Component Replacement in Vintage Synth Repair

[thermionic]

Steve – as we say here in London, you are a diamond geezer :guinness:

I have a full service manual (found it through Google - some kind soul scanned all pages and hosted it).

That is the PSU I see in my schematic. I have replaced tants with tants on all main PCBs. I’ve increased the voltage ratings on some (I had them in the drawer), but have adhered to SCI’s values capacitance-wise. I measured all tants that I removed for safety, and found 2 were dead short.

As for the PSU, I have replaced the tants with electrolytics. The 10uF tant is now a 47uf electro. All 2.2uF tants are now 22uF electrolytics. Each electro is bypassed with a 10nF polyprop cap. Apart from the higher-voltage rail, all other DC rails appear very clean on the 'scope (unloaded).

One Tech I spoke to talked about the "30v rail"... I think I may have been stupid here - and he could be talking about the P-P value of the +/-15v rails? I measured +28 (unloaded) and +24 (loaded) from the white / orange rail (unregulated on the 'schem - appears to come straight from the rectifier with one cap... Is this kind of loss normal for a raw DC supply?)... Unloaded, my voltmeter classes it as DC, but my scope shows around 5mV of jagged corruption… I put this down to a quirk in the P5’s PSU – it has never had a good reputation.

For what the Vregs cost, and bearing in mind how it’s nearly impossible to get a probe in there to look at the PSU’s behaviour under load, I think I’m going to replace all VRegs as par-for-the-course.

I noticed that the mains transformer is very noisy mechanically, but put this down to 50Hz and general noise from old E/I units. Is it possible for a TX to get funky under load?

FWIW, I'd say I'm 90% that the PSU is quite normal for a Rev 2. If it is ok and the P5 is behaving this way, what are the possible culprits? I was told a blown SSM2020 in the VCO would prevent turn-on?? What really makes me nervy is the sight of potted 'cocoons’, amalgamated into the top faces of ICs, with wires and passives spewing from them… Whatever were SCI thinking with regards to future repair? A P5 wasn’t cheap was it? I wonder if the 'cocoons' are in the schematic…

Thanks again,
Justin (edited about 10 times!)

 

[Steve Jones]

By "potted cocoons" I think you are referring to the Tempco resistors stuck on top of the VCO's? They are a couple of resistors (from memory one Tempco and one regular resistor) for thermal stability, so no big deal.

I would expect the +/- 15 volt rails to be just that, particularly the +15 when adjusted correctly. I am working from memory here, so forgive me if I am vague on some things, although I do have a Rev 2 voice board sitting here on my desk.

Perhaps the first thing to do is to is to check the PSU unloaded and adjust it to the correct analog voltage, then disconnect the voice board power and ribbon cables and run the PSU with only the processor board connected. Then see if the power rails still run at their correct spec. if the rails are OK, then connect the voice board and see if the rails sag. (Powering down each time of course!) You could expect some small drop, but not much.

If the voice board drops the PSU (excessively) then you need to find what's loading it down. Look for hot IC's with your finger, at least pretty much every chip should be socketed on the voice board. Same thing if the CPU board pulls the supply down, look for hot op-amps.

Sorry for the slow reply, but London and Sydney are kind of out-of-phase timewise :grin:

 

[thermionic]

Steve – thanks!

If the ‘cocoons’ are just tempco resistors, that puts my mind at rest – I was concerned I could need the services of an X-ray department.

I will try out your suggestion and report back here when I get back to the workshop.

Sorry for the slow reply? You’re joking! It’s a privilege to get advice from you, particularly when you consider that my options for engaging a synth pro here are somewhat limited!

Cheers!

Justin

 

[Steve Jones]

Well, lets just make it an online project, we can work through each issue one by one and get it going eventually. I have a CPU and voice board here, with some faulty and some missing parts, they are basically my spare parts store for rev. 2's, so if you need a chip I probably have it.

