John Bowers Active Control Unit preamp: Info sought

[entropyagent]

Hi DIYAudioers


I hope my question is appropriate for this forum and this subsection - if not, please delete.



I have one of these preamps that appears to be unwell. I have no electronics experience, and the technician I took it to, says it seems to be impossible to disassemble so that the bottom of the pcb is visible. The top and side covers seem to be a single folded sheet which is easily removable. However, the rest of the case (bottom with front and back panels) is also a single folded sheet, with various pcb-mounted switches and jacks attached to it. It might be possible to rip components out of the board. but would be difficult to solder replacements into position. I have been assured that this will make it impossible to isolate the fault, without major surgery to the case.


So.....are there any arcane tips or tricks, magical incantations, secret levers or switches that could unlock this ancient and mystical device? Or was it designed to be disposable?


Even if we were able to expose the entrails, is there a service manual or schematic available that could be used to interpret the omens and portents thus revealed?



I seem to recall that it was quite fine-sounding, when it was working, but is it actually worth the struggle, perhaps removing the front (with glued-on faceplate) and rear panels with an angle-grinder?


Also....it has a 2-pin 'wall-wart/power-brick' box with captive lead. This has half a faded sticker which seems to mention a number of voltages, i.e. 220/240 V AC and 15-0-15 V AC. Does this mean the brick is merely a transformer? What could I replace it with? (One of the mains plug pins is wobbly)
Is 15-0-15 V AC the same as 30VCT? They seem quite rare.


The label says something like
CAUTION
Do not tamper with this unit. Lethal voltages internally.
?5VA Transformer (15? 115? 75? probably 15VA)
For? use only with John Bowers Active Control Unit 1
?ary : 220/240 V AC
?ary : 15-0-15V AC (I'm almost certain the 3rd number is 15)
The rest is illegible/missing.



Can anyone please help with documents, solid info, pointers or experience?

I would be eternally in your debt for at least 5 minutes.


Thanks and Regards
Paul

[Mooly]

Maybe post some pictures of it internally for a start. Unless its been welded together disassembly should (famous last words) be straightforward.

The PSU could be a 15/0/15 Vac tranny. Have you done any measurements on it to see what voltages emerge ?

[entropyagent]

Hi, thanks for the response. Struggling a bit to load pictures, looks like the process crashed while trying to create an album on diyaudio, and loading pictures into it.

oops....just saw the "Manage attachments" button....and the "max filesize" section....some resizing needed.

I don't know how to measure the output of the power supply. It has 3 wires, red, blue and yellow. Should I just set my DMM to ACV 750 and expect 15V between..er...red and blue, and between yellow and blue?
Should I disconnect the power supply from the pre while doing this? It will involve some desoldering, it's hardwired.

[Mooly]

It looks very straightforward... if it needs pulling apart we can come to that later.

Measure initially with all the wires intact.

From what I can make out there appears to be 3 pins on the PCB (near the on/off switch) marked as 0, 15 ?? and 15 with a yellow wire going to 0 and two blue wires to the pins marked 15.

So first test... and I can't see any rectifiers (diodes) near this area so we have to test to see if either AC or DC is present.

Set your meter to an AC volts range (750 range is OK to start) and measure between the yellow to each of the blue wires on those pins. Keep the black lead of the meter on the yellow pin for these tests. If its AC you will see around 15 volts on each blue pin with respect to the yellow.

If you don't measure anything or the readings are low then we see if DC is present.

Set your meter to a DC volts range (say 100 volt range) and again measure between the yellow to each of the blue wires on those pins. Keep the black lead of the meter on the yellow pin for these tests. If its DC I would expect (from the markings) a reading of plus 15 or so volts on one pin and minus 15 on the other.

So that's the first step. Is the unit dead ? I don't think you mentioned what the actual fault was.

