IanB

Keep us posted @Rookierafa🙂

So here is what I'd likely do: connect your meter, measuring voltage, securely to the output of the 7815 voltage regulator- you may have or may need to obtain crocodile clips that slip onto the multimeter probes. Starting with the main board disconnected, turn the power on. You should be reading -15V. Now plug the main board in and see if the voltage drops straight away to zero.

Hi @Rookierafa are those resistance readings? This isn't something I'd really be checking at this stage. I'm speculating at the moment that there is a short circuit somewhere on the main board causing the negative regulator to shut down. Likely culprits would be either a physical solder "bridge" somewhere (have you soldered anything on the main board?) or a capacitor or possibly transistor which is short circuited. Try doing a continuity test on the main board, while it is unplugged, between the incoming regulated voltage rails and ground. If either is shorted (it "beeps") that will be useful although it probably won't be that simple. It's a bit strange that this all started with the PSU suppression cap blowing but that might be a red herring; possibly blew open circuit years ago.

A good Easter to you also @Rookierafa and I'm sure we'll get this sorted. Will post more thoroughly later (just got home from work, rather tired) but two types of transistor are "ordinary" bipolar ones with the base etc and FETs with drain etc. The power amp has its own supply that doesn't run through the 7815/7915 regulators. It's the outputs at the top of the PSU diagram on the right labelled +VA and -VB on the diagram (but apparently +VA and -VA on the actual PCB.

OK so that's progress. As I said previously, the 7815 will shut down if it sees an overload/short on its output (they are well designed to avoid blowing up). So likely there is a short on the main board somewhere. This might be a physical short circuit, or may be a failed capacitor, transistor or some other component. I'm about to head out to work but I'll take another look at the circuit diagram this evening when I get home. The power readings on the main board will be doing strange things because the negative is basically disconnected when the 7815 is in shutdown.

Hmm. The output of the fuse, the capacitor and the input of the 7915 are all directly connected on the circuit diagram so should all be at the same voltage, it's the -VCC fuse. You said you have measured the voltage at the fuse but didn't mention checking the fuse itself. Is this voltage at both ends (i.e. is the fuse good or blown)? If you have your 27V at the fuse out terminal, then either the PCB track is damaged or the 7815 has a dry solder joint. Take out the fuse and check its continuity with your meter. With power off check you have continuity from the fuse terminal to the 7915 input (centre pin) and also to the capacitor's -ve terminal (I assume it's the capacitor nearest the 7815). Can't think of anything else to suggest right now...

Ok, so it looks like you have no output from the 7915 you replaced (it's a voltage regulator not a transistor btw). Check what voltage you have on its 3 pins. The input is the centre one (2) the output is the right one (3). Is it bolted to the case or a heatsink? Its metal tab is connected to its input so if the heatsink is not isolated or it's bolted to common metalwork it should have an insulating washer. If it has no input, check the fuse before it and the bridge rectifier before that (feel free to post a photo of it). You probably have +15V on both pins of the op amps etc because there's not voltage supply on the negative supply pins at all so the +15 is just backfeeding through them. Also note that the 7915 has internal short circuit protection so it may be fine, but is shutting itself off due to a short circuit on its output, so it may not be the culprit but we will start there. If you can disconnnect the PSU outputs from the other board(s) to check the voltages that would be useful to do for that reason. If there's -15V on 19/brown when it's not connected to anything else, the problem is further down the circuit...

Just to clarify, what's the voltage on pin 19/brown coming out of the Power supply?

Maybe not even corroded, just not making connection properly. Some problems can be fixed just by unplugging and replugging, or waggling, connections. Such problems can be hard to declare "fixed"!

Such problems can be hard to track down, let's see how it performs in a soak test i.e. leave it on for a long time and see what happens.

Have you had a look for the output relay?

"Anthem" is the right word!

If memory serves, there is no power relay, the PSU has a lot of circuitry to test before powering up but it's all transistors and logic gates. The relay just connects the audio output after power up to prevent a "thump". So it sounds like there is something dodgy with the power which is also killing power to the relay as it is intermittent. Seems likely to be a bad solder joint or connector if it works sometimes...

The main weakness of the original implementation was the lack of real time parameter control, you had to repatch to change anything. That would be easily do-able now.

Very, very good question. As a Phase Distortion Synthesis fan myself I'd love to see this. And with modern technology, it would be possible to make it much more flexible- the original implementation was limited by the digital circuits that could be run on a chip fabricated at that time, it's all counters and look-up tables if you look at the patents. I've written a plug-in (not released yet) that has my take on a more flexible PD synthesis with real time control, variable "resonance" etc. I'd love to see Casio release a new CZ.