Juno-106 Restoration story/Power Supply woes

[The LT]

Chips were removed from the board. They were installed into Unit #1 and while some of them are dead, a few of them work fine there now.

The cut traces were also all over the CPU board. He even cut one of the Master CPU legs. The CPU was replaced, some of the ceramic capacitors there were also missing. I've restored them along with the resonators and the CPU board started up. Weird.

Having the board disconnected and on the table I am getting:
Short between pin 30 and pin 25.
93 ohms between pin 23 and pin 25.
Around 13 kOhms between pin 27 and 28. (rising due to capacitors, I guess)
Around 5 kOhms between pin 29 and Pin 28. (rising due to capacitors, I guess)

Module board pictures. Hopefully hi-res enough, I may need to grab a DSLR from a friend for them to be more hi-res.

Show Spoiler

Trace damage/lifted pads on IC12. It was me who accidentally lifted those two from the PCB. Both go to C52 though. The lower one to R98 wasn't me, I swear. Still good though.

[Yekuku]

Hmm, it looks like my assumptions were right, the previous owner figured out that he had the digital power line (5v) shorted to ground thats why he cut the decoupling caps.
Do not power up the synth without finding the short first.
Do you have any experience in findings shorts ?
let me do some research , and I ll get back to you.

[The LT]

Hmm, it looks like my assumptions were right, the previous owner figured out that he had the digital power line (5v) shorted to ground thats why he cut the decoupling caps.
Do not power up the synth without finding the short first.
let me do some research , and I ll get back to you.

Yup, from what poor understanding of electronics that I have, I am guessing that 25/30 short isn't good and probably the initial problem that the previous owner had. Thanks a lot for helping me out on this! Unfortunately, I don't have a high resolution ohm-meter, only a basic multimeter.

[Yekuku]

Hi LT,
there are two 5V power lines, one is named 5.0V (connector pin 30) and the other is 5V (connector pin 23), both read low resistance so they are both suspects.
The 5V is for powering the digital circuit , the 5.0V I am not quite sure what it does to the CPU and to IC30,31,32.
Concerning the 5.0V , please check the resistance between pin 30 and and analog ground pin 28, according to your previous measurements it should be low too, which is not good.
So we are looking for shorts on the 5V lines and ground.
For finding shorts usually low resistance meters are used , which unfortunately you dont have .
Another way is to use an external power supply , inject power in the circuit and have fun finding which component is overheating (the one that is causing the short). Absolutely not recommended method , unless you are experienced.
Another way , which is the hardest , is to start lifting components until the short is gone. I guess thats our pick.
Let me check what components are suspects, and I ll get back to you in few minutes.

[Yekuku]

My guess is that an IC is causing the short and not passive components.
I dont see any tantalum cap which is the usual suspect.
I have seen electrolytic short, but I dont think it is the case here, I could be wrong though.
So if you feel adventurous , you should start lifting IC's one by one, until there is no low resistance readings between the 5v lines and ground. Use dip sockets for all the IC's you are removing, It will be hard to find a socket for the CPU though, I have never seen such a thing.
If I was in your place, I would start lifting components with this order:
1st) CPU IC29
2nd) IC 33
3rd ) IC 34
4) IC 30
5)IC 31
6)IC 32
7) IC 39, 37, 33, 34
8) IC 35 , 38 , 23 ,24 , 25
9) IC 10, IC 36, IC 40
The pcb is double sided so it might not be easy to desolder these multipin ICs
I know this is a h**l of a work, please dont hold me responsible if the short is not on the components I have recommended, as i have mentioned I am no expert, I am just trying to help.
If any fellow VSEr has any other opinion/advise , please chime in.
Good luck LT !

[The LT]

Hey, Yekuku, thanks for the IC suggestions! I will begin the laborious process of desoldering those. I have an automotive compressor in my workshop and I am using a fine needle to blow 12-bar air pressure directly on pins using a fine-tipped temp-controlled soldering iron from my somewhat cheap soldering station. I try not to use a fan when desoldering those old ICs.

About that CPU socket: I am using 4 rows of 2.54mm pitch IC panels, like these. The master CPU fit nicely into those. I've test-fit it first, then pushed the pins firmly into place and now it sits snugly in there.



I've had to refit C10 to C14, C5 ceramic cap and a CSA-type 12Mhz resonator. They were ALL cut from the board. I could not find the Mask ROM CPU, so I got another D7811G and refitted it for an external EPROM.

I spent a couple of hours looking at the schematics and I kinda see your logic. That means I am gaining experience. Yay! Thank you for bearing with me. I will report my findings tomorrow. )))

I will start desoldering tomorrow, CPU first, then the 14050 gates. All ICs will probably take me a couple of hours.

[Yekuku]

Well, we learn new things everyday!
I have never tried your desoldering technique and I have never thought of making this kind of DIY socket, great ideas !
Do not desolder all ICs at once, remove them one by one and check if the short is gone on both 5v power lines , so that you can figure out which is the faulty component.
Let us know how it goes, good luck.

