Mashall MG15CDR just went up in smoke!

[charliefarley96]

Hi wondered if anyone could give me some advice about the possibility of fixing my marshall MG15CDR which blew up this morning. When I switched it on it emitted a constant loud low hum. I left it on for about 1 minute while I fiddled with the controls and then noticed smoke coming from the back of the unit. I switched off and removed the control chassis and looking at the board I can see a burn around the pins of the op-amp. It has burned the board a little but the tracks are ok. I believe this is a LM1875 based amp. Is it likely that I can just replace this chip to repair or is it likely something else is wrong? Thanks.

[J M Fahey]

*Basically* it's the chip, but it may have taken a few parts out around it.
Search for the schematic and post a link, so we all speak about the same codes and parts.

[charliefarley96]

I have looked at the chip and it is fried - cracked and burned. The rest of the circuit looks in good shape visually. Unfortuntely I cannot find a schematic for this board. I have searched but to no avail. Maybe someone knows of its existance? Anyhow I have ordered repacement LM1875 and will try that first then. BTW I wonder what caused it. This amp was second hand and has always made the biggest pop when switching on and off. I have only had it for less than 3 weeks so maybe the guy who sold it knew something was up...

[bry melvin]

Are you sure that's an 1875  I THINK that's a TDA 2030  they are USUALLY interchangeable however if you've ordered a new chip

If that's the only thing looking blown... The heat sink tab often needs to be isolated (mica washer and plastic nipple)and not grounded. If the screw loosens they can short and blow the chip...can't be sure without a schematic though.

If there was a thin film square washer under the chip it is to isolate it. Most versions of TDA20xx need to be isolated with a split power supply...which your amp probably has because of production costs(Single sided needs bigger caps and an additional cap on the output)

Also as this is the output chip any short in the output can blow it or even an intermittent speaker connection.

That all said you probably use the application notes for either the TDA or LM to trace out components associated with it...the layout difference is usually only 2 diodes for the TDA which are left out on the LM.


Here's a schematic link:
http://www.amparchives.com/Amp Archives/Marshall/Schematics & Layouts/MG Schematics/Marshall MG15CDR.pdf


Looks like it uses TDA2050.

[charliefarley96]

You are right it is a TDA2050 I can just make it out on the chip. Don't think I can use the LM1875 as direct replacement without modifying the board so will have to buy another chip. Thanks for the info and the schematic. After further reading I think the problem might have been caused by plugging my iPod into the CD-In port as others have said the TDA2050 is a poor quality chip that cannot take the input and is easily blown. Will let you know if fitting resolves problem. Thanks.

[charliefarley96]

I replaced the chip and it worked... for a while. But after about an hour or so playing I switched it off and when I switched it back to test it the hum was back. I didnt wait for it to burn. The TDA 2050 was held with a clip onto the heatsink. I replaced the thermal grease with expensive silver thermal compound. So I shouldn't think it overheated. Anyone able to shine any light on possible problems or other components that may be at fault. Thanks.

[balaboo]

Check the Zobel network (resistor-capacitor in series across output) - I have found the capacitor blown on a number of the Marshalls that us the TDA2025 chip...

[charliefarley96]

Hi not sure where to look on the circuit diagram (link above). Perhaps you could suggest appropriate capacitors to check on the diagram. Note that it is possible that it is a problem with headphone output as it happened after I was listening on the headphones. I can't be sure about this but it was ok for an hour without the phones then it blew after I used them and the second replacement chip also blew almost immediately and I was using headphones from the start then because I thought it might be the loud pop I get with the amp when I switch it off. Thanks.

[balaboo]

C25 and R34 are the Zobel network - Marshall is NOT using the datasheet recommended values, but you can get the parts from RadioShack.

[phatt]

Hi CharlieF,
               I'd say zobel is; R 32,38,39,40 and C25.
But I doubt that is the issue.
My guess is something strange as *JS3* (Headphone socket) lifts the defined imp (R34) *out of circuit*
Also looks like it lifts R31 from loop?
And that does not look right to me. (headscratch?)

Especially since you noted that trouble happens when HP is used
(or not used).
I would be looking into that part at least.

If R34 gets bridged then the current will rise in the power chip.
The HP jack might be at fault.
Sorry I can't be of more help.
Phil.

[Enzo]

I don't get what you are saying about R34.  R34 is the ground return for the speaker.  The headphones jack completes teh speaker circuit to R34.  When plugged into the phones jack, the circuit to the speaker is broken, but R34 remains as the path to ground for the load.

R31, R34 are where they are regardless of which load is selected: either the speaker or the phones with 680 ohm resistor.

I have no idea what it means the 2050 is poor quality and can't handle the input.  Is the MP3 player TONS louder than a guitar when plugged into the CD jacks?  The chip just amplifies music, it has no idea where the music came from.  Your guitar, or a recorded guitar are all the same to the 2050.

If the amp worked when you got it, then it worked.  How would someone know when the IC was about to die?  I have made a living fixing this stuff for decades, and I have to say I sure cannot tell you that an IC will die in a week.

Is your expensive silver compound electrically conductive?  That would be a problem, since the whole point of the insulating mica is to isolate the chip electrically from the heat sink.  And no matter how good such stuff might be, it doesn;t prevent parts from ove3rheating.  WHat it does do is keep normally operating pqarts from overheating due to lack of dissipation.  But if the part is being overdrivien, or faces excess voltage, or faces a shorted load, or any one of a bunch of things, it will get hot.

[phatt]

Hi Enzo,
          Yep I see that,, Until I see that the *Spk Neg* terminates at the Switching terminal *Inside the HP socket*,
**Not directly to R34**.
That is why the head scratch.
Consider What's going to happen if that sw inside HP socket goes open???

The wire obviously goes to a Sw inside the socket (although it may not be mechanically used as a switch).
Those terminals inside those cheap plastic sockets are dodgy to say the least,,, I'm sure you are familiar with the questionable reliability of them?

In my understanding if the FB is lost (open cir) then
Bang = dead chip.
Phil.

[charliefarley96]

Cannot see anything physically wrong with either C25 or R34 so will get and fit replacements to test.

I have checked the HP socket- seems ok, no dry joints, good continuity. But I was checking it with my HP adaptor (small jack to large) and I remembered that this adaptor is a bit suspect. Could it be that a intermittent headphone connection could cause the amp to blow (I was using this adaptor on both occasions)?

There was no inslating mica on the chip when I removed it so I didn't put any in on the replacement. Is that going to cause a problem? ...I meant to say about that the expensive thermal grease would stop it overheating on its own sorry about confusion, but didn't consider electrical conductivity... however the second time I used standard thermal grease for the replacement and it still blew (after only a couple of minutes). Interestingly both times it blew only after switching off and back on.

About the "maybe the guy who sold it to me knew something" thing... I was just ranting in anger... you're right Enzo noone could know when something is going to fail... unless of course it had failed before and he hadn't mentioned this concern to me.

[bry melvin]

just a thought here...you probably created a secondary problem with the "expensive silver compound"

take a look at the spec sheets.... those silver compunds are advertised as "highly conductive thermally and electrically" (BTW mouser sells it in bulk...not as expensive those little tubes are aimed at computer fanboys)

This WOULD work for To3 cases though... or circuits where to TO 220 case is grounded ...ususually single sided supply...

did you clean it off everything meticulously with solvent?

[balaboo]

No need to worry - the chip IS bolted directly to the heatsink, which is left UNCONNECTED to any wiring traces on the pcb. I just use regular white lithium grease - works great.