Ashdown ABM500 bass amp

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?

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)

take

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

now

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arrangement.

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 ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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)

electrically

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then

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arrangement ?

not

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

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

now

arrangement.

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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?

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on