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Recapped Sansui AU-719 Protection mode help!

RR--

RR--

Active Member
Hey everyone! long time since I've been here but I recently came into a problem depriving me of my favourite Sansui.

Back in 2013 I recapped my Sansui AU-719 with all new Nichicon, Panasonic and Gold Tune caps. I also replaced the main fuse for the Australian 240V system. Original post here: http://audiokarma.org/forums/index.php?threads/finished-recapping-my-au-719.528233/#post-6918224

Since then I've also replaced the big four capacitors. I've been listening to the amplifier nearly all day every day since then and loving it.

Recently this summer on hot days (like today) the amp is having issues coming out of protection mode when turning it on. Today it wont last more than a minute before tripping back into protection mode. If I play around with the switch after a minute I can get it to come back on but only for another short while.

I figured it was a heat issue so I replaced the thermal compound on the output transistors and adjusted the bias and DC offset again just to be sure but it's making no difference to the problem.

FYI the sound quality is fantastic for the minute before it trips into protection, no scratchy noise, crackles, pops or anything, when the protection clicks on it sounds normal.

Any ideas on how to bring back my pal? I'd really appreciate any help.

Thanks again
 
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You could measure the DC offset before the speaker relay contacts during 'warm up' - best with 2 meters, one for each channel, but not essential - and BE CAREFUL with those meter probes! - best use 'hook tips' if you have them.

Look for high DC offset, the most likely cause of entering protection mode, but not the only one.

Try turning the Tone Controls off (front panel switch), OR try engaging the Subsonic Filter (front panel switch). This will A). remove the control amp from the signal path, OR B). AC couple the input to the power amp (thus removing any DC coming from the control amp).

If the above doesn't change things then your fault is most likely with the power amp, OR less likely the protection circuit itself. But not ruling out possible power supply issues at this point.
 
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Thanks John.
I adjusted and DC offset and Bias in the time before the protection relay trips so it's dead on what's recommend in the manual. I tried powering it with the tone controls off and again several times with the subsonic filter on but it doesn't seem to make a difference as it only successfully makes it through protection about half of the time.
The weathers cooled down now and it's still not working so I don't think that's the cause. it's happened several times in the past but its always come good. It's never been this bad for this long.
Any more ideas?
 
Well, you appear to have a fault in the power amp stage, but if this is the case you should probably only see high DC offset in one channel (hard to believe it would be both :idea: - unless..... some mistake or omission was made in the original recap - thus both channels could be affected ) - by measuring before the speaker relay as previously suggested.

You seem to have eliminated everything before the power amp, amplification-wise, so now we look at both power amps, PSU voltages, and protection circuitry - does that make sense?
 
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Ok plot twist. I thought I'd double check the Bias and DC offset. The DC offset on both channels was still dead on 0v after the amp had time to warm up and before the protection tripped. When I went to check the Bias it was a little off 23V but the amp was still warming up, after about 10 seconds there was a bright light, smoke and the back 7A 125V fuse blew so something's definitely wrong.
I'm sure there wasn't any problems with the original recap as I replaced every part one by one with photos and have been using the amp daily for 4 years since then with little to no problems.

I'll go grab a new fuse from Jaycar tomorrow. Anything I should try? I'm not great with this sort of thing, replacing capacitors is as advanced as it gets for me. Thanks again for the help
 
Oh dear, that doesn't sound good, I think we are in 'Dim Bulb Tester' territory now - I would be removing and measuring the OP transistors with multimeter, and checking for shorts in the PSU, but not powering up after the fuse change on full mains because that could cause more damage.

Sorry to hear of this turn of events...
 
I'm afraid that's a bit out of my league. I don't have a DBT, also are the OP Transistors the main four on the large heatsinks? To be honest I could probably measure the parts but I really don't know what the values mean so I'd probably be better off looking for a pro. Funny though that it wasn't the newest fuse that blew but an older one. Any chance it's just the fuse that was the problem? I've had a good look at the internals of the amp and none of the capacitors, transistors or resistors look out of the ordinary.
 
Well, fuses do 'age' - and sometimes blow for no apparent reason, but in view of the circumstances around when your fuse blew, I feel this possibility is slim in this case.

Not a bad idea to have a look around, to see if you can see any stressed components, burn marks, bulged capacitors and the like.

Where are you - you're in Australia I think? - do you live anywhere near any of our other illustrious Australian AK members?
 
Yep I'm in Hawthorn, Melbourne, Australia. I know Skippy did me a huge favour by sending me a couple hard to find capacitors back in 2013 but he's up in Sydney. If anyone's keen to help I've got a six pack of your choice with your name on it!
 
Ok got some slightly better news I think. I figured out the fuse blew because I was an idiot with the multimeter and shorted out the amp. I readjusted the bias and now it's spot on 23mv but now the protection constantly clicks when turned on. Any thoughts?
 
Well, that is good news on the fuse front, a lesson learned.

As said before - check the DC offset before the relay contacts, so you can see what it is doing even when the amp is in protection - do you have a service manual?

We need to establish whether the amp really does have excess DC offset OR maybe the protection circuit itself has a fault.
 
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You can measure the offset at the bias test points,

With the meter set to volts,measure from either of the bias test points to chassis.

Cheers

John
 
So I thought I'd add a conclusion here. I took it to a specialist and apparently one of the power supplies had a fault due to a lot of minor issues such as old solder joints and bad fuse connectors. It cost as bit but it's back up and running smoothly. Thanks for all the help everyone.
 
RR-- said:
So I thought I'd add a conclusion here. I took it to a specialist and apparently one of the power supplies had a fault due to a lot of minor issues such as old solder joints and bad fuse connectors. It cost as bit but it's back up and running smoothly. Thanks for all the help everyone.
Click to expand...

Nice - great to hear!

I had intermittent fuse connector issues on a AU-417 once. Drove me absolutely nuts trying to diagnose the fault, and it was staring me in the face!
 
I like to think it was more complex than that! it's been having little fits for a few years but I thought it was bias and DC offset drifting too much. Hoping for another reliable 40 years now!
 
RR-- said:
I like to think it was more complex than that! it's been having little fits for a few years but I thought it was bias and DC offset drifting too much. Hoping for another reliable 40 years now!
Click to expand...

Oh, I have no doubt that it was!! I just enjoyed the coincidence re: the fuse holder issue.
 
Ok time to reopen this thread, unfortunately after almost exactly a year of reliable use this amplifier doesn't want to co-operate again and keeps tripping into protection mode. Coincidentally just as the weather has begun to cool down.
I can make the amplifier consistently power on and work when warmed up with a hairdryer so cold weather definitely seems to cause the protection mode to trip, also switching it on and off several times can sometimes make it turn on for a moment, Bias and DC offset are good and all capacitors are new.

Any new ideas on what's causing the problems with this? The 'professional' tech I paid a small fortune to a year ago didn't fix it. I'm tempted to just replace the relay but it doesn't look bad.
 
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