Ashdown/Klystron Bass Magnifier (ABM500) first gen schematic

[mightysween]

Hi,

First post here after many years of lurking and learning.

I am repairing two "vintage" Ashdown Bass Magnifier heads that I bought DOA for $35 each.

I have one already running and gig-tested after a few simple repairs, but I am searching for a schematic from this era to help diagnose a preamp issue on the other unit.

There is a resistor (R25) blown... and the one I removed is a 150K ohm, but both the EVO ii and EVO iii versions of the amp use a 3M9 in the same position. The working unit has a 250K. So, obviously there was either a major change in the design between 1997 and 2000, or there was a reason the lower ohm resistors were substituted for the 3M9.

If anyone knows of a schematic for Clive Button's original ABM500 preamp (dated 1997), I would love to see it.

Then again, I still have DC on the speaker output anyway, so the preamp wont matter for awhile

[J M Fahey]

What do you mean by "same position"?
If physical position in the PCB, or even part label (say, R34), that means nothing, there are 1000 ways to layout and label the exact same circuit.
What matters is where-to where it goes, follow the tracks to see where they lead and draw just that litle bit of schematic, then we can probably guess its value with some certainty.

Also, what do you mean by blown?
pretty hard to do that to a 250k resistor, which is happy with some 250V across it and would take some 400 or 500V to actually *burn*.
And why donīt you just substitute another 250k there?

[mightysween]

The actual circuit trace is identical to the version from the year 2000 that has been posted here a few times. I attached a detail of it.

The 150K that I described as "blown" is showing infinite... not literally "burned". Yes, I will probably just test the preamp with another 150k, though I have no idea if it ever worked in the first place as configured.

I was just hoping to see if the original schematic shows a 150k (as the bad one has) or a 250k (as the fixed one has), or the 3M9 that shows up in all other iterations... obviously a huge difference in values there!

Edit... just noticed I wrote 250k in the OP instead of 150k. Probably makes more sense now

[Jazz P Bass]

The circuit as shown is called a 'common source amplifier'.

R25 gate resistor to ground serves two purposes:
1: it keeps the gate at 0Vdc.
2: It's large value prevents loading of the signal source.

Typically the value is in the megaohms.

[J M Fahey]

Agree and add: it is a non critical value, you choose it based on other preferences, all will work but sound slightly different.
47k/100k is fine for active Bass, 220/250K is as low as I would go with a passive one, , 1M is typical in most tube amps so most SS preamps also use that, specially thanks to Fet input TL072, amd 3M9 up to 10M is good for raw passive Piezo pickups.
Are you sure itīs open?
It might ba a misread 15M which of course is way off scale for most multimeters.

In any case use 3M9 and be happy.

Check Tr4 (J112) source (pin 2, check datasheet) voltage ,you should have about +2VDC there, give or take a Volt (yes, J112 is a cheap nondescript FET)

One doubt: donīt you have 2 equal heads, one working properly?

[Dave H]

I was just hoping to see if the original schematic shows a 150k (as the bad one has) or a 250k (as the fixed one has), or the 3M9 that shows up in all other iterations... obviously a huge difference in values there!

Check the value of the coupling cap C23. It's only 10n on the schematic. If R25 is 150k it will roll off the bass compared to 3M9.

[mightysween]

Check the value of the coupling cap C23. It's only 10n on the schematic. If R25 is 150k it will roll off the bass compared to 3M9.

Thank you everyone for the responses.

The C23 is, in fact, a 10n.

I got a nice email back from Clive Button himself, who confirmed that the intended value was 3M9... Ashdown may have been attempting a final "tuning" of the preamps using different values, which would explain why two amps 4 serials apart have two different resistors.

Plan is to start with the 3M9 and compare the signal to the working unit.

Thanks again

[mightysween]

Agree and add: it is a non critical value, you choose it based on other preferences, all will work but sound slightly different.
47k/100k is fine for active Bass, 220/250K is as low as I would go with a passive one, , 1M is typical in most tube amps so most SS preamps also use that, specially thanks to Fet input TL072, amd 3M9 up to 10M is good for raw passive Piezo pickups.
Are you sure itīs open?
It might ba a misread 15M which of course is way off scale for most multimeters.

In any case use 3M9 and be happy.

Check Tr4 (J112) source (pin 2, check datasheet) voltage ,you should have about +2VDC there, give or take a Volt (yes, J112 is a cheap nondescript FET)

One doubt: donīt you have 2 equal heads, one working properly?

Thanks, Juan. I double checked and it is indeed a 150K, (4 band, Br/Gr/Y/Gold). Isolated from the PCB, I get an open reading.


Yes, I do have one working to compare to... but it had a completely different resistor, which made me start wondering. The working one has a 250K, and seems to work very well with passive pickups. I am going to test a few different ones and see what changes, and hopefully learn something along the way.

UPDATE: measured +2.73 V on pin 2 of TR4, so all well there. Put a 3M9 resistor in R25 and it works... I am getting clean signal from the preamp output and jumpered it to the working head's output circuit to test, and it sounds great. Just need to diagnose the 60-70V on the speaker out and should be in business. Thanks again, everyone.

FYI for anyone interested, the behavior that was exhibited with the bad resistor was a loud "hum" and no signal from the input.

[J M Fahey]

Cool..
Most probably a shorted MosFet, measure them with power OFF and if two on the same side, then at least pull one to know which one is actually bad.
If none shorted, then next possibility is them being *driven* to +70V because of a driver malfunction, not too common but not impossible either.
Check you have both +/- rails too and that they actually reach the transistors.
Just for amp porn, add a couple gut pictures.

[mightysween]

Well, another step forward and two steps back.

Checked all transistors on the output board and found a shorted BF423 at TR7. Replaced, and no more 70 V on the speaker out..yay! But now that I have it all back together, the darn preamp isn't working again! It just has a constant 60Hz hum both on the preamp output and DI out, and no apparent signal from the input (or anything else down the line like the tube gain or tone controls) whatsoever. Darn. Guess I need to start over and check everything again.

Back to the drawing board... but getting closer to figuring it out.

Will post some pics... here is the full schematic, too.

[J M Fahey]

Well, good catch, because a shorted TR7 will do exactly that: pull output to the +V rail.

Be happy that expensive and not easy to find BUZ*** are fine.

I donīt worry too much about the preamp, if it was working previously then you probably lost sme ground or plugg some connector the wrong way, check that.

First make certain the power amp works, because thatīs the expensive repair.

[mightysween]

Well, good catch, because a shorted TR7 will do exactly that: pull output to the +V rail.

Be happy that expensive and not easy to find BUZ*** are fine.

I donīt worry too much about the preamp, if it was working previously then you probably lost sme ground or plugg some connector the wrong way, check that.

First make certain the power amp works, because thatīs the expensive repair.

The power amp is indeed working, though I haven't put it under much load yet.

Still totally baffled by what happened to the preamp. Tested it with the other known-working power amp and it behaves the same. Just dead, with that constant buzz. Maybe I physically damaged something moving it in and out of the housing so much.

Anyhow, thank you again... it may take awhile, but I will get this puppy running!