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A&R A60 fuse issues

M

MNUGENT

Member
Hi all,

I'm new here but recently found the forum very helpful in diagnosing an issue with my dad's A&r a60. We got this out of storage a few years ago and have been enjoying it since.

Recently however, the input fuse blew when we turned it on. Replacing the fuse did nothing (it immediately blew when turned on). On reading through posts on this forum, replacing the power transistors was recommended which I did and this initially seemed to solve the problem. I also checked all other components for any degradation or leaking but didn't see anything.

However, now the amp turns on the when the fuse is replaced, works perfectly for the duration of the session and is then turned odd. If I turn it on immediately it turns on and operates fine. If I go back a few days later, it will not turn on and I find the input fuse blown. If I replace the fuse, this process is repeated.

I'm not bad with soldering electronics but my knowledge of the inner workings of an amp is minimal and I'm lost for where to start with this problem. Does this sound like anything anyone else has experienced and if so, can you give me some pointers to solve this problem.

Any help/advice appreciated!
Cheers, M
 
Do you know if the electrolytic capacitors have been replaced ? Although they are not leaking, they might be at the end of their life if they are more than 10 years old.
 
As far as I'm aware, no other work has been done other than replacing the power transistors, are there specific capacitors that need to be replaced or is it all of them?
 
MNUGENT said:
As far as I'm aware, no other work has been done other than replacing the power transistors, are there specific capacitors that need to be replaced or is it all of them?
Click to expand...
All of them if they are more than 20 years old. The two large reservoir capacitors are the most frequent replacement to do though. There might be a date code on it. The one below is from the 26th week of 1997 :

 
Perfect, cheers I'll have a look at both the power supply caps and large reservoir caps (unless they're the same thing?) and replace them if I can.

I also remember reading about adjusting trim pots (rv2 I think?) after replacing the power transistors, I didn't do this as I didn't want to mess around with too much but could this be causing a problem? Does the amp need to be tuned into the new power transistors?

Cheers
 
Yes, they are the same thing. If you have replaced the power transistors already you should find it a pretty straightforward job. You should adjust the trim pots in order to get the best performance (see Robert’s guide in the reference section) but I don’t see that being the source of your problem. They can be a little fiddly to set correctly though, which is why some people replace them with multi-turn versions.
 
Your mains inlet fuse is supposed to be a T1A250V anti-surge type (aka 'T'ime-lag surge, aka surge-withstand). These allow higher inrush current during switch on, which would especially be the case after a few days of none use, as the power supply reservoir capacitors will have mostly discharged by then (especially so original ones).

Certainly, if these capacitors are original they are likely to be performing less well than when new, however, they'd have to be in very bad condition to be causing a 1A anti-surge to blow. I'd double check both fuse value and especially type before blaming the caps.

A 1A fast-blow will blow fast here, IYSWIM.
 
Last edited:
Craig B said:
Your mains inlet fuse is supposed to be a T1A250V anti-surge type (aka 'T'ime-lag surge, aka surge-withstand). These allow higher inrush current during switch on, which would especially be the case after a few days of none use, as the power supply reservoir capacitors will have mostly discharged by then (especially so original ones).

Certainly, if these capacitors are original they are likely to be performing less well than when new, however, they'd have to be in very bad condition to be causing a 1A anti-surge to blow. I'd double check both fuse value and especially type before blaming the caps.

A 1A fast-blow will blow fast here, IYSWIM.
Click to expand...


This was the problem! Thank you! For some reason, I replaced the mains fuse with a 630mA anti-surge instead of the 1A one when replacing the power transistors. No idea why, must have seen that number somewhere. Anyway, working perfectly now. Thanks again!
 
Marchbanks said:
Yes, they are the same thing. If you have replaced the power transistors already you should find it a pretty straightforward job. You should adjust the trim pots in order to get the best performance (see Robert’s guide in the reference section) but I don’t see that being the source of your problem. They can be a little fiddly to set correctly though, which is why some people replace them with multi-turn versions.
Click to expand...

I haven't replaced reservoir capacitors yet as its working away happily now, don't fix it if it ain't broke n all that. Thanks for your help though
 
MNUGENT said:
This was the problem! Thank you! For some reason, I replaced the mains fuse with a 630mA anti-surge instead of the 1A one when replacing the power transistors. No idea why, must have seen that number somewhere. Anyway, working perfectly now. Thanks again!
Click to expand...
Glad to hear that it is working for you now.

Re: your "Don't fix it if it ain't broke", I'll see that raise you a "Don't start today by doing yesterday's work". Testing/resetting the quiescent current is easy to do with just the lid off and a multimeter.

From the service manual...

"Precautions when replacing faulty components in the A60 power amplifier

If any transistor has had to be replaced in the power amplifier, the following procedure must be used to reset the output quiescent current."

http://ukhhsoc.torrens.org/makers/Cambridge_AandR/A60/Service_Manual_Nov_1985.pdf​
 
Having left the A&R A60 with the lid on for 30 minutes, I check R63 and R163. Within a few seconds, the current drops a little and then steadies.
Should I rely on the reading just after the lid is removed, or the steady reading once the drop has finished?
Thanks to all who post about this amplifier. I have just completed my third renovation, but I may have to look again at the quiescent current.
Thanks again.
 
Markrecord:
It should be close enough with the reading just as the lid is removed.
If you are still adjusting with the original one turn pots that as good as you will probably get :)
 
Great, thank you. For some reason, that is what I have been trying to do, but that drop off in the reading made me wonder..
I did change the trim pots, which does make the adjustment easier.
Thanks for your generous help.
 
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