Marshall JVM 410, 2006, PbF

[N_Cook]

Early and bad PbF but is it the problem here, most of the ECC83 socket
solderings were very iffy .
Volume dropped and owner changed the valves. Volume came up but a distortion
on the overdrive channels emerged soon after. Redoing those socket
solderings and usual suspects on front controls board has not cured the
distortion problem.
schematic on elektrotanya
Harsh cut-off more like clipping and no sign of any smooth saturation,
rounding of the peak/s, just launches into cut off on the peaks (not
troughs). With all pots at mid posistion the signals ,clean and OD, are
sinusoidal and amplitude ratio of about 2:1 OD:Clean in the gain and tone
block. That preceeds the valve V7 which with mid pots and order 10mV amp
input 400 Hz has on pin2 , grid, about 100mV pk-pk for clean but 7 volt on
OD thence clipping.
All valves test fine, all R in relevant area are fine. A cap problem ? but
coinciding with valve changing? I've not inspected underside of that
wired-in board but I'm assuming a cap PbF solder problem. Vol drop could
even be the bad PbF at the input socket but problems appearing /
disappearing at "intimate" handling suggest PbF problems. There is loads of
relays in this design also .

 

[N_Cook]

And check that a send/return loop isn't engaged with the Mix pot on anything
other than fully on or off - not sure which way drops the volume with no
effects plugged in.



Gareth.

Will do.
I removed the main board and no obvious PbF problems found but treated all
the usual suspects. I did note that the 1 1/4 inch fuse holders would have
failed solder in a short while.

 

[N_Cook]

not S/R loop problem
Just in case it was an odd batch of ECC83 , I tried with others and the
same.
Keeping same input and changing frequency
4KHz , 26V pk-pk on V7a grid , pin 7, and ok normal soft roll top "clipping"
2KHz, 48V pk-pk , bad trace hard clipping
1 K , 35V , bad
200, 15V bad
100 Hz, 5V, normal trace shape

so to find an errant cap that is involved in OD1 and OD2 channels

 

[N_Cook]

Tomorrow I think the 1uF on the cathode of the first triode needs exploring
as its output is upped in the 400 - 1000 Hz area. Of course a bit
exagerated, in that frequency domain in the clean channel, could well go
unnoticed.