Also, can you make out the device type markings of those two black, 8 legged integrated circuits behind the volume control. If we know the type we can identify the supply pins and see whats what

[entropyagent]

Hi

Thanks for your interest. This must be like playing multiple games of blindfolded 3D chess via snail-mail.

Some responses :


the 2 arachnids seem to bear the legends "NE5534AN" and "S 8419". There is some white script on the PCB near them
I think the relevant bits are IC2R for 1 and IC2L for the other. (There's also a C27R and C27L, close by, I hope that refers to
the adjacent little orange M&M thing.)

There are 4 other arachnids, all are inscribed with "M340" and "LF353N".


Looking from the front....LHS is LeftHandSide, RHS is RightHandSide

The 'power' lead from the power supply has 3 wires, a red, and blue and a yellow.

The red goes (via the On/Off switch) to 15V closest to the middle of the device (a.k.a. RHS 15V)
The blue goes (via the On/Off switch) to 15V closest to the edge of the device (a.k.a. LHS 15V)
The yellow goes direct to 0V.

DMM is MajorTech MT845 with a new, freshly changed battery

200 ACV:

Yellow to LHS '15V' 3.8V sometimes 3.9V
Yellow to RHS '15V' 3.5V sometimes 3.8V


20 DCV:

Yellow to LHS '15V' -9.15V (negative 9.15V ??)
Yellow to RHS '15V' -9.2V (negative 9.2V ??)

I don't know how trustworthy that '-' minus sign is....even just switched on, with no measuring happening, the '-' comes and goes(-0.00 to 0.00)


The voltages I measure, seem to fluctuate.....I'm assuming the 'highest' number (disregarding the '-') is the most accurate, with me making better contact. I am scared of slipping off the solder and spearing vulnerable bits with the probes.


The (2-pin) power brick has a loose(ish) pin....and, being 2-pin, it can take a little while to settle it firmly in the socket.
The umbilical has a break which was messily soldered up long ago(not by me). Does it seem as if it is worth tidying it up?

[Mooly]

Your meter flipping between - and + when showing approx. zero (0.00) is normal.

The NE5534 and LF353 are "opamps" and have their supplies on pin 4 and pin 8.

The readings from the power supply seem odd (and it may well be a faulty PSU) so the next check is to put your meter on the 20 volts DC range and measure between ground (use that big knurled nut on the rear panel) and pins 4 and pins 8 on those opamps. Keep the negative lead of the meter on the ground point.

What voltage do you measure on pin 8. What voltage on pin 4 ?

(those voltages from the PSU look wrong because if it were AC from the adapter then 3.9 volts AC is far too low to work with, and if its DC then there should be a plus and a minus rail with respect to the yellow. And your measuring two negative voltages. Everything points to the PSU being faulty at the moment)

[entropyagent]

I looked up the pins here http://circuits.datasheetdir.com/25/...534-pinout.jpg

and got this in the 20 DCV range, for both opamps - there is some unsteadiness, perhaps due to my trembling hand:

Pin 4: -14.1V <> -14.3V
Pin 8 + 2.6V <> + 2.7V



OK, some random fiddling and slight change in symptoms.

Turning the power brick over in my hands, seeking an attack vector, I noticed what appeared to be a movement of a weight inside. I would not be surprised if it turned out to be the alien larva that secretes the magic smoke stirring in repose, or perhaps a loosely mounted small transformer. Pure conjecture, of course. Deciding to pursue the loose transformer theory, I runned the power strip upside down and pressed it down on the power brick, which lay on its back with the plug pins pointing up. I then did the measurements over.

0V 15V 15V wires:
----------------

200 ACV:

Yellow to LHS '15V' +15.2V
Yellow to RHS '15V' +15.2V


20 DCV:

Yellow to LHS '15V' -0.14V approx
Yellow to RHS '15V' -0.14V approx



NE5534AN" and "S 8419" Opamps :
------------------------------

20 DCV range, for both opamps - there is some unsteadiness, perhaps due to my trembling hand:

Pin 4: -14.1V <> -14.2V
Pin 8 +11.8V <> +11.9V (LHS seems slightly higher)


While moving power strip and brick into position, the pre (with nothing else connected) sometimes makes a gentle but disturbing 'tick' sound, perhaps as bits jostle for position inside the brick. However, once everything does settle, the pre is silent.

