Bush DAC10 - V5 Base Failure

[DonaldStott]

Started work on restoring yet another DAC10 and had just got to the stage where I was doing some initial checks - no power on just yet! Cleaned off all the usual muck, dust and cobwebs and started to remove valves as there seemed to be a problem with continuity across the heater chain?

Pulled V5 from its socket and heard that distinctive sound of small parts falling from underneath the chassis - this is what I found on the workbench:-



Fortunately this valve base is bolted to the chassis and not riveted so was easy to remove. Never had this sort of failure before but it seems that the valve pins are not recoverable along with the rest of the valve base!

What's the current recommendations and best practice for replacement valve bases?

Only have the heater pins along with the Anode and Cathode to solder while this part of the under-chassis area is easily accessible.

[Sideband]

Langrex do B8A valve bases for £5 https://www.langrex.co.uk/products/b...e-socket-x1pc/

Should just fit straight in.

[DonaldStott]

Anyone used these:-

https://www.cricklewoodelectronics.c...e-Socket.html?

Any reason why it has to be B8A MCMURDO RIMLOCK as those metal rings won't be going back on again?

[ms660]

If it's for your DAC10 then the one in your link looks like an Octal (International Octal) not B8A.

Lawrence.

[Sideband]

Yes, you have linked to an OCTAL socket. No good for your DAC10 which uses B8A valves. Nothing wrong with those Langrex holders, they don't have a metal spring. All they have is the usual skirt with a keyway. No locking spring to break the glass pip off.

[DonaldStott]

I'm always amazed by the way that Threads seem to grow organically as Restoration proceeds!

Manged to retrieve and transplant a B8A valve base from one of my donor sets.

As I mentioned in my opening Post, V5 in the Bush DAC10 only has the heater pins along with the Anode and Cathode to solder while this part of the under-chassis area was easily accessible - so job done?

NO! - got everything ready to reassemble and put back in the case and gave the set one last test! Glad I did as I was presented with a loud buzzing noise that was constant irrespective of the volume control setting. Some faint radio station signals could still be heard but overwhelmed by the buzzing. My first instinct was that something had come loose or I had a bad solder joint, so all my recent work was revisited. I tend to associate hum in these Bush sets with the smoothing and reservoir caps in the dual can but these are normally very reliable, in my experience. So a quick check with the capacitor tester showed that the reservoir capacitor (32uF) was measuring around 5228pF, yes pF, or as we say in Scotland 'gubbed'!!! If it had been a DAC90A I could have connected up some new caps underneath the chassis but with the DAC10 the dual can tags are on the top (!) so another re-stuffing exercise?

So job done - NO!

After getting the DAC10 back up and working I was just about ready to put it back in its case when it stopped working - again! No sound from the speaker whatsoever, no hum, dial lamps not working, no glow from any heaters etc. - just a very faint electrical ''fizzing' noise if you know what I mean? Here we go again! Ran through all the usual checks again but nothing seemed out of the ordinary? As I had previous issues with continuity across the heater chain I checked each valve again - all beeping nicely with my DMM when removed from their bases, all except the UL41 which we know will show a resistance but doesn't beep! I found that V1 (UCH42) showed continuity across the heater pins when removed from its base but nothing when testing the same pins from underneath with the valve replaced! On closer inspection it appears that part of the valve base has come apart, much like V5, and will also need to be replaced.

V1 is a bit more difficult to access than V5 and it has connections to every pin, some of which are multiple connections. So I've carefully photographed and noted which wires connect to which pins on V1 and then to work up a plan for its removal and replacement. Pins 1-4 are accessible from the rear of the set while pins 5-8 are more 'easily' accessible from the side but that involves working with my soldering iron perilously close to the dial cord. I've enough work here without adding further self-inflicted woes!

But meantime a quick question ...

The close up picture below shows a black wire from the tag board (R3 and C4) which is connected to the central metal ring of the valve base which is connected in turn to a chassis tag. What's the purpose of that intermediate connection and is there any reason why I just can't take the black wire straight to the chassis tag. Space is at a premium at this location but I'm assuming that this was connected in this way for a reason?

[vinrads]

It was common practice to earth the spigot ,if you don't have a spigot just earth the wires , Mick.

[DonaldStott]

Just found this older Thread from 2010:-

https://www.vintage-radio.net/forum/...ad.php?t=55949

In Post #5 we have "The spigot, if grounded, acts as a screen between pins so reduces capacitive feedback even when no other screen is present. If not grounded then it adds to feedback, so it should always be grounded."

