DAC 90A Smoothing Capacitor replacement

[JasonKillbourn]

Hello there, I am probably just a bit out of my depth here, but I found an old DAC 90A in a loft, and, having given it a tentative clean with a paintbrush and a squirt of contact cleaner, was surprised to find it still works. When I say works, the lights are dead, and there's an audible humming, but it managed to find three stations on MW (no life on LW though), before I probably quite sensibly decided to switch it off, pending a bit of research and restoration.
I've since had a good read online, especially this forum, and, from what I've seen, it would appear to be pretty original inside, as all the ancient toffee coloured wax capacitors are there, and it doesn't look like it's been touched in half a century.
Anyway, I seem to be catching the bug, and would like to give it a new lease of life, so I've got a repair set of replacement capacitors and bulbs on the way, but obviously they don't include the electrolytic smoothing caps in a can, so I've been searching for a suitable replacement for that particular arrangement, and I think I've found some, just down the road in North London (I am in Potters Bar). However their spec isn't 100% identical to OE, as they are rated as:
1. Axial Electrolytic Capacitor. Low leakage. 16uF 450V Ripple Current 130mA @ 120Hz
2. Axial Electrolytic Capacitor. Low leakage. 33uF 450V 15 x 35mm. Ripple Current 205mA @ 125Hz
So, what I wanted to ask was, would these two be a workable replacement for the can, if I unsolder it, bundle the two replacements together with heat shrink and connect all negatives together, with the new 33uF positive to the wire that was feeding the red terminal, and the 16uF to the wire that was feeding the yellow terminal? Apologies if I am a bit short of the mark here, but I am usually more up on mechanical as opposed to electronics.
Thank you in advance for any replies...

[TonyDuell]

Firstly, let me say that I wish all 'first posts' were like this. You have given enough information -- model of the radio, possible faults, and so on -- that we have a good idea what you are doing.

And let me guess that the supplier you mention is Cricklewood Electronics. A lot of us use them and they are generally reliable.

As for the smoothing capacitor can, electrolytic capacitors are never close tolerance components. The 33uF would be a perfect replacement for the original 32uF. And you can always fit replacements with a (sensibly) higher working voltage. 450V ones will be fine here.

You have to connect the -ve leads of the new capacitors together (as you suggest) _and_ connect that to the radio chassis. The original capacitor had the -ve terminal connected to the metal can, and thus to the chassis. Some people here re-stuff the old capacitor, they extract the innards from the can and fit (here) 2 new capacitors inside. It's up to you.

As for the other capacitors, the one you must replace is the one from the anode of the UBC41 to the grid resistors of the UL41. It's C18 on the Trader Sheet (1161) if you are working from that. After doing that, replace the other capacitors one at a time and check the set after each one. Then if you make a mistake you'll know which capacitor you mis-connected.

You might want to remove the capacitor across the mains input (C22 on the Trader Sheet) for the moment, If it shorts the result is spectacular and messy! Replace it with a class X2 capacitor.

In general capacitors under 1nF (= 0.001uF, 1000pF) do not need to be changed. In fact changing them is a bad idea, many of them are used to tune various parts of the circuit and if you replace them you will have to do a full realignment of the radio. This is not impossible, but probably something you don't want to have to do.

A couple of other points about this set. Firstly, it is a 'live chassis' design. There is no mains transformer, the high voltage for the valves is obtained by rectifying the mains directly (that's what the UY41 valve does). The metal chassis of the radio is directly connected to one side of the mains, it might be the live side. So do not touch any part of the insides when it is plugged in, unless you are using an isolating transformer.

Secondly, the valves used in this set have what are known as B8A rimlock bases (that's what the '4' in the valve type number implies). They can only be plugged into their sockets the right way round due to a little bump on the glass that goes into a groove on the metal skirt of the socket. There is a spring ring round the top of the skirt that is supposed to hold the valve in position. Unfortunately it often breaks off the glass 'bump' I mentioned when removing or inserting a valve which ruins said valve. I recommend you remove these spring rings. The valves will not fall out.

[Cobaltblue]

Hi Jason

Welcome to the forum

The DAC90A is a great set but please look after your personal safety since the Chassis may be live.

The main smoothing capacitors on these often reform without too much problem but the hard worked output valve UL41 can be a source of hum as can the grid coupling capacitor.

Do a forum search on DAC90A there are more threads about this set than any other.

The bulbs are a bit of a problem being unavailable new for nearly 2 decades but again there are many solutions.

