Unregulated power rail - Neotek Series 1E PSU

[fazeka]

Hi,

I have a Series 1E console 12x8. It's around a late '84/early '85 build. It has a TPS-05 power supply that I just recapped. Also, gave the PSU a good cleaning and even reapplied thermal compound to the bridge rectifier bolted to the front panel and the power transistors bolted to the heatsink and even replaced the two MV5753 LEDs.

Here are some pics:

So after completing all that, I checked the voltages and I get -17.94V and 18.06V for the +/-18, 48.0V for the phantom. So far so good.

However, I get 40.5V on the unregulated ("AUX" on the PCB). The TPS-05 (8510181) schematic shows a note for that rail as "+30 UNREGULATED":

Not sure if there's a problem with my PSU (I'm doing my checks before plugging into my console and potentially damage something) or if I am misunderstanding the note on the schematic.

Perhaps that note on the schematic means that rail is unregulated and when plugged in with the load of the meters/LEDs, the voltage will (should) be at around 30V.

Or is there a problem with my PSU?

Thanks,
Chris

 

[JohnRoberts]

You may be Ok, just seeing unloaded voltage rise.. 

What is voltage rating of C119 on that rail? Hopefully more than 40V.

That circuit is simple enough, hard to imagine a fault that could cause too much voltage there.

JR

 

[deva]

Can someone explain me the purpose of Q105, part of the +48V voltage supply? (or Q106, can't read clearly - the transistor whose collector goes to the output, and has base and collector connected through a 4R7 resistor).

I was initially thinking some kind of current limiting, but I can't see how that would work without swapping collector and emitter, so I guess it serves some other purpose.

Sorry for hijacking the topic!

 

[fazeka]

Hi John,

Honored to have you answer my question. I have one of your (older) designs in my rack, Loft 450. =)

What is voltage rating of C119 on that rail? Hopefully more than 40V.

C119 is 10,000uF/50V, as are C102/C103/C104/C104. The originals were 10,000uF/35V.

Thanks,
Chris

 

[fazeka]

Hi John,

One question, though.

Will the ICs in the meters like/handle the extra few volts if their load doesn't quite pull the voltage down enough?

Thanks again,
Chris

 

[JohnRoberts]

Can someone explain me the purpose of Q105, part of the +48V voltage supply? (or Q106, can't read clearly - the transistor whose collector goes to the output, and has base and collector connected through a 4R7 resistor).

Looks like it is just drawn incorrectly... with emitter and collector reversed from drawing it forms a simple roughly 100mA current limit. 

Bipolar transistors will actually work with emitter and collector reversed, but things like beta suffer and collector breakdown would now be the reverse emitter zenering. For this circuit I don't see any hypothetical benefit from the rather esoteric trick of running the transistor backwards,

Mistakes like this are actually pretty common with old hand drawn schematics that do not have any logical connection to PCB layout software... this keeps you on your toes, when looking at other peoples designs.

JR

I was initially thinking some kind of current limiting, but I can't see how that would work without swapping collector and emitter, so I guess it serves some other purpose.

Sorry for hijacking the topic!

 

[JohnRoberts]

Hi John,

One question, though.

Will the ICs in the meters like/handle the extra few volts if their load doesn't quite pull the voltage down enough?

Thanks again,
Chris

it's a neotek who cares... 

Seriously looks at the PNs and data sheets for the ICs. I have no idea what is in there... Most common IC processes can handle 36V or so...

If the design had a 35V cap in there I suspect it loads down to 35 or less. (it better).

JR



JR

 

[fazeka]

Will the ICs in the meters like/handle the extra few volts if their load doesn't quite pull the voltage down enough?

it's a neotek who cares... 

D'oh!

[quote author=JohnRoberts]
Seriously looks at the PNs and data sheets for the ICs. I have no idea what is in there... Most common IC processes can handle 36V or so...
[/quote]

Six µPC324 and one TL074 per channel of LEDs.

[quote author=JohnRoberts]
If the design had a 35V cap in there I suspect it loads down to 35 or less. (it better).
[/quote]

I hope so, µPC324 datasheet states +/-15V split or 30V single, max.

Thanks again,
Chris