Just picked up an Adcom GFA-5800 (broken)

Systems · October 2009

mmadden28 wrote: »

Question--just curious--since the output FETS are setup parallel, I'm assuming they chose 8 for each N and P channels to achieve a certain output power.

Yes paralelling devices will increase output potential especially into lower impedances and lowers the output impedance (thus increasing damping factor).

...I wonder if I could test with just lets say 3 N and P FETS installed, obviously not driving the output for normal use, but wouldn't that at least let me know if the input and driver circuits were functioning normally?

For the definative answer I would consult Nelson or the service shop in NJ you mentioned before trying ,but for testing purposes I believe you could do so.

disneyjoe7 · October 2009

Well I'm out

Ok you all Pro amp repairing people Go Go Go.

Systems · October 2009

disneyjoe7 wrote: »

Ok you all Pro amp repairing people Go Go Go.

We are merely trying to be helpful within our knowledge base.As mentioned above consulting with the real experts for the final word is highly recommended.;):)

megasat16 · October 2009

ben62670 wrote: »

Transistor matching is a huge pia. If it ain't broke don't fix it. If there is a fault that happened to blow the transistors instead of a short then you may end up blowing a whole bunch of new parts. I don't remember if there are any protection circuits in the 5800. I know that in the older Adcom's that I had there were none. I had a 5800, but never really played with it. I wish I did so I could be of more help. It is not easy via pictures over the internet.

Many guys at DIY Audio are anal about transistor matching and did in several ways to be almost identical. I also tried my best to match the transistors for my amp rebuilt but there are a lot of difficulties when dealing with discontinued trannies.

I don't know how the GFA-5800 is designed but I like to think there will be DC protection and overload protection circuits (fuse) in there for safety sake.

I also don't know what's what from the pictures. And there is no Service Manual to even look at what's what in the schema. Hopefully, Adcom is generous enough to let OP a SM for this amp.

megasat16 · October 2009

mmadden28 wrote: »

I only made that decision based on Anatech's comment:

But what transistors do those need to match with?
Well I'll at least do the zeners once I figure them out.

I'm not buying any parts until I get the Service Manual-But I'm still ordering a Variac in the meantime.

No matter what happens, at the very least I am definitely going to replace the known bad parts and give it a whirl.

I believe what Anatech meant is that he don't recommend you to swap all parts out all the old parts with the new ones (called shotgunning) but for your case, you have very little choices due to the lacking of tools and instruments. When you swap out many parts without knowing which one (or many) is faulty, you are just compounding the problems. It may work but you never really know what you replaced was OK or faulty.

I would wait for a full SM to see of there is anything said about transistor matching. But at the very least, there are differentials pairs in the driver stages that should be closely matched.

megasat16 · October 2009

mmadden28 wrote: »

I wonder if I could test with just lets say 3 N and P FETS installed, obviously not driving the output for normal use, but wouldn't that at least let me know if the input and driver circuits were functioning normally?
I'm just thinking like there's a 3 pack of 9240s on ebay for like $12--If there were something wrong with the driver or input circuits and these got shot , the I wouldn't be out much and I would know before killing the good matched set.

Thoughts?

1. Be careful of the transistors purchased on eBay. You have very little experience about counterfeit transistors and I don't know if you can bench test the little sucker to see if it's real or fake. I've learn that lesson a hard way before and I didn't even buy my transistors from eBay. I bought them from a well known electronics supply store and they were fake!

2. I've said it many times before - you really need to get a service manual or we are just playing with the fire blindfolded. You should install every single components before powering up the amp. It may fire up with 3 pairs of transistors but the idle current and biasing adjustment will be crazy. It's all thermally compensated so all transistors need to be dissipating to get to the correct biasing level. If the biasing level is incorrect, I don't know what may come.

megasat16 · October 2009

You should thanks Audio Circuit for their excellent manual exchange program.

ben62670 · October 2009

Thanks Mega. Where is their web site? I need the Denon 2910 manual?

megasat16 · October 2009

www.audiocircuit.com

I don't think they have Denon 2910 manual.

mmadden28 · October 2009

megasat16 wrote: »

You should thanks Audio Circuit for their excellent manual exchange program.

Sweet!!
Now that is much more comprehensive.
And the schematic is all on one page--no more illusions

Thanks!!

ben62670 · October 2009

megasat16 wrote: »

www.audiocircuit.com

I don't think they have Denon 2910 manual.

