Welcome Jez and thanks for your post.
I'll check those resistors (seems easy to do)
Cheers/Michael
I'll check those resistors (seems easy to do)
Cheers/Michael
exactly the same problem with A100!
A8,
you will not believe this. I bought a second-hand A100 this day and got it cheap because it was not ok. Now I have worked on the thing for a whole day and have exactly the same problem is you have.
The fault is that one channel distorts heavily most of the time. Every now and then this channel works okay and draws current. I put in new caps and new power resisostors. Next I changed the BD139 because when it is distorting this one draws less current also (the 140 seems to compensate for this and starts to draw more current).
I resoldered all leads from the power transistors etc etc.
The problems stays the same!
What discoveries did you make since your last posting.
Rudy/rmgvs
A8,
you will not believe this. I bought a second-hand A100 this day and got it cheap because it was not ok. Now I have worked on the thing for a whole day and have exactly the same problem is you have.
The fault is that one channel distorts heavily most of the time. Every now and then this channel works okay and draws current. I put in new caps and new power resisostors. Next I changed the BD139 because when it is distorting this one draws less current also (the 140 seems to compensate for this and starts to draw more current).
I resoldered all leads from the power transistors etc etc.
The problems stays the same!
What discoveries did you make since your last posting.
Rudy/rmgvs
power transistors
I did this a few days back and this was the problem in my case (I send my findings to A8). Apparently transistors have internal structures worn out in many years of daily use that can be off or on for no apparant reason.
I did this a few days back and this was the problem in my case (I send my findings to A8). Apparently transistors have internal structures worn out in many years of daily use that can be off or on for no apparant reason.
I changed mine yesterday...so far so good , I'll do a couple of days testing before I decide it is fixed.
Just an assumption regarding C10 & R17
Both halves of this symmetrical design have their own NFB network. Tolerances within their components might cause different gain for each half !
Maybe this network helps balancing this out.
Regards
Charles
Both halves of this symmetrical design have their own NFB network. Tolerances within their components might cause different gain for each half !
Maybe this network helps balancing this out.
Regards
Charles
Conclusion?
Hi, A8 and other experts.
I bought a second-hand A100 just a month ago and was impressed by its clear mid/high range sound. I decided to love this black beast.
Tonight, I noticed something was wrong with my amp as the speaker clicked and one channel suddenly faded out. It sounds like quite similar symptom as yours, and I'd like to see the conclusion of your long journey. I know you have replaced so many stuffs and checked the circuit thoroughly, but what was the final touch that revived your amp?
When one channel faded out, I slightly rotated the selection knob (I was in phono mode.. moved it slightly clockwise) and the lost channel came back. (it comes and goes from time to time though..) Perhaps, the A1 preamp mod illustrated by mhennessy could also help the whole thing?
Hi, A8 and other experts.
I bought a second-hand A100 just a month ago and was impressed by its clear mid/high range sound. I decided to love this black beast.
Tonight, I noticed something was wrong with my amp as the speaker clicked and one channel suddenly faded out. It sounds like quite similar symptom as yours, and I'd like to see the conclusion of your long journey. I know you have replaced so many stuffs and checked the circuit thoroughly, but what was the final touch that revived your amp?
When one channel faded out, I slightly rotated the selection knob (I was in phono mode.. moved it slightly clockwise) and the lost channel came back. (it comes and goes from time to time though..) Perhaps, the A1 preamp mod illustrated by mhennessy could also help the whole thing?
Well, in my and rmgvs case we believe it was the output transistors that was faulty.
Your issue seems to be slightly different, when you say faded out...do you mean that the volume slowly decreased or did it just go down instantly?
If it helped fiddling with the input selector it may be as simple as bad contact in the selector.
There is an easy way to test if it is the input selector, just activate tape monitor and connect your source to tape out and set the input selector to an open source (not phono).
If it works this way but not the normal way you have a semi broken input selector.
Your issue seems to be slightly different, when you say faded out...do you mean that the volume slowly decreased or did it just go down instantly?
If it helped fiddling with the input selector it may be as simple as bad contact in the selector.
There is an easy way to test if it is the input selector, just activate tape monitor and connect your source to tape out and set the input selector to an open source (not phono).
If it works this way but not the normal way you have a semi broken input selector.
Read this thread carefully, all answers are there. Please realise that the input switch and volume potentiometer tend to break down because of heat build-up. While your at it please replace all e-caps for 105 degree types and replace the power transistors as well.
In fact it's better to replace all parts that are sensitive to heat ....
In fact it's better to replace all parts that are sensitive to heat ....
thx!!
Thanks a8 and jean-paul. Perhaps my description was not precise. It produced some tick-tick-tick sound from the speaker and one channel instantly stopped. I've read through the posts and even saved them in my local harddrive to keep them as technical troubleshooting manaul.
I'll try your suggestion as well! Thanks!!
Thanks a8 and jean-paul. Perhaps my description was not precise. It produced some tick-tick-tick sound from the speaker and one channel instantly stopped. I've read through the posts and even saved them in my local harddrive to keep them as technical troubleshooting manaul.
