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Old 17th May 2017, 1:41 pm   #1
Chris Wilson
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Join Date: Nov 2011
Location: Whitchurch, Shropshire, UK.
Posts: 612
Default Farnell H60/50 blowing pass transistors, output goes unregulated.

I am having a bit of a saga with my Farnell H60/50 linear power supply I mended years ago. It's taken to blowing pass transistors. What happens is I power it on and often it works fine. I often run it at the full 60V output with a current draw of about 35 to 40 Amps. Occasionally, and seemingly more frequently, it will have uncontrolled voltage output, quite how high it gets I haven't managed to catch, but the 60V meter goes off scale. If I am very quick to turn it off it may work fine again. If I am not a pass transistor fails shorted and takes out D1 and R5 on the control board. These machines seem to have undergone several revisions, and although I have schematics that almost match, the manual I have is for an earlier version that used all discrete components, whilst this has some IC's. I think the functionality is similar for all though. I acquired another one at the weekend, but it's sat in my estate car until I can bribe my pal to help get it upstairs to my shack. I took a look at the boards in that and whilst very similar D1 on the control board has a 10nF film cap across it, as shown on the schematics, whilst my control board does not.

I could really use some help with fault finding as I am still pretty raw at diagnostics.

I think I need a way to protect the PITA to change pass transistors whilst I try and see if the voltage rise is due to the SCR board, or where the trouble lies. Am I right in thinking the primary winding SCR's should keep the secondary no higher than say 10V above the voltage dialled in, to keep the heat in the pass transistor assembly down? My own thoughts are I should try and isolate the pass transistors somehow and see if I can find why the input to them rises uncontrollably, checking if the SCR's are being fired. I may be well off though, so advice is sought! I have reasonable test gear including a decent scope. I have changed all the boards electrolytics a few years back. On another forum someone made this statement about why a shorted pass transistor takes out D1 and R5, which lights up like a bulb before opening:

Quote:
Hi Chris. You didn't say if D1 was open or short but (they usually fail shorted due excess voltage but they can fail open if you have put a large current through them )

Anyway I would say yes a shorted pass transistor could cause R5 and D1 to fail (and possibly p2,p1 voltage adjust pot if your unlucky) due to excess current through those devices .

Explanation :- If a pass transistor is shorted then V output would then be at max and someone would try to reduce output by turning V adj to minimum value ,now since the +-15V aux supply gnd rail is tied to main V+ out (so you will now have 60 V between this and -Vout) so now an excessive current will flow through D1,R5 via P1-P2 voltage adj control (which now is set to a low ohmic value when you tried to turn voltage down )back to -Vout .

Note that a failed D1 on it's own ( D1 is shorted) could also cause R5 to fail because the output could again be at max V (this time the inputs of IC1 would be shorted together by D1 and so IC1 output would be stuck at one of the supply rails due to it's own input offset voltage ). Again trying to adjusting the output voltage down would cause large current through R5 ,P1-P2 to -Vout.
I found my actual board didn't have C10 as mentioned above. But I am loathe to add components whilst trouble shooting in case I make matters worse.

I can link to the schematics for both my machine, which seem to match it 99%, and for the earlier one with all discrete components instead of op amps and IC Schmidt triggers. I have the manual which explains the topology of the earlier version too.

Having rebuilt the pass transistor tunnel heat sink assembly several times, and it being costly and time consuming, I need a better way to diagnose this!!

Thanks for looking.

Schematics that match my faulty unit 99% fully, and the manual that explains the board functions, but is for an earlier version that uses all discrete components are at:

http://www.gatesgarth.com/supply.zip

The schematics for the earlier all discrete components version are at:

http://www.gatesgarth.com/H60-50_Diagrams(2).pdf

All help greatly appreciated. I am as yet unsure if the new one I got at the weekend is functional or not, i just saw it powered up and the voltage control and meter working with zero load on it. I don't really want to mess with it and end up with two knackered ones <LOL>
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CW. 2E0ILY
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