Another Bush DAC 10

[DonaldStott]

What I've done is to remove the cross-braces and the long hexagonal pillar that hold the Tuning Unit in place. This gives me enough wriggle room to better access the tag board , especially those difficult to get at connections at the rear!

Working my way along I'm finding that almost every resistor is at least 20-25% over and will need replacing. So far I''ve replaced (in order) R1, R5, R6 and R8 - just steeling myself before I tackle the closely packed row of waxy capacitors on the right hand side of the tag board!

Just another quick question, how do you remove the knobs to facilitate cleaning:-



Thanks for your continuing interest and support.

[Herald1360]

The knobs simply (!) pull off. In principle..... in practice they may be reluctant

If you can apply force using a tool that pushes directly against the back of the boss rather than just pulling on the knob or its skirt it'll be less likely to end up in tears!

[DonaldStott]

Thanks again Chris - I had assumed that the metal clip would have to be removed and that would reveal a grub screw holding the knob onto the shaft?

I've given both knobs an exploratory pull using my hands - sorry, no special knob-pulling tool here - and they are going nowhere! I'll try some PlusGas overnight and if that doesn't work then the knobs can stay put.

[DonaldStott]

It's three cheers again for PlusGas - the two knobs have been liberated from their respective shafts!

The important technique was to lever from the back (thanks Herald1360 and David G4EBT) and not to pull from the front - and of course a bit of this

[DonaldStott]

I posted this in the Sets, Parts and Service Information Wanted section but thought I'd repeat it here for the sake of continuity: -

"I've extensively replaced all the perished and exposed wiring in and around the Dropper on my Bush DAC10.

Unfortunately I seem to have lost or deleted the detailed photos I had showing the wiring and connections in this locality.

Does anyone have such photos of the front and rear of the Dropper before and after restoration??"

You will see from the first picture in my Post #3 above the poor state of the wiring which basically just crumbled away when it was removed! Situation is complicated as I have also swapped round the LIVE and NEUTRAL connections on the switch.

I know that the LIVE feed comes from the switch to the top of the Dropper i.e. RED wire to 250 connection and that then loops back down to the 230 connection?

Not 100% sure from the photographs I have remaining where the other lower down connections should go?

From the Bush Radio Service Instructions circuit diagram I have surmised that R19 is wired between the 210 connection and Pin 2 (cathode) on V5 (UY 41) - a YELLOW wire.

There is also a BROWN wire that appears to run between the 210 connection and Pin 1 (Heater) on V5 (UY 41).

I'm fully prepared to be shot down in flames - rather have that than the DAC10 going up in flames - any help would be appreciated!

[ms660]

Replies in bold

Originally Posted by DonaldStott

I know that the LIVE feed comes from the switch to the top of the Dropper i.e. RED wire to 250 connection Yes, that's correct. and that then loops back down to the 230 connection? Only if it suits your mains voltage, it's the voltage tap link wire.

Not 100% sure from the photographs I have remaining where the other lower down connections should go?

From the Bush Radio Service Instructions circuit diagram I have surmised that R19 is wired between the 210 connection and Pin 2 (cathode) on V5 (UY 41) - a YELLOW wire. Almost....Pin 2 is the anode not the cathode.

There is also a BROWN wire that appears to run between the 210 connection and Pin 1 (Heater) on V5 (UY 41). There shouldn't be, pin 1 should be connected to the free end of the 950 Ohm dropper section only.

I'm fully prepared to be shot down in flames - rather have that than the DAC10 going up in flames - any help would be appreciated!

Lawrence.

[DonaldStott]

Thanks Lawrence.

Don't know why I referred to Pin 2 of V5 (UY 41) as the cathode when it's clearly the anode!!!

As my mains voltage is 246V then I'm assuming that I don't need the mains selection flying lead between the 250V and 230V tapping points on the dropper?

I will ensure that Pin 1 (Heater) of V5 (UY 41) is connected to the free end of the 950 Ohm dropper section only - physically the bottom connection on the dropper.

