Marshall MG250 DFX. Clean channel not so clean

[Memphis]

I have the above amp here. The clean channel works but is very distorted. Not nasty distorted but just not clen like it once was. Everything else works just fine in the amp. Schematic attached. Any thoughts ?

Bruce

[Memphis]

No bites at all on this one. The clean channel sounds very similar to the OD channel. It is switching. I checked pin 8 and pin 1 on the DG212CJ and the chip is switching correctly.

Bruce

[52 Bill]

Have you checked IC1? Read the voltages on all of the pins and look for any anomalies.

[Memphis]

I checked the voltages as follows. I am cutting my teeth here. The ss stuff is new to me.
Pin V
1 .03
2 .03
3 .03
4 -15.2
5 .03
6 .03
7 .03
8 1

[pecorporation]

Pin 8 should be around +15VDC. REG1 (7815) is not producing the necessary 15VDC possibly caused by a shorted op-amp IC. I'd look at the tops of all the ICs to see if any display a crack or burn mark on top. This may be a quick way to determine which IC is shorting that voltage to ground.

[Memphis]

No cracks or urns found. I did check a couple others. IC2-IC7 and all show -15 on pin 8
I am testing this with DVM to chassis ground.

Bruce

[pecorporation]

Pin 8 should show +15VDC on all ICs, so if IC1's pin 8 only shows +1VDC, I'd suspect a burned trace to IC1 around pin 8. All the ICs share the same +/-15VDC voltage rail, so if one goes down, they all go down. With the unit on, carefully see if you detect excessive heat coming off IC1 by feeling the top, careful, it can get quite hot.

[Memphis]

OK, so I traced it back to reg2 same -15v which is what I'm seeing on pin4 --Also reg1 1v which corrosponds to pin8 on the IC's. So I guess I keep backing up from there.

Bruce

[pecorporation]

One of those ICs is pulling REG1's voltage from +15VDC to +1VDC. Carefully, touch the top of each IC to see which one is hottest, these generally run cool so a hot one will be your indicator of which one needs replaced.

[Memphis]

I may have found it here. I checked W8 and W10. 11V AC. Is that correct ? R108 11v one side 1V other AC were talking. After the bridge 1V DC.
R109 11V AC in 9V AC out.

I think I got that right. I also see R108 looks a bit burned. R109 meters at 2.9 and R108 is a bit iffy at best. moving it around give me different readings. I am going to replace that before moving forward.

Bruce

[pecorporation]

Sounds like you're on the right track. Good luck!

[Memphis]

Got it. Success. R108 was bad. Thanks for pointing me in the right direction pecorporation and 52 bill. What a learning experience. The IC's can be intimidating after reading tube schematics for so long. I think I put in a 40 hour work week since noon. But I think I am headed down the right road.

Thanks a million.

Bruce

[Enzo]

No circuit works without good power supply. If a tube preamp doesn't work, you check the power supply to the tubes. Heaters, B+. If they are wrong or missing you go after the powr problem. SS and ICs are not different. Without both power supplies, the IC cannot work right.