PCS-Electronics

Marco,
I pulled apart an old VCR and yanked out the power supply. Found that it had multiple outputs. 1@45v, 3@14v, 1@12v, and 1@30v(reversed polarity). These measurements are w/o a load. When I hooked it up to my MAX II X-mitter, the voltage dropped from 14v to 12v.

I would like to use the 45v output with a variable resistor in series to knock it down to no more than 16v. What would be a good resistance or "part#" to use safely without blowing the exciter up? If I remember, the current wouldn't change if I run the var-resister in series...right? (it has been about 25 years since my education & somethings I just forget...sometimes).

BTW, this is one CLEAN power supply. I was using a home-made one I built off the web, kind of a sloppy job too, and had a few interference problems (mainly it would cause some wavy lines on TV channel 4 over about 1,000 feet). With this PowerSupply the interference is GONE. The signal sounds better to me also (but this may just be an audio "placebo" effect).

Any help I would appreciate. By the way, thanks for taking time for things like this and all the help you give others too!!!
RadioMan

Why not use those 14V outputs?

Seems easier than getting 45V down to 16?

That is the way I have it now...it works that way, but there is a method to my madness (or madness to my method)...when I hook it up to 14v it drops to 12v with a load (exciter). I would like to experiment with volts vs. power out (=more range?).
rm

Did you say there are two 14V outputs?

actually, there are:
3 (three) @ 14volt outputs.
1(one) @ 12volts,
1 (one) @ 45volts,
1(one) @ 5volts,
1 (one) @ -30volts (the polarity is reversed on that output).

This power supply came from a very expensive Panasonic "PRO-LINE" vcr.
I had two of them that I bought in 1989 (for $1200) for S-VHS editing. The transposrt on BOTH broke down two years ago. With the price falling on that type of gear, it is cheeper to buy new that fix old....hence, it is time to pull them apart for parts etc.
BTW...my multimeter is a small cheap one but it works good for most uses. Drawback is the amperes scale....very low! Like 0.5 milliamps to 250 milliamps. I fried its brother,an identical meter, trying to measure a current rated at 188milliamps.

At what point(s) on the board should I measure for things like E, I, R, if I want to figure out Watts or any reading for that matter?

Or am I just being an idiot?

How about connecting all 14V together? It might be from the same
source anyway, just connected internally.

excuse my ignorance, but what would that accomplish? If I remember, in series the voltage would be equal to the sum of all voltages, in parallel the voltage would be the same at all points...i.e. 14volts. Hooking all three together would be the same as in parallel. I couldn't go from (-) to (+) (serial) if all outputs share a common ground (could I?).

I thought you said the voltage drops when you connect
load to 14V rail, possibly meaning you need more current
on that output. Connecting a few in parallel would improve this
situation, possibly. But they're probably connected internally
anyway, so this would probably accomplish nothing. Besides,
paralel connection of power supplies should usually be done
via summing diodes or something similar, which would drop
0.3V-1V of voltage as well.

I think it may be better to build a new small DC series regulated
power supply. Fiddling with switchers can be lethal, and you would
have to fiddle quite a bit to get that 45V down to 16V (which I think
is a bad idea). Increasing 14V to 16V might be easier, but you'll
still have to figure out which resistor to change. What's worse is
that increasing 14V to 16V might increase 45V to 55V or so at the same
time, meaning you could exceed some capacitors voltage ratings
and set yourself for a small explosion. Not to mention other components,
which might not survive increased voltage (fast rectifiers usually have
quite low max reverse voltage rating).

A couple simple power supply schematics are here, but there are
thousands more available via google.

http://www.pcs-electronics.com/en/guide ... owersupply

A good robust power supply is an absolute basis of any electronic
lab. I've built my own 20A 5-15V unit many years ago. Its a plain
DC series regulated type, with all components overdesigned by about
100%. This means it easily survives a short of the outputs, even though
the isolation starts to melt on the wires which are causing the short.
One needs to be carefull with such stuff, as a short can easily burn
even thick traces on the PCB, not to mention chips and transistors.
Its easy to detect the short this way as you literally burn whatever
is causing it

Its also very good to have at least an A meter. This gives you a general
idea of what's going on with your devices. Too much current tells you
something's not quite right before you smell the burned silicon

thanks for the help & suggestions. As you can tell, I love "fiddling" with things.

In this case, however, I think I will take your advice and start with a new power supply. It's sure not worth blowing up my MAX II trying to get an extra 2 volts into it, or frying out 2 good power supplys that I can certianly use in the future for something else.

Thanks again for the help!!
RM