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Old 10th May 2018, 8:06 pm   #1
DonaldStott
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Default Yet another Bush DAC90A

Picked up a DAC90A at a local Auction as it appeared to be in good condition, case unmarked, back intact and original 2-pin p!ug - but I know that appearances can be deceptive! Loads of useful information, advice and guidance on this Forum which even extends to modifications, so I know that I'm in safe hands.

Initial batch of pictures shown below - looks like this set hasn't been touched for quite some time?

The third picture of the chassis underneath clearly shows that C18, "that capacitor" has seen better days!

The fourth picture below is a first attempt at a close up of C22, the Mains filter capacitor, which has undergone some trauma in the past and has acquired a layer of dust since then. The Output Transformer is also lurking in the background under dust and cobwebs!

I'll need to get all that dust hoovered up and everything cleaned before I can even contemplate taking any readings!

Wish me luck - hope I have better luck than the Bush AC91.
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Old 10th May 2018, 9:04 pm   #2
HamishBoxer
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Default Re: Yet another Bush DAC90A

Just carefully change those wax caps and the mains filter cap and all should be well.
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Old 10th May 2018, 9:35 pm   #3
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Default Re: Yet another Bush DAC90A

Looks nice and original. You can't really go far wrong with these. Good luck.
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Old 10th May 2018, 9:41 pm   #4
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Default Re: Yet another Bush DAC90A

Quote:
Originally Posted by HamishBoxer View Post
Just carefully change those wax caps and the mains filter cap and all should be well.
Thanks for your optimism Hamish and Nick.

As C18 and C22 look very distressed I'm concerned about the impact on the Primary of the Output Transformer. I've also read that this sort of trauma can also adversely affect the volume control track on/off switch!

But the damage does appear to have happened some time ago and no one has attempted a repair.

Need to wait until the weekend to give it all a proper clean.
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Old 11th May 2018, 3:31 pm   #5
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Default Re: Yet another Bush DAC90A

Is there not a capacitor lurking under the tagboard?
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Old 11th May 2018, 3:49 pm   #6
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Default Re: Yet another Bush DAC90A

I've no idea - which cap would that be?

Will need to wait until I find the time at the weekend to get things cleaned up and investigate further.

Hopefully someone with DAC90A restoration experience will be able to answer?
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Old 11th May 2018, 4:00 pm   #7
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Default Re: Yet another Bush DAC90A

C13...100pf, probably ok.

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Old 11th May 2018, 4:15 pm   #8
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Default Re: Yet another Bush DAC90A

Thanks Lawrence - the Bush Service Instructions list C13 as a Silvered Mica type so it will be staying put.
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Old 11th May 2018, 6:19 pm   #9
David G4EBT
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Default Re: Yet another Bush DAC90A

The DAC90A has really been done to death in so many threads so I've written these wordy notes with newcomers in mind to try to help maximise their chances of ending up with a safe working set, and minimising the risks of inadvertently causing damage. As often as not, it's a first set, albeit not best suited for newcomers due to it being live chassis. Too often, the thread starts with: "I've been after one of these for some time and have just won one on e-bay/found one at a car boot sale. I plugged it in to see if it worked and it went bang. It seems to light up but doesn't make a sound - where do I start"?

Well whatever the make and model, you don't start by plugging it in under any circumstances until some proper tests have been carried out with the set disconnected from the mains.

Everyone has their own approach to restoration - some just want to 'mend it/get it going/do it up a bit'. Fine, but that's not 'restoration' and it's not what I'm into.

For me, the first step is to make a simple cradle on which to mount the radio so you can work on the underside of the chassis in safety.

The guys who worked on DAC90As and other sets in customers homes back in the late 40 to late 50s were under time pressures and were not restoring the sets - just repairing them and would be out of the door in half an hour. We're not under time pressures - we're hobbyists - so if we're going to restore a set, we'll be working on it for many hours spread over several days or weeks. It's just asking for trouble to be balancing it on a bench, standing it on end, propping it up on blocks of wood, etc, while soldering or live testing.

A simple cradle for a DAC90A is easily and quickly made by using four 300mm strips of flat iron, about 22mm wide, with an inch at the bottom bent at right angles. The can be bolted to the sides of the chassis, and screwed to a piece of scrap plywood or whatever at the bottom, about 45cmas x 30cms. There will be space on the plywood to rest the speaker during testing. I've attached a pic below.

The DAC90A has several known weaknesses and shortcomings, some due to design limitations, others due to the ravages of time.

