Bush DAC90A repair.

[Cobaltblue]

Its just a series circuit.

The Chassis is not connected directly to either pole of the mains on this chassis.

You can clearly see that in the circuit.

The current passing through the whole circuit causes a voltage drop across the bulbs and its parallel resistor.

You might consider doing some simple parallel series circuit experiments, it might just click then.

Cheers
Mike T

[electrogram]

You could regard the whole radio, EXCEPT the the bulbs and their shunt, as one circuit, and the bulbs and their shunt as another circuit. All that is happening is that these two circuits are connected in series, so whatever current is passing through one of them must be passing through the other. The radio chassis is connected to the junction of the two circuits, not to mains neutral. Neutral is (or should be!) connected to the non-chassis connected end of the bulb/shunt circuit only. The chassis is only a few volts above neutral as the bulb/shunt circuit is a much lower resistance compared to the rest of the radio, assuming mains neutral, not mains live, is connected to the non chassis end of the bulb/shunt circuit

[G6Tanuki]

Yes, the radio chassis sits at a slight voltage above the mains-neutral because of the action of those bulbs and the resistor.

[In turn, your mains "neutral" as delivered to the customer is never at 0V either - there is a voltage-drop due to the resistance (a frw fractions of an Ohm in a good installation)of the neutral wire running back to the power-station (or your local substation) so the mains 'neutral' supplied to the customer can sit several volts above 'earth'. Neutral and Earth are joined together at the substation.]

[John G8MWF]

I think of it as if the set was connected to an isolation transformer feeding the it instead of the mains.
Then there is no live or neutral on the set - just an AC supply and the set doesn't 'care' which way around the supply is because there is no reference to earth.

Hope that helps!

John

[Heatercathodeshort]

Similar arrangement to obtaining 'automatic' grid bias in battery receivers. J.

[sobell1980]

Thank you all for your replies and help clearing this up for me. I've had time to read and digest all your thoughts and comments and process in my mind of what's happening and how that part of the circuit works. It explains the dimness of the scale lamps and why they brighten and dim on switch on. I won't be carrying out any modifications to the scale lamps as I like my sets to be as is and original. If it was good enough for Bush at that time then it's good enough for me. If it's all wired up correctly and original why shouldn't it work hey? Sometimes I think we are all guilty of expecting too much from these sets with modern audio surrounding us.
I will take a look at that spare pin on UL41 to see if it is causing the hum. Does this mean the valve is leaky and faulty if it is the cause? Moving the wires from the spare pin is masking the root cause problem? Or is better too move the wires from the spare pin on valve UL41 and replace the valve also? Is this wiring tag causing the valve to become leaky as it offers the valve a path to find? What actually fails inside UL41 to cause the hum?
Many thanks.
Dave.

[Boater Sam]

Yes!

[Heatercathodeshort]

Originally Posted by sobell1980

I like my sets to be as is and original. What actually fails inside UL41 to cause the hum? Dave.

Then please leave the original 5amp mains input connector in place even if disconnected.

As far as the leakage between the pins. This is caused by evaporation of the materials employed to produce the electrode assembly usually evident by black stains within the valve.

This is not a problem if the unused/unconnected pins are not used as anchorage points in the circuit. Leakage from the heater or in fact any other connections to the audio circuit [if connected to the 'unused' pin 4] will take place via the evaporated deposit causing the hum you are experiencing. J,

[Sideband]

...and the reason we like to isolate pin 4 at the valveholder is because it's a LOT cheaper that the purchase of a new UL41 if you can get one. These are now getting very scarce and a lot of the usual suppliers are showing zero stock. Getting as much life as possible out of the original valve is paramount.

I isolated pin 4 of the UL41 in my DAC90A about 10 years ago and it's still OK

[kalee20]

Exactly. Slow vaporising of materials inside the valve, followed by them landing and condensing where they shouldn't, is the cause of internal leakage in the UL41. If we can live with that (isolating pin 4, and also keeping the grid-leak resistor reasonably low is the key), then the UL41 should otherwise have a reasonably long life.

[sobell1980]

Ok thank you. All useful great stuff. Many thanks,
Dave.

[sobell1980]

Had 10 mins on the set this evening. Was hoping to disconnect pin 4 on the UL41 but this has already been done! See photos. I've also given the mains reservoir and smoothing cap another earth or negative to chassis in case this was bad but I still have the hum. The top of UL41 is very black . Is it possible this has gone more than leaky to pin 4?
On low volume I can still hear the hum through the audio. I suspect I may end up trying to source a valve for this set.

[sobell1980]

Here is a link to a video of the hum. Just sharing so we are all singing from the same hymn sheet so to speak.

https://youtu.be/WvpwulkGrxg

Dave.

[sobell1980]

I should also add that I have replaced R15 which is a smoothing resistor . I replaced the carbon type for a wire wound type . It had gone high in value.

Dave.

[John G8MWF]

Originally Posted by sobell1980

Had 10 mins on the set this evening. Was hoping to disconnect pin4 on the UL41 but this has already been done! See photos. I've also given the mains reservoir and smoothing cap another earth or negative to chassis incase this was bad but i still have the hum. The top of UL41 is very nlack . Is it possible this has gone more than leaky to pin4 ?
On low volume I can still hear the hum through the audio. I suspect I may end up trying to source a valve for this set.

Cleanest set I've seen - so far!
Is it me or has pin 3 been cut as well?

John

[sobell1980]

Pin 3 is an ic connection according to valve data which should have no external wiring connected to it. ic stands for internal connection.

[John G8MWF]

Originally Posted by sobell1980

Pin3 is an ic connection according to valve data which should have no external wiring connected to it. ic stands for internal connection.

Sorry - Quite so - just looked at the valve data sheet. I was previously looking at Trader sheet 1161 pin-out picture that showed a, linked, connection to pin 3.

[Geoff 555]

Hello, now this is where my stupidity shows. If the vapourised material 'lands on' the pins then what would happen if the valve was operated up side down?
Going to get my coat.
Goodbye.
Geoff.

[Boater Sam]

Its not a material landing due to gravity, nor is it just pin 4. It's a build up of vapourised muck spraying due to the voltages present.
There are many threads about this effect and its cure. Try joining all the valve pins together except the grid 1 and sparking the valve either across a gas igniter or a large value capacitor charged up to 80+volts - carefully! Grid 1 to all the rest.
In a Bush DAC90A a 10P13 is a good sub without any changes to bias resistors.
Or rebase/make adaptor and fit a UL84 but change the cathode resistor value.

[sobell1980]

Good morning all. Thank you for your helpful and interesting to read replies. I managed to source a UL41 valve that was new old stock from eBay. It had been tested and guaranteed to be good. I fitted it yesterday and I now have zero hum on the set so I am very pleased. Obviously whoever did the mod of snipping pin4 before me either didn't get it in time or the valve progressively got worse. The hum if you didn't see my video link was pretty bad and could be heard through the audio very clearly. Interesting to have experienced this fault as to my ears it sounds lower frequency to mains born hum, but hey that might just be me. Something I've learned on this repair is leaky valves and the slight difference in it's frequency perhaps so I can listen out for it in future.
The valve had very bad black almost oily deposits on the envelope all the way down which again something to note for future observations. I didn't want to zap the valve, one because I would like to see it done first and secondly this is going to be a gift to live in my friends nailers cottage she has just bought. Now the set is working I will run through the valve voltages and leave it on test throughout the week.
Thank you for everyone's advice and involvement. I will post some pictures in my thread in it's new home.
Dave.