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Techman 19th May 2016 1:28 am

Roband Varex 60-5 power supply
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Another of the 1 'bargains' from the recent 'silent key' sale here in the wilds of rural Lincolnshire. This is a real 'boat anchor' of a power pack and it does work but has one fault. I've read a couple of previous threads on this power supply and downloaded the service information from the link in one of these threads that covers the 'twin' version, but should be near enough the same, and I've read about the problem with the current limit control in one of these threads.

The current limit controls and circuitry seem to work perfectly on this unit, but the problem is with the voltage control. As long as the voltage control RV101B 'fine' outer ring is set fully anti-clockwise, the inner control RV101A can be used to set the voltage without problems. However, if the 'fine' control is moved more than three divisions from its end stop, the voltage will go straight up to 105 volts! This is a bit of a worry should I happen to forget NOT to touch the 'fine' adjustment, that this voltage increase can suddenly happen like this. I've not opened the power unit yet, but on listening to the control as it's rotated I can hear it making that familiar sound of a wire wound pot with a damaged section of track just in one place - probably caused by battery charging with the settings too low as warned about in the manual. I'm thinking that the best way forward would be to hard wire this 'pot' out of circuit so that it no longer 'adjusts' anything as there's little chance of replacing it or easily repairing it - any thoughts?

Techman 19th May 2016 1:47 am

Re: Roband Varex 60-5 power supply
I've just found the thread with that service manual link for the 'twin' version as mentioned above:-

dave cox 19th May 2016 9:24 am

Re: Roband Varex 60-5 power supply
You could replace the concentric pots with a single 10 turn pot if the current is low enough.


About 4mA if its like the twin, ie 6V2 / 1K5

Techman 25th May 2016 5:59 pm

Re: Roband Varex 60-5 power supply
5 Attachment(s)
Thanks Dave, that's certainly a thought. Having got quite a good selection of power supplies (admittedly not a stabilised one with exactly the voltage and current rage of this exact one) I'm not going to spend a lot of time on it.

Like a number of pieces of test gear at this sale, it had a 'strange' two pin plug connected, but with a side earth connection. There was some debate in the room as to whether these were for American 110 volt mains. I thought this very unlikely and the 'different' plug was probably so that they would only be used on a 'dedicated' (probably isolated or otherwise protected) workshop supply - I have to admit that I kept quiet as this was likely to put other folk off items that I may be after:devil: After all, these were all apparent 'radio hams' (not that that would necessarily mean that they would be particularly knowledgeable about such things), anyway, at an auction it's every man for himself. Having said that, I did call out "let him have it" on a number of items when I felt that the person bidding against me was perhaps in more need than me (and also because I'm a bit 'tight' and also trying not to buy too much old junk to take home).

Today I had a look inside the power unit - it's been opened before as there's a washer missing from one of the case screws and another only finger tight. I think the voltage pots have been out and dismantled before as
the 'orientation' of them was different to the current ones, causing stress on the wiring to the green LED, dislodging it from its holder, so there's been a history of trouble in this area. I note that it was mentioned in the thread that I linked to about the fact that the 'fine' voltage control wasn't working, so linking this pot out could have been what was done on that power unit.

The pictures below show the unit 'as-is' and with the pots removed from the front panel - the values are different from those shown in the circuit, so suspect there'll be some other production differences if one were to look closely. Having now moved the red/green wire onto another terminal on the coarse pot the unit works perfectly with no sudden doubling of voltages. It has to be said that there's something to be said for the 'bog standard' un-regulated power supply with just a transformer, rectifier and smoothing capacitor when it comes to the risk of component failure causing catastrophic increase in voltage in these complex regulated supplies. I would think that this power unit would have been very expensive in its day.

Pictures below of the innards of the unit - once it stops raining and dries up outside and before I put the case back on, I'll take it out and give it a blow out with the air blower and fine paint brush to remove all the years of muck, dust and the large dead spider that I noticed behind the rear heat sink.

dave cox 1st Jun 2016 11:36 am

Re: Roband Varex 60-5 power supply
These can be programmed for voltage with a single resistor if you're worried (as you should be!) about the voltage jumping about as you twiddle the pot. For sensitive circuits I would also avoid using the ON switch on the front panel as well, as you may get some overshoot ... The other thing I try and avoid doing is using the set "I" option as this shorts the O/P, if the current pot is noisy you will very likely blow the pass transistors !

What I do like about these units is the absence of a fan, thanks to the thyristor bridge, but this also has the downside of making the unit rather useless in current mode !


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