Cheers

Mike T

[JasonKillbourn]

Thank you Tony and Mike for your kind replies, I should be able to make it down to Cricklewood on my travels either tomorrow or Wednesday, so I'll definitely try to get those capacitors.
As regards the X2 rated capacitor, that one is included in the repair kit I ordered the other day, which should be turning up shortly, along with a C18 replacement. In fact, here's the spec.

5x 0.047 (473) to replace 0.05
3x 0.01 (103)
1x 0.001 (102) this is to replace the 0.003 for improved sound
1x 0.1 (104) this is x2 rated 275 volt ac

So, hopefully with the addition of the 33uF and 16uF ones, that might cover the capacitor situation adequately? Then of course there's the question of resistors, so I should really get busy with the multi meter and draw up a shopping list, especially if I am going to Cricklewood, as it looks like they might be a one stop shop for those too. Speaking of which, has anyone got any advice on the choice of resistors?

Other than that, there's only some disastrously degraded insulation on the wire from the mains dropper that clearly needs replacing, but I've got an old iron in the cupboard that I didn't get round to throwing away, which I can probably salvage some suitable wire from its power cord, so that shouldn't be a problem.

Then of course there's the valves themselves, but I guess I'll see how the radio performs after the caps, resistors and wires are taken care of first. However, I haven't removed any of them yet, as I did give them a gentle pull and they seemed pretty firmly stuck in place, so I'll definitely bear your advice in mind about the spring rings and proceed with caution, especially as there's a fair bit of rust in evidence and those springs might possibly be corroded and sticking.

On the subject of the lights, I've got two bulbs coming with the repair kit, but the advice on those seems quite varied, and seeing as they're getting a bit thin on the ground, I think I'll stick with the standard 75ohm resistor, as a dim glow doesn't bother me too much, though I might toy with the idea of removing the flammable celluloid diffuser screen and replacing it with a bit of light frost lighting gel, as I am pretty certain I've got a roll of that somewhere in my garage from when I closed my old studio (I was a studio photographer for years). I suspect that it will do a perfectly good job of diffusing the point source of the bulbs, but will allow a little bit more light through.

Finally, there's the lack of LW, but I guess there's a good chance that might come back with a thorough clean up of the selector switch, so fingers crossed on that.

As regards the issue of safety and the live chassis design, that was what actually what lead me to this forum, as the first thing I noticed about the radio was the peculiar, and rather worrying, reversible mains lead plug and two pins on the back, which started me googling the DAC 90A in the first place.

Anyway, thank you again for all the advice, it's much appreciated. I shall certainly let you know how I get on

[Boater Sam]

You can check the resistors as you go replacing capacitors, BUT bear in mind that you may need to lift one end to avoid an incorrect reading due to a parallel resistance, often studying the circuit will tell you.
I would be surprised if any are too far out of spec, most were only 20% tolerance and are not critical. The most critical are any high value eg. over 1M ohms which tend to go high and any cathode bias resistors to chassis which will be low values and take a bit of current.
Restuffing the smoothing can is my prefered method if need be, cut around the bottom where the clamp sits then it doesn't show when refitted. I keeps it looking neat rather than a hemorrhoid underneath. With modern smaller capacitors there is plenty of room in the can for 2, wrap in plastic to insulate and bring the negative wires out to one chassis connection, don't try to connect to the can, its alluminium.
Valves will likely be OK, UL41 is the hardest worked and prone to drawing grid current due to debris on the inside of the base button. Leads to over-running and audio humming which is not due to the smoothing capacitor. Sometimes this can be flashed off, search under "UL41 valves recovery" "sparking UL41" etc. will give you some ideas.
Long wave not working, could be switch contacts, frame aerial wire disconnected, oscillator coil. Look for components and connections that are only used on long wave.
Agreed, its a pleasure to help someone who starts out correctly and has provided all the relevant info at the outset.

[Radio Tech]

Hello Jason,

Welcome to our lovely forum, you will get lots of advice and tips here on how to restore any set. If you do not want to fit axial electrolytic caps in place of the main smoothing can type, you can obtain dual can types and other capacitors from Jelly Fish Audio, this is where I get most of mine. And yes, please as advised by Sam and Mike, make sure you are aware of a potentially live chassis, even when you are injecting a signal make sure you have a capacitor on each lead of the signal source.

