Farnell H50/60 uncontrolled V out!

[Chris Wilson]

I had big problems with an elderly Farnell 60V @ 50 Amp linear adjustable bench supply a long time ago and thanks to help here and elsewhere I fixed it.

Sadly it went bang the other night again, blowing a 50A internal fuse between the full wave bridge rectifier / smoothing caps and the pass transistor block. I replaced the fuse with a temporary 10 A one and on turn on see over 70V DC at the output, with no voltage control. I believe if a pass transistor was shorted the fuse between the bridge and itself would blow? I attach the schematic of the bridge, I changed all the pass transistors from 2N2055 to MJ15003 about 2 years ago, which have a much higher 120V DC rating, as last time this thing had an uncontrolled output from the bridge it blew some 2N2055's.

Note, the input to the bridge is supposed to be controlled via a circuit that uses mark space from the main transformer, the idea being to not have a high drop voltage needed on the pass transistor block. This may have gone berserk but i am not sure if a pass block issue could be feeding the control circuit bum info. So I thought I would try and test it in chunks first seeing if I can use an external supply on the base and wherever on the pass block to see if it will control the output. Then work backwards towards the input of the supply.


So how can i externally control the pass block from a separate bench supply? Where to connect what to DISconnect, and what voltage to apply?

Needless to say I am a relative beginner here But I do have some decent test gear and have the correct service manual and schematics, at long last.
Many thanks!

[See_Mos]

I remember this.

The diode and capacitor across the output, or whatever was connected to the output, are the only things that would blow the fuse. Check them for short before proceeding.

The pass block is easy to test. Fit a new fuse or bypass it with a lower value one for the purpose of testing.

Connect your power supply across the big electrolytics. Connect a resistor across the output, anything from 10 to 100 ohms will do, just consider the wattage which should be at least 5 watts.

Make up two resistors in series, 100 ohms each will do. Disconnect terminal C and connect the junction of the two to terminal C. Connect one free end to positive and the other to negative on the big 'lytics. Connect your meter across the output load and wind up the external supply.

The output should be about half of the input voltage. If you disconnect one or other of the two 100 ohm resistors the output should go to zero or close to the input voltage depending on which one you disconnect. Alternatively if you have a wirewound pot up to 5K use that instead of the two 100 ohm resistor and use that to wind the output voltage up and down.

[See_Mos]

Is the copy of the diagrams now in your possession better then the ones posted previously?

Looking at the SCR control board diagram, what are the three terminal numbers at the bottom left?

If you disconnect the series regulator, with F1 in circuit, and power the Farnell the voltage across the big capacitors should be quite low, around 10 volts if I remember correctly. If it is very high the fault is probably on the SCR control card.

[See_Mos]

Where is terminal 10 on the SCR control card? I can't see it on the board diagram but it's there on the main diagram.

[See_Mos]

Terminal 10 is only there on one version. If you have the correct manual for your equipment that would help.

BTW did you check if the primary side SCR's are S/C ?

[Julesomega]

The model we are discussing is actually the H60/50, 60V 50A version. Your schematics show this, but if you need any more of the manual scanning let me know

[See_Mos]

I found the original thread and the rest of the diagrams.

https://www.vintage-radio.net/forum/...d.php?t=138777