The problem with JSD faders is the fader itself. If people push in on the knob, it will push the internals apart. The encoder is a low-cost one. I don't have its part number handy but we sourced them not too long ago. As for changing them...the worst part is resigning yourself to cutting the old one off the the sub board (particularly the ground tabs) and then you can easily remove the solder the remaining pin without the control acting like a heat sink. There are normally some jumper wires from the sub board to the main board that you want to not stress. Once you've done 1 or 2, they go pretty fast. Again, just cut the tabs on the old control before worrying about desoldering.
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If someone was in the used market for a single screen or hobby, not going through a tech or dealer, and not versed in electronics repair, I’d consider budgeting for 2 CP750s if that is the unit of choice.
The trick there is tons of the already dying ones are probably in that market. Definitely make sure it is photographed powered on, and ideally listed as some kind of working pull.
The prices seem to be in the $800-1200 range. Mileage may vary. Definitely make sure they are cooled well and consider the external power supply. I had no idea that was an actual Dolby part! I also don't know if that is a fair used price, not knowing what techs/dealers are charging for that discontinued item.Comment
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steve could I get the part number for the encoder on the JSD80. The powersupplies I have had no problem getting and the other big failure which i cannot figure out is when it is finished booting it starts a hooting sound on all channels and all the pushbuttons flashComment
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A hooting, oscillating sound is a good indicator of a capacitor failure, probably near or in the power supply if it affects all channels.Comment
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The mechanical quadrature encoder used in the JSD series was always a challenge. The firmware ATTEMPTS to deal with the contact bounce but, as the encoder wears, it just gets too bad for the firmware to deal with. As Steve points out, it's best to cut the encoder mounting tabs, then desolder stuff. I know early models (probably JSD-80) had an issue where you could push on the knob and push the back out of the encoder. In later models, we dealt with that by mounting the PCB the encoder was mounted to to the front panel so the board would hold the back of the encoder in place.We also spaced the encoder a bit further behind the front panel so if you pusehd on the knob, it would hit the front panel instead of pushing the back out of the encoder. As I recall, on the JSD-80, the encoder and the rest of the front panel (display and buttons) needed to be mounted in different planes, so the encoder had its own board with a short cable between the boards.
On SMT soldering, I've been using a YIHUA 959D hot air rework station with Essmetuim ZB628 solder paste on projects like https://w6iwi.org/rtty/DspTU3/ .
Automated SMT assembly has really reduced manufacturing costs (my father's company had people hand assemblng vacuum tube equipment). Now, repair costs can exceed manufacturing costs. But, I wonder if someone could make a living buying some model of equipment with a known failure mode (such as bad aluminum and tantalum electrolytics), shotgun replace them all, test the unit, and sell it. Testing can get a bit involved. At USL, we had several Audio Precision units that automatically tested each unit. We also had about a dozen techs that did production bringup, test, and repair.
Also, electrolytic capacitors are available with different temperature ratings. Keeping the ambient temperature substantially below the rated temperature vastly improves the life of the capacitor.
The first USL processor I worked on was the JSD-80. As we developed later processors, we tried to simplify the designs. The JSD-80 could probably have been done on a single PCB (plus a front panel board), but tradition called for separate plug in boards. The JSD-100 brought the system down to a main board, a front panel board, a mezanine board to handle another layer of rear panel connectors, and an optional AC-3/DTS decoder board plus an output board. There were several output boards available: analog, AES3, and BluLink.
As we designed each processor, I learned stuff. The JSD-100 had fairly complicated audio switching in the DSPs. As I was discussing this with the DSP engineer as we worked on the JSD-60, I came up with the idea of making the box as dumb as we could and just have the format buttons load different configurations, such as the input matrix mixer. We made each processing channel the same and had it drive one output. What drove that processing channel and what it did was up to how the system was configured instead of our trying to imagine what someone may want. The input mixer allowed for 5.1, 7.1 (with locally generated HI and external VI), drive in mode, local generation of HI, simple matrix surround (surround was 100% L + (-100% R). The complexity was moved from hardware to software, and that software complexity was only in the configuration. We could add features without a firmware change. It was fun!
I don't remember if it was the JSD-60 or a later project where I figured out that Ethernet magnetics could be used for AES3. Since Ethernet mag-jacks are quite inexpensive, we just used one of those for AES2 input or output.
The ideal design has zero parts...
HaroldLast edited by Harold Hallikainen; 01-20-2026, 06:37 PM.Comment
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Gordon, they used the same rotary encoder for all of them. It is a CUI (aka "Same Sky") part:
ACZ16NBR1E-15FD1-24C
That is a 15mm "D" shaft that you will need to trim down to about 11mm
image.png
It seems to be available from the usual parts suppliers.