[DonaldStott]

One step forward and two steps back...

Reassembled the dropper and all it's associated connections and thought I'd take some more measurements first before attempting a live switch-on with the lamp limiter.

Here is the dropper back out of the set and I've labelled the connections 1 to 4: -



This is what I found: -

1-2 or 1-3 or 1-4 - no readings!

2-3 = 150.4Ω

3-4 = 968Ω

2-4 = 1,119Ω

So clearly we have some problems with that top section of the dropper which should also read 150Ω - maybe that explains why the mains selection flying lead was connected between the 250V and 230V taps (1-2 in my picture). If this is the case then it may mean that the set has been overrun in the past with possible repercussions further down the line (along the circuit).

[HamishBoxer]

You might be able to find a 150 Ohm RadioSpares section to replace the o/c bit.Try a wanted ad.

[ms660]

It looks like somethings slipped and taken a chunk out of it.

As said a 150 resistor will do, minimum 7 Watt rating.

Lawrence.

[ajgriff]

As an example a 10W 150Ω part from here should be fine:

https://www.ebay.co.uk/itm/Wirewound...jvqo8KlXtL472Q

Alan

[DonaldStott]

Many thanks to Hamish, Lawrence and Alan - 10W 150Ω resistor ordered from eBay.

I'm assuming I can just attach some tags to the legs of the resistor and connect it across the 250V and 230V taps on the dropper - with some heat shrink of course. I read in an earlier post that there is a risk that the dropper could "reform" in some way - how likely is that?

[HamishBoxer]

Silly me,forgot about ebay.PS yes it is worth removing the middle bit of the o/c piece just to make double sure in the unlikely looking at it,that it could link.

[DonaldStott]

Originally Posted by ms660

It looks like somethings slipped and taken a chunk out of it.

Just for the record I thought I would publish a photo of the Dropper damage. The image below should show where the resistor wiring has been fractured although I do appreciate that all Forum image files are reduced in size as a matter of course.



If anyone wants a copy of the hi-res original (1.53MBytes) then drop me a PM with your email address.

[DonaldStott]

I"ve now rebuilt and rewired the Dropper and it's associated connections including the addition of a new 10W 150Ω resistor to replace the damaged top section and a new strip of Paxolin. From the top, the three sections of the Dropper are now measuring 156Ω, 152Ω and 970Ω.

It was a bit of a struggle wrestling with the the new 10W 150Ω resistor as it is so large and had to use some solder tags on the legs and silicone wire to get it to fit!

I ordered up some new MES E10 light bulb holders and the replacement bulbs I have now fit perfectly. Another new strip of Paxolin has replaced the original to accommodate the new bulb holders and I had intended to take the opportunity to replace the existing 75Ω resistor with a 180Ω resistor to give more brightness as per the Bush DAC90a Modifications by kalee20. Unfortunately some one (?) ordered the wrong value of 5W resistor so final reassembly, rewiring and refitting of the scale lamp assembly will now have to wait until early next week when the correct components should arrive!

Meantime it's more cleaning and I've removed the Output Transformer and Loudspeaker to allow me to properly clean the cabinet.

I have a question about the Loudspeaker which has accumulations of dust, rust, grime and cobwebs! The picture below illustrates what I mean and I'm unsure if unscrewing the two large bolts in the centre would allow me to remove that top plate and access the actual cone which desperately needs cleaned!

[HamishBoxer]

No,that will loosen the magnet.Possibly just a jet of air to rid the dirt or a dry brush and vac it out.

[DonaldStott]

Thanks Hamish - that would have been a bit of a mistake to loosen the magnet!

A soft brush and the vac brush attachment has cleared a lot of the dust.

[DonaldStott]

So as well as completely rebuilding the Dropper assembly I have now reassembled, rewired and refitted the scale lamp assembly - all looking nice and clean now with its new bulb holders, Paxolin strip and 5W 180Ω resistor.

