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[polo16mi]

Hi JM

You are a guru of amp repair!.

i had mounted it as you suggested time ago, and it save me several fuses, and now i happy, running  my LM3886-SansAmps Clone pre. (comming soon pictures and samples).

It is a "to do" homework for any that want throubbleshooting amp help.

Regards.

[J M Fahey]

Well, thanks !!!
The only problem with that is that Gurus are required to wear a towel wrapped around their heads, sleep often on a nail bed but, worst of all, weigh around 40Kg.
In that case, either I am 60Kg over limits *or* I should be properly called "Oh 2 1/2 Gurus".
And , even worse, Gurus are strict Vegetarians, can´t eat "asado"(BBQ) or "choripán" (coal grilled spicy sausage sandwich). No way José. !!

[joecool85]

Well, thanks !!!
The only problem with that is that Gurus are required to wear a towel wrapped around their heads, sleep often on a nail bed but, worst of all, weigh around 40Kg.
In that case, either I am 60Kg over limits *or* I should be properly called "Oh 2 1/2 Gurus".
And , even worse, Gurus are strict Vegetarians, can´t eat "asado"(BBQ) or "choripán" (coal grilled spicy sausage sandwich). No way José. !!

Juan, you should add that as your Personal Text in your Profile, "2 1/2 Guru" hehe.

[noddyspuncture]

What are your thoughts on adding an AC Ammeter to the circuit..?
Would that be useful in telling you anything extra to what you can find probing with your meter..?

Some images:

[J M Fahey]

You already *have* one ... it's the Lamp brightness.
Normal is dark orange, red or almost invisible; a shorted amp is bright shining as if it were connected to illuminate the room, it will usually blink in rhythm with the music.

[noddyspuncture]

Yes I get that... but I am still a little confused... I jumped in here  from my own thread about my faulty amp so I'll give you a scenario...

What would happen in this case:

OK, so fix a shorted output transistor fault. You put the amp on via the bulb - the lamp lit bright, so you knew there was a short. Got your meter out and found both o/p trasistors shorted. So you replace both transistors but you don't yet know if that's the fix. And, unknown to you there is "another fault" which would blow the transistors right away - normally. So, you reconnect the amp via the bulb and switch on. Now there is no short, as you've just fixed it - so what would you expect to happen here then... a dim bulb? - making it "appear" that everything is OK? Because, if so then as soon as you remove the bulb - those new transistors would blow.

And it wouldn't be a bright bulb as you've just fixed the short!

Sorry, but do you see my confusion?

Cheers,
Tom

You already *have* one ... it's the Lamp brightness.
Normal is dark orange, red or almost invisible; a shorted amp is bright shining as if it were connected to illuminate the room, it will usually blink in rhythm with the music.

[J M Fahey]

So you replace both transistors but you don't yet know if that's the fix.

True. The bulb is an auxiliary device, but your measuring instrument is the multimeter. Use it.
1) Measure both power rails, they should be somewhat lower than normal, because the bulb "eats" some "wall voltage"
Example: normal rails are +/-40V.
If you find something between 25 and 35V each, fine. Reasonable.
If you find , say, +/- 5V .... *who* is eating so much?. Abnormal.
In another Forum a guy's Aguilar 750 amplifier (over +/-70V rails) had perfect symmetical ... 1.9V rails. *Big* trouble.
2) measure DC voltage at the speaker out. You should have close to 0V there. Less than 100 mV in any case.
3) the fire test.
If your amp passes tests 1 and 2, hook the speaker and have it play some music.
You should be able to get at least 5 or 10W out of it, which is quite loud in a room.
If it passes the #3 test, only then plug it straight into the wall, without limiter.

Do not skip repair stages, or you may make an expensive mistake.
You *may* get lucky of course, but don't count on that.

[noddyspuncture]

Thank you..!
Now I understand...;c)


[/quote]
True. The bulb is an auxiliary device, but your measuring instrument is the multimeter. Use it.
1) Measure both power rails, they should be somewhat lower than normal, because the bulb "eats" some "wall voltage"
Example: normal rails are +/-40V.
If you find something between 25 and 35V each, fine. Reasonable.
If you find , say, +/- 5V .... *who* is eating so much?. Abnormal.
In another Forum a guy's Aguilar 750 amplifier (over +/-70V rails) had perfect symmetical ... 1.9V rails. *Big* trouble.
2) measure DC voltage at the speaker out. You should have close to 0V there. Less than 100 mV in any case.
3) the fire test.
If your amp passes tests 1 and 2, hook the speaker and have it play some music.
You should be able to get at least 5 or 10W out of it, which is quite loud in a room.
If it passes the #3 test, only then plug it straight into the wall, without limiter.

Do not skip repair stages, or you may make an expensive mistake.
You *may* get lucky of course, but don't count on that.
[/quote]

[DrewV]

I'll just make a couple comments here. Regarding the level of bulb brightness, I have found that  different amps draw more or less current while "idleing" causing the bulb glow a little brighter or dimmer. Second, to prevent a situation like JM described always use a variac to bring the voltage up gently. If the amp is drawing excessive current you'll see it before you raise the V to high.

[noddyspuncture]

Ahhh! So you are saying it's a good idea to use a combination of Variac and Light Bulb..!?

That makes sense, I take it you would start with the Variac first, feeding your Light bulb set up from it - and then feed the equipment you are working on from the Light bulb...?

Cheers,
Tom

I'll just make a couple comments here. Regarding the level of bulb brightness, I have found that  different amps draw more or less current while "idleing" causing the bulb glow a little brighter or dimmer. Second, to prevent a situation like JM described always use a variac to bring the voltage up gently. If the amp is drawing excessive current you'll see it before you raise the V to high.

[DrewV]

Tom, Yeah that's how I've always done it. You'll know right away by the brightness of the bulb if excessive current is flowing.

[noddyspuncture]

Which one first though - I was earlier thinking Varic first - but now, thinking about it the Bulb first would seem to make more sense... or does it on matter?

Tom, Yeah that's how I've always done it. You'll know right away by the brightness of the bulb if excessive current is flowing.

[joecool85]

Which one first though - I was earlier thinking Varic first - but now, thinking about it the Bulb first would seem to make more sense... or does it on matter?

It may not matter, but I would put the Variac first, then plug the Light Bulb Limiter into that, and the project amplifier into the limiter.

[noddyspuncture]

Hi again...

I just had another thought on the bulb/variac debate.
It is concerning "switch mode power supplies", and of course any gear using them... it is well documented that using a variac is not a good idea with them!

But what about the bulb-limiter? My initial thoughts were that it would be OK and work in the same way... but I thought I'd just ask and see if there were any other opinions on it?

Cheers all,
Tom

Which one first though - I was earlier thinking Varic first - but now, thinking about it the Bulb first would seem to make more sense... or does it on matter?

It may not matter, but I would put the Variac first, then plug the Light Bulb Limiter into that, and the project amplifier into the limiter.

[J M Fahey]

It depends on the PS design.
Most of them do not like anything blocking current.