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Old 3rd Apr 2020, 12:12 pm   #1
DonaldStott
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Default Another Bush DAC 10

Having had previous experience of both a Bush AC91 and a Bush DAC90A I'm hoping that this will stand me in good stead with this DAC 10?

Before starting any work, I have of course read most of the previous Forum Threads here on this popular set along with other online resources including YouTube videos.

External case in good condition apart from a crack to the rear of the Pre-set button panel - but that's for later. The Pre-set buttons were in a very poor state but Forum member Northyorks1 has very kindly provided a full set of replacements in extremely good condition - thanks!

Managed to extract the chassis from the case and there is very little sign of anyone being in there recently - good news. It's a bit of a rust bucket in certain areas but I won't know the full extent until I clear away the dust, cobwebs and other detritus!

Some early pictures:-

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I have a slight conundrum with the mains cord as I am measuring 77.4Ω between the chassis and the LIVE (Red) wire and 1.46kΩ between the chassis and the NEUTRAL (Black) wire. I expected the values to be the other way round so may suggest that the switch is connected up the wrong way round?? All measured values throughout this thread are made using my ToolZone DMM - my AVO 8 is still on the To-do list.

The good news, however, is that the Output Transformer seems o.k. with 569Ω on the Primary and 0.8Ω on the Secondary. And we are getting some crackling from the speaker when measuring theses values.

Now what to do about that rust - a great number of Threads have already covered this and I am favouring Hammerite 'Kurust' at present having had limited success in the past with Jenolite Rust Remover gel?
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Old 5th Apr 2020, 7:27 am   #2
coopzone
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Default Re: Another Bush DAC 10

Morning, I think it's quite likely that the cable is wired the wrong way (but really there is no wrong way). If you check the trader sheet / manufactures sheet - there is no indication as to polarity. Thats because these radios had no fixed polarity you can connect them either way. The fact that it has red/black cables is almost certainly because is was cheaper to buy it like that. they may as well be both pink.

Having said that, i like you, would always try and get what i consider to be the safer (? very slightly, but not really) option of neutral towards the chassis. It should still be considered live and deadly.

Derek
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Old 6th Apr 2020, 12:59 pm   #3
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Default Re: Another Bush DAC 10

With this DAC 10 there is quite a bit of wiring that is crumbling and/or exposed. Here is an example from around the dropper: -

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And just to complicate things here is a picture of the Pilot lamp shunt, clearly showing 75Ω - I'm reading 77.4Ω although both the Trader Service Sheet 1002 and the Bush Radio Service Instructions both state that this should be 250Ω - anyone??

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Of course both of the bulbs are dead - one of them was correct at 3.5V 0.15A while the second one was 6V 0.1A. Replacements are difficult to find although I do have a number of 3.5V 0.3A bulbs - are these suitable?

Back at the mains wiring, here is a picture of the rear of the switch/pot. You can clearly see that the incoming red and black wires are transposed on the outgoing terminals? Or have I missed something obvious? This means that the black incoming wire (Neutral?) is feeding the outgoing red wire that runs to the top of the dropper - kind of makes sense (to me) that this should be fed by the incoming red wire (Live)??

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Old 6th Apr 2020, 2:16 pm   #4
Leon Crampin
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Default Re: Another Bush DAC 10

Bush revised the value of the lamp shunt from 250 Ohms to 75 Ohms to minimise lamp failure on hot switching. The documentation was not, to my knowledge, amended. Likewise, the rectifier surge resistor changed value early on, which affected the voltage readings, again undocumented.

Scale illumination is always poor on these sets. I prefer to use the specified lamps and accept an authentic dim glow. A higher current lamp will give even less light.

Leon.
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Old 6th Apr 2020, 3:50 pm   #5
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Default Re: Another Bush DAC 10

Thanks Leon - I've experienced similar Bush dim illumination in the past with my DAC90A and AC91, just wish that the required bulbs were more widely available!

Knowing that Bush changed but didn't document component values helps a great deal but doesn't bode well for voltage measurements further down the line!
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Old 6th Apr 2020, 4:14 pm   #6
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Default Re: Another Bush DAC 10

If you do a forum search for DAC90A mods you should stumble across the detailed mods for both scale illumination and sound quality by Kalee20.
https://www.vintage-radio.net/forum/...ad.php?t=35784

It makes for interesting reading and most will be applicable to the DAC10 too. Personally, I retain the original lamps but up the lamp shunt to 180 ohms, this gives more brightness (but still under runs the lamps plus they still don't fail on a hot switch on
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Old 6th Apr 2020, 8:11 pm   #7
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Default Re: Another Bush DAC 10

Thanks to Leon and Rob for their advice on the lamp shunt and rectifier surge resistors - something to implement a bit further down the line.

