Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default HT capacitor balancing Rs , necessity ?

Well I call them HT balancing Rs anyway, in valve amps. Where 2 HV caps are
used in series to avoid using extra high voltage single caps. I always
thought they needed 100K to 200K across each to balance out likelihood of
midpoint voltage drifting up or down and exceeding the rating of one cap of
the pair.
Hiwatt AP CP103 ,2005 bouncing back blowing mains fuse , different set of
output bottles from before.
Worked for about 6 hours in total and over an hour before problem
re-emerged.
Amp lost sound, owner turned round in time to see the mains neon fail , ie
after sound failure.
This amp does not have balancing Rs , unlike its 1970s versions. I will add
a couple of 220K/1W but anything else to ponder? These amps always seem to
have had 3A mains fuses and 3A HT fuses for some odd reason - anyone know
why? so the mains fuse sees more current than the HT fuse in normal use,
same in abnormal sistuations?


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Default HT capacitor balancing Rs , necessity ?



"N_Cook" wrote in message ...

Well I call them HT balancing Rs anyway, in valve amps. Where 2 HV caps are
used in series to avoid using extra high voltage single caps. I always
thought they needed 100K to 200K across each to balance out likelihood of
midpoint voltage drifting up or down and exceeding the rating of one cap of
the pair.
Hiwatt AP CP103 ,2005 bouncing back blowing mains fuse , different set of
output bottles from before.
Worked for about 6 hours in total and over an hour before problem
re-emerged.
Amp lost sound, owner turned round in time to see the mains neon fail , ie
after sound failure.
This amp does not have balancing Rs , unlike its 1970s versions. I will add
a couple of 220K/1W but anything else to ponder? These amps always seem to
have had 3A mains fuses and 3A HT fuses for some odd reason - anyone know
why? so the mains fuse sees more current than the HT fuse in normal use,
same in abnormal sistuations?



You could try having a good old sniff around, trying to locate any
burning/arcing smells, particularly around/underneath the output valve
sockets, and the transformers.
(Making sure, of course, it is not plugged in and all HT voltages have
subsided)

If this is a contemporary re-issue, check to see if it has inrush limiter
devices and that they are not dry jointed - along with anything else in the
mains supply to the transformer including switches, pcb fuseholders, 13A
mains plug wiring etc.
Any kind of dodgy connection here can blow a mains fuse.



Gareth.


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Default HT capacitor balancing Rs , necessity ?

Gareth Magennis wrote in message
...


"N_Cook" wrote in message ...

Well I call them HT balancing Rs anyway, in valve amps. Where 2 HV caps

are
used in series to avoid using extra high voltage single caps. I always
thought they needed 100K to 200K across each to balance out likelihood of
midpoint voltage drifting up or down and exceeding the rating of one cap

of
the pair.
Hiwatt AP CP103 ,2005 bouncing back blowing mains fuse , different set of
output bottles from before.
Worked for about 6 hours in total and over an hour before problem
re-emerged.
Amp lost sound, owner turned round in time to see the mains neon fail , ie
after sound failure.
This amp does not have balancing Rs , unlike its 1970s versions. I will

add
a couple of 220K/1W but anything else to ponder? These amps always seem to
have had 3A mains fuses and 3A HT fuses for some odd reason - anyone know
why? so the mains fuse sees more current than the HT fuse in normal use,
same in abnormal sistuations?



You could try having a good old sniff around, trying to locate any
burning/arcing smells, particularly around/underneath the output valve
sockets, and the transformers.
(Making sure, of course, it is not plugged in and all HT voltages have
subsided)

If this is a contemporary re-issue, check to see if it has inrush limiter
devices and that they are not dry jointed - along with anything else in

the
mains supply to the transformer including switches, pcb fuseholders, 13A
mains plug wiring etc.
Any kind of dodgy connection here can blow a mains fuse.



Gareth.




I thought about putting a telltale 2A fuseholder in line specifically with
the caps and then thought better of it. A high voltage in the output Tx whan
a fuse blows could induce very high voltages across windings but loosing the
smoothing could mean an even higher peak voltage if the HT or Mains fuse
should blow.