Interesting...?

[slidertogrid]

Originally Posted by DonaldStott

involves working with my soldering iron perilously close to the dial cord. I've enough work here without adding further self-inflicted woes!

If I need to use the soldering iron anywhere near the drive cord I cover the cord in tin foil that way it won't break if you just catch it with the iron. You still have to be very careful but it gives some protection. Often with Transistor sets there are electrolytic capacitors that need changing that involves soldering near the cord.
Are those wax capacitors in your picture re-stuffed replacements ? I know sets will work with the old capacitors still in place but the general opinion seems to be replace on sight? I am only thinking it will save you having it apart again.
I would also have a good look at the pin connections on the other valve holders, if two have fallen apart the others may not be far behind them...
Rich

[Ian - G4JQT]

Originally Posted by DonaldStott

Just found this older Thread from 2010:-

https://www.vintage-radio.net/forum/...ad.php?t=55949

In Post #5 we have "The spigot, if grounded, acts as a screen between pins so reduces capacitive feedback even when no other screen is present. If not grounded then it adds to feedback, so it should always be grounded."

Interesting...?

Thanks for that. I think I would always ground the centre spigot in any rebuild, but it's the sort of thing that could be overlooked and might lead to the dreaded difficult-to-pin-down instability.

[ms660]

The grounded spigot acts as a screen between the anode and g1 connections. In some valves, eg: the UF41 there's an internal screen between the anode and g1 electrode connections to the pins that is in effect coupled to ground via a plate (which is part of the screen) and the spigot, the plate is set into a recess in the center of the valve base and is in line with the spigot.

The whole providing a screen between the electrode connections at the electrodes all the way down to the connections on the valve socket, it helps the valve not to turn into an oscillator which it otherwise might have the propensity to do due to the fact that the anode and grid circuits are both tuned when used as an IF amplifier.

The plate that's set into the the valve base is integral with the screen that's between the anode and g1 electrodes and their connections to their pins.

You can see the screen extending down to the valve base in the UF41 here:

http://www.r-type.org/exhib/aai0211.htm

The EF80 is another valve that employed the same, here's a full description:

http://www.r-type.org/addtext/add113.htm

Lawrence.

[Colin Boggis]

As this particular spigot is on the rectifier base I don't think it needs to be connected. I can't see it causing any issues if not connected.

[ms660]

Originally Posted by Colin Boggis

As this particular spigot is on the rectifier base I don't think it needs to be connected.

Post#6.....V1.

Lawrence.

[DonaldStott]

I've now managed to transplant another B8A valve base from one of my donor sets. A bit more tricky due to the number of connections and with a resistor across Pins 4 and 7. I decided to follow the advice in several Posts above and have connected the spigot to the chassis tag as per the original Bush configuration. I will check all the other valve bases for stability.

Switched on and all is well - just need to check the manual tuning on the Oscillator and Signal Frequency Circuits. Page 5 of the Bush DAC10 Service Instructions has details of how to do this using a signal generator and a single turn loop of wire approximately 8-10 inches. I'll rig up something with some MDF and battens of wood - it wont be elegant but it will be functional! Following that I'll attend to the alignment of the pre-selected station buttons.

@Colin Boggis - V1 on the Bush DAC10 is a UCH42 i.e. a triode hexode oscillator and mixer and NOT the rectifier - that's V5, a UY41.

[DonaldStott]

As promised I managed to lash up a single turn of wire using some MDF and wooden battens. It's not that elegant and won't win any prizes in the Forum Fashion Show but it proved very effective when connected to my Heathkit Signal Generator.



Managed to follow the steps on Page 5 of the Bush DAC10 Service Instructions to manually tune the Oscillator and Signal Frequency Circuits. Didn't see (hear) anything that made me want to start twiddling with the IF Circuits so probably left well alone!

So another Bush DAC10 restoration nearing completion with only the case to finish off. All the dents and scratches were filled and sanded while the whole case was primed and resprayed.

[DonaldStott]

Having manually tuned the Oscillator and Signal Frequency Circuits I'm still hearing some background crackle on LW e.g. BBC Radio 4 LW.

I've eliminated all known local sources of interference and don't have any such issues with similar sets in the same room?

Where can I look on the circuit diagram to help resolve the background crackle on this particular Bush DAC10?