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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 20th Jan 2021, 3:39 pm   #1
David G4EBT
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Default Bush DAC90A - Improved Alignment 'key'

As the Bush DAC90A is thought by many to be an attractive 'iconic' Bakelite set and is quite commonplace as so many were out on rental for several year from the late 1940 - mid 1950s, they're often bought as a first restoration project, though being a 'live chassis' set aren't the most ideal set for newcomers to cut their teeth on. That said, so many have been restored and modified that there are countless forum threads and youtube videos (some better than others), and there is lots of guidance and encouragement on the forum to successfully restore the radio to safe working order.

They can be expensive to repair if valves are needed, and if unlucky the output transformer - a known weakness will need replacing or rewinding, the waxy capacitors will need replacing, and the largely unobtainable 3.15V 0.15A bulbs may be blown. All of these issues have been covered many times in forum threads.

Often, people can't resist plugging the radio in, turning it on, to 'see if it lights up, and if they can hear any sound' (other than the RF filter caps (C22) across the mains going off with a bang). It's been said many times that before the radio is turned on, at the very least a safety inspection of wiring etc, and C22 and the audio coupling capacitor C18 (often referred to on this forum as 'that cap' due to its notoriety for being 'leaky' in the electrical sense risking damage to the output valve and output transformer) should be replaced.

There are other pitfalls to watch out for such as some types of valve-holders have retaining springs which - if not removed, risk breaking the locating pip on the glass valve, which is an expensive lesson to learn.

As often as not, once restored and working, unless the set has been tampered with, if we're in luck, the alignment of the set won't need attention, and stations will be present where they should be on the dial. I've restored many DAC90 and other sets over the years, and when I've checked the alignment, in only a few cases has realignment been necessary.

Most newcomers won't have a signal generator or a mains isolating transformer so won't get involved in realignment on the DAC90A or any other radio.

Until recently, with an especially troublesome DAC90A which had an obscure fault (defective silver mica capacitor in an IF transformer), I've never had cause to realign a DAC90A. However, in doing so, the first thing that becomes apparent with the set out of the cabinet, is where to line up and position the dial pointer for the five spot frequencies referred to in the alignment instructions.

To quote the Bush alignment data verbatim:

"Check the position of the pointer in relation to the turning condenser. With the plates fully meshed, the pointer should coincide with the datum line printed on the main and auxiliary scales".

When the set is in the cabinet the pointer can be seen on the main tuning scale [the dial that is] and the set can be aligned from the rear with the chassis in the cabinet. If out of the cabinet, a printed copy of a dial scan can temporarily be placed in the dial plate, but what of the so called 'auxiliary calibration scales'?

The data tells us these calibration points: 'comprise a series of indents on the rear of the scale reflector plate', and outlines the order in which the 'indents' appear from left to right. There is also a green paper sticker with dots and frequencies on it which Bush call a 'Key'. It's marked in Metres, whereas signal generator dials are in kHz and MHz, so you need to convert that by reference to the data sheet, which is a bit of a faff. Even if the 'key' isn't shrivelled up and illegible, those dots aren't the alignment points and don't line up with anything - they're just an indication of the order in which the 'indents' appear if you can actually see them, but it's odds on that (unless you're a forensic scientist), you won't find them. The 'indents' are the tiniest little pinprick dots like centre punch marks along the top edge of the rear of the dial plate.

I had three DAC90A chassis on the bench and on two, the marks were totally invisible. On the other one, using a magnifying glass I was just able to discern them and mark them with a pencil tip. For the radio I was working on, and for future use, by reference to the indents, I decided to make an alignment key with all five points marked in Metres and frequencies. I made it the full length between the two upright struts that hold the dial reflector plate in place so that it lines up exactly. I printed it off on paper, sprayed with three coats of clear auto lacquer and stuck it on the rear of the plate. It doesn't obscure the indents along the top edge of they can in fact be seen.

I made some waterslide decals of the full length key, so that should a DAC90A ever darken my workbench again, if I have the inclination to, I may spray the rear of the plate so that a decal can be applied, though it wouldn't stick well on the bare metal plate on the set I was working on. Hence, I stuck the paper one on with Pritt.

