Music 500 -> Music 5000 and Music 500 -> Music 3000

[hoglet]

Although one thing I have noticed is the PSU voltage on the "good" M500 is 4.65V and on the "bad" it is 5.1V maybe the lower voltage prevents the runt pulses from having an effect? Perhaps more modern TTL is fast enough to "see" them?

These are both very plausible explanations.

[hoglet]

There are runt pulses on that 74138, but the other "good" M500 also has them and that somehow doesn't exhibit this problem.

I think generally runt pulses on the nS0..nS7 signals will not matter as:
- the signal is connected to a syncronous input (i.e. an input qualified by a seperate clock)
- the signal is OR'ed with the Phi1 clock before being connected to an aynchronous input

I think the only expection to this rule is where nS7 connects to the Async Clear input of IC3 (a 74LS74).

Dave

[maniacminer]

There are runt pulses on that 74138, but the other "good" M500 also has them and that somehow doesn't exhibit this problem. Although one thing I have noticed is the PSU voltage on the "good" M500 is 4.65V and on the "bad" it is 5.1V maybe the lower voltage prevents the runt pulses from having an effect? Perhaps more modern TTL is fast enough to "see" them?

Think I'd be wanting to try it out with 5V . . . Just to eliminate the low Voltage being the culprit. - Dave H.

I just soldered a 100pF capacitor on and left it at that. I've got a load more "dead" M500's to fix

[maniacminer]

There are runt pulses on that 74138, but the other "good" M500 also has them and that somehow doesn't exhibit this problem.

I think generally runt pulses on the nS0..nS7 signals will not matter as:
- the signal is connected to a syncronous input (i.e. an input qualified by a seperate clock)
- the signal is OR'ed with the Phi1 clock before being connected to an aynchronous input

I think the only expection to this rule is where nS7 connects to the Async Clear input of IC3 (a 74LS74).

Dave

Ah yes, I was following the schematic with kind of truth table and the window here is very narrow, so the capacitor seems to smooth out runt pulses and extend and delay genuine pulses by a tiny margin (nanoseconds)

Oh, I'm so glad I know I'm not losing my mind (although I'd have to find it first, I lost it in a rabbit hole sometime ago)

What amuses me is how many have gone like this and people don't realise? I think having some music tracks from known, good working systems points out that your unit is bad, rather than the Hybrid M500 being just shitty and chalked up to being 80s inferior technology. As when it is working 100% it is a very powerful synthesizer with lots of potential.

[maniacminer]

Now I have two working M500's (yay!) Here comes the next two... (all the space I have on the bench at the moment)

Another Bad M500.mp3

Another Bad M500 (Syncron)

(3.19 MiB) Downloaded 4 times

Yet Another Bad M500.mp3

Yet Another Bad M500 (Syncron)

(3.45 MiB) Downloaded 4 times

Bad M500s Combined.mp3

Yikes! This combination sounds bad...

(3.09 MiB) Downloaded 5 times

[maniacminer]

I pulled one apart today.

The usual suspects

I desoldered the usual suspects, but amazingly, these were all good. So I moved on to the rest of the logic. Then I popped IC35, IC34, IC33, IC31 & IC30. Cleaned them up and put them through the chip tester - IC31/74LS173 was bad.

First bad IC

I really wish there was a product that could take a board pull IC and then clean and align the pins. It takes longer to do that than it does to remove the IC from the board and clean the PCB...

[maniacminer]

I found the RAM has failed and that would explain the lack of wavetable synthesis.

[maniacminer]

After a long walk and a couple of pints, back to the M500...

Might as well pull the lot

Time to stretch your legs

Just need some glue!

[maniacminer]

Yay! Fixed.

PCB bottom

PCB top, just sockets

Installed into case

[maniacminer]

Onto the next M500...

Chips ahoy!

Chips straightened and cleaned ready for testing

Oh dear IC26/74LS374 bad

SRAM bad in IC25

Good vs. bad

[james]

How many Music 500/3000/5000 devices do you have?

[maniacminer]

How many Music 500/3000/5000 devices do you have?

6½ - three are now working, three and a half to be fixed. The half is just the board with no case (also doesn't work)

[maniacminer]

Whilst looking through the vast amount of M500/0 stuff I found a tune I've not heard since I heard an M5000 demonstrated decades ago, so with my three working M5000's all connected via a mixer, playing simultaneously, here's "Hooked on Bach"

3xM5000 Plays Bach.mp3

Three - if I keep going, I could have a choir! (this is Cambridge after all)

(8.88 MiB) Downloaded 14 times

[maniacminer]

So this is what the next two M500's sound like without doing anything to them. They are both playing the Bach piece from the previous post. Listen and weep

Bad M5000-1 - Plays Bach.mp3

Music 500 #1 plays Bach - yikes!

(8.89 MiB) Downloaded 7 times

Bad M5000-2 - Plays Bach.mp3

Music 500 #2 plays Bach - yikes!

(8.88 MiB) Downloaded 5 times

[maniacminer]

Putting the fourth M500 back together and come across a problem I've never had before... Pushing this IC in with my finger and "crunch"

Subsidence? Should have bought Texas Instruments!

I also discovered one of the 74LS374's popped out of the socket as its legs are too short. Eeeek!

Socket city

Lots and lots of soldering

[maniacminer]

Fourth tested and now all four play simultaneously
I can't hear any audible difference switching between them on the fly apart from slightly different levels and bass response.

The ensemble

[maniacminer]

Something I did wonder, the output filters have a very strong cut-off, making the high sample rate pointless. I'm just guessing that it is really brutal because of the artefacts introduced by having really tiny wave samples and then stretching them out over a long period leading to sharp transitions? I think I'll remove the capacitors in a unit section by section and listen to the difference. It's a shame the sample RAM is so meagre