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From: N_Cook on 28 Jun 2010 07:35
All valve. Intermitant cutting out.
Incidentally are the solder pads gold plated before soldering ?
No mention of PbF but appearance & paper & indent test hints at PbF. Crimps
adequte, switch functions ok, all solder points look good (for PbF) x5
inspection /tugging, preamp valve bases pass needle test. Other than
resolder most likely suspects , any known problem with these amps?


From: Phil Allison on 28 Jun 2010 08:29

"Nutcase Kook"
>
> All valve. Intermitant cutting out.
> Incidentally are the solder pads gold plated before soldering ?
> No mention of PbF but appearance & paper & indent test hints at PbF.
> Crimps
> adequte, switch functions ok, all solder points look good (for PbF) x5
> inspection /tugging, preamp valve bases pass needle test. Other than
> resolder most likely suspects , any known problem with these amps?


** The control pots on the front are absolute junk - one bump to any knob
and the pot back comes loose and easily result in intermittent sound.



..... Phil


From: N_Cook on 28 Jun 2010 10:32
Definitely PbF, resoldering the usual suspect points.
I realised I was using the wrong gauge of cut down sewing needle and 2 of
the preamp valve socket pins are highly suspect. Those glossy white ceramic
mounts with holes just bigger than the valve pins.


From: Arfa Daily on 28 Jun 2010 20:28


"N_Cook" <diverse(a)tcp.co.uk> wrote in message
news:i0abre$2i3$1(a)news.eternal-september.org...
> Definitely PbF, resoldering the usual suspect points.
> I realised I was using the wrong gauge of cut down sewing needle and 2 of
> the preamp valve socket pins are highly suspect. Those glossy white
> ceramic
> mounts with holes just bigger than the valve pins.
>
>
If it continues to cut out, a generator, a 'scope and a meter should tell
all. Valve circuits are very simple and very straightforward to trace
signals in, and troubleshoot. Even if it is a bad PbF joint, it should be
easy enough to pin it down to at least a stage, and possibly to a specific
component, based on valve voltages.

Arfa

From: Phil Allison on 28 Jun 2010 21:03

"Arfa Daily"
>
> If it continues to cut out, a generator, a 'scope and a meter should tell
> all. Valve circuits are very simple and very straightforward to trace
> signals in, and troubleshoot.

** You don't service that many valve guitar amps - do you ??

The ones that were originally well made are now so old they have strange
faults most techs have never seen before - while the ones made more
recently have designed in faults never seen before by anyone.

A few examples.

1. Recent Marshall combo amps with fibreglass PCBs that become LEAKY when
hot - output valve bias goes crazy and even the phase splitter stage goes
way out of DC balance in normal operation because of leakage from anode
tracks to grid tracks. There is simply no fix for this problem.

2. Recent Gibson /Trace Elliot amps that develop mysterious crackling
noises all over the circuit due to the use of water soluble flux that was
never properly washed of the PCB. Removing the main PCB entirely and
washing it thoroughly in solvent is the only fix.

3. Same amps as above use 9 pin PCB mount sockets that SHRINK - yep,
the plastic material used to make the socket shrinks under the heat from
EL84s, reducing the pitch circle diameter so much it is impossible to fit a
new tube in the socket. Replacing all the output valve sockets is the only
fix.

Then there are all the issues with the valves themselves - all new
production now comes from China or the former USSR and most of the valves
have designed in faults of kinds that were never seen previously. Eg.
nearly all 12AX7s now crackle and pop if tapped with the back of a
screwdriver and/or have bad heater cathode hum or are very microphonic -
while octal power types often suffer from bad soldering (PbF ?) on the pins
causing all manner of weird intermittents when the solder joint lets go.

Yep - fixing valve amps is pure joy.


..... Phil


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