In the past there was forum post with a replica of the original (rather pointless) Bush alignment key, but the text wasn't very clear, so I redid that in a clearer font as I thought it might be useful for anyone who would rather replace the original wrinkled up or missing key, which was just a paper strip stuck on with Sellotape.

Pic 1: Rear of the tuning scale plate with the daft wrinkled up so called 'alignment key' which is typical of the state they're found in. For clarity, I've marked the virtually invisible 'indents' with pencil, so you know what you're looking for.

Pic 2: The new full length paper alignment key I've created, fitted to the rear of the tuning scale plate.

Pic 3: The full length alignment key, marked in Metres and frequencies, with the dimensions shown, to print off if desired.

Pic 4: A replica of the original Bush alignment key to print off if desired for anyone who might wish to. (It can be stuck anywhere on the rear of the scale plate as it doesn't line up with anything).

Pic 5: For anyone who wants to realign or check the alignment of a DAC90A out of the cabinet but doesn't want to faff around with printing off and fitting the full width alignment key, a scan of one of several DAC90A dials which can be printed off, cut out and slid onto the dial light diffuser to be used temporarily as an aid to alignment to put the pointer at the five spot frequencies. (It's one of the clearer dial designs).

A bit of a diatribe but hopefully, of use to anyone who might be contemplating realignment of a DAC90A.

A note of caution to the unwary: Never connect any earthed test equipment such as a signal generator, frequency counter or 'scope directly to any live chassis radio, as it won't end well. The radio must be powered from an isolation transformer if that is contemplated.

Hope that might be of interest and use.
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Old 20th Jan 2021, 6:54 pm   #2
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Default Re: Bush DAC90A - Improved Alignment 'key'

This is a great and timely post for me as I'm gathering the parts/information to restore a DAC90A and I have to admit I was dreading the alignment part of it cause as you say I don't have a signal generator. The paper alignment key etc that you have created will definitely be of use. I've taken note of your other posts regarding the DAC90A.
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Old 20th Jan 2021, 7:58 pm   #3
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Default Re: Bush DAC90A - Improved Alignment 'key'

Ahh...I wondered what that wrinkled bit of paper was for! I assumed it was some sort of alignment guide but never bothered to investigate further. My one and only DAC90A was restored a good few years ago and didn't need alignment then and certainly doesn't now. Thanks David, very useful if I should restore one for someone else. I like the sets but have no desire for another unless I rescue one from a car boot or something like that.
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Old 20th Jan 2021, 8:43 pm   #4
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Default Re: Bush DAC90A - Improved Alignment 'key'

Thanks David

I have been much enjoying an ongoing argument with Simon Spiers about the virtues of the DAC90A and the DAC10. I am very much in the DAC10 camp. Thank you for providing yet more ammunition as the DAC10 could hardly be easier to align with it’s perfectly formed auxiliary tuning scale. I appreciate your effort in making the DAC90 slightly less unbearable.

(Only kidding David, Sterling work from you as Always )

Regards Chris
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Old 21st Jan 2021, 11:28 am   #5
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Default Re: Bush DAC90A - Improved Alignment 'key'

David, although I am unlikely to ever restore a DAC90A, what you have taken the time and trouble to design and make available is up to your usual thought out and meticulous standards. Thank you.
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Old 21st Jan 2021, 11:40 am   #6
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Default Re: Bush DAC90A - Improved Alignment 'key'

Excellent work David, as ever.
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Old 22nd Jan 2021, 6:38 pm   #7
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Default Re: Bush DAC90A - Improved Alignment 'key'

It is interesting to read about the alignment of these sets. My Dac90a (the looker of the aforementioned Dac10) needed very slight alignment after replacing the coupling caps. RF alignment was good, as was scale calibration. The only problem mine seems to suffer with is slight distortion on weak signals. When I get around to taking it apart again to investigate this fault I will print out one of David's calibration scales for sure,
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