Fuse Blows on Shutdown-D'mer M500 REPAIRED!

 I was given a Dr*wm*r M500 that I have been trying to restore. It originally blew fuses immediately on power up.
After replacing the + and - 15v regulators and a tant cap that read shorted with a DMM, it works, but now blows the fuse on power down.
  Does this mean I haven't really found the original source of the problem?
  I have no schematic, as D'mer does not wish to give them out.
  Once the parts were replaced, it did a self-diagnosis, and works perfectly, other than blowing fuses. It looks like the front right of the case took a big hit at some point, which bent the rack ear back to quite a degree (now fixed), and seems to have been stored near a window during it's downtime, so I had to cut a notch in some of the lid screws to remove the top and bottom covers, as the screws were a bit corroded. Two (of 4) LED/TL072 assemblies inside the case had popped out of their sockets, but were easily replaced. The memory battery had corroded, but was smartly encased in plastic and did little damage. I replaced this with a coin cell and replaced another 'lytic that was nearby.
  I also replaced all the 'lytics in the PSU, but with so-so quality caps (Xicon), as I wasn't sure I could get the unit running, and didn't want to spend too much.
  Otherwise, the build quality is excellent, and the interior is clean. I have inspected the PC with a magnifier, and can find no cracks or bad solder joints.
  Does anyone have any experience with fuses blowing on power down that they could share? If I can make this reliable, it will be one of the nicer pieces in my humble studio.
  Thanks in advance for any suggestions. This forum is excellent, and has taught me so much.
Jim

Just thought I'd send this around one more time.
Fuse blowing on shut down seems like it should be a PSU problem.
Any clues would be appreciated - I would love to rack this and start using it, it sounds good...

What's the PSU?

Is it linear? (transformer/rectifier/caps)

Which fuse is popping- is it in the secondary centre-tap of the transformer?

Check the bridge rect- could be leaky diodes letting the charge from the reservoir caps back through the transf. windings and popping the fuse. Any strange mains filter in the incoming side? Is it a Slow-Blow ("T") fuse?

Interesting fault!

Mark

Would you believe - bad power switch?  If they are switching both the hot and neutral sides with a DPST switch, the power switch could be shorting, causing the fuse to blow.  Just a wild guess. I've seen crazier things than this!

Joe

Mark and Joe,
  Thanks for your interest and ideas. Here's a photo.
  I tried shutting it off the last time by using a power strip on/off switch, instead of the front panel, and it still blew the fuse. Not sure that's a scientific test, but I had suspected the power switch also, as it was near the side that took the hit.
  160ma Slow-blo fuse at 110V. Only one in there.
  Two 2200uf 'lytics for filtering, and an axial 4700uf that I replaced with a radial, 'cause that's all I had. Two diode arrays for rectifiers (1N4002), one on either side of the trannie, and kinda hard to see in this photo. I suspect the diodes, too, but have been too lazy to unsolder them all to check with my DMM. I guess that's next. None seemed to be shorted when I checked them in circuit.
  I ran it for almost two hours the other night, listening and trying all the front panel buttons, and it seems to work perfectly, other than blowing fuses. Down to my last 3 fuses of 10...

My M500 had a defective display illumination which caused the fuse to get blown. Try to desolder the rectifier bridges for all voltages one after another to find out which part of the circuit is giving you the trouble. If all are removed and it still happens I´d suspect the switches but I bet it´s the display PSU which causes trouble in many M500s.

Thank you jensenman,
  I think you might be onto something here. The display has been dimly lit, but I assumed it was normal for a device of this era.
  Thanks to the suggestions made previously, I was looking closely at the power supply last night, and became aware of the voltage regulator that powers the display (7805). It is attached to the side of the case near where the unit was struck, and I think I will just replace it, since the part is only 80 cents at my local surplus store. The diodes are not that expensive, so I might as well do those also, if I'm going to unsolder them anyway.
 

  It was more of a mechanical problem than electrical.
  Turns out that bracing for the enclosure was touching a solder connection for the data wheel on the bottom of the PCB. This was a result of the shock that the device suffered at some point before it came to me.
  Two screws had been sheared off, leaving the bracing unsecured, and it had slipped into a position where it could touch the solder joint.
  I found some screws that fit, secured the bracing, and it has powered up and down about ten times so far with no problems.
  The display is still a bit dark, so perhaps I need to look into the voltage regulator there, but I am very pleased to have this up and running in my studio.
  Thank you all for your help. You sent me in the right direction, and saved me some time.
  If you're inclined to listen, here is a My Space page with some mixes done on my rig (pre Drawmer). They are live recordings of a band I have that plays (among other things) the music of Frank Zappa.
www.myspace.com/crepusculewithfido
    These particular recordings are a good example of mods I did on a pair of Octava MC-012s that were used to make the recordings of the band, though some overdubs were made later. If you listen to the drums, you should be able to tell which recordings were made with the modded mics. I really liked the mics before I replaced the capsule capacitors, but now I LOVE them...
Thanks everyone!
Jim

Anyone know where best to get the internal software re-flashed on an M500 and is it worth it?

IIRC there were a lot of different revisions of the M500, both on hard and software side. If your eproms are dead, then you need to find someone who has the same Version as your unit has, to be sure that it will work. It´s possible to read eproms out and burn them on new and working chips - if you can source them. Some of the 80s chips are not so easy to find on a budget.
Did you contact Drawmer UK? They are usually very helpful.