Marshall MG15dfx Rapid Speaker Movement Problem

[Perkinsman]

I'm trying to fix a friends solid state amp. He said the problem started suddenly when he turned it on. As soon as it powers on, without even plugging in, the speaker moves back and forth very rapidly until shut off. Any ideas on what is causing this would be appreciated.

[Enzo]

Seems like it is motorboating. First thing I'd check is if R34 is open.


Possible bad TDA2050.

And of course check the power supplies for voltage and ripple.

[Perkinsman]

Seems like it is motorboating. First thing I'd check is if R34 is open.


Possible bad TDA2050.

And of course check the power supplies for voltage and ripple.

Do you know what size R34 is? I can only read 2W and can't find a parts list.

[Jazz P Bass]

R 34 is 0.1 ohm/ 2 watt.
Marshall MG15DFX.pdf

[Enzo]

Did you measure it? Please don't go out and get that resistor and change it just to see if I got it right. Measure the thing. It should measure like zero ohms on your meter, like touching your probes together. If the resistor is open, then it will read some high resistance.

[Perkinsman]

Did you measure it? Please don't go out and get that resistor and change it just to see if I got it right. Measure the thing. It should measure like zero ohms on your meter, like touching your probes together. If the resistor is open, then it will read some high resistance.

Yes, of course I measured it. It's .3 ohm, the same msmt as touching my probes together, lol...but I did find one of it's legs was loose & needed to be resoldered. It stopped the motorboating but now there's no sound when I plug the guitar in....I'm familiar with tube amps and point to point but this circuit board stuff is so tiny. What voltages am I interested in taking here? The ones on the schematic?

[Jazz P Bass]

"What voltages am I interested in taking here?"

Follow the schematic:
V + & V- (about 35Vdc)
+15 & =15 preamp supplies

The schematic does not have any pin designators on the TDA2050.
Here is a crop from the datasheet.
TDA2050.pdf
If you insert a signal at the CD input that will pretty much go straight in to the TDA chip.
Generally speaking if the TDA has power rails & a signal and still no output, then it is bad.

[J M Fahey]

That, or there is no continuity to speaker.

Check for continuity (meaning about 0 ohm) from Chipamp OUT to speaker + and from speaker - to power supply Ground, this with speaker disconnected..

Also check that speaker itself has continuity: some 6 or 3 ohm from + to-, still with speaker disconnected from amplifier.

All these tests with amp OFF

[Perkinsman]

"What voltages am I interested in taking here?"

Follow the schematic:
V + & V- (about 35Vdc)
+15 & =15 preamp supplies

The schematic does not have any pin designators on the TDA2050.
Here is a crop from the datasheet.
[ATTACH]55223[/ATTACH]
If you insert a signal at the CD input that will pretty much go straight in to the TDA chip.
Generally speaking if the TDA has power rails & a signal and still no output, then it is bad.

I need a little more guidance since I’m not as experienced with solid state as I am tube amps. What TP’s do I want to test for +\-15v? I don’t see 35v on the schematic. As an update, the resoldering of the resistor stopped the motorboating BUT as I tap thru the board with my chopstick, the amp powers on, then fizzles out with no particular pattern. Then it stops for minutes & turns on again but eventually fizzling out. Does that sound like a cap?

[Enzo]

No, that sounds like a connection.

[Perkinsman]

Any tips on locating intermittent connections on a circuit board beyond tapping with a chopstick?