Search found 194 matches

by wahee
Thu May 07, 2020 7:17 am
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 CPU voltage rail

just a quick message to say thanks for all the great help on this. I have decided to put this one to bed. I have not been able to locate the source of this unusual issue. I have basically replaced the entire PCB, even 74 IC's which are usually reliable, and checked traces until my eyes hurt,but some...
by wahee
Sun May 03, 2020 1:29 pm
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 CPU voltage rail

Is it sometimes solidly high? yes but mostly not I have replaced all the IC's in the reset area and beyond, without any change unfortunately. I even carefully removed the socket for IC43 to ensure there was no lifted pads/damaged traces etc but all was ok Yes the 6116 was IC17. The other one on the...
by wahee
Thu Apr 30, 2020 11:45 am
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 CPU voltage rail

Well at the risk of looking like a monkey trying to operate the international space station, I decided to do some probing around the reset circuit. IC42 pin 3 always goes high when power switched on As does pin 2 IC43 pin4 MREQ – nearly always has a square wave present when power switched on. Howeve...
by wahee
Wed Apr 29, 2020 8:57 am
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 CPU voltage rail

some images of the PCBs just for info (with the Zilog 8mhz Z80) PENT1772.JPG PENT1773.JPG it's bizarre that the Zilog 8mhz CPU will not boot at all with the Z80 ACTC IC installed The D780-C1 will boot ok with it all will boot with the ATC version installed although very patchy. It's very hard to get...
by wahee
Wed Apr 29, 2020 8:52 am
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 CPU voltage rail

One thing you should check since people have been bodging it is that one of the RAM or ROM pins has not got folded up so it looks like it is in the socket or is touching it intermittently. Then probe the whole address and data bus pin by pin looking for funny mid way levels caused by a short. Once ...
by wahee
Tue Apr 28, 2020 9:59 am
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 CPU voltage rail

I set the scope up as follows. Trigger - single, edge, trigger - CH1 mode - normal coupling DC then used CH1 probe on pin 26 RESET. scale is 1MS x Axis here's what I got OScar.jpeg Thanks Graham for those schematics, they're much better than the ones I have. The crystal is now apparent. P is just th...
by wahee
Mon Apr 27, 2020 4:52 pm
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 CPU voltage rail

I have ch2 BLUE on MREQ pin 19 and CH1 YELLOW on RESET pin 26 trigger normal, trigger CH2 when I turn on, a lot of times there is no reading. Other times there is a reading but no boot then it can boot with a reading here is a video of a successful boot, this may show nothing useful as I really don'...
by wahee
Mon Apr 27, 2020 6:05 am
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 CPU voltage rail

Do you see it go low and high on power up? yes on the scope the there's an initial glitch as the button is pressed and then the line goes high after a very short delay. Clock is always present and stable. Very interesting about IC43 used as a comparator, I remember dealing with LM311's in other syn...
by wahee
Sun Apr 26, 2020 6:13 pm
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 CPU voltage rail

Trigger the scope on /MREQ going low. I’ll check this out about how to rig this. Just got the scope so will need to read the manual No, you are being told this by somebody who doesn't understand themselves. The microcode is fixed in the CPU design. What it will do is initialise the RAM for vectors ...
by wahee
Sun Apr 26, 2020 2:14 pm
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 CPU voltage rail

hi thanks again for all the input. I had a look at D0-D7 and there is data, it's hard to see as it's moving around on the scope (Owon SDS1102) I'm such a massive noob with this scope Battery is good as it's a new Varta mempac 150mAh and was fitted last time the synth was serviced. It reads 3.9v with...
by wahee
Sun Apr 26, 2020 10:05 am
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 Pu voltage rail

Hi, I guess the synth is at least 30 years old using the Z80 processor. I have had a couple of problems with old equipment where the processor crystal fails completely or goes 'lazy' and does not always start up and oscillate, that might be the problem you are experiencing. If you have access to an...
by wahee
Sun Apr 26, 2020 10:03 am
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 Pu voltage rail

EATyourGUITAR wrote:
Sat Apr 25, 2020 10:36 pm
price tracker says $4000

https://reverb.com/p/oxford-synthesizer ... ynthesizer
yep, it's an amazing sounding synth, but horrible for reliability - at least mine has been
by wahee
Sun Apr 26, 2020 6:47 am
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Re: Z80 P voltage rail

Hi That’s so much for all the great replies. I should add that I have swapped out the process for a known good and it was even harder to start?! :doh: The power supply, which is very basic (5v,-5) has had all the 2200uf and 1000uf changed out I have just literally bough a cheapo scope which I used t...
by wahee
Sat Apr 25, 2020 3:25 pm
Forum: Music Tech DIY
Topic: Z80 CPU voltage rail
Replies: 37
Views: 876

Z80 CPU voltage rail

hi all I have an OSCar synthesizer with a weird issue. It will only boot up sometimes. I have done all the usual things like replace/check battery, reload sysex factory patches. The CPU a NEC D780C seems very flaky and prone to not working. I tested the digital 5v rail and it was 4.8v. \this just co...
by wahee
Mon Mar 16, 2020 11:27 am
Forum: Music Tech DIY
Topic: just bought a sick Kork Polysix, progress
Replies: 15
Views: 678

