Hello. Just joined today. I am new to audio electronics and electronics, in general. My adcom had some white fluid leaking from the output transistors. In wanting to clean the thermal paste, I saw that I’d have to take things apart to gain access and thought I may as well test things.
Are there any digital LCR or other meters which test the capacitance of the large, high voltage, high uf rated caps?
I was looking for some vintage test gear, but it is difficult to find. Tried getting help elsewhere and they just said not to bother. They don’t seem to be fans of this amp. I have had the GFA 555’s for 27 years and find them to be excellent sounding amplifiers.
Any help would be appreciated. Thank you.
Are there any digital LCR or other meters which test the capacitance of the large, high voltage, high uf rated caps?
I was looking for some vintage test gear, but it is difficult to find. Tried getting help elsewhere and they just said not to bother. They don’t seem to be fans of this amp. I have had the GFA 555’s for 27 years and find them to be excellent sounding amplifiers.
Any help would be appreciated. Thank you.
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Welcome to diyAudio 🙂
The 'white fluid' is traditional old fashioned thermal paste used to improve heat flow from transistor, through the mica insulation washer and then on to the heatsink. Its fine and should be left alone.
If you have disturbed them (the transistors) then you should use new insulating kits which these days use silicone washers. The old type with the correct paste although more messy offer the best performance.
The caps (if they have deteriorated and which they may well not have done) would tend to suffer from an increase in something called E.S.R. or equivalent series resistance rather than a change or loss of value. Measuring just the value of a cap tells you little in many cases. It can still be essentially non functional and still read the correct value. Big caps like those usually last well, small electrolytic types in hot places on a board are the ones to watch.
Looking at the ripple on the supplies with an oscilloscope would tell you more about the caps but I'd bet they are fine.
The 'white fluid' is traditional old fashioned thermal paste used to improve heat flow from transistor, through the mica insulation washer and then on to the heatsink. Its fine and should be left alone.
If you have disturbed them (the transistors) then you should use new insulating kits which these days use silicone washers. The old type with the correct paste although more messy offer the best performance.
The caps (if they have deteriorated and which they may well not have done) would tend to suffer from an increase in something called E.S.R. or equivalent series resistance rather than a change or loss of value. Measuring just the value of a cap tells you little in many cases. It can still be essentially non functional and still read the correct value. Big caps like those usually last well, small electrolytic types in hot places on a board are the ones to watch.
Looking at the ripple on the supplies with an oscilloscope would tell you more about the caps but I'd bet they are fine.
Thank you for your reply.
Yes, I’ve learned as much in my recent reading.
I have not ordered an ESR tester yet or capacitor tester. Looking for one which will also test for leakage.
If you would recommend some meters that can test the high voltage capacitors, I would appreciate it. The audio repair shops nearest me charge a fortune and scratched up my equipment last time, so I’ve decided to do the work myself.
Do you know of a good meter which can test capacitance in the higher values?
Yes, I’ve learned as much in my recent reading.
I have not ordered an ESR tester yet or capacitor tester. Looking for one which will also test for leakage.
If you would recommend some meters that can test the high voltage capacitors, I would appreciate it. The audio repair shops nearest me charge a fortune and scratched up my equipment last time, so I’ve decided to do the work myself.
Do you know of a good meter which can test capacitance in the higher values?
I don't really know what to advise on a cap meter simply because I don't use one 🙂
A lifetime spent in repair and diagnostics has you developing other methods. If there is an issue with a cap then it shows in other ways. Big caps can be problematic to test anyway on a meter because things like leakage are naturally pretty high (and that's normal) and tolerances can be very wide. If you use a cap meter you will start chasing values and wondering all the time, is that 47uF cap that reads 43uF really bad, a new one reads 50uF so it must be... and so on.
Your best single items of test equipment are a decent oscilloscope and a decent multimeter.
A lifetime spent in repair and diagnostics has you developing other methods. If there is an issue with a cap then it shows in other ways. Big caps can be problematic to test anyway on a meter because things like leakage are naturally pretty high (and that's normal) and tolerances can be very wide. If you use a cap meter you will start chasing values and wondering all the time, is that 47uF cap that reads 43uF really bad, a new one reads 50uF so it must be... and so on.
