Apologies for starting a second thread, but I needed to focus on the actual issuer. I’m in a bit of a fix - I’m hoping it’s something simple but I need some guidance.
Back story: Memorymoog purchased 2 weeks ago, with the understanding it would crash after 2-3 minutes. I pulled the power supply and one of the first things I did was remove the power transistor sockets and solder some extensions on so I could mount them right on the board. Then I happened to notice a cracked solder joint under the shunt resistor on the 15v rail (I had been noticing that my 5v measurement was no good and would keep dropping in voltage until the synth eventually locked up). Fixed that and everything seemed to be in business. I then re-capped the supply and replaced the trimpots, and dialed back in the voltages.
So I started doing the full calibration. I wasn’t 5 minutes in when I was re-checking the supply voltages on the DMUX board and ZAP I slipped with my DVM lead and the synth just started freaking out, doing the long, THX-sounding sweep until I shut it off. Turning it back on presented the same issue so I got worried and spent some time to pull the DMUX board and inspect it. While I had it out I re-capped that board also. I got it back in, and with some mixups and connectors in the wrong sockets (apparently there are multiple connectors in the same row with the same number with only 1 used) I fired it back up and I get just the faintest sound of the synth sweeping again but all the controls are frozen. No key response, no audio. I checked my supply voltages and voltages at a couple of the chips in the area where I dropped my lead… all look good. I do have a shunt resistor and transistor about 2 inches away from where I slipped that are getting wicked hot… the voltages are measuring out correctly on them but I don’t think they should be that hot. I also pulled and reseated the processor just for giggles.
I’m so upset; all I was going to do was calibrate everything and I slipped on the first shot… anyone out there ever had a similar experience or know what I may have damaged? The fact that it’s locking up again and it’s not the power supply (assuming), leads me to believe it’s a CMOS chip or something at the digital side…
Lets start with the Power Supply (PS). You stated you removed the power transistors and and mounted them to the board? Do you mean the TIP41s? These were originally mounted on the PS board as I recall and relocated to the heat sync plate because they get too damn HOT! I was one of the early guinea pigs of this design flaw and well - when those things over heat watch out and pray you don’t have your amp output turned up much past 2!!!
Please identify by board and part designator which transistor and shunt resistor are getting wicked hot.
Your probe slipping may have shorted something and scrambled the CPU. There is a reset on the digital board by grounding a test pin. Check the service manual and schematic and see if that helps.
As to plugging into the “wrong” connector I believe those (2 sets of 3 iirc) are duplicates and should not have affected anything provided you did not reverse the connection.
I’m not the scolding type (I know a few here that are) but I would caution you to not guess at troubleshooting - especially on this board which is tempramental to begin with. No need to recap or reseat anything else at this point other than if you suspect a cables. The good news is just about any Memorymoog can be fixed if you have the time, knowledge and experience. Just about all the parts are available and much less expensive now with the reissue CEMs in abundance.
Well I read your overhaul thread in triplicate just trying to get an idea where I’d stand getting this thing worked out.
So on the DMUX board is where I was getting the hot resistor and transistor. I first identified it with my thermal gun and saw that area of the board was over 115 degrees! Ouch… the parts happen to fall right above where I shorted. It doesn’t seem normal but my voltages are all perfect at the rest points as well as on those hot parts. I believe it’s R172 and the adjacent transistors, Q13-14. I also checked the voltages at the pins of the op amps just prior to those parts- I also scoped them but couldn’t tell if it was anything flaky.
I didn’t know about the reset function, that’s what I was praying someone would tell me lol! Maybe I scrambled the CPU or a ram chip… I’ll get right on that tonight and see if that changes anything.
I really don’t plan on doing any kind of recap job on this… I did do the 4 on the DMUX board while I had it out (with the hopes of never having to take it out again). I was all set to begin calibration and then this happened! The power supply recap just seemed smart while I had it out- and yes, the sockets I’m talking about are the power transistors- they have them mounted in these stupid plastic sockets that when you flex the power supply they short out and lose connection. I re-pasted the trannies to the heatsink and used new mica pads between, then I soldered on extensions to mount them directly into the board so, no more flex.
I had been in good shape with the power supply up until last night! I’ll keep you posted- and thanks for responding! I know you certainly had your fair share!
Ooh - regarding R172 and Q13 - that’s what generates the -7.5V, i.e. the voltage I totally missed when I had intermittent DEAD OSC running the autotune. Kicked my ass for weeks if not better than a month because I totally missed this secondary voltage being used on the VCs. Fortunately the doctors say I should be fine now… as long as I don’t get excited.