[Mooger5]

Yekuku there are 2 5v pwr lines. One for digiyal and one for analog. You can tell in the schematic which is which bby the grounds. One is dg and the other is agThe cpu has analog inputs. The slave on the voiceboard is fed by yhe analog 5v at pin 43. When it dies it shorts the 5v to ground.

[The LT]

Hi Mooger5, how are you? I've read your threads. Inspiring.

Good news, I've desoldered the slave D7811G CPU and I there are no shorts on the above-mentioned pins. Cleaned up the IC29 socket and going to be installing 4 single-row IC mounts for a new CPU. As I don't have a proper Mask ROM 102 CPU, I've prepared the programmed EPROM with a wide DIP-28 socket for IC37 with the B2 dump and a TC40H373P flip-flop for retrofitting.

I will probably need to remove the J1 resistor and put a jumper on J2 to enable external ROM. Can anyone confirm that?

Unfortunately, I ran out of single-row 2.5mm IC panel sockets and isopropyl alcohol. Got 3, need 1 more for the CPU and 6 more for the 80017A chips. My local electronics store doesn't carry 2.0mm pitch sockets for the 5534 chips, which is a shame as I won't be able to test.

I still need to replace R30, R66 and R100 with proper resistors. The ones the previous owner has fitted are wrong size and wrong value. My store also doesn't carry those. Can anyone recommend a replacement?



Getting a bit ahead here, but I also found a few components missing on the Jack board and I am having a bit of trouble finding suitable replacements. It's C3, C53, C57. They look like polyesther and two foil capacitors, not something my local shop has. Here are the reference photos of my first unit which has a working and fully-populated jack board.

[Mooger5]

Hi, how are ypu. Thanks fpr the compliment.

Test the old cpu fpr cpntinuity between pin 43 and 32 and 33. if theres continuity, the cpu chip is busted and the short is gone at the connector, with the cpu chip removed.

The 3 resistors need not have the same size as the originals. they can be larger.
The pcb tracks dont exactly relicate the schematic there. its the only error Ive seen in the 106 service manual. it still works though.

Regarding the jumpers, just move the existing one to the other position. leave the resistors in place.

The caps dont have to be exactly like the originals. C3 can be any regular polyester 100n and the others are polystyrene because of the tight tolerance to match both channels. you can use silver micas if they´re easier to find. Until then 150p ceramic will do the job.

good luck

[The LT]

Roger that, I will test the CPU legs for continuity. Thanks for the cap suggestions, I kinda figured the foil caps were probably there for the matching tolerances. Will try the flea market. Will keep you updated.

[Yekuku]

Mooger5 to the rescue! thanks for dropping in , the value of your experience cant be matched!
Goood work LT , I am glad that you did not have to remove many IC's in order to find the short.
Keep us updated !

[The LT]

Good news, my friends. With your help, I have successfully restored the module board to its working condition. As I only have one MC-5534A waveshaper and two 80012A chips, I have tested only two voices. But they work! The calibration is off but I won't attempt it before I have a full set of 3 MC-5534A chips and 6 80012A.

I've tested the module board in Unit #1. I haven't yet put it into Unit #2 as it lacks the caps on the Jack board.

I've cleaned the board up, restored a couple of broken traces from C52 to 5534 and it's looking peachy. I am quite happy with my work and would like to express my big big gratitude to fellow vintagesynther's Yekuku and Mooger5.

I've been a lurker on the forums for more than a decade and a visitor to vintagesynth for as long as I can remember myself. Great resource and great people! THANK YOU!

But we are not out of the woods yet. I still have to repair the Jack board on Unit #2 and rework the whole bender assembly. I am also waiting for the felt fader cover gasket kits for both units to keep them clean and dust free.
I'm also waiting for the foil caps near the Chorus BBD chips and some 6 fader caps the second unit is missing.

On that note, I haven't recapped my Unit #1. Should I recap the PSU? How about the other boards? Should I redo all the electrolytics or is it overkill?

Some pictures of the restored board.

[Mooger5]

glad you got it working

yekuku your kind words are far from the truth, you did all the work, but well thanks anyway.

cheers guys

regarding recaps, buy or build an esr meter and test away. onboard. saves time and money.

these are selling quickly. waiting for one to arrive myself

https://www.ebay.ie/itm/TFT-Transistor- ... rk:37:pf:0

[Yekuku]

Awesome news ! Good work LT !
About the electrolytics, as Mooger5 advised, you can get an in circuit esr meter and test them.
Mooger5, I have a similar tester, which is a very usefull tool but unfortunately it can't measure alot of things in circuit.
I am hearing good things about this, it can also do low ohm readings so it would be great for finding shorts but i have no personal experience using it, and it is not so cheap:
https://www.ebay.com/itm/MESR-100-V2-ES ... :rk:4:pf:0
There is no need to recap everything, but if you have obsessions about electrolytics, recapping only the PSU might be a good idea;)