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**shadowfax** · 02-06-2009, 04:24 PM

By the way I have also seen a photo of the same amp in a book with a different power switch. Mine is clear red and lights up (once!). The other one is plain black. Could it be a switch problem?

**52 Bill** · 02-06-2009, 08:11 PM

What you describe sounds like a shorted output chip, the TDA2050.

It may look perfect, but it can still be shorted.

**gbono** · 02-06-2009, 08:34 PM

Looking at the manual - plugging in the adapter would have automatically disconnected the speaker. Are you sure you didn't plug into the extension speaker jack?

Rare to see a power switch cause this problem. You would start by isolate the power supply (disconnect secondary side of transformer) and verify that the problem isn't in this section. I would say it's time to take it into a tech.

**shadowfax** · 02-07-2009, 02:42 PM

Cheers for the replies,
52 Bill the TDA2050, I've only scanned over the diagrams and haven't found it yet (will do eventually), this would need a replacement then. Is it possible to explain why this would be the problem, if I have to order componants it's better to get them in one go.
gbono it was in the headphones as there was no sound from speaker. The 3.5mm adapter was left in when I unplugged the 2.5mm headphones.

Can I rule out the transformer as this will be the most costly part I'm sure?
The power switch does light up before the fuse goes so it should be ok but it looks like its been changed.

**52 Bill** · 02-07-2009, 04:37 PM

The chip is the main output amp. It is on the lower left side of the schematic right after the CD inputs.

If you remove the chip from the amp and then power it up, the amp will probably not blow the mains fuse. But, you should check the power supply and related diodes for shorts as well.

These small chips are asked to perform the job of what used to take an entire board of parts, so they are under constant heat and electrical stress. It may have failed due to age or it didn't like a spike caused by the ac line or by plugging in the headphones or the FX unit. Sometimes they just give up the ghost for no apparent reason.

This amp is a perfect candidate for a light bulb limiter. If you have one or build one, you can plug the amp into it and then take voltage readings on the output chip. You'll probably find a dc voltage on the output that shouldn't be there.

**gbono** · 02-08-2009, 12:21 AM

Page 4/6 from the manual you posted:

External Speaker: Connecting an external speaker automatically isolates the internal speaker.

Not familiar with this amp or the output IC so I can not tell you what on-board protection comes with the TD
A2050.

**shadowfax** · 02-09-2009, 03:45 PM

I've now spent some time looking at the diagrams again and I can see the TDA2050 clearly now.

I should say that my Amp was made 2005/6 and looking at the 'HMC30R-CH' this is the correct diagram as there is no R55,R56 as on the older 'HMC30R'.

From what I understand its the main op amp IC and is in the section of the circuit with the headphone output. But is it possible that if the amp was trying to output to the headphone socket (which wasn't draining any power as it was just an adapter) could it have caused the circuit to be overloaded and burnt out the TDA2050. Does it feedback to this op amp if the headphone speakers aren't using the power. I thought that would have been protected against, unless they didn't account for me!

The TDA2050 is connected to a big heatsink, the connection is good, and the amp had been switched off, it's also pretty cold in the UK at the moment so I don't think it would have overheated. I don't know if that's relevent, could it still fail.

This is info on TDA2050, it says it has overheating protection.
http://www.edutek.ltd.uk/Binaries/Da...ar/TDA2050.pdf

I've seen a replacement, but can somebody tell me if a TDA2050 and TDA2050V are the same. Does that last letter just refer to update/improvements. The one I've seen is �3.66 in Maplins TDA2050V probably a ST one as in pdf.

Can I test the circuit with a multimeter to see if the TDA2050 is the problem. Fuse blows very quick but I could take a quick reading before switching off.

Never heard of a 'light bulb limiter' I've search for instructions but nothing detailed found. A 40w bulb in series with power input?
Could I just try a new TDA2050. Would it just blow again if there where other componants damaged. The power diodes are the likely ones?

If you have one or build one, you can plug the amp into it and then take voltage readings on the output chip. You'll probably find a dc voltage on the output that shouldn't be there.

Do you mean use 'light bulb limiter', leave the old TDA2050 in, read DC V at its output (pin No.4).
Where would the neg. probe go.

Thanks for persisting with me its a steep, steep learning curve,
Cheers.

**Fragger** · 02-09-2009, 05:21 PM

'light bulb limiter'

In action

**52 Bill** · 02-09-2009, 08:05 PM

Originally posted by shadowfax View Post

From what I understand its the main op amp IC and is in the section of the circuit with the headphone output.

If you look carefully the speaker connects to the headphone jack in such a way that when the phones are plugged in the speaker is turned off.

Originally posted by shadowfax View Post

The TDA2050 is connected to a big heatsink, the connection is good, and the amp had been switched off, it's also pretty cold in the UK at the moment so I don't think it would have overheated. I don't know if that's relevent, could it still fail.

Yes

Originally posted by shadowfax View Post

I've seen a replacement, but can somebody tell me if a TDA2050 and TDA2050V are the same. Does that last letter just refer to update/improvements.

I believe that the last letter refers to how the leads are formed for vertical mounting. If you search for a datasheet, the different packages will be illustrated for you.

Originally posted by shadowfax View Post

Can I test the circuit with a multimeter to see if the TDA2050 is the problem. Fuse blows very quick but I could take a quick reading before switching off.

No, you will not get any useful information doing this. You'll just waste your time and money.

Originally posted by shadowfax View Post

Could I just try a new TDA2050. Would it just blow again if there where other componants damaged. The power diodes are the likely ones?

