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Old 15th December 2016, 02:27 PM   #1
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Default pre-amp issues.

I am using an old Audiolab 8000c/p combo to amplify my tv,conected to the stereo outs on the tv. The other day the amp shut the sound down but the amp lights were still on. On closser inspection the pre-amp had got very hot on the left side around the source selection switch. When it called down it worked fine,but again after a couple of hours it shut down again . There are no sign of burnt or blown componens inside . So in all i am at a loss.. Any ideas where to start? Thanks....

Last edited by martin1969; 15th December 2016 at 05:54 PM.
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Old 15th December 2016, 05:43 PM   #2
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TV might have sent some nasty DC spikes. They tend to do all sorts of horrible stuff with the audio. Probably killed your amp.
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Old 15th December 2016, 05:45 PM   #3
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Locating the burnt parts you can see and matching them up to the circuit diagram would be the first steps. We could advise as to a possible cause then.
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Old 15th December 2016, 05:57 PM   #4
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Sorry typo. There is "no" sign of burnt or blown parts. It is actually working at the moment.but it will at some point stop as it slowley overheats over the next couple of hours!!
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Old 15th December 2016, 06:02 PM   #5
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No problem

You are still going to have to get the top off at some point and locate the general area of heat. The good news is that preamps tend to be low power stuff and so damage is rarely catastrophic.

I would guess possibly a power supply problem, maybe failed caps or a failed voltage regulator. Get the top off and see where the heat is coming from.
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Old 15th December 2016, 07:10 PM   #6
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The heat is comming from the heatsink in this picture but it is getting far hotter than it used to. The outer case gets to hot to touch !

Click the image to open in full size.
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Old 15th December 2016, 07:29 PM   #7
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Those devices are going to be voltage regulators. I can't make out the device numbers but it looks like it might be 'discrete' i.e transistors rather than an IC type.

It would really help to see the circuit diagram if you have one.

If nothing else in the preamp is getting hot (indicating its drawing to much current) then its possible the fault is with the regulators.

First steps would be.

1/ Identify the device type numbers of those two components. 2SAxxxx 2SBxxxx and 2SCxxxx and 2SDxxxx would be transistors.

2/ Measure the voltage on all three legs of each device and record the result. I would imagine one side will be positive and one negative.

3/ Have a good feel at other components in the preamp and see if any are getting hot. Feel the caps as well.
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Old 16th December 2016, 01:45 AM   #8
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Anything over 5 years old, you're more likely to have fault with the bypass capacitors before and after the regulator, than the regulator IC itself. Those black cylinders next to it are electrolytic capacitors, I'd buy all those I'd order all the orange ones on the same order. Cheap rubber seal lets the water out, then they leak electrically, then FFFFT, your regulator IC just became a fuse.
Mark the minus side with a sharpie, if you put them in backwards they blow up immediately. I like to buy the >5000 hours service life grade so I don't have to do this again. Using TV parts store caps, I replaced the e-caps in my ST70 4 times since 1970. Life is too short. Newark, Mouser, Digikey sell quality Panasonic, Nichicon, Rubicon, vishay caps. Look at the service life, all the above sell **** grads caps too if you want to save 30%. The manufacturers nearly all buy the **** grade.
You can buy a $100 meter to test them, but the cost of all of them might be $10 +$8 freight, I'd just replace them.
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Old 16th December 2016, 03:41 AM   #9
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Think those regulators are for the phono stage and do run pretty toastie in any audiolab c ive had. Maybe they have shorted against the top of the case?
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Old 16th December 2016, 04:24 AM   #10
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If the regulator is getting hot, something else is probably getting hot also.

(An internal reg fault is possible, but mostly it would be excess load and heat elsewhere.)
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