Bush DAC90A repair.

[sobell1980]

Hi all
Well this is my first time at one of these Bush sets. I must say I was delighted to see how easy it is to work on and we'll it came apart. This set is completely unmolested and original which is always a relief to see.
I'm in the process of re capping the usual waxes. I've come to replace the mains smoothing cap and it is rated at 32+16 uF in a dual cap. This arrangement doesn't seem particularly easy to come by . Would it be be permissable to fit a 32+32uF dual cap? I can't think of any reasons why it wouldn't be ok? I've usually been lucky and sourced a like for like value. Next project ensues!
Many thanks
Dave

[kalee20]

Electrolytics like this are usually OK. May be a little leaky at first, but you can wake them up gently.

If there's no sign of leaking, significant bulging, or cracking of the rubber bung, it is very likely good for the next 60+ years.

[ukcol]

I'll second that, the chances are the 32-16uf will be fine.

In general when changing the value of smoothing capacitors you have to be careful not to exceed the maximum value of reservoir capacitor permitted for the rectifier value used. This value will be stated in the valve's data.

The DAC90A uses the 32uF section for the reservoir so if you are very unlucky and need to replace it a 32-32uf will be fine.

[MurphyNut]

My first restoration was a Dac 90a and like you found the experience enjoyable and not as difficult as I thought it might be. But not all radios are as easy to work on as with this Bush as I was to find out later.
Regarding the duel electrolytic capacitor, out of the two Dac 90a's I've fixed in one of them I did have to replace the unit. I actually wired in a 32uf and 22uf separately, it worked very well.

My DAC90A was done when I was new to this vintage radio malarkey https://www.vintage-radio.net/forum/...ad.php?t=56320

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Very usable sets that do suffer from the modern cacophony of wall warts and the like if used in the average house. To cure this I...
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Common enough to be cheap and iconic enough to be a centre point of a working display given a good signal. The various pantry transmitters available give an idea of what was good sound on MW (or LW) before the brickwall transmitter filters came into use. Much as I hate to admit it sounds better than my EKCO A22.

[sobell1980]

The repair is going well. I've changed all the wax caps. The reason I had to order a mains smoothing cap was the original had bulged on the top and blistered with some white powdery residue so I have had one arrive from the tube doctor yesterday along with some nice braided mains cable. I'm going to hard wire it to the mains switch and remove the two pin connection on the rear of the set. Is anyone aware of where I can get the glass rubber channel from for the tuning glass? I was thinking of otherwise using heatshrink with a cut down one-side?
Dave.

[sobell1980]

Some photos of progress.

[sobell1980]

Many thanks to Martin Bush who has kindly supplied the genuine rubber channel to fit to the tuning glass. Many thanks Martin, the project may continue.
Dave.

[sobell1980]

Something has been puzzling me with this diagram and set up of the scale lamps. How are they receiving power being connected on the neutral side of the mains? What is the purpose of the R18 the shunt resistor in parallel across the dial lamps? These are on the negative side of the circuit .
I'm confused by this.
I've also replaced the mains smoothing cap and resistor but ive noticed still an amount of mains hum . However my scale lamp bulbs are not connected to chassis at the moment. Seeing the diagram these bulb holders aid the negative connection of the mains smoothing cap through the bulb holders through to the neutral side of mains. This section of the diagram I find hard to understand. Any help gratefully received.
Dave.

[Graham G3ZVT]

Originally Posted by sobell1980

Seeing the diagram these bulb holders aid the negative connection of the mains smoothing cap through the bulb holders through to the neutral side of mains. This section of the diagram I find hard to understand. Any help gratefully received.
Dave.

Hi Dave, Would it make more sense to you if the scale lamps and shunt were in the live side, to the right of the dropper?
Can you see now that electrically it makes no difference, the current is exactly the same.

As for the purpose of the shunt, it drops notionally 7 volts (probably rather less in practice) to make the two 3.5V bulbs glow.
You should regard the shunt as completing circuit to the radio, not the lamps.


Removing one or both lamps will make little difference to the operation of the radio, apart from the obvious, removing the shunt however will put full mains across the lamps which will blow instantly.

Graham.
G3ZVT

[G8UWM-MildMartin]

Hum is often caused by the UL41 suffering any of its usual faults (a search of the forum will
yield information on improving it, including zapping it with a gas cooker's ignition spark and removing a connection to the pin just used as an anchor point for the grid 1 stopper resistor, if proved guilty by temporarily substituting a known good one or a 10P13 valve.)

[sobell1980]

I'm having trouble still getting my head around this. Neutral should be zero (return back to electricity station) . Any thing after a consumer should be zero. So unless my new smoothing cap is dead short how is the shunt resistor receiving full mains to drop the voltage across the bulbs? I'm sorry if you are repeating yourselves. I just like to be able to fully understand the circuit in working on therefore bring able to fix it. Many thanks.
Dave.