First thing to do is to download the data sheet.

In the case of the DAC90A - indeed most Bush radios - the manufacturer's data is more comprehensive than the Trader Sheet. The Bush datasheet of the DAC90A runs to eight pages - the Trader Sheet (1161) is only two pages. As with almost all Trader Sheets, often, the component numbering doesn't correspond between the two sets of data, so if quoting numbers (R15, C18 or whatever), always make it clear which data you are referring to.

The 'big bang'? That will be C22, 0.1F 500V tubular paper mains filer capacitor which is mounted directly across the mains input.

It may not go bang and disgorge its contents - it might just be that when you plugged the set in and switched it on without doing any tests, it put a dead short across the mains and blew a fuse. Not many sets had mains filter caps, but Bush seemed to fit them quite a lot. Whether it's there or not makes no discernable difference to the performance of the set, so you have two choices. Snip it out and don't bother replacing it, or replace it with a Class X capacitor designed to be self-healing and to fit directly across the mains. 'Class Y' capacitors are designed to go from line to earth and neutral to earth and have no relevance to the DAC90A. (See sketch for clarification). A class X cap is unlikely to be tubular - mot likely it will look like the one in pic 3 below and you'll need to bend the original retaining clip to suit.

The more tests you can carry out with the set disconnected from the mains, the safer you will be, especially with live chassis sets such as the DAC90A. My approach would be:

1) If you have the original reversible two-pin mains plug (To be pedantic, it's not a plug, it's a socket as it's female - the male plug is mounted on the chassis), mark both the plug and socket with a dab of red paint to show which is live, then always ensure that it's fitted so that the chassis is connected to neutral.

2) Don't even think about pulling any valves out of their sockets until you've first removed the spring retaining rings from the valve-holders. If you don't there's a high risk that you will smash the delicate pip off the side of the valve. In a domestic environment, the retaining sprigs serve no useful purpose and given that the DAC90A went into production in 1948, I can only assume that that the valve-holders were intended for equipment used in rough terrain during WW2. If you leave the rings in place, not only do you risk destroying the existing valves, you risk destroying expensive replacements. A damaged valve can be spotted by the white coating on the inside of the valve and the little hole in the pip.

3) The mains dropper 'R15' has settings for 210/230/250 Volts. It will be kinder to the components and valves if you have set on 250 Volts than 230Volts and will probably be closest to your mains Voltage anyway.

4) Before you even think about any live testing, ensure that you replace C18, the audio coupling cap (often called 'that cap'), which is 0.01F. Its job is to allow AC signals to pass from the anode of V3 to the signal grid of V4, while at the same time, blocking DC. If it becomes 'leaky' in the electrical sense, it can overdrive the output valve, risking damage to the valve, and causing a short on the primary of the output transformer, which is a known weakness.

5) Check the resistance of the output transformer to see if the windings are still intact. The primary should be approximately 500 Ohms, the secondary, 0.75 Ohms. If the primary is open circuit, forum member Ed Dinning can re-wind it, or there is an RS replacement, which though not ideal, will suffice. Others can comment on that. Assuming that you're in luck and the OPT is intact, now is as good a time as any to replace C20, 0.01F fixed tone corrector, across the primary of the OPT.

6) Make a list of all of the resistors and check if they're still within tolerance as it's probable that some may have drifted high in value. With the exception of R4, 8 & 14, which are +/- 10%, the permitted tolerance for the rest is +/- 20%, so for example, R11, which is 470K, could be as high as 564K and still within tolerance. Any resistors that are outside the permitted tolerance will have a bearing when you come to measure Voltages at the live testing stage. Apart from the higher wattage wire-wound ones, (which are unlikely to have changed in value over time), most other resistors are 1/4 Watt. If any do need replacing, modern ones are tiny by comparison, so you might wise to use 1 Watt or 2 Watt metal film ones, which look more in keeping.

7) Speaker and pilot bulb wiring: The rubber insulated flex is likely to be in poor condition, cracked, flaky, crumbling, and hence, unsafe so should be replaced. Personally, I only ever use heatproof silicone rubber multi-strand flex which Phil Marrison, forum member SWB 18 is able to supply in a wide range of vintage colours and two gauges. (He usually has a table at the NVCF).

8) The loctal valve-holders are small and it isn't very obvious which is pin 1 of each valve, so when you come to taking measurements, it's wise to first put a blob of modelling paint next to pin 1 of each valve. Be very careful when poking around with test meter prods as it's very easy for a prod to slip and short out two valve-holder tags.

