Bush DAC 90 speaker

All of this could be complete twaddle so best wait for Mike ( Cathovisor) the Bush Guru.

I thought the original DAC90 was a bit of a failed design due to the heat from the large valves and the Dropper. the latter shrouded in its asbestos coffin which needs to be dealt with.

Had your DAC90 retained its back no doubt you would have seen the usual damage all DAC90 suffered, the afore mentioned dropper burns away a fairly large portion of the rear fibre-board. I also thought the subsequent DAC90A was Bush's answer to all these heat related issues.

As to the main question was the performance better than the the DAC90A, I've no idea.

The one to look for is the Bush AC90, looks identical to the DAC90/DAC90A with the exception of a white Bakelite speaker grill and a most important improvement, a mains transformer, yes an AC only version, I've only ever seen one.

I found that the ferrite tuning slugs on the i.f.'s fall to bits and are a pig to Araldite back in place but with care it can be done. You can tell if they are broke by tipping the set upside down and shaking it. If it rattles they are broke. I mentioned that as it could be why some people say the DAC90 is a bit deaf. The ones I have here (with repaired slugs) seem fine.

Just had a look. The loudspeakers are not identical but should be interchangeable. Ra for CL33 is 4.5k and for UL41 is 4.3k so you would also get away with transplanting the output transformer if need be.

http://www.vintageradio.me.uk/info/VALV ... 8/mul7.jpg
http://www.vintageradio.me.uk/info/VALV ... 8/mul9.jpg

The manuals for the DAC90 lists three speakers (Rola 6-Z, Celestion 1022 or strangely, a Goodmans) and the DAC90A lists one speaker, none of which share common part numbers: the impedance of the 90A speaker is given as 3 Ohms, the 90 as 2.6. The main thing to look for is the fixings line up and the magnet assembly doesn't protrude too far back but if a 90A speaker fits, by all means use it.

I think it's a bit harsh to call the DAC90 a "failed design" when so many - over 100,000 between 1946 and 1950 - were sold, and the set was sold in export and domestic guises, as battery, AC mains and AC/DC mains versions. The brief was to make a set that didn't use a line cord but was AC/DC. When the 90A came along, nearly 250,000 of these were sold over a period of nearly ten years but yes, it did neatly solve the heat problem. Considering Bush were avid users of barretters before the War, it's a pity they couldn't have put one in the 90 but you'd still have a degree of heat to lose.

A little note of clarification though: I think Chris might be referring to the 3-band open aerial set, the DAC/AC91 when he refers to the set with the 'drain cover' (and not all of them had it). There genuinely was an AC90 though - I have the service manual for it. I get the feeling, BICBW, that it was actually a model made for export to mainland Europe where the use of live-chassis sets was banned in some countries: I know Bush specifically made versions of the AC81 table set for the Danish market, for example.

In my humble opinion, for some unquantifiable reason the DAC90 "sounds" better than the DAC90A. Yes yes, I know that's dealing with subjectivism and we all know where that can lead, but it does. If you are in a position to, play Glenn Miller's Pennsylvania 6-5000 through both and see what I mean.

Radios are not my bag and well, I did qualify my post with

All of this could be complete twaddle so best wait for Mike ( Cathovisor) the Bush Guru.

I think it's a bit harsh to call the DAC90 a "failed design" when so many - over 100,000 between 1946 and 1950 - were sold.

A little note of clarification though: I think Chris might be referring to the 3-band open aerial set, the DAC/AC91 when he refers to the set with the 'drain cover'

And there we have it proved as thus

I also prefer the sound from my DAC90s to DAC90A's. There certainly is a difference (to me) in favour of the 90s. Maybe partly because the earlier Bakelite was thicker or denser or just because after all it is a totally different set inside the similar cases?

Thanks for your responses, I must admit the case does seem quite robust compared to other Bakelite cased sets I have, and I like the threaded brass bushes for screws which are absent on the 90A.
I have not noticed anything rattling so fingers crossed with the slugs
The set seems to have vent holes in the base (sort of under the tuning gang) and I wondered if they had been added? - eight about 5/16in dia
I would like to keep the original speaker but as I said the voicecoil is solid - anyone tried releasing a rust locked coil?
All in all it looks to be a worthy of restoring set which I will give best!

Alan

Catho will know better than me but I think those holes line up with trimmers and are to allow at least partial alignment with the set still in its cabinet.

Cheers

Mike T

Here is the poor little fella!

