M90 AC power problem (120v version POSSIBLY powered by 240v)

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docs

docs

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Guys, would you think that the first thing to check and replace in this instance would be the Regulator transistors followed by the power amps ?
I'm checking out an M90 for a friends and believe it may have been powered by 240v. The result is that it only works on DC/Batteries and there is no audio ie: no VU meter action at all.
I've voltage checked best I could the regulators and the voltages were zero or next to zero across each E,B and C.
Any help appreciated guys.
 
docs

docs

Member (SA)
Any techies with advice please ?
I'm sure this will have been come across before and is probably a rather common problem.
Help appreciated guys.
 
Superduper

Superduper

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If you're not getting voltages at BCE, then I suspect the fusible resistors that protect the regulators are blown. There are many. Service Manual schematic shows power rails in red. Follow the red and you'll find where power stops.

No power-on with AC probably blown transformer.
 
docs

docs

Member (SA)
Norm, bought the service manual from your site, thanks for the resource.

Looks like FR601 is ok and is passing the right voltage to Q601 collector as the voltage is correct at the collector pin. However Q601's emitter is only passing 0.18v.
Fortunately I could check against Q602 which is passing the right voltage through the emitter (8.8v).

Buying a replacement 2SD439 for Q601, fingers crossed.
 
docs

docs

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Difficult to get in but I believe 0v's are present, after checking several times same result.
 
Superduper

Superduper

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That transistor could be bad. But I'll tell you right now that if you are getting zero volts at the base, the emitter will not pass meaningful voltage/current. The diode tethered to the base is far more likely to fail than the transistor. If you replace the transistor, I'd certainly also replace the diode. Zero volts at the base tells me that either the diode has shorted to ground or the resistor feeding that diode has open circuit.
 
docs

docs

Member (SA)
Thanks Norm, that confirms my suspicions too so I'm going to get into that board to do some checks. I think I checked the resistor and got 680ohms so assumed it was ok. I'll test the diode.
As ever Norm, thanks!
 
docs

docs

Member (SA)
Norm, looks like the diode is shorted as it reads zero in both directions when using DMM with diode test feature.
 
docs

docs

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Just need to figure out how to get the chassis out in one piece as its slighly different to the manual.
Does the deck need to come separately? Seems something on the left as looking at the insides is stopping the chassis lift out. The 7 chassis screws are out and connectors removed.
 
BoomboxLover48

BoomboxLover48

Boomus Fidelis
baddboybill said:
docs said:
Ok, will continue try get the chassis out.
Its a m90C.
Click to expand...

Chassis are all the same and deck comes out with it door need to be opened. It's a simple removal :thumbsup:
Click to expand...


As always there is a hidden screw in the middle. :yes: :yes:

The manual don't show it clearly.

488131_4947977091562_1214198983_n.jpg
 
docs

docs

Member (SA)
BoomboxLover48 said:
baddboybill said:
docs said:
Ok, will continue try get the chassis out.
Its a m90C.
Click to expand...

Chassis are all the same and deck comes out with it door need to be opened. It's a simple removal :thumbsup:
Click to expand...


As always there is a hidden screw in the middle. :yes: :yes:

The manual don't show it clearly.

[ Image ]
Click to expand...

Agreed !
where where where !!
:thumbsup:
 
Superduper

Superduper

Moderator
Staff member
Look at figure #7 on the service manual. You need to remove screw #8. It is the one below the motor. Hard to see but it's there.
 
docs

docs

Member (SA)
To replace D601 am I ok with a 12v half watt zener diode?
The manual rates the current one as 9.7 - 10.1v.
As ever, thanks for all your help.
 
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