I have a Radio Shack 22-504 PS that quit working. I discovered the transformer
leads were all corroded beyond salvaging. The transformer in it is a 18V, 3A. I have
a 24V, 4A that I bought for another project that never happened. I was wondering if
the original circuit would be able to hande the higher voltage and amps?
Also would there be a way to modify it so that it would be variable voltage?
Thanks for any help.

 

Unlikely, without some major changes.
As far as the voltage goes, probably the only things to check are the voltage ratings of the capacitors.
The higher input voltage from the proposed transformer, higher currents, and making it adjustable to lower output voltages will all increase the heat produced in Q4 so a better heatsink would be required.
I believe the principal determinant of the regulated output voltage is ZD1 so some change to the way it works is going to be needed to make it variable.

 

Thanks for the info Albert. I was afraid it might take alot of changes to make it variable.
You said that ZD1 sets the voltage, does that mean if I use the 24v, 4A transformer, it will
still put out the regulated voltage it did before?

 

I would Try changing R10 to 100 Ohm / VR1 to 1k Ohm and R7 to 100 ohm.
that should make vr1 change the voltage much more.
the transformer should work but like Fisher77 said keep an eye on the temps.
Watch for hot spots prior to smoke.

 

It will have the same output voltage as before, assuming nothing is overloaded by the extra volts.
I would expect it to be ok, but there will be more heat generated for the same current as before as now the regulator will have to drop an extre 6 to 8 volts. That can equate to quite a power rise. You may find it works better.

 

Hello,

I would replace C1 and C2 for an higher voltage type as the 24 Volts will create about 34 Volts on those capacitors.
I would use 50 Volts or higher types.

Bertus

 

Thanks for the info Albert. I was afraid it might take alot of changes to make it variable.
You said that ZD1 sets the voltage, does that mean if I use the 24v, 4A transformer, it will
still put out the regulated voltage it did before?

It should (hopefully) regulate as before, but its dumping a lot of extra voltage which usually turns up as heat.

Before anything else; make sure all the transistors are rated sufficiently for the higher voltage. Electrolytic capacitors can cause a bit of excitement if you don't make sure they can take the new voltage - the 24VRMS secondary needs to be multiplied by 1.414 to get the peak value. most resistors are calculated to set the current flowing into some component or other - you probably have to re calculate the values for the same current with the higher voltage. This usually doesn't apply to the resistor divider that samples the output voltage - its the ratio that matters and if the regulation holds; no increase in current.

If increased dissipation is a problem - the fan off an old CPU cooler might do the trick.

 

If it's possible, try to remove a few turns on the new transformer secondary.,

 

Thanks for the replies. I wasn't thinking about it last night when I was posting,
and didn't remember till I pulled the transformer out of the cabinet today, but the
replacement transformer is center tapped. So by using the center tap I can
maintain close to the orignal voltage. Now that leaves the current.
The replacement transformer is a 100VA. But since I would be using the center for 12V,
wouldnt that only be using half of the 100VA?
If that is correct it would be pretty close the original voltage and amp rating.

 

Hello,

Using 12 Volts as input might be a bit short for regulation as the rectified voltage will be about 16 Volts.
Better look for a 15 to 18 Volts transformer.

Bertus

 

The collector of the output transistor could be supplied a lower voltage than the rest of the circuit.

Instead of using just half of the center tapped secondary of transformer, configure the whole of it in a center tap circuit by removing the negative output connection of the bridge rectifier from the circuit and connecting the center tap to the negative instead. That voltage supplies just the output transistor collector.

Then use voltage doubling to get the higher voltages to operate the rest of the circuit. As suggested, change capacitors to higher voltage ones as needed.

Since the output transistor is generally operating off of a lower voltage, it will tend to run cooler than originally. I can't say if under a load the output current will reach the full rating before the voltage of the transformer sags too much to maintain regulation from the output transistor. However, the lower voltage of the replacement transformer will tend to be offset by its having almost twice the VA rating.

 

Well I replaced the caps with 50V. Wired in the new transformer, and the supply is putting out a whopping 33v.

 

Ok, I did some rather dodgy butchering to the original transformer and replaced the corroded
wires for testing purposes. I then reconnected it to the circuit and turned it on. I am now
getting 23v on the emitter, base, and collector of Q4. Does this mean Q4 is toast, or
something in the circuit is toast?

I tested Q4 following this video and it passed the test:

,

 

You can remove Q4 from the circuit and check its diode junctions with a multimeter set on diode check.

 

You can remove Q4 from the circuit and check its diode junctions with a multimeter set on diode check.

Ok from base to collector it reads .023 and from collector to base it reads the same.
From base to emitter, it reads OL in both directions.
Looks like Q4 is toast.

 

Indeed, it appears shorted from base to collector, keeping it from turning off. That explains all the connections to it being at the collector voltage. If Q4 is out of the circuit, it is simpler to check Q3. Usually bipolar transistors fail by developing a short circuit from collection to emitter. While it is out, is is a good idea to check it's driver, Q3.

 

Can anyone explain the circuitry around Q1? It appears to be injecting a half wave rectified signal into the control loop.

 

I think it is to start the regulator up as the control power is from the output when running.
To start power is fed via R6/D8 to turn Q2 on.
When running, ZD1/R10/Vr1/R7 is used, and R2/R5 turn Q1 on to stop the startup circuitry.
At least I think that is what is going on.
A very odd circuit.

 

I was puzzling over it also. My take on it also is that it allows start-up by keeping Q1 turned on during power-up start-up.

However, I think the input of pulses from the transformer also have something to do with prevention of latch-up of the overload protection so that once any overload were to be removed, the circuit could start-up again.

 

That could be it. I was wondering why they startup needs to be pulsed.