I'll try your suggestion as well! Thanks!!
i have ma65 schematics and also fixed the amp. the current on r30,r31 is 330 ma with 36 volt rails.what do i have to change to make current higher at about 800 ma?
Maybe you can show the circuit / schematics. It looks like the A1/A100 but it has some differences I guess. A1 current is for example depending on version 700-900 mA per transistor. There are 2 high value resistors of 1m8 or 3m6 (depending on version): if you make these smaller the current will rise (you get class B operation by leaving this resistor out of the circuit). Also there are these bigger power resistors of .22 or .47 Ohm (3 or 5 Watt), if you make these smaller the current also will rise.
where to buy parts
I bought an A100 back in 1986. I haven't been listening to it for a while because one of the channels is either heavily distorted or or drops out intermittently. This thread provides a lot of good troubleshooting and recommended updating advice.
I have two questions:
1. Does anyone have a recommendation on where to purchase replacement parts online?
2. Does anyone have a parts list, including substitutions for items that are no longer available?
Thanks!
I bought an A100 back in 1986. I haven't been listening to it for a while because one of the channels is either heavily distorted or or drops out intermittently. This thread provides a lot of good troubleshooting and recommended updating advice.
I have two questions:
1. Does anyone have a recommendation on where to purchase replacement parts online?
2. Does anyone have a parts list, including substitutions for items that are no longer available?
Thanks!
Partial Parts List for A100
I found an electrical engineer friend to help me. We opened up the case and dove in.
We ended up replacing the power supply capacitors, adding a low capacitance bypass to the original design (based on a rebuild scope of work from the original supplier). New bipolar transistors (one of mine had gone bad), all new electrolytic capacitors, installed op amp sockets and new op amps, and a fresh coating of heat transfer paste (the single most expensive item).
The whole rebuild cost me less than $100, and wow!!! was it worth it! Sounds better than when it was new.
I've attached a parts list. Unfortunately, the parts list is not complete as some items weren't known until we started taking the amp apart and I didn't update the list with the additional items.
I found an electrical engineer friend to help me. We opened up the case and dove in.
We ended up replacing the power supply capacitors, adding a low capacitance bypass to the original design (based on a rebuild scope of work from the original supplier). New bipolar transistors (one of mine had gone bad), all new electrolytic capacitors, installed op amp sockets and new op amps, and a fresh coating of heat transfer paste (the single most expensive item).
The whole rebuild cost me less than $100, and wow!!! was it worth it! Sounds better than when it was new.
I've attached a parts list. Unfortunately, the parts list is not complete as some items weren't known until we started taking the amp apart and I didn't update the list with the additional items.
Another one.
I just purchased a faulty A100 for NZD$30 and have read all of this thread for tips.
Based on reading this thread, all of the capacitors are going to go (was originally going to keep the PSU ones), all of the semidestructors, and any hot resistors.
I think MJ15024/025 are the best choice of current devices, but will use MJ802/4502 if I have some as they are, in my opinion, better performing (and sounding) devices for lower voltages.
I'm planning to use the amp in my bedroom, which plays at low volumes through a pair of small ESS speakers with AMT tweeters, so looking forward to sounds befitting the environment.
My main system has 1100 WPC for main speakers and 400 WPC for the 18" subs. Planning to upgrade the sub amp when new subs get built.
Thanks to everyone for their comments and schematics.
regards, Marcus.
I just purchased a faulty A100 for NZD$30 and have read all of this thread for tips.
Based on reading this thread, all of the capacitors are going to go (was originally going to keep the PSU ones), all of the semidestructors, and any hot resistors.
I think MJ15024/025 are the best choice of current devices, but will use MJ802/4502 if I have some as they are, in my opinion, better performing (and sounding) devices for lower voltages.
I'm planning to use the amp in my bedroom, which plays at low volumes through a pair of small ESS speakers with AMT tweeters, so looking forward to sounds befitting the environment.
My main system has 1100 WPC for main speakers and 400 WPC for the 18" subs. Planning to upgrade the sub amp when new subs get built.
Thanks to everyone for their comments and schematics.
regards, Marcus.
Hi all
I am afraid that I an the next one in line with A100 affected by distorsions on one channel, disappearing and coming back. I've read the thread and it seams that I need to replace the output transistors. Can anyone be kind to send me the schematics and specification of the parts that need to be replaced?
And my A100 has another problem. One of the fans is not working. I've tried to find another axial fan, but no luck. Maybe someone has some solution for this? Replacing both fans may solve the problem, but with what?
Thanks in advance.
/Goran
I am afraid that I an the next one in line with A100 affected by distorsions on one channel, disappearing and coming back. I've read the thread and it seams that I need to replace the output transistors. Can anyone be kind to send me the schematics and specification of the parts that need to be replaced?
And my A100 has another problem. One of the fans is not working. I've tried to find another axial fan, but no luck. Maybe someone has some solution for this? Replacing both fans may solve the problem, but with what?
Thanks in advance.
/Goran
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