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Topics - noddyspuncture

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Hi folks,

I am struggling with an amplifier I have replace the output transistors in.
The service manual states that it is critical to set the quiescent current when any components have been replaced. Just incase I turn the unit on normally and they all blew again, I used the lightbulb trick and all was (seemed) fine.

The light bulb lit and then dimmed immediately. I also put a known good unit onto the light bulb and it behaved the exact same way - so I deduced that all was well.

When checking the quiescent current I had an imbalance. They say you should set in to measure 6.6 to 8.8mV across R's 4, 14, 44, 45.

One side read "zero" across the resistors ... the other read -76mV.  I could not set them and assumed it was because of the lightbulb.

So I removed the lightbulb - after all, it didn't "light" brightly and behaved in the same way as the known good unit...  I got sparks (from R42) and blew the output transistors again...!

My question is - should I be able to set the quiescent current whilst using the lightbulb trick...? I know voltages would be lower overall but should I be able to get an idea is setting the quiescent current will work...?

Appreciate any help,
Cheers,
Tom



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Amplifier Discussion / TDA2050 keeps blowing...
« on: November 05, 2014, 01:03:00 PM »
Hi Guys,

I've been doing quite well recently with my repairs, but one has just got me stumped so I thought I'd run it past you here. Maybe someone can shed some light...?

I am trying to fix a small fender practice amp that uses a TDA2050 amplifier IC/transistor.

It's getting so that I am wondering if it is worth spending any more time/expense on the thing.

I changed this component 4 times now. Each time I fire it up, check voltages then connect speaker etc.... and it works perfectly! For a while... but then, all of a sudden - *pop* and it starts humming or whatever...

The output leg (pin 4) gets shorted (or very low resistance) to one of the power rails. Everything (there's not a lot) around the IC checks out fine. It is a "Fender Frontman 15R" - I've attached the schematic.

Cheers,
Tom







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Tubes and Hybrids / VOX AD15 valve modelling amp fault...
« on: February 27, 2014, 02:59:41 PM »
Hi guys,

I was wondering if anyone could help me with an amp I have been struggling with for a while now, please..? I have a faulty one and also another a good one to compare to - so in theory it should be really easy... but I'm still struggling....!

I've attached the PDF... it has two power amp schematics... one is the 15 and the other is the 30. Both look the same.... although mine are the 15's.

What I am getting on the faulty amp is basically low level... (and an apparent lack of bass - probably just a side effect of the low level?) It's just not a loud and sounds compressed when turned right up.

My scope confirms that the signal level is much lower on the grids of the valve in the faulty amp. (Everything up to that point is identical on both amps) So, basically on the left side of R23 & R27 (both of which are shorted out by the IC switch) I get a full, identical level on both amps. But the faulty one has a much lower signal level on the grid sides of these resistors. I originally suspected the IC which I changed. There was no difference. Even with the valve removed it is the same. The scope confirms a normal level going into the switch but a lower level coming out of the switch. On both amps, good & faulty, the control feed to pins 8, 16 (and 9) of the IC looks the same on my scope (and elongated square wave) so I can discount that I suppose. I even tried shorting out those IC 'switches' temporarily - the level improved slightly but still wasn't what it should be. This has been baffling me for sometime now.

Does anyone have any ideas please?

Cheers,
Tom

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Amplifier Discussion / Fault symptom - only with speaker connected...!
« on: December 18, 2013, 10:09:41 AM »
Hi Folks,

I've perused the site but couldn't find any pointers to my strange symptoms...

I am working on an old Simms-Watts transistor amplifier. It was blowing HT fuses. I found two shorted transistors which I have replaced. No other components are showing as 'faulty'...

I powered up via a Variac and a light bulb. The bulb lights and gets brighter as I increase the voltage... so I back it off and chicken out...! You would assume there is still a fault.

I do measure DC on the output... BUT only with a speaker connected...!
Disconnect the speaker... bulb don't light and there is no DC on the output.

Now I would have thought that a *fault*  would give me DC on the output all the time, speaker or no speaker...!?

Also, with such a fault which lights the bulb, I would have expected to read a dead short to ground from say the HT lines or maybe some where around the output transistors...!?

Does anyone have any ideas please...?

Many thanks,
Tom

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Amplifier Discussion / My Problem Amplifier And Symptoms...
« on: April 16, 2012, 07:09:10 AM »
Hi everyone - I just joined and have a request for help please...

I am trying to fix a combo amplifier and wondered if anyone could advise me? I am normally quite competent and sucessfull in finding faults but this one has me currently stumped!

The initial symptom was blowing of fuses. I found two shorted o/p transistors - and nothing else.

So I replaced them and switched the amp on. It worked fine for a week or so - then I decided to spray the noisey volume pots - now this was on the pre-amp panel, and didn't involve removing the power amp side which I had just worked on - as the pre-amp is on the other side of the cabinet, the two panels are connected by a ribbon cable. So I sprayed the pots and replaced the pre-amp front panel.

I turned the amp on - fuse blew again..!

This time I replaced the fuse and whilst monitoring the power amp as I switched it on again I noticed two resistors 'smoking' (100 ohms between the base and emitter of both o/p transistors)

I quickly turned off (the resistors still read 100 ohms!) This time the o/p transistors weren't shorted - but obviously needed replacing (again) as they read a few hundred ohms both ways between certain legs (cannot remember which).

Again, I replaced both o/p transistors - but obviously I don't want to blow them again. And again, as before - there is nothing standing out as 'shorted' anywhere on this simple circuit. With these symptoms you would think it would be staring you in the face..!?

So next I used my Variac... as I slowly wound up the voltage, with a speaker connected and measuring the HT volts I noticed, very early on, almost just off the Variac's end-stop - I am quickly getting volts building but also a 'hum' appears on the speaker.

I suppose I have a question here - as there are no actual obvious shorts anywhere - could the driver transistors (BD537/538) be causing this even though they read fine - out of circuit - with my meter? Could they be 'leaky' or just faulty 'under load'...?

There is also another transistor (BD711) which feeds the drivers which also seems fine out of circuit on a meter...

Has anyone experienced this type of fault and could I be obn the right track?

Many thanks for any help.
Cheers,
Tom

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