Re: just bought a sick Kork Polysix, progress

weird problem is that the scale of the synth seems shifted but perfect i.e. key C is actually D, can't adjust with tuning. I checked the P1501 optocoupler and it seems ok, at least it has been changed at some point and the voltage are ok makes calibrating it a bit of a pig UPDATE, the darn pitch whe...
by wahee
Thu Mar 12, 2020 4:42 am
Forum: Music Tech DIY
Topic: just bought a sick Kork Polysix, progress
Replies: 15
Views: 678

Re: just bought a sick Kork Polysix, progress

well ordered an OWON 100mhz 2 channel scope. Real cheapie but seems ok for my needs. So will have a bash at some calibration. At least the envelopes etc. May need a frequency counter or a app as well
by wahee
Thu Mar 12, 2020 4:39 am
Forum: Music Tech DIY
Topic: just bought a sick Kork Polysix, progress
Replies: 15
Views: 678

Re: just bought a sick Kork Polysix

I checked the caps on the voice board and most of the 10uf ones were 12.5 and heading south, so replaced those. ESR or capacitance? if that is capacitance then that is close to %20 spec it is actually %25 no worries. if that is ESR also I think that would be fine and probably top rate for ESR. capa...
by wahee
Wed Mar 11, 2020 6:25 am
Forum: Music Tech DIY
Topic: just bought a sick Kork Polysix, progress
Replies: 15
Views: 678

Re: just bought a sick Kork Polysix, progress

yes was thinking about a frequency counter, I really need a scope though for some of the other measurements, just to get them spot. Also I could use it on my other synths, specifically the Prophet 5
by wahee
Tue Mar 10, 2020 4:53 am
Forum: Music Tech DIY
Topic: just bought a sick Kork Polysix, progress
Replies: 15
Views: 678

Re: just bought a sick Kork Polysix

hi all and thanks for the replies. Well I have been working hard on the P6. Checked power rails and the were good. Keyboard was 100% The CPU board had already had some repair work done. Battery was changed. It was shoddy but electrically sound. I replaced the wire jumpers with a better job and also ...
by wahee
Mon Feb 17, 2020 3:02 pm
Forum: Music Tech DIY
Topic: just bought a sick Kork Polysix, progress
Replies: 15
Views: 678

just bought a sick Kork Polysix, progress



this

should be a 'fun' project.
There were pictures of the CPU board and the battery looked like it had been changed but maybe not very well.
I should have the synth in a week or 2
by wahee
Mon Feb 10, 2020 4:36 am
Forum: Music Tech DIY
Topic: NOS small value capacitors, worth it?
Replies: 5
Views: 327

Re: NOS small value capacitors, worth it?

canister style radial aluminum jobs.

I knew the caps were NOS, I was just a bit surprised to find out how old they were
by wahee
Thu Feb 06, 2020 8:16 pm
Forum: Music Tech DIY
Topic: NOS small value capacitors, worth it?
Replies: 5
Views: 327

Re: NOS small value capacitors, worth it?

hi thanks for the reply. Yes mostly for my own kit and to repair old gear I have. Not sure on storage, as they were from an electronics surplus supplier. I was repairing an old Soundmaster STIX ST-305 drum machine and it has quite a few in the audio section. I removed them and found they were actual...
by wahee
Thu Feb 06, 2020 6:12 am
Forum: Music Tech DIY
Topic: NOS small value capacitors, worth it?
Replies: 5
Views: 327

NOS small value capacitors, worth it?

I was able to get a large quantity of capacitors with a seeming date code of 1982. I don't recognise the symbol on the case to get a manufacturer ref Small value 1uf 35v polarised electrolytic capacitors I measured a few and capacitance was around 1.1 to 1.2uf Slightly higher than a modern equivalen...
by wahee
Thu Feb 06, 2020 5:59 am
Forum: General Gear
Topic: VINTAGE sTIX st-305 DRUM MACHINE ISSUE
Replies: 4
Views: 285

Re: VINTAGE sTIX st-305 DRUM MACHINE ISSUE

yes just finishing it off and will post a link.

Turns out the TA75558 dual op amp is dead - hence the low volume.It's still too low though and noisey

It's tricky finding out where the voices all sum to go to the op amp with all the bodges on the PCB. All pat of the fun :party:
by wahee
Mon Feb 03, 2020 11:17 am
Forum: General Gear
Topic: VINTAGE sTIX st-305 DRUM MACHINE ISSUE
Replies: 4
Views: 285

Re: VINTAGE sTIX st-305 DRUM MACHINE ISSUE

hi I'm working on a YouTube video detailing my experiences with this machine. So I have managed to get somewhere with it but it's very hard with just the PCB and no schematic. Plus the board is full of bodges and mods, makes life difficult. I may put the video up as a part1 then a pt2 conclusion if ...

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