Can you give me a link to the manual that you found? Mine is an old photocopy on A3 sheets, it would be nice to have a better copy...

 

[thermionic]

[quote author="Steve Jones"]Well, lets just make it an online project, we can work through each issue one by one and get it going eventually. I have a CPU and voice board here, with some faulty and some missing parts, they are basically my spare parts store for rev. 2's, so if you need a chip I probably have it.

Can you give me a link to the manual that you found? Mine is an old photocopy on A3 sheets, it would be nice to have a better copy...[/quote]

The files are listed individually, so you'll probably have a bad case of RSI by the time you've downloaded all 140 of them!!

I can't find the link either... Best save yourself RSI and let me email the lot over as a ZIP.

I'll PM you my email!

J

 

[Steve Jones]

That would be good, then we can reference the same page numbers while we figure out how to fix it. :sam:

 

[thermionic]

Good news and bad news:

Good: it didn’t seem to like being powered via the variac, and when I switched it on tonight, minus variac, the switch lights came on.

There was no smoke, and no ICs seemed to be too hot (they were all pretty cool from what I could tell).

I got a scope probe onto the +15v, -15v and +5v rails on the actual boards. The noise / ripple was below 2-ish millivolts. Not good enough for a ribbon mic preamp, but something resembling DC

Bad: No controls did anything at all – everything was stiffed.

The Eproms were rather warm (I didn’t leave it on for more than a minute or so at a time), but not ‘hot’.

I spoke to Tony the Guru and was told that I may have caused damage by turning it on via a variac… He suggested that by ramping up the voltage, all kinds of weird DC offsets would be created in the logic circuits, which could have pernicious effects… He also said that the behaviour of the synth was in-keeping with what occurs when either a VCO IC is dead, or if a main Eprom has blown.

The mains transformer sounds like a small bandsaw – I’ve never experienced such a noisy transformer in my life. I remember a friend’s rev2 being noisy, but not this bad…

Tony claimed that the unregulated rail typically might be anywhere between 22 and 28 volts, and that several mV of corruption was typical

Justin

btw - if anyone wants the Service Manual go to Hotmail with the following info:

[email protected]

Password: prophet5

(I set the account up to transfer the manual - this is not Steve's personal info!)

 

[Steve Jones]

I don't know if running it up with a variac would do damage, the regulators shouldn't turn on until they have enough input to start regulating, so presumably there wouldn't have been odd voltages generated inside the machine.

Can you tighten or clamp the mains transformer? If it has a metal sheath around it you can crimp it in around the laminations to compress them, or if it's bolted together maybe the bolts are loose.

If the internal voltages are OK, then get your scope out and start looking around the CPU. See if there is clock at pin 6, and that the NOT RESET line is high.

Then look at each of the data and address lines and see if there is any activity. also the NOT RD, NOT WR and NOT IRQ lines. This will determine if the CPU is clocked running.

The CPU diagram is on 3-15a.jpg

Check the battery hasn't corroded and damaged anything also :shock:

 

[thermionic]

Alas, things aren't looking too good... I just turned it on and no lights were on...

When I probed the Z80 pins, all I got was a mV or so of junk (suggesting there's no action going on, just the kind of crap your scope picks up when not terminated to something useful).

:sad:

Justin

 

[Steve Jones]

Is there any sign of clock? That would be the first thing to check, as nothing will work if there is no clock to the CPU, and also if the CPU not RESET line is wrong. I have seen clock circuits fail in these machines before, so don't worry too much at this point, just start at the beginning, and:

Check the power supplies on the CPU board at the chips to make sure they are correct.

See if the clock is running, so look at the 74LS124 (looks like U322 on the drawing) and confirm there is clock there. Don't forget to wind up the sweep speed on your scope to see the 2.6 meg signal.

Look for that [not reset] line in pin 26 of the CPU, if it is not correct it will stall the CPU. See if U309 is OK.