Meantime it's not just the wiring that's crumbling, the Paxolin panel for the Dropper has succumbed as well: -

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I feel that this may be the first of many trips to the Wanted section for this set?

WANTED - Paxolin panel
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Old 6th Apr 2020, 8:37 pm   #8
AD360 Rob
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Default Re: Another Bush DAC 10

Quote:
Originally Posted by DonaldStott View Post
Back at the mains wiring, here is a picture of the rear of the switch/pot. You can clearly see that the incoming red and black wires are transposed on the outgoing terminals? Or have I missed something obvious? This means that the black incoming wire (Neutral?) is feeding the outgoing red wire that runs to the top of the dropper - kind of makes sense (to me) that this should be fed by the incoming red wire (Live)??
Indeed it should be that way Donald although with the DAC10 having a fixed mains lead it was less of a problem if the chassis was connected to the live side of the mains rather than the neutral. However, it might be worth for peace of mind, to meter out the connections from the mains plug to the top of the dropper. I've had these on off switches fool me into thinking exactly what you're seeing whereas the switch is in fact wired correctly despite appearances to the contrary.

Rob
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Old 7th Apr 2020, 11:50 am   #9
DonaldStott
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Default Re: Another Bush DAC 10

Quote:
Originally Posted by AD360 Rob View Post
Indeed it should be that way Donald although with the DAC10 having a fixed mains lead it was less of a problem if the chassis was connected to the live side of the mains rather than the neutral. However, it might be worth for peace of mind, to meter out the connections from the mains plug to the top of the dropper. I've had these on off switches fool me into thinking exactly what you're seeing whereas the switch is in fact wired correctly despite appearances to the contrary.
Thanks again Rob - unfortunately (?) I appear to have continuity from the black wire on the mains lead through the switch to the top of the dropper confirming my suspicions about these connections being transposed.

I appreciate it's not really a problem but, as you say, it's just peace of mind.
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Old 7th Apr 2020, 5:31 pm   #10
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Default Re: Another Bush DAC 10

To facilitate access to allow the essential rewiring, connection cleaning and rust treatment work I've removed the Dropper from this set.

Underneath the Dropper on the upper side of chassis there is some type of white fibre washer that is very fragile:-

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Hopefully someone can advise that this is not asbestos?

What would be a suitable replacement??
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Old 7th Apr 2020, 6:22 pm   #11
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Default Re: Another Bush DAC 10

Sorry but I think it will be asbestos. I'd certainly treat it as such.

Alan
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Old 7th Apr 2020, 6:38 pm   #12
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Default Re: Another Bush DAC 10

Certainly Asbestos,try to Gently put water on before moving it.Do not spray it.
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Old 7th Apr 2020, 7:06 pm   #13
DonaldStott
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Default Re: Another Bush DAC 10

Thanks for the helpful advice even though it's not what I wanted to hear!

If I do wet it and remove it how do I dispose of it?

And a suitable replacement would be??

Just came across some advice from Forum member Mike.Watterson: -

"I'll soak the asbestos in fast setting high strength sill and step repair cement. Expensive stuff, but fire proof and will totally stabilise it."
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Old 7th Apr 2020, 10:56 pm   #14
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Default Re: Another Bush DAC 10

Quote:
Originally Posted by DonaldStott View Post

If I do wet it and remove it how do I dispose of it?

Most recycling sites (when they reopen!) will take small quantities of securely double polythene wrapped asbestos.


It's not toxic, just physically hazardous(!) to one's long term well being
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Old 7th Apr 2020, 11:49 pm   #15
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Default Re: Another Bush DAC 10

Don't disturb it and DON'T get a vacuum cleaner anywhere near it. Just clean up the chassis out in the garden and then carefully put it all back together as it was. Asbestos in some form or another is to be found in many vintage items including old radios, it's nothing unusual.
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Old 12th Apr 2020, 7:58 pm   #16
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Default Re: Another Bush DAC 10

This week we have been mostly replacing perished and exposed wiring around the mains switch/volume pot, the dropper, the scale lamps and the connections to the Output Transformer. I seem to have crumbly wiring almost everywhere - everything I touch just disintegrates and it is clear from a quick visual inspection that there is more to come!

I've also snipped out the old C27, the R.F. Filter cap across the mains, and replaced it with a modern X2 capacitor - I had to rework the original retaining clip to suit the new rectangular profile. The old cap wasn't in too bad a condition, but just pleased it hadn't done the usual Bush trick of disgorging its contents across the inside of the set: -

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Along with a great deal of chassis cleaning (very time consuming and laborious) and rust treatment the set is in an advanced state of disassembly although all parts have been clearly labelled and safely stored in one of those plastic cases with lots of individual sections - lots of pictures as well. The metal part that retains the scale lamps and diffuser was very rusty and that has been removed, cleaned and treated. Following a final clean down with IPA it had a coat of Primer then two coats of Lada Cream spray paint (Halfords) - looks almost new!

