60 cycle hummmmmmmmm

Average Joe Strummer

Hi All

New MM owner here and excited/frustrated with my new baby. Just bought a 410 HD 150, 1980. Played amazingly through input 1 on the reverb channel, then pulled the cable and stuck it into input 2.............BBBBZZZZZZZ

I have some experience with circuits and can work safely inside a chassis. I have swapped out the C54/55 filter caps with correct spec cans. No result. Also, the bridge rectifier diodes looked heat effected but not burnt out, so replaced them as well with 1N4004's (can't get 1N4001's here). When I bought it, C56/59 had been changed with higher rated caps, but the rest of the electro caps were original.

So, when I power her up, then flick the stand-by, V2/4, the pair closest to the OT glow crazy purple and the other pair seem to get no juice. I swapped the pairs of tubes to eliminate them and the same sockets were glowing. I checked pin 4 on all tubes (suppost to have +350V??) and got a silly low reading on all.

Any ideas??? I have a gig in 8 days and would really like to debut the MM.

TIA for any help anyone could give.

Update
Tested all tubes again this morning. Pin 3 has around 350VDC, pin 4 195VDC and pin 5 around 15VDC on all tubes.

Full Name
Lars Verholt
Music Man Equipment
HD-130 head, 210X cab, 210RH cab, RD-50 110 combo, Sixty-Five 112 combo, RP65-112

Member for

11 years 2 months

lmv Fri, 03/16/2018 - 19:04

Check the output (driver) transistors. If one or both is/are defective, replace with a matched pair of 2N6488.

Cheers,
Lars Verholt

Full Name
Scott
Music Man Equipment
Just purchased a 410 HD 150, more to come.

Member for

7 years 3 months

Average Joe Strummer Fri, 03/16/2018 - 22:04

Cheers Lars.

Saw that on another post and was the next point of investigation. They are the 2 inside the heat sinks, correct???

Full Name
Lars Verholt
Music Man Equipment
HD-130 head, 210X cab, 210RH cab, RD-50 110 combo, Sixty-Five 112 combo, RP65-112

Member for

11 years 2 months

lmv Sun, 03/18/2018 - 16:54

Yep, that's them.

Cheers,
Lars Verholt

Full Name
Scott
Music Man Equipment
Just purchased a 410 HD 150, more to come.

Member for

7 years 3 months

Average Joe Strummer Sun, 03/18/2018 - 19:29

Hey Lars.

Thank you for your knowledge and input. You were right, one of the transistors was shorted. 2 new ones installed and biassed and back to sonic nirvana!!

Cheers
AJS

Full Name
Lars Verholt
Music Man Equipment
HD-130 head, 210X cab, 210RH cab, RD-50 110 combo, Sixty-Five 112 combo, RP65-112

Member for

11 years 2 months

lmv Sun, 03/18/2018 - 19:35

Glad to hear the amp is back to working!

Cheers,
Lars