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Default HT capacitor balancing Rs , necessity ?

In article , "N_Cook"
wrote:

This amp does not have balancing Rs , unlike its 1970s versions. I will add
a couple of 220K/1W but anything else to ponder?


N_Cook-

I think equalizing resistors are a good idea. However, you must use
high voltage resistors.

A 220K resistor would dissipate 1 Watt at 469 Volts. A higher value
resistor might be appropriate if DC leakage current through the
capacitors is low. But you will find that ordinary resistors may fail
due to dielectric breakdown, even if their power rating has not been
exceeded!

Fred
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Default HT capacitor balancing Rs , necessity ?


"N_Cook"

Well I call them HT balancing Rs anyway, in valve amps. Where 2 HV caps
are
used in series to avoid using extra high voltage single caps. I always
thought they needed 100K to 200K across each to balance out likelihood of
midpoint voltage drifting up or down and exceeding the rating of one cap
of
the pair.



** Not needed in most cases, the electro caps will simply balance themselves
at a mutually satisfactory leakage current.

The idea come from series connected, high voltage film caps which DO need
them or it is snap crackle pop time.



.... Phil





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Default HT capacitor balancing Rs , necessity ?


"Gareth Magennis"

Any kind of dodgy connection here can blow a mains fuse.


** Had a Marshall 100W head returned to me repeatedly for blowing its AC
fuse in the first few minutes of the gig - of course, it tested perfect
while on my bench, no matter what torture tests I tried.

The cause eventually turned out to be a loose fitting IEC plug that arced
under heavy vibration (when sitting on a cab) and made the AC power surge
over and over till the fuse blew. I used my own IEC lead in the workshop so
initially I never saw the fault.

In another 100W Marshall, I found that if I turned the amp off and then back
on again in 3 to 5 seconds, the inrush surge was much longer than usual and
took out the 2A slo-blo fuse.

This was due to the tube bias voltage dropping fast and recovering slowly in
the particular model, so the bias voltage was very low at switch on but the
4 x EL34 cathodes were still hot and so all drew a heavy DC current for a
couple of seconds.

Marshalls do SOOOO many ****ing weird things I could write a book on them.



..... Phil


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Default HT capacitor balancing Rs , necessity ?



"Phil Allison" wrote in message ...


"Gareth Magennis"

Any kind of dodgy connection here can blow a mains fuse.


** Had a Marshall 100W head returned to me repeatedly for blowing its AC
fuse in the first few minutes of the gig - of course, it tested perfect
while on my bench, no matter what torture tests I tried.

The cause eventually turned out to be a loose fitting IEC plug that arced
under heavy vibration (when sitting on a cab) and made the AC power surge
over and over till the fuse blew. I used my own IEC lead in the workshop so
initially I never saw the fault.

In another 100W Marshall, I found that if I turned the amp off and then back
on again in 3 to 5 seconds, the inrush surge was much longer than usual and
took out the 2A slo-blo fuse.

This was due to the tube bias voltage dropping fast and recovering slowly in
the particular model, so the bias voltage was very low at switch on but the
4 x EL34 cathodes were still hot and so all drew a heavy DC current for a
couple of seconds.

Marshalls do SOOOO many ****ing weird things I could write a book on them.



..... Phil



I have on more than one occasion replaced a Marshall PCB mounted mains
fuseholder that had dry jointed, arced, and blown several fuses the owner
tried.

Occam's Razor again.




Gareth.


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Default HT capacitor balancing Rs , necessity ?


"Gareth Magennis"
"Phil Allison" "Gareth Magennis"

Any kind of dodgy connection here can blow a mains fuse.


** Had a Marshall 100W head returned to me repeatedly for blowing its AC
fuse in the first few minutes of the gig - of course, it tested perfect
while on my bench, no matter what torture tests I tried.

The cause eventually turned out to be a loose fitting IEC plug that arced
under heavy vibration (when sitting on a cab) and made the AC power surge
over and over till the fuse blew. I used my own IEC lead in the workshop
so
initially I never saw the fault.