Anyway if you’re getting +/-7.5 out of that circuit it’s probably fine. 2W resistor on the -7.5V is there because that line pulls some juice. I’d expect it to be hot.
Btw I now understand your power transistor mod - you reminded me those were originally socketed and then they released an ECR to mount directly to the back plate - good job.
That’s what I did with the power supply transistors… see the red wire tubing over the solder joints…
NOTE: please ignore that radial 1000uf cap! I thought I had axials and turned out I didn’t so I didn’t order them so I’ll swap it out before this is all done…
I found the test point (P49, empty 3-pin connector) but am I supposed to short all 3 together? It doesn’t really indicate any of this in the manual.. I found it by looking at the schematic (based on the one empty connector I saw)
EDIT- Ok, I ground the 3 leads together just figuring what else would I lose - and it went silent on boot-up, rate LED flashed a few times… but that was it. Didn’t solve my issue - I’m still sitting at a frozen puppy…
Here’s what I actually did to it, figured I’d make a quick video:
Before we proceed any further do you have the schematics, not just the board layouts? T.P. Reset circuit is clearly shown on the left side of the digital board with each pin of that P49 connector. The reason I’m asking is it will be necessary that you are familiar with getting around schematics and tracing signal paths. If not that’s fine, but I’ll probably suggest a different course of action. Btw turns out shorting all 3 pins was just fine. The next step will be to see if there’s clocking activity on some of these DMUX busses. These are difficult to see on cheap scopes ( like I have) but we’re not looking for any real detail, just enough to know that the processor is cycling through its routine.
As I understand it this behavior is new since you first opened it so I’m fairly confident the digital board is still functioning. But this is the logical place to start. Once we establish this it’s back to the DMUX board where I really believe the problem lies.
Ok - so, yes, I do have the schematics and have been using them so far to try tracing out my issue. For as ‘noob’ a tech as I make myself out to be, that’s purely based on knowledge of complicated digital electronics. I do have fairly good equipment, in fact I just bought a new Siglent scope (but I have an older Tek analog scope as well). My soldering and de-soldering tools are all professional, and I’m very confident with doing that type of work. So, barring that:
I already scoped different points on the CPU (clock seems to be working)
Just a quick digression, I recently had a similar issue with my Prophet T8 and I had swapped out CPUs and reseated / shorted ram chips and anything I could do to get it to boot properly but in the end it wound up in the hands of someone more confident than I am. Hoping this isn’t the case here
And yes, this is totally new behavior since after fixing the initial power supply issue the synth worked perfectly fine. All I did was slip my positive test lead while I was re-measuring voltages on the DMUX board caps (as per the manual) and it sparked and made the sound you hear when you now turn it on (as in the video).
Got it Mike - just wanted to make sure I wasn’t get you into trouble here.
Now I’m thinking maybe its best to start with the DMUX board. I would probe the DAC and a few of the 4051 muxes to the right. These are what ultimately control the front panel functions and their associated read-outs. Since everything is “locked up” check to see that the DAC output shows pulse train activity as well and you see changes on the various address signals on the 4051s (they cycle in the 100khz range iirc).If you’re not seeing anything at the I/O pin of the 4051s then look at the the DAC supporting circuitry. It the address lines or Inhibit line show no activity then it’s back to the digital board.
Btw I’m OK continuing this here if no one objects but if you prefer you can pm me and we can even do some of this over the phone.
I probed around on the DAC and the surrounding circuitry. I’m not getting a super good reading on the scope but as I increased the V I could see the DAC was moving in a continuous way, so I’ll presume that it’s good…
Interestingly, I checked the voltage at pin 16 on the 4051s and it’s supposed to be 5v. I’m pulling 15 from just about all of them (I checked most of them and then assumed the problem was downstream…)
At the 3 caps on the DMUX board I am getting correct voltage from the power supply.
Also, what was weird was when I pulled out a random op-amp (let’s say I pulled U5) the synth booted, obviously no audio but the front panel functioned briefly and then locked up again. Like I could hit autotune.
And if I swap the chip (U5- 351) with something else comparable, or another 351… that ugly screechy sweep that starts high and then goes low when I power it on goes the opposite direction.
That’s a typo on the 4051s supply voltage - 15V is correct. There are many typos on the schematics.
Take a look at the MUX BUS signal coming from the front panel and the Approx Compare signal. That should have activity although if you turn all knobs to 0 it will be significantly reduced.