Sorting a seized speaker cone can be a nightmare. If you can release the dust cover in the centre by picking it off carefully with a craft knife, working a thin strip of plastic down the gap around the centre magnet may release it. Then try vacuuming the debris out of the gap till you can work the cone in and out. Using a 1.5 volt battery on the speech coil in both polarities will drive the cone in and out safely don't leave it connected to long. Put an air break on the vac hose to reduce the suck if its too strong.
If you get it free and its still scratchy, you could try removing the magnet screw if it has one but you will then need to recentre the coil with paper or plastic strips in the gap whilst you reassemble the magnet.
A ticklish job but its definitely possible.
You will possibly find that the centring bellows has detached from the chassis too.
That's just a careful gluing job, I use 'Uhoo' with success. Again, recentre the coil on assembly with strips in the gap and use the battery to keep the cone pulled in for a short while whilst the glue gels, a used ordinary battery is safer, not alkaline or rechargable, it avoids cooking the coil.
If all else fails and you feel that you can, try peeling the cone away from the edge of the chassis so that you can completely dismantle the speaker, unsolder the leads from the solder tags first though. But that can be really difficult.
I have yet to find a way of making new cones for elliptical speakers, has anyone else made any?
Boater Sam.

It might be a good idea to give the white dropper covering a coat of emulsion to keep it safe and secure or is that asbestos the previous owner could do? (Ouch!)

Hi Alan

Something nasty seems to have happened to your dropper.

Here are some pics of a typical 90 needing restroration.

Despite not really being a Bush collector I can never resist these as they are a true classic in terms of original sales and charm.

I must have 15 or so including various variations

Cheers

Mike T

Your right the dropper does look a bit sad! not sure what's gone on there (yet) yours seem much taller? and the asbestos does seem a bit powdery and flakey - is that a recommended repair (emulsion paint)?
I have stuck back a centering corregated piece before which was quite successful
I think the problem with dislodging the rust from within is that it is magnetic so won't want to come out even with a vacuuming, we shall see. I will try and save it because it does look quite sound (no pun intended)!

Alan

is that a recommended repair (emulsion paint)?

Probably highly frowned upon I should think but it keeps any asbestos dust down. You can also replace the asbestos with bits of kitchen tiles.

It looks like the top section of the dropper has burnt out and someone has tacked a resistor in place of it (the wire going to the left side of the dropper case) . I have a scrap set here which I was going to say you could have the dropper out of but when I looked someone has already had it along with the 'speaker

Thanks for the near offer! Stolen from under your nose!
Seriously, I now have the service manual for the set and while it quotes the resistances for the dropper it doesn't give the wattage ratings?
Any idea what (watt) might be required? alternatively I have half read bits about 'capacitor droppers' might this be the way to go
A closer look at the existing dropper fitted shows the top broken off! So cleanly I hadn't noticed with a quick look!

Alan

Hi gents, just confirming an earlier part of the thread, the speaker transformers are identical between 90 and 90A

Ed

Step / lintel repair rapid set cement with no sand added. Comes in Pollyfilla sized card box.
Stops any dust and is refractory. Emulsion paint chars.

Also can be used to recoat droppers, but after it's dry, bake in grill / so residual damp doesn't corrode wire at first use.

Theres a tidy DAC90 chassis on Ebay here

Cheers

Mike T

... it quotes the resistances for the dropper it doesn't give the wattage ratings?
Any idea what (watt) might be required?

Only just spotted this, Alan!

Dropper is 150 + 150 + 950Ω

The 150Ω sections are 20V droppers carrying both HT and heater current. W = E²/R which is (20 * 20)/150 = 2.66W each

The 950Ω carries only the 100mA heater current. W = I²R = 0.1 * 0.1 * 950 = 9.5W

The formulae above are based on W = E * I with the formula from Ohms Law substituted for whichever term is unknown.

You could instead perform the two calculations one after the other. Thus the current though the 150Ω sections is 20V/150Ω = 0.133A, then 20V * 0.133A = 2.66W and 950Ω * 0.1A = 95V, then 95V * 0.1A = 9.5W.

Whichever way you do it, you still get the same answer!

If you want to make one up from scratch, I'd use two 5W resistors and one 10W (unless you can find 3W resistors for the 150Ω sections).

... it quotes the resistances for the dropper it doesn't give the wattage ratings?
Any idea what (watt) might be required?

Only just spotted this, Alan!

Dropper is 150 + 150 + 950Ω

The 150Ω sections are 20V droppers carrying both HT and heater current. W = E²/R which is (20 * 20)/150 = 2.66W each

The 950Ω carries only the 100mA heater current. W = I²R = 0.1 * 0.1 * 950 = 9.5W

The formulae above are based on W = E * I with the formula from Ohms Law substituted for whichever term is unknown.

Ahem... that's a 90A dropper. The dropper in a 90 is 600 + 100 + 100 at 0.2A - or 32W! (Actually a bit more as the HT is carried through the two 100 Ohm sections, but IIRC it'll only be a quarter of the anticipated power due to the half-wave rectifier) That is why the back gets cremated...

Much as I deprecate robbing from Peter to pay Paul, there is a Pye dropper that matches the values exactly, in the model 45A/47A.