TS24 PSU - Incorrect Output Voltages

[blue_thunder]

I have a Soundcraft Series 800B that is powered using a TS24 PSU. I was noticing there was a bit of noise on my input channels whenever I switched on the Phantom Power, so I decided to measure the phantom power voltage. It's measuring about 67 volts, and so I'm assuming that's the reason for the noise.

I've never troubleshooted a psu before and can't seem to find a schematic for this particular psu. I'm guessing the problem is a faulty transistor, and was considering just replacing all 6 transistors in the 48V circuit. Is this a reasonable course of action, or is there a better way for me to do this?

Also, Does anyone know what purpose the variable resistor serves? Is it part of a voltage divider that sets the output voltage, or is it used to bias one of the transistors?

Thanks,
Joe

[blue_thunder]

No responses. :-(

I guess you guys are as stumped by this as I am. Looks like my best bet may be to just start replacing components until I find the culprit.

Thanks anyway,
Joe

[jensenmann]

Without schematics it´s hard to tell anything. 67V could be the unregulated voltage which feeds the regulator circuit. I´d guess that something in the regulator circuit is blown causing the unregulated voltage to pass.
If you don´t know exactly what you´re doing I´d highly recommend to have a real tech repair your PSU. Besides having lethal voltages inside a mistake in the wrong spot can easily destroy parts of your console. I don´t know if you want to risk that for the sake of saving 100 bucks.

[blue_thunder]

I'm more interested in learning how to troubleshoot this myself, than saving $100 bucks. What steps would a tech go through to troubleshoot this? And, why couldn't I do the same thing?

[blue_thunder]

I guess I could just pull each transistor and test the leads with a diode tester to see if they've gone bad.

[GJC Designs]

Whereabouts are you measuring the 67V?
If it's the DC that goes to the console, then it is definitely bad news!


As you have not noted a PCB number for the PSU, this is condensed description of PSUs of the time:

At the time the 800B and TS24 were being made, SC had 3 PSU types in production.
"Small console PSU"..... used for exactly that..... no use at all on an 800B.
"Standard console PSU". 2U 19", compact and heavy with finger-cutting heatsink at the back. Used singly or in pairs to power most consoles. The usual PSU for 800B 1600, 2400, Series 4. Mostly one PSU per console, but 2 would be used for large format consoles. IIRC this also applied to TS24.
"Large console PSU". 3U (or was it 4U?) with a fan and an internal sloping metal tunnel to guide the forced air flow over its internal PCB mounted heatsinks. Used to power TS24 and large versions of consoles noted above that would have otherwise used a pair of Standard PSUs.

Is it the Standard PSU, the Large PSU, or some other model that you have?


SC, like most manufacturers, would re-use existing and known-to-work circuits. The phantom regulator could be the same or similar in the Large and Standard PSUs; it's worth checking. There should be a Standard PSU schematic at the back of the 800B manual.

If it is a large or standard PSU that you have, the preset pot sets the output voltage.

I guess that the PSU is >30 years old, in which case dried-out caps may be the source of trouble. Bad output voltage and noticeable noise are plausible symptoms.

[blue_thunder]

It is the large format psu, with the "sloping metal tunnel". I recapped it about 5 years ago, and there is actually only one electrolytic cap in the 48V circuit.

Since my original post, I have pulled each of the transistors in the circuit and tested them with a diode tester. I found that one of the A970GR & one of the C2240BL transistors were bad. I couldn't find exact replacements, so I replaced them with a A992FB & C1845FT (respectively). Now my output voltage is around 25V. I'm thinking I should just pull each component in the circuit and test them to see if they are within spec.