[ms660]

The shunt resistor ensures that the receiver will still function ok if any of the scale bulbs fail open circuit.

When the receiver is working, the current flowing through the bulb/shunt combination is not the same value as the current flowing through the valve heaters, initially at switch on it is, but as the rectifier starts to conduct and supply HT current to the valves that current also starts to flow through the bulb/shunt combination, in other words when the receiver has warmed up the current flowing through the bulb/shunt is greater than the current flowing through the valve heaters, because it's a combination of heater and HT current.

It's a cheap way of initial switch on surge limiting so far as the bulbs are concerned.

If I've got that right...

Lawrence.

[sobell1980]

I'm sorry that I'm finding this hard to grasp. It's the fact that the bulbs are on the neutral side? How are they receiving current and voltage being after the negative connection or neutral? Are you saying that the bulbs are receiving what's left after the heater chain before being returned to neutral which is why they are so dim?

[ms660]

Originally Posted by sobell1980

I'm sorry that I'm finding this hard to grasp. It's the fact that the bulbs are on the neutral side? How are they receiving current and voltage being after the negative connection or neutral? Are you saying that the bulbs are receiving what's left after the heater chain before being returned to neutral which is why they are so dim?

Ok, rule number one, unless diverted to earth, the current flowing through the live conductor is the same value as the current flowing through the neutral conductor.

In this receiver there are two current loops under consideration for the moment, the first one is the current in the heater loop, easy to trace out on the schematic, the current supplied by the Live flows though the dropper, the series heater chain, through the chassis connection, through the bulb/shunt and on to the Neutral, the circuit is complete, current flows, call that current X.

The second loop is the HT loop, also easy to trace out on the schematic, the current supplied by the Live flows through the dropper, through the rectifier, through the valves, through the chassis connection, through the bulb/shunt and again on to the neutral, call that current Y.

It follows therefore that the current flowing through the bulb/shunt when the receiver has warmed up is current X plus current Y and that the sum current is greater than the individual currents.

Hope this helps.

Lawrence.

[Sideband]

Originally Posted by ms660

The shunt resistor ensures that the receiver will still function ok if any of the scale bulbs fail open circuit.

When the receiver is working, the current flowing through the bulb/shunt combination is not the same value as the current flowing through the valve heaters, initially at switch on it is, but as the rectifier starts to conduct and supply HT current to the valves that current also starts to flow through the bulb/shunt combination, in other words when the receiver has warmed up the current flowing through the bulb/shunt is greater than the current flowing through the valve heaters, because it's a combination of heater and HT current.

It's a cheap way of initial switch on surge limiting so far as the bulbs are concerned.

If I've got that right...

Lawrence.

That is also why the lamps change in brightness as the set warms up. This was quite a common circuit arrangement at one time.

[Sideband]

Originally Posted by ms660

It follows therefore that the current flowing through the bulb/shunt when the receiver has warmed up is current X plus current Y and that the sum current is greater than the individual currents.

Hope this helps.

Lawrence.

Brilliant way of describing it!

[Cobaltblue]

Originally Posted by sobell1980

I'm sorry that I'm finding this hard to grasp. It's the fact that the bulbs are on the neutral side? How are they receiving current and voltage being after the negative connection or neutral? Are you saying that the bulbs are receiving what's left after the heater chain before being returned to neutral which is why they are so dim?

The reason the bulbs are so dim is by design.

They are under run to give them longer life and they are not very powerful to start with.

they are 3.5 V at 0.15A or 0.525W each at full ratings most AC only sets would use a pair of 6.3V 0.3A bulbs or 1.89W almost 4 times as much.

If you want brighter dial lamps you need to follow the well worn path to use alternatives such as LED or fit a small transformer to run higher wattage bulbs.

Cheers

Mike T

[kalee20]

Originally Posted by rambo1152

...Removing one or both lamps will make little difference to the operation of the radio, apart from the obvious, removing the shunt however will put full mains across the lamps which will blow instantly.

Graham.
G3ZVT

Err... No they won't! They would glow rather brightly, that's all.

They can't possibly see more current that the radio demands, which is around 200mA RMS (100mA heaters, plus the rather peaky pulses which the rectifier draws). Which is enough to overload the specified 150mA bulbs and give them a short if brilliant life, but not enough to blow them instantly. In fact until the valves had warmed up, they would still be somewhat under-run!

[sobell1980]

Ok we have discussed about current return path back through neutral. I get this as same as a battery. Current passes through the live lead to the consumer, a bulb , after the the bulb returns through to the negative side of the battery. The reason you get a little spark when rebooting up a car negative terminal. But here is the but. What about voltage. The chassis should always be on the neutral side of the mains . So how is it the chassis is now live to power these bulbs? Does this mean we are seeing 3.5 volts on the chassis to pass the current through the bulbs?