Thorn Automation 40v 50A PSU fault

[See_Mos]

I agree about the confusing drawings! Following it through it looks as if the common rail on one card is connected to the control line on another card.

Set the over voltage pot to maximum then monitor the voltage between B9 and B1 on the pre-reg board while changing the output voltage control from min to max then set the output voltage pot to max and swing the over voltage pot, the result should be the same. If the measured voltage swings from zero at minimum to minus nine or ten volts at maximum then the problem may just be the presets on the pre-reg board.

Calibration is described at the bottom of page 34. Make a note of the presets before making any adjustment.

The voltage across the big cap should be six volts higher than the required output, I read the manual this time!

[dave cox]

Hi Sean,

I think I can rule the thyristors out now - I have disconnected the gate drive from each of them in turn, to see if the operating conditions change - I get the same result with either thyristor connected, so I guess I am back into the drive circuit again, trying to understand why they are not being driven harder into conduction.

What effect did this have, if any ?

I am wondering if the gate drive is just a little weak and can't bring the SCR's into conduction when there is less than a 'certain' voltage across them. In a similar vein, if the gate pulse is getting mis-timed as the voltage increases (WRT to the anode / cathode voltage) the SCR will fail to switch on.

Such an effect will likely be easier to see on a storage scope with a long capture where I imagine you might see a much longer period (<<100Hz) cycling between firing @ 100Hz and then not firing at all! This might not show up on an analogue scope ...

dc

[Sean Williams]

Hi Dave,

This had no effect to speak of - the voltage output from the rectifier pack remained the same, with either SCR disconnected.

I will run through the pre reg set up procedure just in case there is something wrong there.

Sadly no storage scope here - not convinced I want to get one to repair a PSU

Will report back, but probably not until the weekend - I have loads on the go this week

[dave cox]

Attached is my understanding of how these SCR pre-regulators work ...
(only shown with 1 SCR working)

One problem I ran into, looking at something similar (a tiny 600W version!), is that unless you are pulling a lot of current the thing needs to fire the SCR's so infrequently that without careful triggering and a dark room (or a storage / digital scope) it might look like its doing nothing This is also very likely why disconnecting 1 SCR made no difference as the remaining SCR is more than man enough, unless you are close to its max rating.

One quirk I did notice, was that by judicious fiddling with the load I could make it only fire 1 of the 2 SCR's, I wondering if this would be problematic with a toroid transformer

I should confess to not looking at the schematics in detail so take it all with a pinch of salt!

[See_Mos]

Set up of the pre regulator continues at the top of page 36

[Sean Williams]

Been driving myself mad trying to sort this power supply - so, took some direct action.

Thanks to Chris (Onewatt) who had two of these supplies in an untested state.

We powered both of them up, and found there was a set of viable control cards - I borrowed them, and swapped them into my supply.

I can now say that all of the heavy stuff is working, further substitution tests has proven beyond doubt that the Pre Regulator card is at fault.

While this means that I am not much further forward in the scheme of things, it does mean I can concentrate on one specific area, instead of being drawn all around arts that are functioning as they should.

I will take some measurements from the functioning cards now and compare findings with the faulty unit.

[stevehertz]

Did Ultra provide any help or information?

[Sean Williams]

Sadly not - no reply at all

[stevehertz]

Out of interest, what do you plan to use it for Sean?

[Sean Williams]

Hi Steve,

At present, I plan to use it to power up some avionic equipment - A Collins 17L-7 transmitter, a 51x2 receiver, and the VOR localiser and nav instruments.

I also have some other aircraft gear that needs 115V at 400HZ - I have some beefy rotary converters that need a supply this large to deal with the starting current.

[stevehertz]

Should do the job when working. Thorn's 50/40 was a big seller for us, thousands sold. A very good, hefty (as you know!) PS.

[Sean Williams]

I thought I had better post a final outcome on this thread.

The fault has been fixed.

After much head scratching, and a lot of replacement parts (based on age alone) TR11 on the Pre Regulator board had gone "leaky" - replacement of this component has returned the power supply to full health.

Thanks to everyone that have offered help and assistance in getting this beast back into full health again.

I can now drive everyone potty with the noise of rotary converters, and HUM! (at 50A this supply isn't exactly quiet!)

Cheers
Sean