Yes, if there are other problems it could damage the new chip. If the output chip is not the problem, then the power supply diodes would be most suspect.

Originally posted by shadowfax View Post

Do you mean use 'light bulb limiter', leave the old TDA2050 in, read DC V at its output (pin No.4). Where would the neg. probe go.

Yes, all voltage readings should be referenced to ground. Attach the negative lead to the metal part of the main chassis.

I suggested the light bulb limiter, as it will allow you to power up that amp without blowing the fuse. Once powered up, you will be able take voltage readings that will indicate where the problem(s) lies.

Instead, you could just replace the output chip and see what happens. Or you could just remove the old chip and see if the amp will power up without blowing the fuse. If it does, then you'll know that the chip is shorted and that the power supply is ok.

Of course, doing any of these things will require you to work on the amp while it is plugged into the mains, and can present a real danger to your well being. So if you are unsure or unqualified, you should consider using professional help.

**shadowfax** · 02-10-2009, 12:23 PM

Got the link Fragger, thanks.

Thanks for explaining the option's 52Bill.

Regards TDA2050V I'll bet the V stands for Vertical, refering to the pins -as you said. That position suits the heat sink.

Neg is Earth is the metal casing 'obvious' sorry, it was there in my mind, but electrics always makes you worry if your not experianced.

I appreciate the warning, I'm always careful with electrics. I think just keeping your workspace clear, thinking and planning help alot.

I found this problem with a fender amp, quite similar (though it wasn't starting up like mine does) I was interested to see that it was the power diodes -the zener diodes.
I've read up on how Ac is stepped down (trans) then smoothed with the Caps and Diodes so I see their role now, very interesting.

http://www.electro-tech-online.com/g...eshooting.html

**gbono** · 02-10-2009, 02:17 PM

Assuming you have a soldering iron - remove the connections at BR1 (bridge rectifier) that go to C45/44. Carefully isolate these wires from contacting anything and power up the amp. If the fuse doesn't blow you know the transformer and bridge rectifier are okay. You can hook up your DMM as a "fuse" by using it in lieu of the fuse. This way you can watch the DMM (in current reading mode - amp meter) to see if the current is rising sharply. This works well when using a VARIAC but in your case it will just save fuses - make sure you use the correct current setting on the DMM. You are acting like the fuse in this case by disconnecting the power if the meter shows a high sustained current flow - should be used with light bulb current limiter.

You still need to check the D7/D9/D10 to see if they are damaged. If all this checks out. I would remove U5 (TDA2050V) and see if I still have a short circuit condition. I would also check switches SK5/4 to make sure there isn't a problem there.

If removing U5 still doesn't solve the problem - you have a short some where between the input and U5.

Good luck and remember that you should always keep one hand in a pocket or free of the chassis when working on live electrical circuits - the current needed to de-fib heart muscles is .05A.

**shadowfax** · 02-11-2009, 04:53 PM

OK, as I'm having to use library for net access the firewall doesn't like the link to the 'light bulb limiter' link. I'll get that info sometime soon.
It seems that the simplest way for me would be to
1. Desolder TDA2050, start amp. If fuse ok then solder new TDA2050, and hopefully starts ok.
2. If fuse blows then replace the power diodes, restart. Hopefully fuse ok and then replace new TDA2050. If fuse goes ... rethink.

But ..Question. with (1.) starting up without TDA2050 wouldn't harm the circuit?
With (2.) I think I would replace all 3?? diodes as its simple enough and they're cheap.
gbono you say D7,D9,D10. I think thats D15,D16,D17 on the HCM30R_CH diagram, which matches my later circuit.
So thats D15 [15V 1.3W zener diode], D16 [15V 1.3W zener diode],
D17 [5V 0.5W zener diode].

**52 Bill** · 02-11-2009, 06:20 PM

Originally posted by shadowfax View Post

Question. with (1.) starting up without TDA2050 wouldn't harm the circuit?

Should not be a problem.

Originally posted by shadowfax View Post

With (2.) I think I would replace all 3?? diodes as its simple enough and they're cheap.
So thats D15 [15V 1.3W zener diode], D16 [15V 1.3W zener diode],
D17 [5V 0.5W zener diode].

No the diode that you may need to replace, will be one of the actual power supply diodes D10, D11, D12 and D13. If any of the Zener diodes had shorted their related limiter resistors R55, R56 and R57 would be burning before the fuse would blow.

**shadowfax** · 02-13-2009, 04:45 PM

Ok got the lightbulb limiter info now.
I'll strip down the board this weekend and take out the tda2050 and then power up. This is a good easy starting point. I'm reluctant to take the nobs and screws off the front till I'm certain as it will invariably scratch off so paint etc. and don't want it in bits for to long.
I'll build a limiter if I have to go further.

Question. For replacing the TDA2050, am I going to be OK regards the soldering heat on the replacement. I'd considered cooling the new IC and heatsink in freezer then fitting and soldering. This would help stop the heat travelling up the pins. Have read that semiconductors very delicate. That goes for the diodes too then?

You still need to check the D7/D9/D10 to see if they are damaged.

gbono
So was this wrong? or just worth checking. (understandable if you've got the meter out)

I've got photo's to post but I didn't bring them, I'll post tomorrow.
Also spent some time looking at circuit yesterday and have noticed it is ver.9346-3. It's just before the diagram
http://tremolo.elektroda.net/Firmowe.../HCM30R-CH.pdf
but the relevant parts look the same, only Laney direct would supply the actual one as I've looked everywhere.

Thanks for your very patient

help.

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