I have just become the proud? owner of one of these beauties. - ex Ferrymead RPS. It is in quite good order but needs a full restoration which I will undertake soon. I have found quite a bit of info on the net but the only circuit I can find is very murky. Can anyone help please?
I have found a couple of errors in the circuit as I checked it against the VCT. A revised circuit & component list are attached. I have also drawn out each function circuit and that's when I found the errors. It turns out that my unit doesn't have the vibrator supply fitted. I'm getting there with the restoration. I have repaired the meter which had a loose glass & very dirty scale plate, repacked the two can condensers and started on the front panel prep for a repaint. I am going to have the hinges etc nickel plated so if you want yours done I am happy to do them all together.
Many thanks for the additional circuits, the individual function ones are most helpful. My VCT is in reasonably good condition although it has been “got at” a bit over the years. Two holes have been drilled in the front panel to add an on/off switch and a switch that reverses the polarity of the meter. I will retain the on/off switch, but have removed the other switch and will put a fuse holder in its place. Inside a few of the caps had been replaced along with R36 and R35 has been changed to a 500k which making the 1000V range into a 500V range (which was not noted on the front panel!)
I did notice a couple of differences between your schematic and my unit. R21 is 20ohms rather than 20k, and C3 is a 0.25uF instead of 5uF. My VCT is the model VCT-V and is fitted with the vibrator.
The hinges and clips on mine are painted back and in good order, so I’ll pass on the nickel plating offer, but thanks all the same.
R21 is a meter shunt on the Electrolytic checks only. My meter is 100 ohms and R21 is definitely 20 ohms. This, in shunt with the 100 ohm meter resistance, will give a full scale reading of 6mA so 20 ohms seems more likely than 20k? C3 is really just a filter cap. I have replaced the 5uf one with a 10uf one so we'll see what happens. - should be fine. I have replaced all the caps and all but 2 of the resistors were more than 10% out. It took a bit of time playing with serial & parallel resistors to get the correct values! On to the repainting of the front panel now.
You have R21 listed as 20,000ohm on the component list. It's odd though that there is such a difference in the C3 values. I'm pretty sure the cap is original, its a old "Chanex" branded 0.25uF. Possibly this value was used because of the vibrator in my unit to filter some of the hash rather than than the AC ripple from rectified mains.
Here is a photo of the front panel of my VCT before I started work.
Interesting to note that there is a second UX5 socket on the left of yours. My unit has a 5 pin side contact socket there which requires the 2 mounting screws. I've only ever come across one 5 pin side contact valve. A VY2 half wave rectifier in a German Kleinempfanger radio which I repaired recently.
Yours appears to be an earlier serial number to mine, maybe the side contact 5 pin was dropped in later models. Or buyers may have been able to specify which socket they wanted in that position when ordering a VCT?
I have now finished replacing all the bad resistors and caps in my unit. Most of the resistors were out of spec and it also took me a while to put together the required values, fortunately I had some 1% tolerance resistors left over from a VTVM restoration that were of use here. The advertising for the VCT mentions that the resistors were “hand selected” so colour codes don’t necessarily match the actual value of the resistors in the series / parallel networks which I found confusing. This process would have taken me a lot longer if not for your circuits and component list!
As I mentioned earlier, my VCT-V uses a 0.25uF instead of a 5uF as C3 (filter cap) 0.25uF in this position results in the test voltages for the two electrolytic tests, paper condenser test and Megohms tests being low with lots of AC ripple present. I tried installing a 10uF in this position, but this results in the test voltages going too high (42V on the 25V electrolytic test) I will order some 4.7uF caps and see if that brings the voltage down a bit.
That's very interesting. A closer look at my C3 shows 2uf, so I may have to alter the value later when I get to powering it up. I too have spent a long time selecting & matching resistors. I have had a disaster with the refinishing of the front panel which has set me back. I had to strip & repaint it because much of the old paint had flaked off. I was all done except for the final clear coat but despite a satisfactory test on a piece of wood, the paint bubbled and crazed almost as soon as I had sprayed it. - there were a few very bad words! I'm ready to reapply the black and have a suitable clear coat ready which I have satisfactorily tested on a scrap of metal this time.
You wouldn't believe it! I have prepared, undercoated & top-coated the front panel. It looked excellent and took a long time to achieve. I then applied the final clear coat and b..... me some of the black paint has crinkled. All that work down the drain and some very bad words to go with it. I don't know why this has happened as I specifically asked the MITRE 10 person about compatibility AND I did a trial run on a scrap piece of aluminium which was perfect. Strange that it only happened in 3 small places, the rest is fine. Anyone have any ideas as I don't want to go through the whole process again to end up with the same result?
Could it be contamination? - the reaction is likely nothing to do with the two coats, but something else - for example silicones can cause this on timber when spraying lacquers - if you use something like shellac as a 'primer', it will go on fine but the lacquers will fish-eye. Contaminated surfaces? Brushes? Paints?
That sucks, and I feel for you - been there done that and got the T-shirt (then used the t-shirt dipped in Acetone to try and remove all the hard work!)...
There are no personal problems that can't be solved with the liberal application of high explosives
That's a real shame you are having such trouble with the repainting. I have had a similar experience when restoring an 18th century longcase clock dial. I refilled with engraving with new black wax after re-silvering the dial, then when I lacquered the dial (the last step) the lacquer made the wax run! I was not happy! I had to redo the whole dial and I ended up sealing it with beeswax as they did "back in the day" rather than lacquer.
My VCT is up and running now. I had to do a bit of experimenting with C3. I put in a 2uF after taking out the 10uF. However 2uF was still too large a value. 25V electro test was still about 40V and the the line check was pegged full scale on all positions. I then worked out the original value in my unit was 0.47uF. They used two 0.25's in parallel, one hidden under the wiring at the top of the transformer. After putting in a 0.47uF the line check worked correctly and the electro test voltage was between 27 and 30V depending on the line check setting.
I am yet to check the AC volts and current functions, but everything else is working and I'm impressed with the DC voltage and resistance accuracy. I like the neon paper condenser test, its quick and easy and a bit of fun to test some old caps. The megohms range takes a bit of getting used to as you have to multiply the ohms scale by 500.
Now I need to move on to the case. Its in pretty good shape, but the inside compartment for storing the power cord is damaged and I'll have to rebuild it.
I hope you have some success with the front panel, I'm sure it will look great when its done!