Spent some considerable time working my way along the tag strip underneath the Tuning Unit and have replaced all the waxy caps and over-tolerance resistors - wow, what a test of dexterity and patience that was but all done now! Replacing and testing one at a time really dragged things out but at least I now have a working radio with a couple of strong stations on LW and several stations on MW. Not much in the way of any hum which is surprising but the usual noisy hash between stations which we can investigate later.

Only real moment of excitement was after the first live switch on with the lamp limiter when the Dropper started smoking - reassured myself that it was only dust and other contaminants and I've read somewhere on this Forum that the Dropper material is hygroscopic so some of the smoke may in fact have been steam! No matter, after about 10 minutes the smoke/steam stopped.

Checked the Valve Pin voltages and all looking fairly good - only real issue is that the HT is lightly low across the whole set? Looking more closely at V5 (UY41) both the Trader Sheet and the Service Instructions state that Pin 2 (Anode) should be 220V A.C. and Pin 7 (Cathode) should be 205V - I'm reading 190V A.C. and 184V respectively. Both documents also show the V5 Pin 2 feed comng from the 210V tap on the Dropper via the 150Ω resistor so I'm thinking of moving this up to the 230V tap? My mains is about 246V A.C. and, of course, varies slightly during the day.

I've still got a number of resistors and the V3 Grid / A.F. Coupling capacitor (another paper waxy) to replace which leaves a couple of outstanding issues:-

1. The preset levers are very noisy when operated and it's clear that there are problems with the switch contacts! I'm not sure how I can sort this out without removing the Tuning unit to get at the switch contacts at the rear. Don't want to indiscriminately spray the unit with contact cleaner - very hit and miss!

2. The loudspeaker is scratchy and sounds like there is "stuff" trapped in the workings or a misaligned suspension? The main magnet is an obvious source of "rust dust" and it would be good to get "in about it" to give the speaker a proper clean and check. Apart from removing the two large bolts holding the magnet in place I don't see any other way of disassembling the speaker? I have replaced the Tone correction cap across the Primary of the Output Transformer.

[DonaldStott]

It was all going too well ...

I had a working set with a couple of strong stations on LW and several stations on MW so proceeded with the first stage of reassembly which was to replace the cross-braces and the long hexagonal pillar that hold the Tuning Unit in place. Really difficult to access the bolts that attach the Unit to the chassis but perseverance saved the day.

And the result - a set that is now silent on both LW and MW

I have a couple of things to check so I won't list my findings just yet but I must have disturbed something when carrying out the work detailed above!

One thing I did notice was that the connection from one end of the V3 grid earth return (2MΩ resistor) had come loose - a black wire attached to the 2-way tag strip - not sure where that came from? The diagram below details the two connections from the capacitor that runs from the the VR1 wiper - one connection (RED) goes to the 100kΩ resistor and then to Pin 2 on V3 and this is all present and correct. The other connection (BLUE) goes to the 2MΩ resistor and then to earth, but I do not know where that connection should be made, physically. I've sort of assumed that this should go the central connection on the V3 valve base??



Need to go back and check my valve pin voltages, heater chain etc. as there may be something amiss there - probably just trying to avoid the likely outcome that I've disturbed something in the Tuning Unit when struggling to refit it!

(I've found it really difficult to access the valve pins in this set to take accurate readings and wondered if anyone could recommend a pair of long Multimeter Test Leads for such jobs?)

Hope someone can help - especially when today is my Birthday!

[DonaldStott]

Thanks to Forum member Keith956 for his advice and guidance on the stray black wire as the set is now back up and working. The additional good news is that there appears to be no damage to the Tuning unit as I first feared.

The switch contacts of the preset levers have all had a good spray with Deoxit Fader F5 - incredibly expensive but very effective - and any previous noise from these when operated has all but disappeared.

Still issues with the speaker (scratchy/buzzy) and lots of hash between stations so hopefully some advice may be forthcoming on these points? It's still amazing how quieter things become when all other electrical equipment in the vicinity is turned off - one of the prices we pay for our modern, connected society!