Your best single items of test equipment are a decent oscilloscope and a decent multimeter.
And just to be clear, I’m not saying there is an issue. I simply wish to be able to test. I agree, capacitance is only one value, but I still wish to test it.
Also when ordering new ones, just to see if they are within the stated tolerances.
Also when ordering new ones, just to see if they are within the stated tolerances.
No, I'm not saying that. I'm saying that I prefer other methods than a cap meter to diagnose faulty caps.
You need the experience to be able to interpret what a meter might tell you and to decide whether the result is correct or not. Caps, and particularly electrolytics are not like measuring voltage or resistance and so on where the result is more of an absolute. Different meters may give differing results depending on how they test the cap (test voltages and frequencies used). Its probably fair to say the bigger the cap and the more unreliable readings might become.
If you are set on getting a meter then do so 🙂 but familiarise yourself with what to expect. 30 years ago a common tolerance on electrolytic caps might have been +50% and -20% of the marked value, modern parts are better. So you need to know what to expect before making a determination.
If you are measuring E.S.R. then excepting the case where a bad cap shows up as obviously faulty you would need to know the spec of the actual part being tested and what the test conditions were.
Hopefully someone else might be able to suggest a suitable meter for you based on experience using it.
I'd leave well enough alone unless there was an issue with operation of the amp.
And heat sink grease doesn't "leak", its just cream-like consistency.
That's my advice.
Heatsink grease does dry up though it takes 20+ years to happen. The first time
I found it I cleaned it all off and replaced the grease. Later I switched to silpads
but managed to find some with excellent thermal conductivity. Unfortunately they
also had excellent electrical conductivity. Those didn't work out so well so beware.
G²
Silpads have good isulating properties if not mechanically damaged by sharp hole edges on the sinks or over-tightening.
I stick to the good old mica insulators with grease, never had a problem with them.
You can keep those dumb silicone pads, I've seen too many punch-through shorts over the years.
You can keep those dumb silicone pads, I've seen too many punch-through shorts over the years.
Actually, you are incorrect.
It did leak. All the way out the bottom of the amplifier.
I’m a bit awestruck
I just want to get in there and clean it all and then replace. I did smell a burning odor a few weeks ago from this amp. Not sure if it was related. But the amp is running perfectly and sounds great. I would like to use the same thermal paste that was originally used. Does anyone know where I may procure that?
Ok. Do you know of anyone with experience with the old Adcom gfa 555 amplifiers?
One of the reasons I am doing this is because I had one go bad on me last year and it set one of my speakers on fire. Literally. Not good.
I am trying to be proactive now and this white fluid leaking from the bottom definitely prompted investigation. I figured since I am dismantling the rear to get in there and clean everything, I may as well test what I can and familiarize myself with some tools. Between shipping back and forth to a repair shop and the costs of service just to look at it, it is simply too expensive to pay someone else. So I am just trying to get my feet wet. I have owned the GFA 555 model amplifiers since the 90’s and have found them to be excellent. They pair well with my Magnepan speakers.
Thanks for your input. I found a meter online and am awaiting delivery.
I’d like to use what was originally used. Do you know where I may purchase this?
No, I'm not "incorrect".
I'm a certified 45+ year professional audio/video service technician.
Whoever slopped that heatsink grease in there used way too much of it, and so it can drip and make a mess.
Wipe the excess carefully away.
I stand by my earlier comments.
High Wattage power amps are not really a good place to learn about
the quirks of electronics. The 555 runs +/- 80 V at 10(?) Amps. This
CAN kill you. You admit you didn't know what heatsink grease is which
is one of the most elementary things. MY recommendation is to learn
more about amplifiers using lower power amps, and THEN tackle the
Adcom. It's a fine unit (I have one) and is currently working OK so it
isn't broke. You're not ready to 'fix' it yet. I keep thinking "I changed
the oil in my car. Time to do a valve job". I know that sounds
condescending but if you break it you'll feel a lot worse. I know as
I've done it myself.
If you're determined to do it anyway you can get heatsink grease by mail
easily. Google "gc electronics heat sink compound" and you'll get plenty of
hits.
Good Luck
G²
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