uF caps, 100v, that would probably work. Here's the data sheet for the brand I found (which you can apparently buy at www.mouser.com):

http://www.cde.com/mpdf/cgs.pdf

If you want both amps to sound the same, you may want to replace the caps in both. You may never hear a difference though.

IMHO, power supply caps, in this capacity, don't affect the sound as much as caps in the signal path, so brand may not be as relevant as it would be elsewhere in the circuit. You just need the caps to smooth out that AC voltage and store energy, something most basic caps will do well.