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Old 16th Oct 2006, 3:03 pm   #1
DoctorWho
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Default Neon Tuning Incorrect Operation (Bush SAC5).

I have a Bush SAC5 which, unfortunately I have not been able to find any service data for.

The set has now been fitted with new condensers and it works well on both medium and long wave.

There is a neon tuning indicator, a Cossor bayoney fitting unit, this is in working order as, when the set warms up, it starts to glow when tuned to a station, however as the set fully warms up the tuning indicator stops working and does not glow at all.

Has anyone any ideas/suggestions as to what would be causing the neon tuning indicator not to be working properly and how this may be overcome?

It's an attractive set and when the tuning indicator glows it illuminates a glass Bush symbol, so it would be nice to have that working properly.

Any suggestions greatly appreciated, similarly if anyone has service data for this set I would be very grateful.

Peter.
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Old 16th Oct 2006, 8:13 pm   #2
Ed_Dinning
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Default Re: Bush SAC5 Neon Tuning Incorrect Operation

Hi Peter, neons usually work with about 90V across them, so it may be that the HT is falling as the set warms up; check the usual suspects and also any high value resistors in the neon circuit.

Ed
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Old 17th Oct 2006, 6:54 am   #3
PWH
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Default Re: Bush SAC5 Neon Tuning Incorrect Operation

I don't know about this set, but I have a Pye which te tuning indicator goes out when it is on tune, and comes on when you de-tune it

Perhaps it is one of these? Just a thought.
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Old 18th Oct 2006, 12:23 pm   #4
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Default Re: Bush SAC5 Neon Tuning Incorrect Operation

Thanks Ed, I'll check this out. The neon only glows early on, as soon as the set is fully warmed up it goes out and will not come on in any position.

Thanks again,

Peter.
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Old 18th Oct 2006, 2:10 pm   #5
Biggles
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Default Re: Bush SAC5 Neon Tuning Incorrect Operation

Has the neon got two or three electrodes? If it only has two then the voltage across it as Ed says will be a clue to the problem. If it has a third electrode then the problem may be a bit more complicated as I'm sure I read somewhere once that the third electrode is used to control the length of the visible discharge column.

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Old 18th Oct 2006, 3:40 pm   #6
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Default Re: Bush SAC5 Neon Tuning Incorrect Operation

I can confirm the use of the third electrode as I have such a one in a Cossor. If the neon is used to illuminate the Bush logo it would be surprising if the variable length type was used so your problem might not be too complicated.
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Old 18th Oct 2006, 5:59 pm   #7
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Default Re: Bush SAC5 Neon Tuning Incorrect Operation

Hi Peter
I have a Cossor neon in an HMV radio. It has twin contacts plus a connection to the shell, thus presumably three electrodes. One electrode goes near the top of a potential divider across HT. One goes to a pre-set pot a little further down the potential divider and the third goes into the anode load of a dedicated double diode triode amplifier (strapped diodes) which rectifies the IF signal and amplifies it. The pot is used to set the length of the discharge column on zero signal. I guess everything depends on the balance between these various HT-derived voltages. Since in many sets with directly heated rectifiers HT rises to a high initial value before the other valves warm up, that possibly explains what you are seeing. I would check the emission and bias conditions of the valve driving the indicator tube as well as the HT itself, which may be low if the rectifier has seen better days.
Cheers
Nigel
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Old 18th Oct 2006, 8:01 pm   #8
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Default Re: Bush SAC5 Neon Tuning Incorrect Operation

Thanks for the input here. The neon does indeed have 3 electrodes, two to the base and one to the outside.

The neon is used to illuminate the Bush logo, but I've removed it from the housing to see what's going on and whilst it does glow whilst warming up as previously mentioned it is completely out when fully warmed up. It is a long neon though, so I imagine it may well be variable length, but there is no long electrode inside the glass that I can see, just around the base inside.

Thanks for the tips about the rectifier valve itself and the valve driving the neon, I'll check both of these and see what I can turn up.

All the best,

Peter.
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Old 19th Oct 2006, 7:27 pm   #9
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Default Re: Bush SAC5 Neon Tuning Incorrect Operation

Hi again Peter,
I have sent you an e-mail with a picture of the part of the HMV 800 circuit which drives the neon TI. It may help.
Regards
Nigel
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Old 20th Oct 2006, 3:36 pm   #10
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Default Re: Bush SAC5 Neon Tuning Incorrect Operation

Thanks for that Nigel, you've helped me cure the problem!!

There was a 40K resistor connected directly to the neon, I reduced it by 5K and now it works perfectly!!

Thanks for the help.

Peter.
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