Best wishes

Ken

[JasonKillbourn]

Thank you Sam and Ken for your replies. The advice I've received so far on here has been fantastic, and things are starting to move at this end, as the first lot of capacitors arrived today, and I am hoping to get down to Cricklewood tomorrow to get the two electrolytic caps, then the wiring and soldering can begin in earnest.
In the meantime, this evening, I've built a lamp limiter, taken the chassis back out and given everything a more thorough clean, including the cabinet, which responded quite well to a quick cut and polish. I also took the time to photograph everything in detail for future reference on reassembly, so this is the current state of play...

[Station X]

Wrap that tuning scale glass in tissue paper, put it in a jiffy bag and put it somewhere safe. If you leave it in your working area it's sure to get scratched or broken.

[JasonKillbourn]

Oh yes, I know that's exactly the sort of thing that would happen to me, so don't worry, I'd just finished cleaning it in that shot, and it's now already wrapped, boxed and well out of the way on a high shelf I get a touch paranoid with such things, having once spent hours carefully grinding a 10" x 8" glass focusing screen I'd made for a camera I was building, left it on the work bench for just a minute, while I rummaged for some 1/2" screws, and one tiny screw tumbled off the shelf and landed perfectly, point first on the glass, shattering it to pieces. At least in that instance, I just had to cut another piece and start over, but, as soon as I saw that dial glass, I thought that's something I really don't want to be breaking, as I imagine they're pretty rare to come by and it's in fairly good condition. The radio was in a cold, damp loft for a long time, so it's got a few small areas where the black paint is missing, but fortunately nothing much encroaching on the lettering or details, so I presume a careful dab of black paint here and there with a fine brush should make it look presentable?

[Radio Tech]

Hi Jason

very clear photographs of the set, plenty of detail for reference, I believe the capacitor that is in picture 4 near that dropper resistance is an X2 type.

Best wishes, keep at it, good luck

Ken

[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. This was especially the case in ye olden days because as well as having a two pin connector on the radio many mains wall sockets were also reversable two pin. You might reverse one connector making it safe and someone else might then reverse the other bringing the chassis alive again!

I don't know if such neon screwdrivers are still available.

[Oldelectronics]

These Dual cans seem fine and they do a 32-32 also If you dont mind them being black and gold. http://www.tubeampdoctor.com/en/shop...amp_mount_1182

Kev

[Herald1360]

If it's any help, the degraded insulation round the dropper isn't in itself a problem- air is a pretty good insulator and rigid wires don't move. They're also safely inside the cabinet anyway. Dress them a bit to maximize clearances but they don't have to be replaced.

Unless underchassis appearance is paramount, it's much easier to replace the wax capacitors with modern, smaller ones by snipping the leads close to the old capacitor bodies and using the leads to make small loop tags to attach the new capacitors to. This also avoids the pain of trying to unsolder and unwrap the old wires without damaging anything else in the process.

[Ed_Dinning]

Hi Jason, the chassis looks in normal condition for one of these sets so it should restore well. Glassfibre sleeving is easily obtained and make a good heatproof insulation for the dropper wires.
Be sure to do the mod that involves cutting the leads from pin 3 or 4? on the UL41.
This is used as a tie point and on some makes of valve can lead to problems. It is documented in the notes on the forum.

Ed

[kalee20]

Welcome to the DAC90a world!

You may even find the big electrolytics are fine. If you've had the radio switched on, and they haven't exploded or got hot, they won't be leaky. So, the only other thing is that they could be low capacitance.

If you get your new electrolytics, just tack them across the existing can terminals and try it. If the hum is vastly reduced, then yes the originals are shot. But if there's not much difference, then the originals will be quite serviceable, and the hum is coming from elsewhere!

You've got the capacitors to replace the waxed originals, that's brilliant. There is one which MUST be replaced (between the UBC41 and the UL41), but you're on the case anyway!

Looking forward to news, with interest!

[David G4EBT]

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.

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.

[HamishBoxer]

Sorry! it is UL41 output valve. On subject of Bush smoothing caps, I have only had to change one out of all the sets I have restored. BVWS do an excellent one I must add, top quality if needed.

[Boater Sam]

Originally Posted by David G4EBT

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.

David had a typo, it should of course be UL41 valve. (hope that's OK David)

Remove all pin 3 and 4 connections on this valve base, Bush liked using them as make do tag strips but it causes problems with some UL41 valves

[David G4EBT]

Ooops - thanks for spotting that. EL84? Huh - I don't know how that slipped in!

I guess it's what used to be terms a 'senior moment' or 'losing the plot' but in these PC times in which we live, the preferred term now seems to be 'intellectual interlude'.

[Boater Sam]

You're lucky, I have mental aberrations. Anytime. Where was Lawrence?