My latest addition - Radialva Groom

[Joe]

Can't see any reason why not. The first DAC90a I did had 1000V caps in and I can only think they were all they had in the factory when it was made and it was more economic to use them rather than await delivery of more.

- Joe

[Yorkie]

I think that when you get to the testing stage John, you'll find that the actual working voltages when you test the set will be way below the WV of the caps - often just a few volts. Same with most sets - the working voltage of the caps is often way over that dictated by the circuit. Cheaper for the makers to buy in large quantities of high voltage caps and gain the economies of scale, than to order caps in several voltages - say 150, 250, 300, 350 etc. It's for a similar reason that the keypads on self-service petrol pumps have Braille markings. The same keypads have other applications and it's easier to make all keypads with the Braille dots added than to make special ones. (Besides which some drivers seem to need Braille, judging by how they drive!).

I assume that the WV of a cap is dictated by the type and thickness of the dielectric material. If thicker material is used, to get the same value of capacitance will mean that the cap will have to be larger in diameter and longer. Having said that, I have some 1,000VW yellow polyester caps that are smaller than 630V ones of the same make.

[Geaney]

Thanks guys, I was sure it would be OK but thought I'd get the team view in case of holes in my knowledge. I've hit another little quandry with the two electrolytics - the circuit diagram clearly shows them as 100uF but the ones fitted are 50uF. I don't have any 100uF caps on hand but the 47uF ones I have will just about fit in the available space. No idea how I'll get 100uF types in there. Anyway, I've ordered some up so I'll see when they arrive.
Working on those older sets is definitely 'different'. The markings on the components are very different from what I'm used to but it's all good fun learning new stuff!

Cheers, John

[Joe]

100µF seems hellish big, especially in a set so old. Assuming that it is correct, are there two fifties connected in parallel to make up the value? With a bit of luck the old ones will be OK anyway.

- Joe

[tiv48]

As it has a valve rectifier 100mf looks far to high if you can find the valve data it should show the maximum value normaly around 16mf for the resevoir and higher for the smothing section.
rob t

[AlanBeckett]

John,
What's the rectifier? There's often a limit value of Reservoir quoted in the data. However, with a big value resistor - Line Cord - in the way it may not matter.
Alan

[Robert Darwent]

I acquired a model 41 Groom some time ago, it is still awaiting my attention to be restored. I can however, confirm that the value of 100 uF for C24 is correct as per the circuit diagram.

C24 is actually two 100 uF electrolytics housed in a large square screening can. One is connected after the 25Z6 rectifier between HT and what looks to be some sort of bias arrangement for the output valve, with the other connected between HT and the chassis.

Regards

[AlanBeckett]

That's an interesting Rectfier - http://www.r-type.org/pdfs/25z6g.pdf
It does say that if more than 40uF is used then the 'it may be necesary to add additional Plate Supply impedance'. Clearly that's the case with a Line Cord, especially on 240V.
I'm not sure how that works with a Capacitive Dropper.
Alan

[Geaney]

Thanks for all the help guys - this is definitely a learning exercise for me - and it keeps going - there appear to be capacitors measured in 'cm' whatever that means. Based on the values I assume pF but I've not come across these types of markings before.

Again, all help and device is welcome.

Cheers, John

[AlanBeckett]

Pretty close - http://mysite.du.edu/~jcalvert/phys/caps.htm
Alan.