Not to defend anyone but, how much fan noise can you tolerate? They might have been saving cost or trying to avoid fans. That circuit tries to control the fan speed based upon the level of heat. When they noticed that it ran ballsout all of the time they may have thought the circuit was not functioning properly and cancelled it.
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I've never considered noise to be a problem in a projection room.
Where do you see the temperature control on the circuit? It looks like a filter/regulator to me. It gets 15V, it outputs more or less 12V. CP500 and CP650 were also not temperature regulated AFAIK.Comment
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I don't see a control input to that from any processor. I could have missed it. Instead (I quickly assumed that) the diode in the circuit biasing the base of Q7 likely is designed to keep that transistor off until things warm up. I did not do a thorough analysis.
I have a Pioneer surround system where I think there is a similar circuit and in my opinion it doesn't activate the fan enough. That could also have been the issue here. The fan never came on so yank the circuit to save cost. That, instead and figuring out the correct resistors to make it useful. But probably by that point the original engineer was nowhere to be found. I've seen situations where when you go back over a design you actually can't figure out who was responsible for some part of it. And, if you decide the resistors should have been different, isn't that a warranty issue?
[Edit] I am impressed by how hard you guys are trying to keep this CP750 alive.Last edited by Bruce Cloutier; 01-25-2026, 09:59 AM.Comment
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Ahh, I was hoping they just nixed the header and fan itself. Seems they nixed the whole circuit for BOM costs, and as noted maybe it was not activating when populated.
I guess the more painless approach would just be to take 15v off the supply and power a fan independently, perhaps with an added dc to dc voltage regulator... always on. (perhaps not needed if the PSU is externalized).
Edit: if a DC to DC board seems like a weak link, apparently 15v fans are a thing too.
Example: https://www.aliexpress.us/item/3256804632895262.html
Note I don't trust any 15v fan to be anything other than a 12v fan with it's own step down circuit. Or worse, as 12v fan being rebadged as a 15v fan, just more spinny. But at least could be replaced easily with minimal troubleshooting.Last edited by Ryan Gallagher; 01-25-2026, 09:43 AM.Comment
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It would be great if someone could built a public Digikey or Mouser list of replacement components for the CP750. People can then just "one click order" as many sets as they need instead of having to look up all those parts. And on capacitors, use those with the highest temperature rating available.Comment
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Digikey links:
Capacitors
https://www.digikey.ro/en/products/d...C1V100R/672773
https://www.digikey.ro/en/products/d...26M035/7393940
https://www.digikey.ro/en/products/d...5A9GAA/3083104
https://www.digikey.ro/en/products/d...ML035M/5423044
Power supplies
https://www.digikey.ro/en/products/d...0AShgEIgC6AvkA
https://www.digikey.ro/en/products/d...RT-65C/7706376
https://www.digikey.ro/en/products/d...RT-85C/7706380
Connector
https://www.digikey.ro/en/products/d...-F-BK/15287449
Protection diode
https://www.digikey.ro/en/products/d...KE22A-B/714908
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Great! Now if somone could make a Digkey List ( like https://www.digikey.com/en/mylists/list/F7IU3FFGT6 ) that includes quantities of each part along with reference designators, someone could one click order kits as needed and know where to put each part.Comment
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It looks like someone scored a schematic!Comment
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I doubt that noise (fan) was a factor. You don't want to just plop a fan on your 15V supply with a DC-DC unless you address the noise it is going to put on your analog audio power. Note they called it a buffer in their circuit design.
Remember too, the chassis doesn't have a place for the fan though on the side with the outputs (opposite of the power supply) there is sufficient space to locate a fan if one were to drill some holes for air and mounting.
The next thing that gets all things that move are is dirt. Once there is a fan, there is a dirt-pump in the system that will start to coat the MB...which could bring other thermal issues.Comment
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I'll go through my notes and order history from back then, to see what I ordered. No guarantee they're still around, but I tend to keep such stuff.Comment
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Is a failed LCD a common failure mode on the 750s too?
I personally picked up a very cheap parts one on ebay just to play with. Power supply was definitely bad, swapping that brought it back to life, but the LCD never boots.
Any known failure points there? I haven't tested functionality or the web interface yet, need some days off to dig deeper.
I know there are deeper failure modes and this one may need a full recap too if I feel like an adventure.Comment
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The one I recapped was completely dead, as in, it didn't boot and also didn't show anything on the LCD. There is a difference though between not booting at all and a possible defective LCD.
Does one of the Input Select LEDs eventually light up or not?
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Is the backlight out? That is not normal. The contrast going wonky is semi-common. Adjust it one tick in either direction and it come back. The front panel board can definitely go bad though. I've seen that.Comment
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Since this thread has come back up...here is a recent one (and I have multiple examples of this:
The power supply capacitors were failing (one of the ones at the bottom is domed but I've seen any of them, including the main filter dome up). As the supply fails, it tries to regulate the 5V output but, by consequence it ramps up the +/- 15V rails too. If you look, you'll see that Z14 is cooked. That is to protect the +15V rail. It is an 1N6277A. It's nominal breakdown voltage is 18VDC (17.1V minimum, 18.9V max). So, those analog chips got to see a bit more than +/- 15V. Are they rated up to 36V rails (+/- 18)? the Supply voltage on the SM2142 line amplifiers is +/-18V MAX. the MC33078 Op amps used leading into the line amplifiers are also showing a +/- 18V MAX.
So, you can kind of see how the failing power supply eats the output section (and possibly the analog input too as the +/- 15V rails climb due to the power supply failing.
image.png
Also note the connector itself gets cooked as it moves towards the top.Comment
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On the power supply connector overheating, I had that happen in a product I designed in the 1980s. The switching supply had two pins for +5V, two pins for ground, and more pins for +/- 12V. The 5V and ground pins would overheat. We replaced the normal "crimp and poke" Molex pins in the connector, which have one point of contact per pin, with Trifurcon pins which have three contacts per pin. The rpblem went away.Comment
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