That is a pretty cool site. I am reading about DIY ESL speakers for kicks.
Thanks
Ben

mmadden28 · October 2009

So, I know this is too early for this part but, on the Bias adjustment...:
The 80kHz low pass filter that's mentioned--Is that something I need to provide or worry about?
And how do I measure the 83.3W at the output?

ben62670 · October 2009

Don'y worry about the 80khz filter that was used just for distortion readings. As for the 83.3 watts you use an 8 ohms resistor and measure the voltage across the r. Us this with an ohms law calculator to figure the watts.

mmadden28 · October 2009

megasat16 wrote: »

...
I also don't know what's what from the pictures....

FYI,
The entire Page 1 of the schematic (at least of the two I originally showed), is in this section of the PCB:

Page 2 is the output MOSFETS and resistors.

Its almost layed out as it is in the schematic. Not quite , but pretty close.

mmadden28 · October 2009

ben62670 wrote: »

Don'y worry about the 80khz filter that was used just for distortion readings. As for the 83.3 watts you use an 8 ohms resistor and measure the voltage across the r. Us this with an ohms law calculator to figure the watts.

But what Voltage? AC or DC? I'm assuming AC, but I always figured my meter was tuned for 60Hz AC, not varying Audio Frequency AC or in this case 1.5kHz. Of course I guess the meter is probably just reading RMS anyway

ben62670 · October 2009

Set it for AC and turn up the volume of a 60hz test tone, and note the volume position when you reach 83.3 watts. Then change the frequency to 1.5khz and turn it to the same position. This isn't super crucial. There are many things to consider like room temp and case temp. It is also fan controlled colling so that can effect the settings.

mmadden28 · October 2009

I guess I'll have to try it at 1.5kHz and 60Hz and see if there is a measurable difference at the same volume setting.
I may just try this on one of my working amps--fun stuff!

ben62670 · October 2009

One thing is for sure you will be surprised at how loud a 1.5k test tone is a 83 watts! Wear hearing protection;)

mmadden28 · October 2009

, yeah I'm sure--I'll be wearing my chainsaw gear when I do it in case any parts pop

.

Systems · October 2009

Awesome find on the SM mega!

mmadden28 wrote: »

But what Voltage? AC or DC? I'm assuming AC, but I always figured my meter was tuned for 60Hz AC, not varying Audio Frequency AC or in this case 1.5kHz. Of course I guess the meter is probably just reading RMS anyway

I read 25.7 VAC on my Fluke 73 across an 8 ohm load resitor at 83 watts.

ben62670 wrote: »

One thing is for sure you will be surprised at how loud a 1.5k test tone is a 83 watts! Wear hearing protection;)

You should'nt hear anything other than maybe a low level mild buzz driving a load resistor.You wouldn't want to drive a speaker with test tones at that power level,very bad things would happen.:eek:

megasat16 · October 2009

Cool, you have the better pictures of the PCB. You see a lot of resistors showing signs from heat related stress, right? Just to be safe, you should replace all the resistors in the driver stage. They are cheap enough and buy the exact same type as in the SM.

You'll regret not replacing them! I saw a few of them have the coatings cracked or color changed (a good sign for resistors) on R22, R23, R7 and may be more which I can't see.

Also, replace electrolytic caps (yellowish canister type caps) while you are at it.

Transistors may or may not be OK but I saw one with White color on the far top in the picture, it's also may be a sign of extreme heat before it give up. I would check all transistors but you don't have a transistor tester.

If I were you, I will approach a local tech and ask him to check the transistors for me if he could and pays him for his time. Or just buy the matched pairs and replace them.

megasat16 · October 2009

mmadden28 wrote: »

Sweet!!
Now that is much more comprehensive.
And the schematic is all on one page--no more illusions
Thanks!!

You are very welcome but all credits owed to Audio Circuit and their excellent ideas of manual exchange program!

ben62670 wrote: »

That is a pretty cool site. I am reading about DIY ESL speakers for kicks.
Thanks
Ben

You are very welcome too, Ben!

mmadden28 · October 2009

GV#27 wrote: »

...

You should'nt hear anything other than maybe a low level mild buzz driving a load resistor.You wouldn't want to drive a speaker with test tones at that power level,very bad things would happen.:eek:

Doh, well duh--guess that's what happens when we think tech stuff at almost 4 AM.

mmadden28 · October 2009

megasat16 wrote: »

Cool, you have the better pictures of the PCB. You see a lot of resistors showing signs from heat related stress, right? ....I saw a few of them have the coatings cracked or color changed (a good sign for resistors) on R22, R23, R7 and may be more which I can't see.
...
Transistors may or may not be OK but I saw one with White color on the far top in the picture, it's also may be a sign of extreme heat before it give up. I would check all transistors but you don't have a transistor tester.

