Just picked up an Adcom GFA-5800 (broken)

mmadden28 wrote: »

Yeah I'm not sure I'd go much higher that about $225 for parts, but its still a fun project either way so I'd be getting some satisfaction and learning experience out of it.

If this were at a repair shop and that was just the parts costs and I still had to pay for labor??? No way. At least this way I'm doing the labor and it seems likely these parts will fix it.

I gotta ask! Are you sure that's all it takes to fix the 5800? I would check the driver board too just to be safe.

When you are testing the amp, use the VARIAC if you have one. Or use the 100W light bulb in series to the power plug (to limit the current input to the amp).

megasat16 wrote: »

When you are testing the amp, use the VARIAC if you have one.

Good advice and IMO it is almost essential.Also be sure the bias is set correctly and checked several times after the amp has had ample time to warm up.

I don't have a VARIAC so what I have done is use 2amp FB fuses in place of the larger rail fuses.

GV#27 wrote: »

Good advice and IMO it is almost essential.Also be sure the bias is set correctly and checked several times after the amp has had ample time to warm up.

Thanks GV! Excellent point about biasing adjustment.

ben62670 wrote: »

I don't have a VARIAC so what I have done is use 2amp FB fuses in place of the larger rail fuses.

I think $100 for a reasonable Variac is a good start instead of spending a few bucks for 2 amps fuse everytime it blew.

mmadden28 wrote: »

Well, I'm basing that assumption on the fact that the other half of the channel and all the other components on that board test OK, also I did have it powered up already. Looking at the schematic the damage doesn't go all the way back, and it appears that the source of the failure may have been a short at the binding posts or speaker wires somewhere. So I'm more hopeful more than anything. At least I'm not dealing with any ICs or logic gates here....

No Variac.
How do you do the Light bulb hookup thingy?

I am thinking it would be OK too but you really need to check and make sure the driver stages are OK. At minimum, I would check to make sure there is no high DC V (should be only a few mV) at the Gate of the MOSFET. It seems like a good start without having oscilloscope.

The light bulb needs to be hooked up in series to the live wire of the amp plug. If you have an old light bulb socket, use one end of the wire in series with the live wire of the amp, use the other end to plug into ac outlet. Use 100W or less light bulb. The light bulb should dim after a few seconds when the caps are charged and if there is no short between transistors or any mishaps. If the light bulbs never dim after quite a few seconds, there may be a short somewhere.

Ok, there is limitation - there are amplifiers that consumes 200W or more at idle power so using only one 100W light bulb does not really work and the light bulb may never really dim. You'll need to use two or more of light bulb and sockets in parallel to increase power (more watts) in series with the amp ac plug.

mmadden28 wrote: »

Any suggestions on a decent Variac?
Sounds like a useful tool, especially with anything AC related--I never liked working with full AC and the sudden hums, pops and then the subsequent dimming of the lights on the whole block .

Variac is very useful and pays itself off in a couple of years of regular use. You can find one on eBay or in many electronics store too. But make sure it's rated for at least 500W or more.

GV#27 wrote: »

Your probably aware but just a reminder that Mosfet's are sensitive to and can be damaged by a static charge,handle with care.

Yippie! MOSFETs are super sensitive to static discharge.

mmadden28 wrote: »

Not sure if you can see the detail it he schematic above but it only says 8.2v. Nothing about a part # or wattage rating. I'll have to see if I can see a # on the part--but they are pretty small. I don't have the servioce manual--Only the schematic above and a small section of the service manual that provides the Bias setup instructions.

How does one determine if the existing resistor is non-inductive? Apparently this makes a difference in this kind of circuit...

I can't tell you what it is from looking at schematic you posted there but there will be the parts list in the manual and you can look up the part number in the parts list (Bill of Materials) to determine the part number for zener.

Some Wirewound resistors could be inductive if the wattage is too large but I don't think you need to worry about it as long as you put the replacement with the same wattage.

mmadden28 wrote: »

yep--although I never really got why storing them in styrofoam was a good idea. I mean I do but then again I don't - if you follow me.

At work I'm a big fan of antistatic precautions--people always walking around, esp in the winter, with hard drives or memory chips in their hand exposed. :mad:
Some people don't understand that static doesn't always kill right away, but that it can weaken a part for failure later, and when it does fail they don't realize it was because of the static damage they did awhile back.

The power MOSFETs may be a bit more forgiving to the static discharge than smaller MOSFETs but I would wear wrist wrap and grounded my skin (and body) before I even touch it.

I like the VARIAC or light bulb in series for testing anything which can be powered up and blow something else. The VARIAC can be reused where the fuse isn't

Yes using dimmer, but a bulb is fine in my mind. Not as handy as a VARIAC but handy yes, dimmer NO.

Ben,

The idea of using Variac and Light bulb in series or fuses are not the same and does not work the same.

Variac limits the voltage to the device under test (DUT) so it'll protect the DUT while you make some critical measurements from the components. Thus, it allows you to troubleshoot problems and keeps the DUT from possible damages.

Variac can also be used to reform old electrolytics caps in the old and unused amplifiers and bring them back to life safely (but this method is not always successful to reform electrolytics and replacement is always better than reforming).

Lightbulb idea is very similar to Variac but it limits current (not voltage) so it'll still protect the DUT but has limitations. It's something you need to use only if a Variac is not available.

Fuse Only protects for a short time and blew so you can't really troubleshoot anything if it blew.

But if you use the Variac like once a year, may be you don't need one and better send the broken item to the Tech for professional repair.

megasat16 wrote: »

Yep! The guys in DIYA knows their stuff and have many many years of practical experience that money can't buy or learn in school.

I would definitely listen to some gurus from DIY and AK.

I've followed Chris's posts for a long time and he has helped me with some specific questions so I know he's one of the good, no BS types that post over there.

H9

P.s. and he knows Adcom, Carver, Nakamichi forwards and backwards

megasat16 wrote: »

Ben,

The idea of using Variac and Light bulb in series or fuses are not the same and does not work the same.

Variac limits the voltage to the device under test (DUT) so it'll protect the DUT while you make some critical measurements from the components. Thus, it allows you to troubleshoot problems and keeps the DUT from possible damages.

Variac can also be used to reform old electrolytics caps in the old and unused amplifiers and bring them back to life safely (but this method is not always successful to reform electrolytics and replacement is always better than reforming).

Lightbulb idea is very similar to Variac but it limits current (not voltage) so it'll still protect the DUT but has limitations. It's something you need to use only if a Variac is not available.

Fuse Only protects for a short time and blew so you can't really troubleshoot anything if it blew.

But if you use the Variac like once a year, may be you don't need one and better send the broken item to the Tech for professional repair.

I am not disagreeing with what you are saying, but for people who do this once in a great while they become dust collectors. Again if the fuses are low amp pieces that cost next to nothing to use, and they don't blow with a true power on with (like flipping the power switch) then you are good. So say you do get a VARIAC and you power it up slowly I would still use lower amp fuses till truly tested good. Again you still should use some low amperage fuses for testing. Those output trannies are not cheap. Bottom line when it comes to amp repair listen to Chris(anatech) over anyone here.
Ben