DAC90A Beginner

[Ed_Dinning]

Hi, I agree with the point about changing all the waxies. There are 2 on the AVC line that will not cause any damage, BUT, will severely reduce the sensitivity of the set if only slightly leaky.

Once the set is up and running you could consider the mods outlined in the articles by Kaylee on the other forum, this will improve its performance further.


Ed

[Nick]

Hi,

When I started doing DAC90As a while ago, I produced a Visio of the current (at the time) "Best practice", including the mods mentioned above. I find "Visio Foo" is useful for this sort of documentation...

The Excel (.xlsx) file gives all the component mappings from the original Bush Service Data & Trader sheet designators to the new layout. I also give the suppliers & part numbers for the replacement components.

I hope that these are useful - personally, I find pictures far better than words! Please don't redistribute this as there may be mistakes, but I believe it to be OK - I'll put a definitive link up on my site in due course.

All constructive feedback welcome.


  NMdS DAC90A1.pdf (Size: 231.79 KB / Downloads: 41)

  DAC 90A.xlsx (Size: 24.75 KB / Downloads: 29)

[Terry]

I was wondering if any of the range of  '100 Volt' audio transformers sold by CPC might be suitable

No, Mark. A 100V line transformer would be used with a public address or similar amplifier feeding a range of remote speakers. The high voltage minimises losses in the distribution wiring and each speaker has its own line matching transformer, usually with several taps so that the appropriate power level required at that particular location can be selected.

Nothing at all like the output transformer required for anything you are ever likely to come into contact with.

The ratio of the output transformer is the square root of RL (the valve's optimum load impedance), in this case 3k, divided by the speaker's impedance which with sets of this vintage is usually ~3 ohms.

Thus we have 3000/3 = 1000 and √1000 = 31.6. If it is a 4Ω speaker, the ratio would be 27.4 so, ideally, you are looking for a transformer with a turns ratio of ~30:1. However, for the moment, a suitable mains transformer as suggested by Jeffrey will suffice to get the set working. If it has several secondary taps, a 9V tap will be the closest to what you want.

[markandrewberry]

Nick, that's amazing work, thank you for sharing it.

I assume that the components identified as "New component or changed value" are the P. F Vaughan (kaylee20) modifications. In your experience are these worth undertaking? There is something slightly unsatisfactory about diverging from the intended design. (Not that I'm hung up over originality, it isn't as if I've got the last one!)

I've read about not using the UL41 pin 4 as a soldering tag, to help preserve the valve. Is this worth considering (and adding to the layout)?

Terry, thank you for clearing that up for me. If I don't come across a spare I will ask Ed to rewind mine. But for now I'm in no hurry to farm out work, it isn't as though I'm in any rush.

Off topic: the hardest part of starting a new hobby is not having loads of stuff, tools and spares, lying around. In the past I've wasted joyful hours in working on old motorbikes, Land Rovers and old clocks. Now I have no rusty frames or oily gearboxes hanging around, I hope that my other half will be a bit forgiving it I happen upon a few old radios.....

[Murphyv310]

With all due respect, Sam, in this case, I would change all the waxies in a DAC90/90A.  They'ra all likely to be leaky, and on the 90/90a being mounted on a tagboard are easy to get at and replace.  I have a 90A in my 'roundtuit' pile, so will be replacing the caps. in that.  I will however re-iterate the advice often given to newcomers, which is to change one cap. at a time, and check your work as you go. Another tip would be to snip out old components, leaving as much lead as possible, then solder the new item's leads to that, rather than try to untwist & remove the old component's leads completely.  More time-consuming and can lead to broken tags as well!

Hi.
Well although you "can" replace all the wax caps there are some that can be safely left in place with no detriment to the set or performance. The capacitors in question would be those across cathode resistors in the Frequency changer and IF amplifier. You can of course replace everything if you wish but wax caps will not be an issue in some sections of the set. You rightly see that bunging mains across a leaky filter cap will give some interesting results and of course the audio coupling capacitor to G1 of UL41 should be replaced as a matter of course. I also agree pin 4 of the UL41 should NOT be used as an anchor pin.
I also have to say joining a new component to a "stub" of wire is poor practice, there should be a mechanical joint that is then soldered, I'd not consider doing a "repair" or restoration like that. I use a good quality solder vacuum sucker and braid to remove excess solder, then it is easy enough to unravel the wire from the tag strip or valve base and then feed in the new component.

[Terry]

I also have to say joining a new component to a "stub" of wire is poor practice, there should be a mechanical joint that is then soldered, I'd not consider doing a "repair" or restoration like that.

I think that there are sometimes exceptions that break the rule, Trevor.

The Bush TV75/85 and 95 series of sets had two 21Ω 4W resistors for HT voltage selection. Apart from things like solder suckers and flux impregnated braid not being around in the heyday of these sets, they were soldered onto tags of thin tin plated brass sockets on the voltage selector panel. With the punishment they received from the constant heating and cooling there was the distinct possibility that they might snap off if they were not treated carefully, turning a simple repair into a major one.

We dealt with them as follows: the resistors (we always changed both) were cut off close to their bodies and cutters run over the wires to scrape them and reveal bare metal. The wires were then bent double, as were those of the new resistors after cutting them back slightly and the resistors hooked over the original wires. The 'hooks' were then squeezed tightly shut with pliers to produce good mechanical joints, after which they were soldered.

Even equipped with a solder sucker, I wouldn't do anything differently today as I doubt anybody is sitting on a stockpile of Bush voltage selector panels ...

Incidentally, we used RS 5W resistors as replacements and, obviously, large numbers of these resistors in various values were used throughout the trade but I've never known one go open circuit! Has anybody else?

[Murphyv310]

Hi Terry.
My point is that a mechanical joint is essential, sparky certainly didn't make it clear that he made a mechanical joint prior to soldering. I certainly agree if the original component is cut at its body a loop of wire can be used to loop in the new component and then squeezed with long nose pliers prior to soldering. If sparky does do that all well and good but when giving an opinion to a novice its always a good idea to give the full info as in this case he or she may think a non mechanically strong joint can just rely on solder.

[Old Sparky]

I apologize for not making it clear that I do make the kind of mechanical joint outlined above by Terry, Trevor, and would add that, wherever possible, I do completely remove the faulty component, but, because of the possibility of solder tags breaking off, I often cut and join as already described. The other point is that, with my 'serviceman's hat on', and i still do repairs for payment, the old adage of 'time is money' comes to mind. As is well known, often, for various reasons, more time is spent on a job that can be justified by the repair charge, and obviously these instances should be kept to a minimum.

[Terry]

... I do completely remove the faulty component, but, because of the possibility of solder tags breaking off ...

Except in the sort of case I described, tag strips and boards are usually pretty sturdy with strong tags firmly fixed.

By all means cut the faulty component out leaving just a short piece of wire which is easier to manipulate, then a couple of seconds work with a solder sucker and a quick tweak with needle nosed pliers is all you need to get the wire off the tag. Then you also get the satisfaction of a nice neat job, even if you are the only person who ever sees it!

It certainly shouldn't take a long time as you suggest.

[Stitch1]

...

I've read about not using the UL41 pin 4 as a soldering tag, to help preserve the valve. Is this worth considering (and adding to the layout)?

...

I recently restored one of these but it had serious hum. I suspected the smoothing caps but these were fine so I tried the pin 4 mod and the difference was amazing. I’d say get it working first then try the mods and see for yourself if they make a difference. 

John