9) With the set still unplugged from the mains, check that both poles of the on/off switch on the volume control have continuity in the 'On' position. If not, the switch on the pot is defective. If yes, check the continuity of the valve heater chain. The heaters are in series, so all must be intact. Still with the set not plugged into the mains, if you put one Ohmmeter prod on the chassis (croc clip is better if you have one), and the other on the live input to the set, that will test the mains dropper (950+150+150 Ohms), plus the heater chain. If you get no reading (open circuit), check across pins 1 & 8 of each valve in turn to see which one(s) is/are defective.

10) When you've got this far, before you start replacing the other tubular caps, it's a good time to give the set a preliminary 'MOT' - Moment of Truth! Plug it in via a lamp limiter to see if it works. Hopefully it will, if only after a fashion. If it doesn't work, take Voltage measurements as indicated on the circuit and in the datasheet to try to diagnose where any faults lie.

If you have a signal injector/tracer, inject a signal at the slider of the volume control (or just scratch it with an insulated screwdriver, to check if the audio stage is working. Use a signal tracer on the slider of the volume control to see if you can hear tunable signals on both bands. If not, there's a problem in the frequency changer/IF/Detector stages.

CAUTION: Remember that as the DAC90A is a live chassis radio, unless you are using an isolation transformer, you must not connect test gear such as a signal tracer/injector, signal generator directly to the set. You must have isolation capacitors in the test leads.

If the set isn't working at this stage, think twice about replacing the other tubular caps just now as it's unlikely that they are the cause of the faults. If do you replace those now, (which are often not leaky anyway), you risk introducing more faults rather than curing those that already exists.

Take before/after photos and make notes as you go along.

Looking on the bright side, and assuming that the set works, when replacing components, it's always wise to do a bit/test a bit. Take a photograph of the tag-strip, replace the caps one at a time, testing after each cap is replaced. When replacing those caps, if you snip one out, you'll see that there is a disused tag lying flat on the Paxolin board at each end. Bend it upright carefully, scrape the tarnish off the tags, and fit the new caps to this spare tags. (Well they were spare, but they're not now!). It makes a much neater job that trying to solder the new caps to where the old ones were wired. See pic below.

Hum: If you get hum, it's tempting to assume that it's the reservoir/smoothing cap, but the DAC90A twin can is very reliable and it's much more likely to be the UL41 output valve that's the culprit.

Wave-change Switch: A known weakness, but fortunately not too common. It can be replaced by a Lorlin switch, but it's a ticklish task, covered elsewhere.

Faulty volume control/mains switch: The set uses a 1 Meg Ohm logarithmic potentiometer with a double-pole mains switch. Now note commonplace, Bowood Electronics can supply a suitable replacement if switch cleaner doesn't do the trick.

I hope these rather copious notes will be a useful reference for any newcomers or novice restorers. It's just my take on things - I don't mind if other adopt a different approach.

In closing, you might think I'm a fan of DAC90As - I'm not - they're probably my least favourite set, but that's a personal thing. I was nine years old when the set came out in 1948, so for me, the brown Bakelite ones are a drab reminder of a cheerless post-war era of austerity, shortages, queues, ration books (till 1954 when I left school), power cuts, no coal, and food parcels from Canada. But if I see one for under a tenner, I end up buying it, so over the years, many have passed through my hands as they have through countless others, and all of the points above are based on hard lessons, expensively learned.
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Old 11th May 2018, 6:56 pm   #10
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Default Re: Yet another Bush DAC90A

Lovely write up David, I absolutely love the Dac 90a, I have a cream one in the bedroom and feel rather pleased with myself as it being one of the first radios I ever managed to fix and not having any electrical background.
It sounds remarkably good, gets used regularly and very pleasing to the eye.
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Old 11th May 2018, 8:12 pm   #11
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Default Re: Yet another Bush DAC90A

Thanks David - much appreciated again.

I think your Post should be a Sticky on the Forum as it will save myself and others a great deal of time doing numerous Searches.

Your idea of a cheap and cheerful jig is a splendid idea!

Hopefully I've more than enough now to be getting on with ...!
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Old 12th May 2018, 10:02 pm   #12
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Default Re: Yet another Bush DAC90A

Did some preliminary cleaning up tonight after liberating the chassis from the case - speaker wires snipped off as they will be replaced in due course with heatproof silicone rubber multi-strand flex supplied by Phil Marrison, Forum member SWB 18.

