Oddball Regulator in Sony Amp...

[EchoWars]

0.5 watt should be plenty for the zener. It will normally run at just a few milliamps at most.

Thanks. I did not order any zeners, since I have a number of 9.1V 1/2W devices here and figured to use them.

Q334 could see quite high voltages (you need to scope the collector to see just how high) so I wouldn't go much below that. That said, if you can confirm worst case voltage then 2N5551 might be OK. What about some of the MPSA devices ?

Well, I figure the Panasonic devices I have on the way will do the job (the 2SC3941).

These type of circuits can be twitchy on the exact devices used, particularly things like the SCR's. Gate trigger sensitivity is a big factor.

Yeah, here's where I did some digging to find something appropriate.

NEC 5P2M -
Gate Trigger Current: 15mA max
Gate Trigger Voltage: 1.5V max

Littelfuse S4010LTP -
Gate Trigger Current: 15mA max
Gate Trigger Voltage: 1.5V max

So we may be OK there. I hope.

 

[Mooly]

You can only try the Littelfuse SCR and see. Is the Zener a 9.1v device ? I don't recognise the number but it needs to be correct as it is the reference for the error amplifier.

 

[EchoWars]

Sony uses this TO-92-style zener in a number of units of this vintage. On the power supply of the TAN-8550, for instance, a 1T243M is on the emitter of Q333. No voltage is given at that point, but the schematic says '10V' on the base (that's just one example).

As is was (with the circuit broken), the voltage on the cathode was a hair over 10V, but probably because everything else was so out-of-whack. ilimzn has likely seen a number of these devices from working on VFET amps and could probably confirm if he'd peek at the thread again.

 

[Mooly]

Fair enough I just wanted you to be 100% sure over that part because its absolutely mission critical. If another amp uses it in a similar way as you mention then yes, its going to be around 9v1.

 

[EchoWars]

Holiday season had pulled me away from my work, but this evening I managed to install the new parts in the regulator. For the time being, I left the original zener in place.

I have the driver boards out, so there's really no load on the supply. I'm getting about 70V at the caps, but I get NO change in this voltage when adjusting the trimpot (which is new).

I'm relieved to see that nothing blew up, and that I'm no longer overvolting the caps, but I'm concerned that I cannot get any voltage variation. FWIW, the zener voltage is sitting at around 11V...dunno if that's too high and causing problems or not.

Failure to plan ahead means yet another order placement before I can complete the driver boards, but once in place I'll see if the load test makes any difference.

 

[Mooly]

I would add a little load, even if its just something like a mains bulb.

 

[EchoWars]

Well, I'll have the parts I need by the end of the week.

I tried to put a bulb in the mains (60W), but after the caps charge there's so little current draw that the voltage to the caps still hits 67V, so I think I need a real load...

 

[Mooly]

I meant for the bulb to be across the rail actually

 

[EchoWars]

Lol! Hadn't thought of that. I have a 25W bulb with soldered leads to discharge caps, but too small for this. A 40W bulb from V+ to V- might not last long. Eh, parts will be here in a day or so.

Betting that I'm going to have to tweak resistor values. Thinking that I'll monitor the B-E voltage on Q331 and monitor the voltage on the 10K caps. With a resistor sub box connected across R333, slowly lower the combined value (resistor || sub box) till I see that Q331 will turn off and the SCR can fire. This could be a slow process, so I'd like to have both amp channels running and set for 25W output on each, rather than 50W output on one. The heatsinks on these amps are not what you'd call 'beefy'.