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Messages - noddyspuncture

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I am going to guess you had a load on the amp.  You should always make sure the amp is stable and NOT producing DC before leaving teh bulb, then try UNLOADED with full mains before continuing.

No, it was not loaded and I checked and it was not producing DC either...!
I was OK on the lightbulb... bulb dimmed and stayed dim. But it was instant fireworks off the bulb...


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,

Amplifier Discussion / Re: TDA2050 keeps blowing...
« on: November 07, 2014, 07:07:36 AM »
Many thanks for all the input folks... :)

I filed an 'unhappy claim' on eBay for these IC's... just in principle - they were after all, only a few dollars.

Today I got a refund...!


Amplifier Discussion / Re: TDA2050 keeps blowing...
« on: November 06, 2014, 12:58:10 PM »
Well it looks to me that your were right... mine actually seems a bit smaller.
I attached a couple of photos...

So I was duped..! At least I now know... :-\

Amplifier Discussion / Re: TDA2050 keeps blowing...
« on: November 06, 2014, 11:06:42 AM »
Thank You guys, for all the help and advice...!


Amplifier Discussion / Re: TDA2050 keeps blowing...
« on: November 05, 2014, 07:48:32 PM »
Yes, sure I can... when you say 'break' - just exactly how do you mean please...?


Can you break one of the failed ones and post a picture on here?

Amplifier Discussion / Re: TDA2050 keeps blowing...
« on: November 05, 2014, 02:48:08 PM »
Thanks for the reply.

Aha... OK.
Well, they were bought from China - you think that might be the problem...?
I suppose this could be common?


Are you sure your replacements are not fakes?

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.


Tubes and Hybrids / Re: VOX AD15 valve modelling amp fault...
« on: March 01, 2014, 05:07:42 PM »
Thanks teemuk.... you gave me quite a bit to go on there.
I shall try a few of your pointers and see what happens...!

Tubes and Hybrids / Re: VOX AD15 valve modelling amp fault...
« on: February 28, 2014, 10:57:44 AM »
I'm not quite sure... this is a 'modelling amp' - and those switches just seem to short out a 15K resistor feeding the grids of the valve...! I was comparing what was on the control to the IC (pins 8 & 16) on other 'good' amp I have - and that looks identical... an elongated square wave, so I just assumed it's correct, as the other amp is working perfectly.

Basically it's the fact that the signal level just drops on the grid side of those IC switches that is stumping me. Even with the valve removed... the signal is much lower on both grids and then there is only a capacitor and that -15volt feed left to investigate on both lines...?


I was concerned about the control pins for the switch IC, you said there was a slow square wave???  Ought not the voltagr sit at one level or the other and stay there as long as the channel is selected?

Tubes and Hybrids / Re: VOX AD15 valve modelling amp fault...
« on: February 27, 2014, 07:24:01 PM »
Thanks for the reply Enzo,

Yes it is -15volts on the grids.

I was looking past the 'switch'... it's only the valve grid next... and with or without the valve I get the same 'smaller' signal there on the faulty unit.

This is what has had me stumped.


The AD15 has 20v rails while the AD30 has 30v rails.

The AD15 uses an LM1875 power amp IC, the AD30 uses an LM3886 power amp IC.  The AD30 has the variable power feature.

Look past the switch,  Instead of the switch being bad, maybe the switch simply dumps the signal into some low impedance it doesn't like.  Is that -15v on those grids?

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?


Amplifier Discussion / Re: Fault symptom - only with speaker connected...!
« on: December 23, 2013, 08:31:08 AM »
Many thanks to everyone for their help and advice....  all sorted now. I was just a overly cautious - probably not a bad thing - and thus ended up asking lots of questions just to make sure. Everything was fine and it's working a treat.... ;)

I've learned a lot on this one.

Thanks again,

Amplifier Discussion / Re: Fault symptom - only with speaker connected...!
« on: December 22, 2013, 11:55:49 AM »
Many thanks for the detailed explanation Enzo!
Today I sat down and was going to go through and perform all the checks as you listed... but then there was yet another spanner in the works, so I've delayed..!

It was mentioned before about what wattage bulb I was using... I used a 100watt bulb, which I've since changed for a 60watt bulb. Now, whereas the 100w bulb didn't light with no load... this other 60w bulb DOES light - albeit dimly with the variac wound up to full volts. I treble checked and put the 100w bulb back in and it still does not light - at all!

So, a 100w bulb doesn't light - but a 60w bulb does light, but dimly.
On both counts there is no DC on the output.

So I am now confused... again.... :-\


Understand what the bulb does.  The point of a bulb is you can turn the circuit on and IF there is a problem, instead of blowing fuses, the bulb lights.  It offers some protection to the unit under test.   But the amp often will not work normally with a bulb.   Once you can power it up with the bulb remaining dark, then get rid of the bulb.

Another problem is that amps are not always stable at low voltage.  An amp designed to work with 50v power rails will not necessarily function with 5v on each rail.  If you can turn it on and find no DC on the output with the bulb and without a load, then eliminate the bulb and turn the amp on, still no load.   And I mean just flip it on, no variac-ing.   Does it come up with no DC?   

Just reading between the lines, are you bringing it up on the variac each time, load or no load?   With no load, very little current flows, so the amp is able to keep hold of itself.  But when there is a load present, it then has to control the currents as well as the voltages.  ANd when loaded, if you bring it up slow on a variac, you will go through those stages where there are only a few volts present, and the amp will not be stable there.  It will swing over to one rail , and likely stay there, which causes large currents in your load.

So once you find - unloaded - no DC on the outs WITH a bulb, then try WITHOUT a bulb.  If that works with no DC on the outs, then put the bulb back on it, connect a load, and then flip it on.  No variac.  Does teh bulb come on bright?   or does it blink maybe but otherwise stay dim?   If it stays dim, then lose the bulb, and flip it on loaded.

The bulb is a great thing, but it only does what it does.   A whick is great for making whipped cream, but lousy at carving a turkey.  Always get the right tool for the job.

Amplifier Discussion / Re: Fault symptom - only with speaker connected...!
« on: December 19, 2013, 04:46:30 PM »
Thanks for the info DrGonz.

OK, so could/should bite the bullet and wind up the volts... after all, the bulb (even at full brightness) *should* protect my new transistors... right?


Cool amp! Also I remember another time I plugged an amp in, via light bulb limiter, with the speaker connected and it made the light shine bright. Without the speaker it was fine, but yeah I went through the whole amp to make sure there was nothing shorted. Turned out the bulb only shined bright when the speaker was attached. I guess it is how the amp is designed and I have read about this before. Mostly I rely on the dim bulb test without a speaker attached, but still a good idea to check both ways.

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