Also, are you CERTAIN that the power supply is OK? I have been caught before on Yamaha and Roland machines where 1 of the 4 bridge rectifier diodes fails and the voltages seem to be OK at first glance, but they have a sawtooth wave in them. SH-5's, OBXA-s and Yamaha GS-1's are a bit notorious for this.


If you are ONLY seeing zero Volts on any of the CPU pins then it is a power supply issue, even a stuck processor will have 5V sitting on some of the pins. So recheck all your PSU lines.Don't forget that even if the machine is dead in the water, it may be be a simple problem in a critical place. It's pretty rare to see a Sequential synth come in that is so faulty that it isn't worth fixing, some can be difficult, but unless there has been a catasrophic over-voltage failure in the PSU you just need to work through the issues one by one, starting with the PSU, then the CPU/ROM/RAM, but none of these will work unless the clock is running and the CPU reset is in the correct place.

 

[thermionic]

Steve,

It will be a couple of days before I can do these tests as I am bogged down by my day job. I will report back soon.

Many thanks for the help :guinness:


Justin

 

[Steve Jones]

That's OK, I'm working on an Oberheim OBXa at the moment with some ugly computer and DAC problems, I have learned that old machines can smell fear, so best to just go ahead with confidence, and if you don't have any luck after a few hours then just go and have a hamburger and some chocolate.

 

[bcarso]

[quote author="Steve Jones"]... I have learned that old machines can smell fear, so best to just go ahead with confidence...[/quote]

I like it.

 

[thermionic]

You beat me to quoting that one, B :grin: These old synths are like sharks, you know.

I haven't had a chance to get into the workshop today, but when I ran in earlier, I turned on the P5 and saw the lights come on again... Sometimes they come on, sometimes they don't... When I scoped the PSU rails before (admittedly, just the +5 and +/-15) I didn't see any triangular-style corruption.

I will try Steve's suggestions when I get time in the next few days.

Thanks again, Steve.

Justin

 

[Steve Jones]

I can't believe how much crud sits on the 5V line of an OBXa and the machine keeps running. It has +/- 2V of fur and spikes on it and it still boots and runs. I replaced the entire CPU/MOD board assemblies with a spare set that I have, just to see, and it is the same. This machine is going to need a lot of work. (it belongs to me though, so no rush).

The other rare treat that I have here is a Moog Sonic 6, which are hard enough to work on at the best of times because of the way the boards are loaded into the suitcase. Some part of the machine is putting a small 1.2kHz sawtooth wave onto the power supply rails, so it is going through everything. You could put the schematic on a dart board and throw a dart into it, and be pretty sure that if you put a 'scope probe on that point you will see a sawtooth wave. The synth completely works, but with this tone overlaid on everything, beating with the VCO's. There is no way to really isolate individual parts of the circuit, so it is going to be like trying to figure out which of the 15 people in the lift did the fart when the smell is everywhere.

 

[dabo]

I had a P5 that developed strange behaviours and then failure. Power rails read OK, nothing burned...
The machine uses long-ish strips of metal with lots of legs for distributing power. (or maybe it was ground, I can't remember) Check to make sure none of the legs have cracked from board flexing. I pulled these buses and replaced them with bus wire to solve my problems.
I hope this helps.

 

[somnium7]

[quote author="Steve Jones"]

The other rare treat that I have here is a Moog Sonic 6, which are hard enough to work on at the best of times because of the way the boards are loaded into the suitcase. Some part of the machine is putting a small 1.2kHz sawtooth wave onto the power supply rails, so it is going through everything. You could put the schematic on a dart board and throw a dart into it, and be pretty sure that if you put a 'scope probe on that point you will see a sawtooth wave. [/quote]

I also repair old synths and build DIY synths for fun. The problem you described here is eerily familiar to me but it was in a mixer.

