OB-Xa Thermal Images

[scottrod]

So I finally bought an OB-Xa and embarked on a nut & bolt restoration. All tantalum and electrolytic caps were replaced, new hardware, voice dip switch, key bed bushings, modulation and pitch pots, Encore midi kit, polish all header pins, paint work, etc. Used 22 uF Elna Cerafines on the voice boards and Panasonic FC's everywhere else.

During the process I though it would be fun to take some thermal images and maybe understand why the rectifiers occasionally fail. Have a look.



The +5 VDC rectifier on the left is definitely working overtime as its temperature is 198 °F. On the right is 171 °F and the middle one is 157 °F.

I replaced all four main transistors, rectifiers, big caps, little caps, IC's; basically everything but the resistors. Reinstalling resulted in the following:


The main rectifier dropped 18 degrees from 198 to 180. The camera is set to autoscale for good contrast, so you can't compare the color of the two photos. The HOT+ was turned on and shows the hottest pixel on the image.

That was still too hot for me, so I got some heatsinks and installed them.



They helped, but not as much as I'd hoped. They dropped the temp on the main rectifier to 174 °F.


So in all, new components and a heat sink dropped the main rectifier operating temp 24 degrees. Not too shabby.

Note the main caps are only running at 90 °F. Replaced those with the same Illinois Capacitor brand parts with the same TTA part number. Still making them after all these years (even the same color), just smaller!

Also, you could probably do those two resistors on the top right a favor and upgrade to 1/2 watt components. They're running at about 136 °F.

[scottrod]

Took a few other shots.

Main Heat Sink. The transistors look cool as they still have the plastic caps on them. Heat sink temperature is around 100 °F.



Upper Control Board. Max temp is 125 °F.



The Voice Cards. CEM chips all doing their thang at around 100 °F.

[roginator]

GREAT JOB MAN!!!!!

I also decided to put heat sinks to mine and clusterchords OBXa rectifier



and that that wont harm is to locate ICs that are used to work on Computer interface ... they should be removed and OBXa 5v rail would make less heat!!

[rokuez]

Wow great thread
Lotta sick info here

Gotta do this thermal imaging with other gear!!

[tyler477]

I would hate to see how hot the PSU gets in my polysix

[atma]

that's crazy.. what do you use to take the thermal images with? it's like the Predator fixing a synth

[The Real MC]

When I was repairing the power supply on my OBX, I replaced the rectifiers with bigger ones - double the amperage capacity. Heatsinks aren't enough.

When a power rail goes down on an old Oberheim - such as failing rectifiers - it will take more damage with it. If the wrong one goes down, you have fried CMOS. ZAP!

[seen-da-sizer]

Originally Posted by atma

that's crazy.. what do you use to take the thermal images with? it's like the Predator fixing a synth

Yeah, next is to use Kirlian Photography to banish the evil spirits that plague my broken XPander!

That is actually a very cool (or literally hot) idea to track problems caused by overheating! Some more technical background please!

[Akh]

Originally Posted by seen-da-sizer

Yeah, next is to use Kirlian Photography...

I realize this was a joke, but from what I understand Kirlian photography could be used as a means to determine exactly how and where cross-talk between components is occurring in some devices.

[clusterchord]

hahaaa this is AMAZING, roginator and me discussed this very problem not 3 days ago...


what did u use to take the pictures ?

[golden beers]

there's a similar (probably worse) problem with Pro1s. one of the rectifiers takes to much juice which is why they fail. on the pro1 schema you can see it's taking way to much. a crap design made that way to save a few pennies on extra circuit and components.

i also added heat sink but the thing still only lasted 7 years or so. what i've done to solve this problem is to cut the track before the rectifier and add a huge 4W resistor there in series. The effect of this is that the resistor gets hot instead of the rectifier. a little cowboy but it stops the rectifier overheating. why dont you try that?

[scottrod]

The images were taken with a Fluke Ti45 FlexCam owned by the company I work for. We use it for imaging deice systems on aircraft propellers.



It was around $25,000 when we purchased it years ago. I think the equivalent is about half that now. I had read about the rectifier issue which prompted breaking out the camera. When I measure the load on the rectifier, it's pulling 0.75 amps and the rectifier is rated for 1.5 amps, so it's right in the middle of its operating range where it should be happy.

Looking at the data sheet for the MCC brand W02M (which is what I used for the replacement, from Digikey), at 0.75 amps it should be operating at 185 °F, so it's doing exactly what it's supposed to. Should be good for years to come. Sure seems hot to me though...

[LWG]

Hello,

Synth tech, Kevin Lightner mentioned this some time ago, on
another forum. He raised the issue of the bridge rectifiers on
the X and 8-series Oberheims being under-sized for the task.
The SX doesn't run as hot as the others.

Regards,

-L

[maisonvague]

Originally Posted by scottrod

It was around $25,000 when we purchased it years ago.

I was just about to ask if I could get one of these at Radio Shack, but I don't think I'll bother now.

Very interesting stuff. Thanks for posting!

[Oli]

Great thread, thanks. Also, seems like a really good job on the restoration.

BTW, do you particularly recommend those caps you used? I'm not really aware of which caps are best for audio circuits.

Cheers

[flat earth]

Originally Posted by atma

it's like the Predator fixing a synth

Yes, those intergalactic beasties, when they aren't ripping out human spines and chasing verbally challenged austrians through forests, actually have a side interest in turning up in peoples studios and insisting on doing a complimentary thermal service's of vintage Oberheims.

...but seriousley, fascinating shots scottrod.

[roginator]

Since THAT Ultra hot SPOT on OBXa Thermal pictures is 5v Rail for digital ICs on upper and lower CPU board ... i was thinking a bit!!!


ENCORE OBXa MIDI is actually replacing a bunch of ICS on Upper PCB!!!

I thing big problem and power eating devices from past are RAM ics and Eproms!!! with Encore midi kit that is replaced with NEW components that usually eat less power ... so I think that is one step to low down temperature of 5v rail rectifier , also removing ics from Computer interface save some power too!!!

heat sink on Rectifier is way to save it and also original rectifier is W02M ... 1,5A

to replace it with 5A rectifire is not a bad idea!!!

D

[pjwhite]

Very interesting photos. Thanks for posting.

Oberheim bridge rectifiers are prone to failures, and not always in a catastrophic way. I've seen machines randomly lock up or reset for no immediately apparent reason. It turns out that when these W02M bridges get really hot, they will sometimes just drop out for one or two line cycles -- enough to cause the DC voltage to drop and confuse the Z-80 CPU, but not enough to trigger the "PUP" reset circuit.

I usually drop in a 3 to 5 amp bridge (the square type that is about 0.75 inches across) and have no further problems.

You could also use a Schottky diode bridge replacement which, having a lower diode drop voltage, will result in lower power dissipation across the bridge and a lower temperature.

[gordonmerrick]

This is an awesome thread. I'm sure I'm not the only one who would wile away a few internet-hours in a gallery of thermally-imaged synths.

[Electric Motel]

What a fantastic idea! I have access to a thermal camera as well and never thought to do this. No doubt this will help in my polymoog keyboard restoration.

[pendejo]

What part did you use for voice dip switch replacement? I mistakenly used those switches when stereo panning the voices on my Obxa and I was almost unable to turn some of them back on. Had to mess with it for ages.

Also, the signal is much lower when I use the 2-pole filter. Is that normal? I couldn't find any adjustments for that in the service manual.

Thanks.

[JEBEQ]

Scottrod
Glad you finally found an OB-XA. I just wasn't really prepared to get rid of mine just yet even though you made a nice offer. Hope yours works out well for you. They are great synths