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Thankyou for that informationComment
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Steve is on point on the topic opening, most of the CP750 processors will suffer first from the power supply failure (I have not seen lately the first issue, with blown varistors on the input) where due to the excessive heat, the ESR of the main filter cap and of of the output caps will rise, and the power supply will compensate, lifting the +15V to almost 19 - 20V. This will trigger the 19V transil protection diode DZ2 to shortcircuit and protect the motherboard. If you have a CP750 that will not power on after a replacement PS is installed, thinking it is a dead motherboard, check this diode and either cut one leg (in a pinch) or replace with:
https://www.tme.eu/ro/details/1.5ke2...cdil/1-5ke20a/
Generic replacement external PS with (+5V, +15V and -15V over 50W) are available at most of the main electronic suppilers. One would have to find a 5 wire core piece of cable like 5 x 0.75 mm or 5 x 1mm(shielded will be better) and wire in a standard XLR 4 pin connector like:
https://www.tme.eu/ro/details/fc6151...ori-xlr/cliff/
Caveat - the original PS is outputting specifically 5.4V, to compensate the loss through the double diode D4. It will work fine with PS generating 5.0V only (fully tested), but if your generic PS will have an adjusting potentiometer, use it to lift the voltage to 5.4V. Example of generic PS:
https://www.tme.eu/ro/details/rt-50c...rat/mean-well/
Pinout is:
PIN 1 = +15V
PIN 2 = +5V
PIN 3 = GND
PIN 4 = -15V
For those with more advanced soldering skills, who would like to cure their processor of the second most reported issue, low output on various channels, you will need to replace (for cinema operation channels) the capacitors highlighted in the attached photos.
There are a number of 9 caps of 10uF/35V - size 5mm diameter and 5.8 mm height to be replaced, and 34 caps of 22uF/35V same size. These are more or less in the signal path of the audio.
Additionally you can replace 2 x 47uF at 6.3mm diameter and 5.8 mm height.
Panasonic has a good variety of them, easily found on Digikey / Farnell / Mouser/ TME
For removing the dried one from the board, I found the gently twisting the caps with a needle nose pliers to be the most effective methode.
I hope that the on point informations above will help some, save some units from reaching the landfill.
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For an external supply, I have used a Mean Well GP50A14E-R1B. It does have a DIN connector that needs to be removed and a Neutrik NC4FXX (or equivalent). To retain the ferrite core requires some careful cable prep as there isn't much space but it is doable.
Moving to the external supply removes heat from the chassis, which is one of the goals so I wouldn't bother with replacing the internal supply and would just unplug it, if the opportunity presents itself.
Thanks for the photos and the capacitor information Adrian. Also note, some CP750 MB have SMT protection diodes instead of the through-hole (like they have on the +5V rail). The SMTs were prone to fail.
Have you found that the other capacitors (not boxed in yellow), that are exposed to the same heat, were unaffected?
For those doing some exploring. The CP750MB power rails all seem to have a similar component layout of three electrolytic capacitors with a zero-ohm resistor next to them. The zero-ohm resistor is the test point to check that power rail. Look in Adrian's left photo above, towards the top. The power regulator is often near them too. There are something like 5 or so power rails in it.
Oh and to add insult to injury, Adrian's left photo shows the abandoned fan connector (down and to the right of the yellow boxed diode. You can see via the traces, the capacitors, not stuff were for that too. There is no evidence that the chassis ever had the fan planned for, however (too bad).Last edited by Steve Guttag; 01-25-2026, 08:00 AM.Comment
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Skilled folks would be able to slap a fan header on there. I'm curious if the system would know what to do with it. Wonder if it probes a standard 12v there? It's not a PWM fan header. Next test would be a fan on some test leads? Caveat, there may be other components missing to make that circuit useful...
Amusingly I personally scored a "working pull" unit this week for $200 best offer this week. (The guy has a 2nd one on ebay still). Will definitely be trying the external supply mod, and thanks Adrian for the specifics on other repair routes. Mostly just wanted to spend more time learning the config interface, since I almost never get to touch ours in the small theatre. But having a practice or spare one is nice too.
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I have used also the Meanwell GP50A PS, it looks neater, like a laptop power brick, however i didn't find enough in stock, so I have tested the more generic type al well, like in the example above. Also the Meanwell does not have 5V adjustemnt potentiometer inside, but the CP750's that are in service with it are working just fine (~ two years now).
Regarding the capacitors on the MB, I have found that almost always, the capacitors from the DC IN area, and those that are near the various rails (3.3V, 1.8V, 2.5V ) and arround the FPGA's, are ok, and still in spec.
On photo 2, indeed you will find more out of spec, or down right dried up capacitors (the ones that are not inside a box - above the Non Sync connectors), but for the cinema use, (Main Audio Output) channels 1- 8 , replacing the ones indicated will bring the correct levels and solve the issue.
Indeed there was a fan circuit design on the motherboard, but not populated in the end. According to the circuit, I gues one could use a 12V fan, if installing the missing components.
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"Do not stuff"
I'll stop here to avoid getting rude!
This actually shows that the fan circuit was designed and ready to go but it was actively cancelled because...?Comment
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Who knows, from circuit design until the actual production of the MB, a lot of stuff can suffer modification
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Yes, just a coincidence that the fan circuit was scrapped and 15 years later it turns out that the main cause for their failure is "excessive heat"?Comment
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