Marshall AVT150H No lights no fans no Sound... need your Help

[phil1988]

Hello everyone,

I'm Philipp, 21 years old and from Germany. I'm studying mechanical engineering an now i need your help. Sorry for my bad english, i try my best.

My Marshall AVT150H is brocken. At a single blow there was now sound of the fans and no one of the LEDs shined. The evening bevore it worked as ever!

I made a classical troubleshooting but nothing workes. Than i opened the case.
With my multimeter i meassured the tension after the transformer.

- Tension after the Transformer : W104/W103 33,4V AC ; W105/W107 18V AC;
- Tension after bridge rectifier: L100 meassured: Voltage 91V DC
- Tension after bridge rectifier: L101 meassured: VOltage 4,59V DC
- Tension after voltage regulator: REG101 ( IC 7815 com) meassured: Voltage 0,5v
- Tension after voltage regulator: REG100 ( IC 7915 com) meassured: Voltage ,5v


I thing that one of the brigde rectifiers are brocken because the voltage between the exits of the REG101/100 normally has to be +15V / -15V


Does anyone of you have some idea to fix the problem? I really need you. Your my last chance.

Philipp

[jswinue]

Check the resistors R202 , R223 (2R2,1W)

[Jazz P Bass]

Marshall AVT150

Test point W105/W107 must measure more than 18 Vac.
I would remove L101 & then measure W106/ W105 & W107/ W105.
You should read about 45 Vac on each winding.

[phil1988]

Good Morning,

Thanks for your Help,

I meassured the resistance of R202 & R223. The R223 Has exactly 2.2 Ohms how it has to. But the R202 has 4 Ohms! How could that be??? Magic??
I meassured it in implemented condition. But there was no voltage on the Amp!!

@Jazz P Bass: i got a understanding problem. How could the voltage at W106/ W105 & W107/ W105 be more than 18V? On the transformer stands 18v- 0 -18v ???


Thanks a lot. I think together we get the problem

Philipp

[Jazz P Bass]

Power Supply Check

Sorry for the bad info.
W106/W105 & W107/W105. If they both measure 18Vac that is good.
Check L101 for a short with DVM on diode check.
Check both + & - regulators for a short.
Remove the regulators & measure the output pins on the board to ground.
You want to see a high resistance reading.
If either is low, then you have a bad component down stream.

[phil1988]

hey ho,

i checked the Bridge rectifier in implemented condition with my Multimeter with diode-test. I think its ok!

Than i removed the two Voltage regulators REG101 /100 and measuered the resistance between the pins on the board and the ground.

REG100:

Pin1 0Ohm (ground)
Pin2 110KOhm
Pin3 44.3kOhm

REG101

Pin1 707Ohm
Pin2 0Ohm ( Ground)
Pin3 1.3MOhm

How can i interpretate this results? in my opinion the capacitor C221 on Pin1 of the REG101 is damaged??? Normally i would have to meassure an infinite resistance when a capacitor is in the way to ground or not??????


Thanks for your help.. Philipp

[jswinue]

Check the input DC voltages on PIN1 of REG 101 and on PIN2 of REG 100

[Jazz P Bass]

Reg 101

707 ohms to ground on pin 1 of the +15 Vdc regulator does not look good.
Remove C221 & try again.

[phil1988]

Hey guys

I think i got the problem. I checked the resistors r202 & r223 wich musst have 2.2 Ohm ( right?) But one has 600 ohm and the other has 2kOhm... do you think so to??

For what purpose are these resistors???



Many thanks for your help, Regards Philipp

[jswinue]

They are as fuses.

[Enzo]

Those resistors sem to measure different each time. When you measure some odd value for a resistor, reverse your test leads. You may have the power off, but that does not mean some voltage is not still in the circuit. If you put your ohm meter to a cap, the meter will charge the cap up to a volt or so right there. If a 10k resistor measures 20k, swap ends with your meter leands and see if the thing now reads 2k or even "negative" resistance. If the readings are different it is because or residual voltage in the circuit.

If in doubt, unsolder one end of a part and lift it out of circuit and remeasure it.

[phil1988]

Hey guys,


Thanks a lot for your help! I fixed the amp last night. The two resistors where brocken! Now it works as fine as before :-)
Thanks a lot for your help. Your great. Without you, i got no chance!


Regards Philipp