Ashdown ABM500 bass amp

[N Cook]

Had been seriously dropped in a flight case and a number of problems, now
sorted so it now works but there is a fairly low level background hum
problem with no input.
I have the schematic from Ashdown, but is anyone familiar with them ?
All inputs and outputs and pots (exept the pot casing) are electrically
isolated from chassis, on schema and in reality.
Just the mains inlet earth bond point to the chassis.
The pa has a nominal "grounding" point of // 100nF + 10K, 1/3W R
and the pre has the same to chassis and the signal line "ground" between
them.
So about 5K between real ground and nominal ground.
Disconnecting the signal lead, the pa o/p is clean.
Maybe a smoothing cap problem on the pre-a secondary ps , but before I take
it all apart again is there anyone familiar with this sort of arrangement.

 

[Ron(UK)]

Had been seriously dropped in a flight case and a number of problems, now
sorted so it now works but there is a fairly low level background hum
problem with no input.

How low level? They are not the worlds quietest amps, if might be
normal. Does it still hum with a shorted input connected? Does the hum
level change when moving the volume control?

I have the schematic from Ashdown, but is anyone familiar with them ?
All inputs and outputs and pots (exept the pot casing) are electrically
isolated from chassis, on schema and in reality.
Just the mains inlet earth bond point to the chassis.
The pa has a nominal "grounding" point of // 100nF + 10K, 1/3W R
and the pre has the same to chassis and the signal line "ground" between
them.
So about 5K between real ground and nominal ground.
Disconnecting the signal lead, the pa o/p is clean.
Maybe a smoothing cap problem on the pre-a secondary ps , but before I take
it all apart again is there anyone familiar with this sort of arrangement.

You can easily bridge the smoothers to prove them.

I`ve seen lot`s of dry joints on Ashdown stuff, it`s a fair bet that
there could be hairline cracks in the print, also check pots for true
continuity across the outer terminals, I`ve seen tracks go o/c at one
end through flexing

Ron(UK)

 

[N Cook]

Had been seriously dropped in a flight case and a number of problems, now
sorted so it now works but there is a fairly low level background hum
problem with no input.
I have the schematic from Ashdown, but is anyone familiar with them ?
All inputs and outputs and pots (exept the pot casing) are electrically
isolated from chassis, on schema and in reality.
Just the mains inlet earth bond point to the chassis.
The pa has a nominal "grounding" point of // 100nF + 10K, 1/3W R
and the pre has the same to chassis and the signal line "ground" between
them.
So about 5K between real ground and nominal ground.
Disconnecting the signal lead, the pa o/p is clean.
Maybe a smoothing cap problem on the pre-a secondary ps , but before I take
it all apart again is there anyone familiar with this sort of arrangement.

I forgot to say that shorting the preamp 10K then the hum goes.
The input 1/4 ins shorting contacts, with no input jack, are ok

 

[N Cook]

How low level? They are not the worlds quietest amps, if might be
normal. Does it still hum with a shorted input connected? Does the hum
level change when moving the volume control?
arrangement.

You can easily bridge the smoothers to prove them.

I`ve seen lot`s of dry joints on Ashdown stuff, it`s a fair bet that
there could be hairline cracks in the print, also check pots for true
continuity across the outer terminals, I`ve seen tracks go o/c at one
end through flexing

Ron(UK)

Driving into 8 ohms , with shorted inputs then at max volume setting then
0.7V on DVM ac range, dropping to 0.002 V for min vol.
It is more intrusive than I would normally expect but I don't have any
figures for what is normal.
Putting 1K across the pre amp 10K then the hum level drops to a much more
acceptable 0.1V. What are the safety issues with this sort of arrangement ?
They are only 1/3W , 10K so if either end should go to 240V they would not
last long or is that the point. They are not marked on the schema as
fuseable resitors nor on board marked ! in a triangle.
With this sort of arrangement is static discharge leading to electronics
destruction if guitarist has plastic shoes on nylon carpets ?

 

[Ron(UK)]

Driving into 8 ohms , with shorted inputs then at max volume setting then
0.7V on DVM ac range, dropping to 0.002 V for min vol.
It is more intrusive than I would normally expect but I don't have any
figures for what is normal.
Putting 1K across the pre amp 10K then the hum level drops to a much more
acceptable 0.1V. What are the safety issues with this sort of arrangement ?
They are only 1/3W , 10K so if either end should go to 240V they would not
last long or is that the point. They are not marked on the schema as
fuseable resitors nor on board marked ! in a triangle.

