Dropper dissipation in Bush DAC90A.

[ms660]

Originally Posted by bobhowe

Autotransformers wont fail & a motor run capacitor would go open circuit as long as you use a branded type

One would think....Try shorting out the tap on an auto transformer, eg: simulating a possible fault condition....

Lawrence.

[bobhowe]

Yes if it was my set it would blow the 1A fuse in the plug ( that is the first job that should be done remove the 13A fuse fitted to the plug ) before anything else on DAC90 & DAC90A

[ms660]

Originally Posted by bobhowe

Yes if it was my set it would blow the 1A fuse in the plug ( that is the first job that should be done remove the 13A fuse fitted to the plug ) before anything else on DAC90 & DAC90A

One would hope so.

One thing I've learned having served many years in the trade is to never say never....

Lawrence.

[bobhowe]

Yes you are right there seen one Bush DAC mains lead with a bayonet plug on . In the 50s that would have been plugged in to a lamp socket and the fuse box that was supposed to protect it shorted out with wire . Will start a new thread about these plugs kind regards Bob

[pentoad]

Originally Posted by ms660

You could crow bar the output of the dropper diode with a crow bar diode and suitably fuse the dropper diode (the crow bar diode acts as a short circuit should the dropper diode go short circuit thus blowing the fuse and protecting the heater chain) If that's done the dropper diode (and crow bar diode) should be at the input side of the dropper resistor with the cathode of the crow bar diode connected to the cathode of the dropper diode and the anode of the crow bar diode connected to the switched Neutral (If powering the heaters with a +ve voltage (recommended))

Lawrence.

Would this work if the mains input plug was inserted upside down (+ chassis)

[ms660]

Originally Posted by pentoad

Would this work if the mains input plug was inserted upside down (+ chassis)

Yes, AC is AC.

Lawrence.

[pentoad]

Originally Posted by paulsherwin

Yes, personally I prefer diode droppers to caps because it's a simple job to make them fail safe like this. If a suitably meaty crowbar diode is used, then there's no need for additional fusing as the plug fuse will blow. You can also increase the resilience of the diode dropper by simply fitting two or even more in series.

"Crowbar diode" do you mean the addition of a single diode (and possibly a fuse) or a circuit involving a number of different components?

[ms660]

Originally Posted by pentoad

"Crowbar diode" do you mean the addition of a single diode (and possibly a fuse) or a circuit involving a number of different components?

4th schematic (D2):

https://www.cool386.com/dropper/dropper.html

Lawrence.

[paulsherwin]

Lawrence described the operation in #4. Basically, the crowbar diode is 'backwards' and does nothing in normal operation. If the dropper diode should fail short circuit, the crowbar diode will pass a lot of current, both protecting the heater chain and blowing the plug fuse (and probably the failed dropper diode too).

[murphyv310]

Although I'm an advocate of capacitor droppers I agree that with a Diode dropper a simple reversed polarity Diode is a simple and effective crowbar. The down side of Diode droppers is there is still often the need in some sets for a resistive dropper. The other downside is with the reduction of the dropper value switch on surges are higher and there is more chance of thermal shock on heaters. Yes a thermistor can be fitted but it's yet another heat source. In fact the auto transformer is also guilty of this too. The cap dropper is calculated for valves at normal running temperature so cold valves don't get the thermal shock as current is limited, just observe the heater voltage as the set warms up, very similar to how they behave with the original dropper.

[G6Tanuki]

Diode droppers are a good way of doing things - and I'd hope that these days a suitably-rated diode would be a vanishingly-small failure-issue.

[I'm reminded of a certain 1960s TV that used a diode-dropper in the heater chain - and the cunning placement of an additional capacitor provided, in the event the diode went S/C, catastrophic frame-roll so the thing would be turned-off and a service technician summoned rather than continuing to work with murderous results to the valve-chain]

[stainless]

Originally Posted by G6Tanuki

[I'm reminded of a certain 1960s TV that used a diode-dropper in the heater chain - and the cunning placement of an additional capacitor provided, in the event the diode went S/C, catastrophic frame-roll so the thing would be turned-off and a service technician summoned rather than continuing to work with murderous results to the valve-chain]

When I was young, the set was rented; I insisted on having it on for Children's Hour, even if only the sound and a rolling picture.

The service chap wasn't very happy when he finally got to the set a few days later!

[murphyv310]

I modded my Murphy V659 with a Diode as the dropper went partially open circuit. I made the heaters negative going and fitted an extra Diode as a crow bar. Never had any issues in the last 8 years.
I used 1N5408 Diodes. As in the above article having heater and HT diodes in opposite polarity helps with not putting DC on the mains.

[Heatercathodeshort]

I encountered this on many occasions. Tube and all the valves written off due to excessive over run. J.

[bobhowe]

Hi John was this caused by over run on first switch on kind regards Bob

[Heatercathodeshort]

Hello Bob, Not quite clear what you mean but generally a service call was booked due to breakdown. On checking the valves and CRT would be glowing a bit on the bright side.
Further checking would reveal a dropper section bodged across the high resistance section of the heater dropper. This would typically be a 33 ohm RS section across a 150-180ohm section.
Surprisingly the receiver would work like this for some time until finally breaking down under the strain. On fitting a makers replacement dropper you would discover that all the valves and the CRT had virtually no emission writing off the chassis that may have been only 3-4 years old. Enquires to the customer would establish that the TV had gone dead a few months back and they had called 'someone out' to repair it.

Regarding the heat problem with the DAC90/90A I'm still a supporter of the autotransformer. This would rid the chassis of that dropper and it's very nasty Asbestos overcoat. With the much reduced accumulation of heat it would allow the inside of the back to be covered with fine nylon mesh similar to the type that was used on the first BEAB approved TV's such as the G8. This would make the Bush safer but not SAFE in unexperienced hands. Just my thoughts. John.

[bobhowe]

Hi John thanks for the info about service call with out going OT the same used to happen to me in my job someone else had done a job then i would get a call when it broke down . The autotransformer is the way to go as Bush used it on the AC91 & SUG3 models & the Ed Dinning autotransformer has a 6.3V tapping for the dial lamps so easy & cheap to get bulbs & they dont glow like cigarette ends . Like the idea of the mesh on the back that would have been sensible John

[ms660]

Just a note about the cold switch on current for these valves, they will take 6 times the hot current at cold switch on no problem, eg: 0.6 Amps.

Lawrence.

[bobhowe]

Hi here is a picture of Ed Dinning Autotransformer ( 20p coin to scale ) . This transformer is wired for a DAC90 as the Valve heater voltage for a DAC90 is 79 volts . The colour connections as follows Brown 240V Switched Live in Blue Radio Chassis Neutral Red 202 V HT DAC90A OR DAC90 . Supply White 6.3V Dial Lamps ( Remove shunt resistor ) Yellow 79V Valve Heaters DAC90 ONLY . TOP left tap ( white paint ) 117V Valve Heaters DAC90A ONLY . The rectifier anode resistor may be removed or varied to get the HT rail voltage correct ( on my sets i have removed the resistor with no issues . This transformer will go in the same place as the mains dropper kind regards Bob

[pentoad]

Is this how the diode dropper would be incorporated into the Bush DAC90A?