Hello all,
I recently acquired a "serviced" Adcom GFA-545 (gen 1) from my technician for $80. It turns out that he forgot to check everything and there are a few issues.
The DC offset was insanely high (105mv) on the left channel and was an acceptable (21mv) on the right. I opened it up and checked the bias using the test points, and adjusted the bias back down to 6.5mv (from 13mv). I attemped to adjust the bias (it was 20mv) on the left channel (the one with high offset) and the trim pot disintegrated. The part that the screwdriver goes into (I think it's called the wiper) just fell off of the pot. The bias skyrocketed to around 45mv. I shut the amp off.
I also noticed that there was a TON of rust and corrosion on the left side. Almost everything was corroded. The screws, some legs of components, and a ton of corrosion "dust" all over everything. There is also a bulging/corroded cap. 47uf 100v (I'm pretty sure it's C609 in the service manual)
So basically I am planning to take it back to my technician and ask him to fix it and get rid of the corrosion. I am planning to replace the trimpots with sealed Bourns ones, and both of the caps. I am also planning to replace the binding posts and RCA jacks with Cardas parts.
I wanted to ask you guys for some advice and to make sure my parts list is good:
I have narrowed it down to 4 trim pots. I'm pretty sure it is a 2k trimpot, so here are the ones I found. Which would work best?
3386F-1-202LF
3386P-1-202LF
3362P-1-202LF
PV37W202C01B00
If there are any others that would work better, that would be greatly appreciated.
I am planning to replace the capacitor with this:
Elna SILMIC II cap. Seems to be a good audio-spec cap.
I am also planning to get these Cardas RCA Jacks and these Cardas Binding Posts
Please let me know your thoughts/suggestions.
Photos:
Good (right) side
Bad (left) side see bulging green cap.
Whole amp
Thanks so much,
ocinn
I recently acquired a "serviced" Adcom GFA-545 (gen 1) from my technician for $80. It turns out that he forgot to check everything and there are a few issues.
The DC offset was insanely high (105mv) on the left channel and was an acceptable (21mv) on the right. I opened it up and checked the bias using the test points, and adjusted the bias back down to 6.5mv (from 13mv). I attemped to adjust the bias (it was 20mv) on the left channel (the one with high offset) and the trim pot disintegrated. The part that the screwdriver goes into (I think it's called the wiper) just fell off of the pot. The bias skyrocketed to around 45mv. I shut the amp off.
I also noticed that there was a TON of rust and corrosion on the left side. Almost everything was corroded. The screws, some legs of components, and a ton of corrosion "dust" all over everything. There is also a bulging/corroded cap. 47uf 100v (I'm pretty sure it's C609 in the service manual)
So basically I am planning to take it back to my technician and ask him to fix it and get rid of the corrosion. I am planning to replace the trimpots with sealed Bourns ones, and both of the caps. I am also planning to replace the binding posts and RCA jacks with Cardas parts.
I wanted to ask you guys for some advice and to make sure my parts list is good:
I have narrowed it down to 4 trim pots. I'm pretty sure it is a 2k trimpot, so here are the ones I found. Which would work best?
3386F-1-202LF
3386P-1-202LF
3362P-1-202LF
PV37W202C01B00
If there are any others that would work better, that would be greatly appreciated.
I am planning to replace the capacitor with this:
Elna SILMIC II cap. Seems to be a good audio-spec cap.
I am also planning to get these Cardas RCA Jacks and these Cardas Binding Posts
Please let me know your thoughts/suggestions.
Photos:
Good (right) side
Bad (left) side see bulging green cap.
Whole amp
Thanks so much,
ocinn
With the trim pots its important to get ones that physically fit the board. Also make sure you understand which position of the wiper will give minimum current and be sure to set accordingly before powering on.
If the pots are of a poor type then replace them all.
The suggested cap should be OK although many now opt for 105C rated parts rather than 85C.
If the amp is a few years old or is showing signs of aging then its standard practice to replace all electrolytics.
Pictures... try attaching them to the forum rather than hosting on a 3rd party site 🙂
If the pots are of a poor type then replace them all.
The suggested cap should be OK although many now opt for 105C rated parts rather than 85C.
If the amp is a few years old or is showing signs of aging then its standard practice to replace all electrolytics.
Pictures... try attaching them to the forum rather than hosting on a 3rd party site 🙂
Ummmm I hate to tell you but trying to help an inexperienced person repair something like a power amplifier remotely is something an experienced person would rarely do....