In another 100W Marshall, I found that if I turned the amp off and then
back
on again in 3 to 5 seconds, the inrush surge was much longer than usual
and
took out the 2A slo-blo fuse.

This was due to the tube bias voltage dropping fast and recovering slowly
in
the particular model, so the bias voltage was very low at switch on but
the
4 x EL34 cathodes were still hot and so all drew a heavy DC current for a
couple of seconds.

Marshalls do SOOOO many ****ing weird things I could write a book on them.


I have on more than one occasion replaced a Marshall PCB mounted mains
fuseholder that had dry jointed, arced, and blown several fuses the owner
tried.


** The magnetic surge of the power transformer alone is about 50 amps peak
with a 240V supply - a 2A slo blo fuse can take this once in a while but
NOT if repeated rapidly.

A loose fitting plug in the AC outlet or IEC inlet is all it takes and the
blown fuse that results sends most musos into a panic.



..... Phil




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Default HT capacitor balancing Rs , necessity ?

Gareth Magennis wrote in message
...


"Phil Allison" wrote in message ...


"Gareth Magennis"

Any kind of dodgy connection here can blow a mains fuse.


** Had a Marshall 100W head returned to me repeatedly for blowing its AC
fuse in the first few minutes of the gig - of course, it tested perfect
while on my bench, no matter what torture tests I tried.

The cause eventually turned out to be a loose fitting IEC plug that arced
under heavy vibration (when sitting on a cab) and made the AC power surge
over and over till the fuse blew. I used my own IEC lead in the workshop

so
initially I never saw the fault.

In another 100W Marshall, I found that if I turned the amp off and then

back
on again in 3 to 5 seconds, the inrush surge was much longer than usual

and
took out the 2A slo-blo fuse.

This was due to the tube bias voltage dropping fast and recovering slowly

in
the particular model, so the bias voltage was very low at switch on but

the
4 x EL34 cathodes were still hot and so all drew a heavy DC current for a
couple of seconds.

Marshalls do SOOOO many ****ing weird things I could write a book on them.



.... Phil



I have on more than one occasion replaced a Marshall PCB mounted mains
fuseholder that had dry jointed, arced, and blown several fuses the owner
tried.

Occam's Razor again.




Gareth.



Switches, wiring and fuses seem ok
I dummy load test at 8 ohms and this amp is used at 16 ohms. When I set it
for 8R no sound from the attenuated quality-monitor speaker. Seems ok in the
16R setting but perhaps when warmed from current over an hour goes open.
That event at more than half full output would presumably blow a fuse if not
an o/p transformer. These rotary sw are rivetted in , the 1970 version were
sockets and double pin jumper selector type.
2 balancing Rs added . If it bounces back again then I think I will try a
tell-tale in line with the main caps of a 1.6 amp fuse in parallel with
vitreous 1R 10W . Difficult to gauge what values so that the fuse will blow
in aberrant condition but not burn out the R if either mains or HT fuse
holds and not blow in normal seitch on and use. The minor HTs supply is more
straightforward , for such a telltale, as tempered by 100R dropper anyway
and cladding of that vitreous looks as new


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Default HT capacitor balancing Rs , necessity ?

Before adding the balancing R in the first 10 minutes the difference between
the 2 caps voltage went from 25V to 35V and still increasing 1V / min or so
before I decided that was evidence enough for some balancing.

The 4/8/16R switch is made by SWP with logo of a linearised S in a
rectangle. I would not want to rely on them for their supposed 250V/8A
rating. The fixed contacts are just punched holes in the tinplated terminals
and the moving contact, just a dome, path has 4 crude sprung surface
contacts along the way




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Default HT capacitor balancing Rs , necessity ?

I've just realised the mains voltage selector is the same make and design
just different markings.
That too is unreliable I now find concentrating on it, so now permanently
wired for 240V , no bad thing, in case someone mistakes this selector for
changing oputput for different cabs and in the UK.

At least 1970s and earlier loop-thru-tag and solder assembly has gone by the
board by 2005


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