Interestingly enough.....the left channel on the pre now seems to work. The right channel is still silent. Except......
Both left and right 'pre-out' channels give the standard horrible 'missing earth' shout through the speakers, if the pre-to-power
plugs are re-arranged by someone too lazy to switch off the power amp. If the RHS pre-out is connected to the power amp, there is no sound...except...if I touch the multimeter probe to one of the copper bars in the pre-out internal chassis-mount assembly AND the one of the copper bars in the adjacent 'tape-rec' internal chassis-mount assembly, then I get a faint musical signal through the speaker. This signal does not seem to be affected by the volume control.

I tried to show this bridging with a red line....please ignore the blue wire which refuses to move. I'm guessing the bridge is Pre-out's Right channel
hot to Tape-rec left channel hot. Just a guess, though.


So...does this support your theory that the power brick is unwell? I'm scared to move it....
Is it safe to hypothesize that I could benefit from a new 15 - 0 - 15 ACV brick? Is there anything special I should look out for in seeking a replacement?

Or is there still a concern that there should be a -15V somewhere? (I interpret your earlier comments to mean that this would be only if the brick was supplying DC)


I have the idea that the 'missing channel' problem is why I stopped using the pre, long ago. Will I need access to the bottom of the PCB to fix this? I suspect there's no happy ending here, if that's the case.

[Mooly]

There's a lot going on with this one There may be more than one problem too.

Something loose in the PSU isn't normal so that needs looking at. Is it a glued together thing or does it come apart ? Any screws might be under stickers etc

I think the PSU has to be the first line of attack. From what you have shown you should be seeing -/+ 15 volts from the PSU and you are not... but it sounds like there are other problems too.

The 5534 opamps should have "equal and opposite" supplies. It seems as though 15 volts is the voltage this preamp runs on and so you should see plus 15 on pin 8 and minus 15 on pin 4 measured with respect to ground.

The PSU isn't an off the shelf item as it appears to be a -/+15 volt unit (that's DC).

[entropyagent]

Thanks for the feedback, sorry about the delay, been bit distracted recently.. You say the brick is broken, but....is this best-case measurement not a sign that sometimes the brick is working OK?

0V 15V 15V wires:
----------------

200 ACV:

Yellow to LHS '15V' +15.2V
Yellow to RHS '15V' +15.2V

OK, anyway, trying to open the brick. It was obviously sealed by someone who hates me. It seems to have a bottom plate with a top shell, both plastic. The bottom plate has 3 'spots' which might be screw-holes filled with plastic, like one might use plastic wood to the heads of woodscrews in furniture.

I used a grinding bit attached to an electric drill to try to get to one of the screws - looks like I found one. Bit late to be operating a drill now - hope to get to the others later this week.

[Mooly]

Lets try and get a handle on what is going on here because some of the readings seem very variable.

Your saying with the meter on 200Vac range that you measure 15 volts between the yellow lead (using that lead as the common) and the other two from the brick. Those readings sound on the face of it reasonable, but are different to the 3.8/3.9 volts reported in post #5. Have we an intermittent problem with the brick or not ?

Lets say that as long as you can read 15 volts from the brick, then we'll say that its working "at that moment in time" and we can try a different approach.

Do you see the two 3 legged devices (that have metal tabs on them) located between those four large blue electrolytic capacitors. They look like voltage regulators.

Can you make out the type numbers on them ? If we know the devices we can see if the voltages are correct or not. The regulators could be something like 7812, 7912 or LM317 and LM337 as an example.