I've attached a couple of close up pictures showing each end of C22, the 0.1F tubular paper Mains filter capacitor which is mounted directly across the mains input. Thought I'd share these pictures as this is what a "big bang" means when the capacitor disgorges its contents!!! In the case of this set it must have happened some time ago as all the components on the chassis have an equal distribution of dust, muck and spider's webs!

The preliminary cleaning allowed me access to the Output Transformer where we have the Primary reading 1 (!) on the multi-meter and the Secondary reading 0.8Ω. So options are to place a request in the Wanted section for a replacement Bush DAC90A Output Transformer, look at what's involved in the RS replacement or send a PM to Ed Dinning. Of course all these options would be adding costs and time to the restoration.

I'm assuming that C18 the 0.01F tubular paper Coupling to V4 capacitor i.e. "that capacitor" has also given up the ghost so I'm concerned about the impact of all this on the UL41 itself. Any tests I can do on this valve at this stage without powering up?

Lastly, for the moment), how do I remove the spring retaining rings from the valve-holders?
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Old 12th May 2018, 10:25 pm   #13
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Default Re: Yet another Bush DAC90A

Quote:
Lastly, for the moment), how do I remove the spring retaining rings from the valve-holders?
A small screwdriver underneath the spring will be enough to see that it can be just eased out of the locating tabs...I found a couple on the floor tonight when I was sweeping up.
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Old 12th May 2018, 10:43 pm   #14
David G4EBT
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Default Re: Yet another Bush DAC90A

Typical DAC90A then - C22 gone bang, C18 leaky, so UL41 overdriven and hence, OPT primary is open circuit. (Presumably your multimeter reads '1' for open circuit?). Looking on the bright side, at least you've made a start in identifying the culprits.

The retaining springs are just rings of wire with overlapping ends, so if you poke a small screwdriver or pocket knife blade between the inside of the ring and the skirt of the valveholder you can open it up enough to be able to grip the wire with pliers and open up the ring, then just slide it up over the valve. Be very careful not to damage the fragile glass pip in the slot of the valveholder skirt.

If the heaters of all of the valves are intact, you may as well wait till you've got the set working to see how the valves perform. The only way you could test the valve without a valve tester is if you've got another set which uses a UL41 in which to try this one. It's always handy to be able to substitute valves with known good ones to see if the existing ones are past their best. It's the UL41 that has to work the hardest - they're starting to get a bit expensive these days.

Good luck with it!
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Old 13th May 2018, 10:57 am   #15
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Default Re: Yet another Bush DAC90A

PiSome success - spring retaining rings removed and all valve pips intact.

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And here's C22, the Mains bypass capacitor snipped out - RIP

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Old 15th May 2018, 9:48 am   #16
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Default Re: Yet another Bush DAC90A

Having checked out the loudspeaker and VR1 (the potentiometer) which are both good I wondered why I was getting intermittent or no readings when checking continuity on valve heaters and making sure that the heater chain is intact?

See image below:-

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Time for some DeoxIT D5.

Meantime, out of tolerance resistors identified and capacitors ordered - I made a small mistake with the capacitor order that was sorted out quickly by Syd at Cricklewood Electronics at no extra cost even though the parts I needed were more expensive - great Customer Service!

Made contact with Ed Dinning so just need to extract the Output Transformer, and package it up for posting. Making good progress with this DAC90A but the costs are starting to mount up!
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Old 15th May 2018, 10:10 am   #17
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Default Re: Yet another Bush DAC90A

Can I just say,that the De -Oxit is worth the price.Works for me were others do not.

Another point the pleasure of this little Bush will long outlive the cost!
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Old 15th May 2018, 4:12 pm   #18
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Default Re: Yet another Bush DAC90A

And yet more expense - no readings across the heater chain so checked each valve heater individually for continuity.

Of course it's the UL41 with no heater continuity!
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Old 15th May 2018, 4:28 pm   #19
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Default Re: Yet another Bush DAC90A

Is that using the continuity function or the ohms function, if the continuity function it might be worth checking the heater on the ohms function as well.

Having said that I have no idea what the cut off value of R is that stops the continuity from giving an audible output.

Apologies for any diversion...!

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Old 15th May 2018, 5:11 pm   #20
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Default Re: Yet another Bush DAC90A

Thanks Lawrence - certainly not a diversion.

The valve heater was checked with the continuity function and there was no audible output - switching to the ohms function I am reading 72.5Ω across the heater pins for my UL41 valve.
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