Part of learning how to repair anything electrical or mechanical is learning how things work so you can test to see if they are bad. If something looks obviously bad like a bulging capacitor or burned resistor replace it. For semiconductors you need to learn how to do in circuit testing for the majority of components. When you measure something odd you need to remove one or more leads to confirm. Unless you have a curve tracer you can never be sure something like a transistor, mosfet or diode is 100%. An IC is more problematic.
Unfortunately a power amplifier is similar to a rocket engine. If you miss one bad component you run the risk of the unit going tits up once again and blowing up all your new components or much worse.
My technician is going to be the one who installs these parts, he just asked me to make a list of the specific brands/models of the parts I need.
I do not plan to install these myself. I only came here to verify that my replacement parts would work with my amp.
I am going to ask my technician to double check the components one more time to see if there is anything else that needs replacing.
My experience mostly lies in speaker design, not as much with amplifiers.
Ok. I'm going to have my technician install these components, as he should have checked the amp out the first time.
I'll take the physical measurements of the board and current trimpot and check to see whether the new one will match up. I plan to replace both trimpots (there is one on each side, only 1 is broken) to keep everything matched.
The amp is 27 years old now. It will be a secondary amp, and won't see much use, but I'll still check all the electrolytics to make sure nothing is wrong.
I'll see if there is an audio-grade Nichicon or other cap that is rated for a higher temp. The current one is an 85c part, and the amp won't be seeing tons of high-load use, so I doubt it would be a problem.
I got an error message when I originally tried to upload to the site. I just tried it again, and it worked, I'll make sure to upload to the site in the future. Sorry about that. 🙁
Erm... well tbh and if you don't mind me saying 🙂 your tech doesn't sound up to much if he's getting you to locate suitable parts. And these are all issues that should be picked up when the unit was 'serviced'.
I'm tempted to say you might be as well fixing it yourself...
Pictures. I have this as a copy and paste 😉
To add a photo, files or non standard files.
First click "go advanced" in the box below the "quick reply" message box. Doesn't matter if you decide half way through a message to do that, it carries it forward.
Then click "Manage attachments". Maximise the new Window so that you can see all the text.
Click browse in the first box at the top and find your picture. Repeat for any more pictures.
Click upload... a message appears "uploading"
When complete the files will show as being attached. Now click the small text that says "close this window"
The pictures should now be attached and when you submit your post they will appear.
Make sure your pics aren't too big, a couple of 100k is plenty, and many members object when they are massive and it alters the margins
It tells you in the attachments window what max sizes are allowed.
If you want to attach a file that has a non standard format for example excel, circuit simulation etc then try putting the files in a zipped folder and attaching that.
I'm tempted to say you might be as well fixing it yourself...
Pictures. I have this as a copy and paste 😉
To add a photo, files or non standard files.
First click "go advanced" in the box below the "quick reply" message box. Doesn't matter if you decide half way through a message to do that, it carries it forward.
Then click "Manage attachments". Maximise the new Window so that you can see all the text.
Click browse in the first box at the top and find your picture. Repeat for any more pictures.
Click upload... a message appears "uploading"
When complete the files will show as being attached. Now click the small text that says "close this window"
The pictures should now be attached and when you submit your post they will appear.
Make sure your pics aren't too big, a couple of 100k is plenty, and many members object when they are massive and it alters the margins
It tells you in the attachments window what max sizes are allowed.
If you want to attach a file that has a non standard format for example excel, circuit simulation etc then try putting the files in a zipped folder and attaching that.
I live in a pretty small town is this is one of the 2 acceptable techs we have. One of them isn't too good. For example he installed a 5v LED in a 24v circuit in a friend's VU meter. He is great for smaller jobs and is a lot cheaper. I bought the Adcom from the other tech.
Me and my friend dropped off his amp for repair, and I noticed the Adcom sitting in the tech's "warehouse". I asked him whether it was available for sale and he told me he'd check it out and make sure it was good. My friend's repair ended up being very laborious and expensive in parts, so he gave me a deal on the amp and said it was "serviced". He actually replaced a binding post, so I know he did something.
I let him know about the problems, and how I wanted to get sealed pots. He told me it would be helpful if I found the specific parts I wanted since he would have just installed a plain pot and capacitor.
He's actually a really good tech, and he has 40+ yrs experience. He was one of the only techs in the state qualified enough to repair my friend's Conrad-Johnson and Phase Linear gear.
My speakers are very difficult to drive and have some insane impedance dips, so many amps have trouble driving them, and many go into protection. There is a chance that I could have caused the caps to blow.
Thanks a ton for the info on the pictures.
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