I've managed to source some s scale lamps with the correct values and a section of Paxolin also arrived in the Post - those are jobs for this week

And of course there is always a BUT ...

When I was connecting the new wires for the Output Transformer it was extremely difficult to get the soldering iron safely to the back of the underside of the chassis to solder one wire. This made me realise that it will be almost impossible to replace any components on the tag board without disturbing something or inadvertently touching the soldering iron against a wire or similar! Notwithstanding that risk, how on earth are you supposed to take voltage readings on the valve pins?? I note that The Trader Service Sheet 1002 has instructions on pages 2-3 on "Removing Tuning Unit" which I think is almost essential with this set if I have to replace any caps or resistors on the underside?

So what's the collective view of on how easy it is to remove and, more importantly (!), replace the Tuning Unit or can it simply be unbolted and pushed to one side to allow access?
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Old 12th Apr 2020, 8:50 pm   #17
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Default Re: Another Bush DAC 10

Jumping back a bit.... it wasn't unusual for live chassis sets to have a single pole mains switch in the neutral line or at least in the line connected to the chassis. Looking at the DAC10 circuit, though, it should have a double pole switch anyway.

Whatever the arrangement, treating the set as live unless it's physically disconnected from the mains is the only way to go.

Replacing caps I have managed without removing the tuning unit nor the tag board though maybe resistors can be more of a problem. I was also just "repairing" the set not "restoring" it so old caps were just cut out and new ones fitted to the old ones' tails.
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Old 12th Apr 2020, 9:21 pm   #18
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Default Re: Another Bush DAC 10

Quote:
Originally Posted by DonaldStott View Post
So what's the collective view of on how easy it is to remove and, more importantly (!), replace the Tuning Unit or can it simply be unbolted and pushed to one side to allow access?
It's quite easy to remove it, its putting it back that's the problem!

What I did was cut each of the 9 (I think) wires to the tuning unit one by one, about halfway along their length. Then labelled each half of the wire with a label made of masking tape with the wire number.

After replacing components etc. refitting was a case of fitting new wires, again one by one, to replace the two halves of each cut wire. Easy apart from 2 - the capacitor C6 to the grid of V1, as C6 is directly connected to the tuning unit and the valve base pin. I used a short extra length of wire spliced on, and some heat shrink sleeving to cover the splice. The other wire that was tricky went to one of the coils and again was easier to splice in an extra length.

I can't stress the importance of labelling the wires enough - I was able to reconnect them without a mistake, just trying to reconnect based on the schematic would have been far more error prone.

Keith
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Old 12th Apr 2020, 11:57 pm   #19
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Default Re: Another Bush DAC 10

I have 'repaired' quite a few of these, I would not go to the trouble of removing the tuning unit unless absolutely necessary. I have never removed the tagboard. As Herald says you can just attach new caps to the tails of the old ones. You can have a 'repaired' set that will work just as well, be just as safe, and be just as reliable, as a restored set. It just may not look as pretty.
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Old 13th Apr 2020, 5:50 pm   #20
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Default Re: Another Bush DAC 10

Thanks to everyone for their helpful and informative responses - much appreciated.

Firstly, I have confirmed that I do indeed have a double pole switch installed - when switched off, both the Live and the Neutral are disconnected and vice versa. I'll swap round these connections from the plug to the switch later on but will heed Herald1360's sensible advice to treat " ... the set as live unless it's physically disconnected from the mains."

Secondly, I note that there are different views on removal of the Tuning Unit, as expected, but appreciate that the level of skills and expertise you guys have is probably way beyond my own. I made a start by replacing some of the easier components at one end of the tag board including R13 (28% over), C23 the Audio Coupling capacitor and C22 the I.F. Filter capacitor.

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A bit of a challenge (for me) to get these fitted and don't think I'll win any prizes in the Elegant Soldering Competition!

Thirdly, I appreciate that most if not all of the paper tubular caps and some of the resistors will also have to be replaced but I like to replace and test one at a time as I go. Difficult to progress any further with these as I have the set disassembled so I can put back any decision on the Tuning Unit removal to a later date. Those other components further along the tag board do look challenging!

Plenty to do meantime in the cleaning department and even more wiring to replace. I have a section of Paxolin to cut and then reassemble the dropper and all its associated connections - hope I took enough pictures?!
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