Anyway, I finally managed to trace the source of the of the sawtooth into the power supply where I found some large spikes on the base of one of the pass transistors. The transistor had been replaced with a different part. The original was a 2SA unit and it had been replaced with it's 2N cross reference. I found the original 2SA part and after swapping it in the sawtooth was gone.
The 2N part tests good on it's own.

I'm not certain yet but I believe the 2N part may have done something bad to the impedance of the regulator circuit and created a resonance in the smoothing filter.

It was a weird problem. Had me pulling out hair for almost two days.

 

[Steve Jones]

[quote author="somnium7"][quote author="Steve Jones"]

The other rare treat that I have here is a Moog Sonic 6, which are hard enough to work on at the best of times because of the way the boards are loaded into the suitcase. Some part of the machine is putting a small 1.2kHz sawtooth wave onto the power supply rails, so it is going through everything. You could put the schematic on a dart board and throw a dart into it, and be pretty sure that if you put a 'scope probe on that point you will see a sawtooth wave. [/quote]

I also repair old synths and build DIY synths for fun. The problem you described here is eerily familiar to me but it was in a mixer.

Anyway, I finally managed to trace the source of the of the sawtooth into the power supply where I found some large spikes on the base of one of the pass transistors. The transistor had been replaced with a different part. The original was a 2SA unit and it had been replaced with it's 2N cross reference. I found the original 2SA part and after swapping it in the sawtooth was gone.
The 2N part tests good on it's own.

I'm not certain yet but I believe the 2N part may have done something bad to the impedance of the regulator circuit and created a resonance in the smoothing filter.

It was a weird problem. Had me pulling out hair for almost two days.[/quote]

It is a really nasty problem, I set it aside for a few days to work on the CS-80 power supply. I bet something has drifted out of spec and turned itself into a little oscillator, there's got to be a capacitor aiding and abetting it somewhere. It;s saturday here, so I am going to pull the Weed Whacker apart today and do a little internal combustion maintenance.

 

[Steve Jones]

[quote author="dabo"]I had a P5 that developed strange behaviours and then failure. Power rails read OK, nothing burned...
The machine uses long-ish strips of metal with lots of legs for distributing power. (or maybe it was ground, I can't remember) Check to make sure none of the legs have cracked from board flexing. I pulled these buses and replaced them with bus wire to solve my problems.
I hope this helps.[/quote]

That's a good point, that voice board is very long and flexes easily.

 

[thermionic]

Ok, this is a bizarre one…

I had a spare hour last night, and – more for peace of mind (neurosis if you like) than engineering logic – I decided to replace all the Vregs in the PSU. I felt that, after having found 2 shorted tants, and bearing in mind the cost of the Vregs is minimal, it would enable me to move on, without worrying that the Vregs were compromised in any way. The PSU board was very messy anyway.

So, I replaced the Vregs and put the PSU back in place. However, I reversed the 2 identically-coloured red leads on the secondary. I couldn’t see how this could have an effect, as the blue denotes CT and the red leads just denote each side of the secondaries…

Upon power up, I was rather horrified to take a reading of +90v from the LM320T-15… (is supposed to put out -15v…). Being totally without an explanation for this, the only option at my disposal was to reassemble the PSU, but this time, EXACTLY as it was before… Voila! Everything was fine…

The transformer is listed as a 36v CT unit on the schematic, which I assume is 18vAC per secondary. The readings from the raw DC supply confirm this. How on earth could it put out 90v DC from a Vreg? Makes no sense at all to me. Reversing the secondaries makes no sense…

BTW – I haven’t had a chance to try Steve’s suggestions yet, but with the rebuilt PSU, there is NO difference… The lights come on, but nobody’s home.

Thanks to all that have advised :guinness:

Justin

btw - here's the PSU again (thanks Steve): http://moonpony.net/images/psu2.jpg

Note that I tested the PSU via a variac when I saw the +90v - fortunately, it was not connected to anything...