Also does pushing the autotune button do anything or is it locked as well?
I’m guessing that removing the Op Amp breaks the closed loop sample and hold circuit where the CPU is writing the value it has stored in part of it’s timing cycle and sampling the parameter for updates in another part. I wouldn’t make too much of this observable.
I also just realized I was probing the Z80CTC, not the CPU - Glad I looked at the schematics! I want to recheck that tonight and make sure things look right.
I still feel like the problem is at the DMUX somewhere.
If I probe something and it goes high on the scope to the point of disappearing, should I assume that it’s still good or is that also an indication of a failed or failing chip?
EDIT: MB I just saw your response about the chip removal. Cool… I just ordered a ton of chips, pretty much everything to cover the DMUX board in the event I need it, plus a couple Z80s to keep on hand.
Cant answer that last question - too many variables. But I agree it’s still looking like the DMUX. At this point it looks like the CPU isn’t seeing the fact that a button is pushed or a pot is turned…OR… it seeing it but when it trys to send an updated value or state to the DMUX it’s not getting read. All the front panel information gets back to the CPU via the MUX BUS signal going through the U6A comparator. So again, try to look at that MUX BUS signal and the address and the inhibit lines of the 4051s.
And trust me, this can be difficult to troubleshoot even if I had the board in front of me.
I’m getting readings on U6, both the comparator line and the MUX line. Just for giggles I pulled the chip and fired it up, and sure enough I get no front panel leds (except for the octave switch on the mod wheel). I tried replacing the chip with a TL082 and it made no difference (though I don’t know if the chips I have are good). I’m also following the same reading to the connector on the digital board. I also reseated the CPU and tried the TP reset again but again, nothing. All 4051s on the DMUX are getting reading on the INB lines. Voltages still look right in various places I check.
An interesting thing happened though. After I reseated the CPU, I fired it up and noticed I had no more noise on boot and functions started working, except no audio. Then it froze again. A minute or 2 later. I thought I was getting somewhere but then realized my 15v line came out of the power supply. So I plugged it back in and was back to where I left off. When I shorted the board I was testing the voltages at the caps on the DMUX. I am still getting correct voltage everywhere but would I be right in presuming whatever I blew out was on the 15v rail?
Is it possible I just damaged the CPU somehow? I’m getting readings from it (reset, busrq, etc), just don’t know what I’m looking for. I do have my new parts coming in the next day or two so if I have to start throwing stuff at it, so be it…
OK - let back track here for a second. I watched your posted video again and wonder did you power it off at the end? In other words if you leave it on does it stay on with the same LEDs on each time and the character display showing MOOG? If so than I think your original problem is still present. In other words your probe shorting may have done something downstream affecting the audio (or not), but it’s unrelated to the lock-up condition. Could be your intermittent problem elevated to a permanent problem which on the bright side should make it easier to troubleshoot
This being the case maybe your rework on the power supply didn’t really fix anything - certainly worth a closer inspection of what you believed to be a cracked solder joint. Have you checked all the secondary power signals including the sense lines? With this train of thought I would look carefully at the Digital Board reset circuitry and battery back-up circuit. Also good descriptions in the service manual - may be of help. By the observable symtoms it appears that things are stuck in Reset mode. Again it would be helpful to know whether all LEDs come up the same each power up .
I’m starting to run out of ideas here Mike -trying to do this remotely is very difficult. Also (and as I said I’m not the scolding type) I highly encourage you to not swap out parts randomly. I’m betting there are a few very knowledgeable regulars here lurking and rightfully shaking their heads because they’ve been here too many times! This board is 35+ years old. Those socket connectors suck even when they were new, and they oxidize along with the ribbon cable connectors. Every time you pull a part you risk causing another problem compounding the issue. This make it impossible to troubleshoot logically. The chances of you getting lucky are not good. If you end up bringing to a repair shop your’re potentially looking at much higher cost. Well, I never said I wasn’t the lecturing type . But seriously- just trying to help.
I may be off line for a few days- After a replacement Mother Board install Microsoft decided last night to automatically upgrade me to Window10 Pro (was Home edition) and now my Access code doesn’t match up. Got to do a clean install. Damn - I should have activated right away…UGGGH
so I’ll swap it out before this is all done…
So I plugged it back in and was back to where I left off. When I shorted the board I was testing the voltages at the caps on the DMUX. I am still getting correct voltage everywhere but would I be right in presuming whatever I blew out was on the 15v rail?