If I understand you correctly, one end of the 10K resistor is connected
to mains/chassis ground, so it would be most unlikely to get 240v across
it. If it were a fusible, in the event of it failing it would remove the
earth connection, which isn't what you want in a fault situation!

With this sort of arrangement is static discharge leading to electronics
destruction if guitarist has plastic shoes on nylon carpets ?

Shouldn't happen, because the guitarist should be grounded via his
guitar strings. I've never noticed any manufacturer of group equipment
address a problem of static discharge emanating from the muso.

Ron(UK)

 

[N Cook]

If I understand you correctly, one end of the 10K resistor is connected
to mains/chassis ground, so it would be most unlikely to get 240v across
it. If it were a fusible, in the event of it failing it would remove the
earth connection, which isn't what you want in a fault situation!


Shouldn't happen, because the guitarist should be grounded via his
guitar strings. I've never noticed any manufacturer of group equipment
address a problem of static discharge emanating from the muso.

Ron(UK)

At least with that 5K for dissipation, then any obvious and can be alarming
perfectly harmless static discharges, won't be even noticed which would be
an advantage
Looks as though its a hairline track failure, rather than rivet/track
failure on the most awkward to replace miniature double track 50K log ,
solder tag pot and slap up against the tone control board for even more
physical constraint for replacement.
Probably be a double 100K with 20K "log" resistor added for this clean/grind
pot.
Another sort of problem with this faux ground arangement.
I thought it odd how long it took to drain the 450V caps and the main
reservoir caps before I previously dismantled all the boards. This time I
connected the 20 ohm resistor to faux ground rather than through 20 +
10k//10K to real ground

 

[Meat Plow]

Had been seriously dropped in a flight case and a number of problems, now
sorted so it now works but there is a fairly low level background hum
problem with no input.
I have the schematic from Ashdown, but is anyone familiar with them ?
All inputs and outputs and pots (exept the pot casing) are electrically
isolated from chassis, on schema and in reality.
Just the mains inlet earth bond point to the chassis.
The pa has a nominal "grounding" point of // 100nF + 10K, 1/3W R
and the pre has the same to chassis and the signal line "ground" between
them.
So about 5K between real ground and nominal ground.
Disconnecting the signal lead, the pa o/p is clean.
Maybe a smoothing cap problem on the pre-a secondary ps , but before I take
it all apart again is there anyone familiar with this sort of arrangement.

Familiar with a dropped amp? I've worked on a few but certainly wouldn't
expect anything I've come across to be anything besides random damage.

 

[N Cook]

I forgot to say that shorting the preamp 10K then the hum goes.
The input 1/4 ins shorting contacts, with no input jack, are ok

I've probably found the source of generic hum noise problems in Ashdown amps
with this faux ground structure.
The problem pot marked with a "B" meant it was linear and not log so a
replacement easily found.
Taking the original apart there was no broken track or rivet /paxolin
failure, stressing/bending paxolin and measuring etc showed nothing
untoward.
But I did notice the track was offset, ie not coaxial to the paxolin base.
The noise intrusion was most prominent at about 2/3 and at that point due to
the off centre track and slight ovalling of the outer pot casing the
track/wiper at this point was very close to the casing.
For the replacement pot I bent back the four tangs that hold the middle
casing/housing section of the pot to the bushing plate, introduced four bits
of insulation, closed back on them and glued in place.
So the casing is now isolated from the spindle and bush which is still
properly grounded to the chassis.
Now normal hum level with shorted input and maximum vol/gain settings
varying from .04V clean to 0.07V ac on full grind setting. So factor of 10
or better improvement just from isolating that one pot casing and presumably
more centrally aligned pot tracks perhaps to lesser effect.
The 2 x 10K faux to ground resistors still as the original design.

I wish there was a proper techie/muso translator somewhere ,ie full
translator techie-speak for techies and muso-speak for musos . What is grind
? in terms you can see on an oscilloscope for instance, not the usual muso
speak.

 

[N Cook]

eg such as this useless gobbledy-gook
'Grind' is designed to deliver a modern rock sound and emulates the sound of
a distorted Sansamp PSA1 fed into an Ampeg SVT, but with the ability to use
the Drive control to mix in the clean sound of the guitar.

I want to know, in this instance, if you feed say a triangle wave in what
comes out on full grind?

 

[N Cook]

One of those annoying self-made, in a sense, problems also associated with
the faux earth.
Trying the amp with the equalisation board free of the chassis was fine but
mount it back in and horrible mains noises.
To save messing about I was temprarily placing the board back in without the
3 spacers on the 3 pots. These pots have the casing to chassis earth but the
4 slider pots have the cases connected to faux earth. Without the spacers
then the slider cases came too close to the chassis.