Help fixing Memorex amp

[DRGL]

Hi, somebody refered me to here as you may be able to help-i hope so! I have a Memorex STAV-3100 amplifier/receiver. All was working fine when it decided to go pop! The volume wasn't on high at the time, when i tried to switch it back on it blew the fuse in the plug, the fuse in the back of the amp next to the PSU was fine. I checked the output transitors and ALL were short along with one driver transistor, one of the biasing transistors had the side blown off! I have replaced all of those & the amp powered up OK. I noticed at this point that the volume meter was reading on the left channel even though the volume was off. I also noticed the amp seemed to be giving out massive(but clean!) bass! I switched it off and had a look over it, when i switched it back on it was dead & has blown the fuse in the plug again! I have the tech manual but the schematic is missing. I'm now at a loss, i can't see any physical damage or shorts, i need to check the transistors again but fear it has fried them again! There is no bias adjustment pot. Any help on where to go from here would be great! Thanks for reading.

 

[willyrivero]

Make sure the transistors you replace are reliable, be careful with fake ones or under rated. Also check caps, they also could have gotten damaged. they do strange things when leaky.

 

[DRGL]

Thanks. I've read about the fakes but how do you spot them? How would i check a cap with a multi meter? There is no sign of bulging or leaking.

 

[willyrivero]

Try this place it helped me: sound.westhost.com/counterfeit.htm

You'll find good points to spot fake transistors there. As for checking leaking caps your multi meter won't do. You have to get an ESR (equivalent series resistance) meter. Caps might look OK but this meter cleared my path to repair many "tough" ones.

 

[DRGL]

Thanks, i'll take a look. Do fakes tend to fail quickly?!! The big output transistors came from a large UK supplier, the smaller drivers came from a smaller company but they have been in business for years. What else can cause them to get going short?(assuming they have again).

 

[Nigel Goodwin]

Thanks, i'll take a look. Do fakes tend to fail quickly?!! The big output transistors came from a large UK supplier, the smaller drivers came from a smaller company but they have been in business for years. What else can cause them to get going short?(assuming they have again).

You generally can't just replace the drivers and outputs, it's often something earlier that causes them to blow, so if you don't repair what caused them to fail, they will fail again. As a professional service agent I always replace all transistors in the DC chain, which helps to prevent repeat failures.

 

[DRGL]

Thanks Nigel. When you say DC chain is that likely to be on the main amp board? I have the service manual which i can email to you if you can take a look? It has some schematics but not the manin ones. I'm learning..............the expensive way!!!

 

[Nigel Goodwin]

Thanks Nigel. When you say DC chain is that likely to be on the main amp board? I have the service manual which i can email to you if you can take a look? It has some schematics but not the manin ones. I'm learning..............the expensive way!!!

Where it is depends on the design, but it's just the power amplifer that matters, there will be a DC break in front of that.

 

[Chippie]

I think that there is an unrevealed failure if all the semis in the power stages have failed...

Checks in the power supply would be prudent before changing the transistors again....

 

[Nigel Goodwin]

I think that there is an unrevealed failure if all the semis in the power stages have failed...

Checks in the power supply would be prudent before changing the transistors again....

There's nothing in the PSU to cause the amp to fail, PSU's are crude and simple.

Like I said, it's just that not repaiirng the amp correctly causes everything to blow again.

 

[Chippie]

So you dont think defective diodes could cause any problems ?

 

[Nigel Goodwin]

So you dont think defective diodes could cause any problems ?

It will blow the fuses, possibly blow the mains transformer, but it almost certainly won't damage the amp - I've never seen (or heard of one) that did, and I've changed a LOT of short circuit rectifiers over the years.

In what way did you think it could damage the amp?.

 

[Chippie]

It will blow the fuses, possibly blow the mains transformer, but it almost certainly won't damage the amp - I've never seen (or heard of one) that did, and I've changed a LOT of short circuit rectifiers over the years.

In what way did you think it could damage the amp?.

I was thinking maybe if a diode or more was s/c then the ac volts from the transformer could mebbe wreck the semis...

 

[Nigel Goodwin]

I was thinking maybe if a diode or more was s/c then the ac volts from the transformer could mebbe wreck the semis...

How would it get to them? - there's a dirty great big electroytic on the output of the rectifier.

The opposite can be true, failure of the amp can kill the diodes - particularly if the user keeps banging fuses in and blowing them.

 

[Chippie]

How would it get to them? - there's a dirty great big electroytic on the output of the rectifier.

QUOTE]

point conceded...I agree with the last statement too....He should check them as a matter of course...

 

[willyrivero]

How did it go?
Listen, do you know how to use a series lamp? If you don't I would I advise you learn (it's very simple and very handy) this will help you get to the problem easier without having to spend lots of money on breaking down transistors.
When installing transistors put new micas don't re-use the old ones. Once installed check with your ohmeter that transistors are not in short with the aluminum case.
Make a good check to everything related to the output, especially higher watt resistors.
When you turn your amp back on with the series lamp check if the voltages are OK. If it has simetric voltages check that they are equal.
If all looks fine play with the volume slowly watching the lamp, if it goes too bright means you still have problems if it is dim there's a good chance you got it.