DAC90A Beginner

[Crackle]

With some receivers that's not always advisable, replacing en mass is the only practical route if it takes ages to strip down a part of the receiver to gain access, replace one wax capacitor reassemble, switch on and check, dismantle, replace another wax capacitor, reassemble, switch on and check and so on...so many strip down and replacements down the line, wires can snap off, bandchange switches, coils or other vital bits can get damaged in the process.

Lawrence.

That is exactly the situation I have found myself in on a current restoration I am doing of a 1938 KB 750. I had to remove 3 capacitors to get at another.
I made a mistake as a result, so it is imperative to take photos and draw out and label the connections of all the associated components and their surroundings if this method has to be done.
But on a DAC90A I dont think the OP should have too much of a problem with the components laid out in the tag strip.

Mike

[ppppenguin]

The OP's project is a DAC90a, sufficiently simpleand accessible that the replacement methodology hardly matters. Though anyone can make a mistake, however experienced. Hence the invaluable advice to take notes and photos.

Mike (Crackle) has made the point that where access is difficult it's sensible to replace suspect parts in the right order. Or where it could easily damage the set if you did them individually. An example is a Thorn 2000 TV where all the small electrolytics really need changing and you'd wreck the PCB edge connectors if you did them one at a time. But this sort of set isn't (or shouldn't be) beginners' territory.

[Murphyv310]

Hi.
Even after well over 50 years repairing sets etc I still either draw out a diagram or take a picture. I did this yesterday when replacing a B8A 6L18 with a B9A 6/30L2 in a Murphy V230. A simple job but important to note it all down.
https://youtu.be/Y7YLnPEkALI

[markandrewberry]

Hi everyone, I just wanted to say thanks for all your input, it is incredibly enlightening: I feel I'm gaining in days from experience built up by others over many years. I expect that I'll quickly discover some sever limits to this when I have a hot iron in my novice hand. I've had to put things on hold for a week (OFSTED) but I look forward to getting my mind back onto the important things in life next week. Thanks again, Mark

[Kalee20]

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 agree. However, there is also evidence of cost-cutting in the intended design, such as the UL41 Pin 4 used instead of sticking in another tagstrip. Saves cost, but builds in a latent problem. The mods were aimed at getting better results, for a bit of time and expense, without changing things massively.

Who knows whether the original designers wanted to design it exactly as the aforementioned mods, but were overruled because it would have put the DAC90a cost just above their nearest competitor, instead of just below? We will never know. Maybe that's why the DAC90a is so plentiful! We do have something to thank the bean counters for, after all!

Agree with most of the previous posts. The valves will probably be OK. If 'that capacitor' waxy has leaked, it may have given the UL41 a hammering, and the rectifier too, but unlikely to have killed them.

You can operate the radio without the mains filter capacitor. The worst that will happen is there may be a bit more interference.

The mains dropper is unlikely to have failed. The dial lamps, more likely - but check they are screwed in properly.

The output transformer is a showstopper! But a cheap mains transformer with 9V secondary will get you going. Make sure, even if just a lash-up, that connections are secure - trying to operate with an open-circuit anode connection to the UL41 means all its current is diverted to the screen-grid, which can overheat. It won't kill it, but it's bad for it all the same.

One thing which has not been said, is the loudspeaker suspension has a habit of becoming unglued in these radios. It might sound scratchy is so.

It's an easy chassis to work on. Have fun!

[ppppenguin]

Designers are not gods. They don't always get it right. I should know because I am one! A designer that is, not a god.

As Kalee20 pointed out, it might be commercial pressure, lack of knowledge or a straightforward error.

i look back over my career in design and realise that I've made quite a few mistakes. Most of them have never made out of the door to customers but I'm sure some have. They may never have casued trouble, or may have been fixed on test by a engineer questioning my parentage. Or various other possibilities.

[pwdrive]

Maybe the pin 4 thing was good practice, remember that the UL41 valve was a relatively new kid on the block at that time and that it had a fairly high value of mutual conductance, the valve manufacturer recommended that a grid stopper when fitted should be connected as close to the valve as possible which to me implies that it should also be located as close to the valve as possible (which it was) There would have been enough room on that corner tag of the tag strip to connect it to along with the coupling capacitor and the grid return resistor if required, so maybe not a bean counting exercise but actually good design practice at that time for a valve that has a high mutual conductance....

Lawrence.

[Kalee20]

Agree! It could have been seen as good practice - and knowledge of internal leakage developing over time was not known, because the time just hadn't been clocked up.

However the reasoning behind putting the resistor close to the valve, is because the length of lead between resistor and valve pin should be short (to minimise lead inductance, and capacitance to anything troublesome). Giving the instruction to mount the resistor as close as possible satisfies this. But so does swinging the resistor away from the valveholder, as long as that bit of wire is as short as possible.

With hindsight knowledge gained since, (Jeffrey's point), using pin 4 is now proved to have adverse effects. So, markandrewberry can decide to stay with the original design intention, but he'd be better advised to do what the designer would no doubt have done if he could have revisited his design after analysing failures of DAC90a's with 10 years service - and that's isolate pin 4! Unless he has found a secret stash of NOS UL41's!

[ppppenguin]

I agree with Kalee20's analysis. At the time, interpin leakage wasn't seen as a problem, possibly not even known about. In any case UL41s were readily available and customers wouldn't have been surprised by the need for a new valve every now and then. AFAIK, Mullard labelled pin 4 as not connected, without any footnote to suggest that it shouldn't be used as a tie point. So why not use it if it's convenient.

[pwdrive]

From a bean counting point of view it would have been better not to use pin 4 as a tag.

Lawrence.