Quote:
Originally Posted by Boom
It used to be the case that absolutely every repairman carried a neon screwdriver and the very first thing they did was to touch it against the chassis to see if it lit and therefore the chassis was live.
I don't know if such neon screwdrivers are still available.
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They're still available, but a non-contact voltage detector is a much better bet. If you place it near say a light switch or power point, typically they flash red, and some give an audible indication too. There are scores of them available online, typically from £5.00 - £10.00, or if you're a 'brand junkie' Fluke do them at around £30, which confer no additional features or benefits, other than the badge. (There are no safety issues, unlike with say a multi-meter)
CPC/Farnell have a range of them from £2.00 upwards:
http://cpc.farnell.com/c/test-equipm...ieCreated=true
This one seems to fit the bill at £3.02 inc VAT:
Spec:
Visual and audible indication of voltage
Green LED indicates tester is operational;
Red LED shows that a voltage is detected
Auto power off feature
CAT IV 1000V rated.
http://cpc.farnell.com/duratool/d030...est/dp/IN07343
As to replacing the caps on the DAC90A tag-strip, if you look at the tags, they're double, with one vertical and used tag paired to one flat against the Paxolin and unused. in my view, the neatest way to replace the caps is to completely snip off the old cap, carefully bend up the unused tags at each side of the top of the board and after cleaning and tinning the tags, solder the new caps to those tags. Best to do one cap at a time to avoid making errors.
I've attached a picture and a sketch to hopefully make this clearer.
As to the filter cap across the mains input, (on top of the chassis - C22, on the Bush data sheets - 0.1uF), as others have said, it must be an x class capacitor, and seems to make do difference whether it's simply clipped out, or whether it's replaced. Many if not most radios don't have one.
The 8-page Bush data is far more comprehensive than the two-page Trader Sheet, and when quoting component numbers, they differ between the two, so always best to state the source of data.
The twin unit smoothing/reservoir capacitors are very reliable, reform well and are rarely the source of hum, almost invariably caused by the EL84 output valve.
Given the amount of work involved in replacing the waxy caps, replacing perished rubber wiring, carrying out live voltage tests on valve-holder tags which are close together - whether it's on this set or any other - with safety in mind and the avoidance of damage, rather than try tp balance the chassis on books, blocks of wood or whatever is to hand, it makes good sense to mount the chassis upside down on a cradle. Simply made from four strips of flat iron and a sheet of scrap plywood or whatever, as shown in the pic below. Why make life difficult and chancy? (The sharp-eyed might notice in pic 3 that at that time, some years ago, I hadn't caught on about bending the disused tags up on the tag-strip).
Others have mentioned that before removing the valves, first remove the retaining springs, and throw the rings as far as you can, because if you don't it's almost certain that you'll break the glass pips off the valves. Even more important if you're fitting new valves. I can only assume that the retaining springs were originally fitted for WD use in ruggedized equipment in WW2 over rough terrain, but they serve no useful purpose in a DAC90A - they just force up the price of UL41s on ebay and increase the scarcity due to broken pips.
Hope that's of interest to someone.