....

The only resistors that appears out of the ordinary were only the 16 Source and gate resistors, and the one for the Error LED. All other resistors in the input and driver stages look and meter fine.
I'll have to have a closer look at the Resistors you mentioned--

The white on them is heat sink compound. These are normally screwed down to the massive heat sink when installed in place. I don't have the pic avail to me at work to dbl check what your seeing, but one of them is actually a Thermistor.
I'll take a hi-res shot of that section when I get home.

I'm building the MOSFET test circuit from Nelson Pass's article, I used mostly stock parts I had laying around, but I need a few more, so they are going to be on order. I'll be testing the FET transistors individually using that. But I can't get ANY of the parts suppliers to return my call at all so far....arrgh.

megasat16 · October 2009

mmadden28 wrote: »

The only resistors that appears out of the ordinary were only the 16 Source and gate resistors, and the one for the Error LED. All other resistors in the input and driver stages look and meter fine.
I'll have to have a closer look at the Resistors you mentioned--

The white on them is heat sink compound. These are normally screwed down to the massive heat sink when installed in place. I don't have the pic avail to me at work to dbl check what your seeing, but one of them is actually a Thermistor.
I'll take a hi-res shot of that section when I get home.

I'm building the MOSFET test circuit from Nelson Pass's article, I used mostly stock parts I had laying around, but I need a few more, so they are going to be on order. I'll be testing the FET transistors individually using that. But I can't get ANY of the parts suppliers to return my call at all so far....arrgh.

I believe you check resistors out of circuit by removing one end to measure it's resistance?

It's cheap enough to replace these resistors with that has colors changing or any cracking on the coating.

mmadden28 · October 2009

Yeah, I intended to replace whatever I could, I noticed a couple of the ones with chipped coatings, and upon closer inspection I am not happy with the looks of at least one of them (R21). Actually I may have chipped two of them doing just that--pulling one lead out for an out of circuit measurement. I only had to do that for a few of them since they were in parallel with another resistive component. most I was able to get a good read in circuit.

Haven't had a chance to get a better pic yet.

I wonder, does anybody know what some of the modders to to mod this amp. I don't want to get ahead of myself, but if I'm replacing parts, and any of these parts are worthwhile to get higher quality parts for, might as well right? How about the caps?

mmadden28 · October 2009

On another note, I built a series in-line Light bulb outlet. Even though I'm getting a Variac annyway, I thought the using the light bulb as a resisitive current limiting load an interesting tool, esp in AC work.

I wanted it to be a very simple project with minimal customizing required. Essentially I took a duplex outlet, split the receptacles, wired it so both receptacles are in series (splitting the Neutral). The top outlet is dedicated for a plug in bulb adapter or a lamp. The bottom outlet is for the load. Put it in a box, added wire and a plug and whalla-And it works!!
It definitely restricts current, wouldn't let my air compressor ot grinding wheels start up. I could hear them trying, adn the light was bright, but...
I bought a few bulbs for, it, higher wattage bulbs have lower resistance-If I recall the 300 watt bulb had about 3 ohms and the 55 watt bub was about 45 ohms. Well how about that Ohms law at work again

When using a light bulb for an amp, what wattage/resistance bulb is best
I'll have to get my list

Of course, at first I split the Hot (as with a regular light swich) but after a quick zap while touching the metal box and the exposed screw on the bottom of the bulb, quickly reminded me that I had it backwards for this scenario.

All safe now

Pics tomorrow.

megasat16 · October 2009

You become more dangerous by the minute.

I would buy 3 prongs plugs and sockets. I would never use bare wires in case I need to mention it (or miss to mention it before). Sparks can be a spectacular show but electrocution is a zapping experience.

ben62670 · October 2009

Yes Mega he scares me. He brings back memories of polarized caps being inserted backwards in my first amp project. Lots of stinky explosions!

mmadden28 · October 2009

ben62670 wrote: »

Yes Mega he scares me. He brings back memories of polarized caps being inserted backwards in my first amp project. Lots of stinky explosions!

hehehe ah the good old days at tech school--nothing like the smell of baked electrolyte in the morning

I've been called Sparky on more than one occasion

My wife was NOT happy when I was finishing up replacing a ceiling fan, and tucking the wires in and tightening it all down (forgot to cut the breaker the last time) I pinched the hot and sparks flew--left a nice comet shot of black across the ceiling and some molten metal from the shroud fell down onto her nice wooden table.......

Yes the marks are still there today on that table. In a new house now, many years later and she still hasn't forgotten about it.

Just picked up an Adcom GFA-5800 (broken)

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