Faulty Drumulator

[ambivalent]

Hi

I've just acquired a Drumulator which won't power up.

I checked the power supply and the 5V rail and 15V rail measure perfectly but the -15V rail measures 0V. I suspected that VR5, the 7915 regulator, may have failed so replaced it but the problem remained. On closer examination, VR5 is outputting -15V when I desolder the output leg from the board but it measures at 0V when soldered in.

I've tested for continuity and there seems to be a short somewhere between the -15V rail and ground. I've replaced C25 (the 1uF tantalum from -15V to 0V) and the local 0.1uF ceramic decoupling caps, C8, C15 and C57. Even with those four caps removed I get continuity between -15V and 0V.

Before I start scouring the schematics for other possible causes, can anyone make any suggestions? If it makes any difference, it's a Rev 2 PCB, 6/5/83. Serial number 5398.

Thanks

Greg

[dr.c]

Take a DVM having a contact function (NOT Ohm meter, contact or diode test), one probe on the ground, and the other at different ponts where the -15V should be.
You will find where it will beep, its going to be the faulty part.

Decoupling capacitors and ICs using the -15V are to be tested.

[Elmbeatz]

Take a DVM having a contact function (NOT Ohm meter, contact or diode test), one probe on the ground, and the other at different ponts where the -15V should be.
You will find where it will beep, its going to be the faulty part.

Decoupling capacitors and ICs using the -15V are to be tested.

The master ...   

[ambivalent]

Thanks. I'll have a go and report back.

[ambivalent]

Closer examination revealed that this problem is actually mentioned in the service manual: "We've had some problems with the 10 microfarad 25 volt blue tantalum capacitors shorting out... C60 for positive supply and C61 for negative supply are the possible causes." Sure enough, C61 was to blame. I replaced it (along with C60 for good measure) and now have healthy +5V and ±15V supplies. I had another spare cap so I also replaced C26 and I'll replace C7, C9 and the decoupling cap on the TO3 regulator when I get some more.

However, my problems aren't over yet. When I power the Drumulator up I now get a short burst of drums as I switch it on (it seems to be the same pattern each time – I can do a quick audio clip if it helps) but none of the LEDs light up and I can't get any more sound out of it. Can anyone offer any further suggestions?

Thanks

Greg

[gertie]

Hi Greg

I think you problem will be solved if you get a new Eprom with the OS in it...seems as if your OS is fried
I had this before too....boot up and hear about 6 different sounds appear in sequence....

you will probably get the same result if you boot the drumulator without the os eprom in it

you cound try resetting the os but i dont think this will help
(clear the memory by holding down ERASE and CASSETTE.  While continuing to hold ERASE press ENTER. A beep confirms that the data has been erased)

theres a guy in the UK selling the chips...
http://www.strellis.com/drumulator.shtml

otherwise if you know someone with a prommer get em to burn the os for you


hope this helps

[ambivalent]

Hi Gertie

Thanks for the advice. I tried the reset procedure but it didn't seem to do anything. Would the faulty OS EPROM also explain the fact that none of the LEDs light up?

I was actually already planning to upgrade to the 3.0 OS so I could use MIDI - if anyone's willing to make a copy of the OS EPROM for me I'd be happy to pay for it.

Thanks

Greg

[gertie]

try taking out the os eprom and see if you get the same behaviour......then you can almost know for sure its the os eprom.....
reset wont do anything if yer os is fried......but it was worth a try.....

[ambivalent]

Just tested without the OS EPROM and it appears to be the same. I've got a couple of leads on possible replacements so I'll try with a new OS 3.0 EPROM and report back.

[dr.c]

Do you have all the required voltages ?

Does the reset AT STARTUP (the automatic one) is working ? Eveytime you start the machine, a reset occurs, telling the CPU to way before cycling and start working. Check this !

[ambivalent]

All supply voltages check out fine. How do I test for the reset on startup?

[ambivalent]

I've now purchased a 2764 EPROM and had a copy of the OS burned to it but I still seem to have the same problem. I get a few sounds at startup (apparently in a different pattern than I did before), then nothing but hum.

When I turn the machine on with the new EPROM, the Value/Seg display lights up like this:



With the previous EPROM it would generally not light up at all, or occasionally just light a seemingly random segment or two. If I turn it off then back on again, the display doesn't light up at all unless it's been left 30 seconds or so. None of the LEDs other than the Value/Seg display have ever lit up with either of the OS chips. The reset procedure still doesn't do anything.

Is the most obvious answer that this new EPROM is faulty? I have no way of knowing for sure whether it's functional, but there's no obvious reason for it not to be. I used the binary file from this page - http://home.earthlink.net/~kerrybradley/id9.html - and had the chip programmed by a local electronics company.

Does anyone have any more ideas?

Dr C - could you explain about the reset on startup?

[ambivalent]

After a break and another restart, the display is now showing this: http://i40.tinypic.com/2earurb.jpg It's probably irrelevant but the fact it's changed may help with the diagnosis?

[dr.c]

THIS is a very good reaction.

What you have is apparently a bad scratchpad memory, or something polluiting the data/adress bus.
Do you have de the diagrams ? Do you have a scope ?

If the memory roms can be take away, shut the machine down, take them away, and restart the machine without them.
Don't worry, you won't kill anything.
If your machine works (or seem to have a coherent display), by putting back the eproms two by two, (any order) untill it fucks up again will give you the EVENTUAL bad eprom.
Did you reset your machine ?

[ambivalent]

Thanks very much Dr C. I really appreciate your help so far. I'll try removing the memory chips tomorrow and report back.

I have the schematics but unfortunately no scope. I've tried lots of times to do the POOF reset by holding down erase then pressing cass then enter, but it's never appeared to do anything. Is there another reset procedure?