Dropper dissipation in Bush DAC90A.

[pentoad]

I have just repaired another one of these radios

I don't like the heat coming off these sets

Is it recommended in order to reduce the heat generated by the dropper adding this circuit:

"Connect the 1N4007 in series with the heater chain somewhere away from the dropper and other high temperature parts and add a 680R 9W resistor (eg Welwyn W23) in parallel with the main dropper section. A 10nF 400V capacitor across the 1N4007 may be needed to shut up its reverse recovery RF noise, or you could use a suitable fast recovery diode instead."

Seems like a great idea and less risk than the capacitor method.

Is it deemed as a modification if you sell the set ( ie the legal can of worms) if so has anybody added a successful simple heat shield to protect the casing?

[Techman]

The modification would be on the 'radio' side of the mains on/off switch, so wouldn't come under the rules regarding altering the mains input termination, which legally should always remain as original with the two pin plug and socket arrangement. However, if you made a bodge job of it and someone tried to blame you for causing their house to burn down, then it might be a different story.

Regarding heat shields round the dropper - I'm sure it's been discussed before, but the problem is that any sort of heat deflector is going to tend to direct all the heat straight onto the cardboard type material back cover in just one concentrated area, rather than letting it just convect and disperse naturally around the inside of the cabinet and filter out through the vent holes.

The diode modification would seem to be a good idea, although I haven't checked out the values. I don't worry about the heat from the dropper in my DAC90A and it has sometimes been on all day, although droppers are horrendous wasters of power and the cause of much heat related damage in old TVs and radios that were on all day and every day.

[Heatercathodeshort]

The problem arises when the heater diode goes S/C. It would not stop the DAC90A from working and could lead to very serious overheating. If it had been sold in this modified state there could be implications.

All old AC/DC equipment is a potential risk to the present day public that do not understand the dangers. Just my thoughts. John.

[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.

[bobhowe]

Hi Auto transformer is safer than the diode route and on the plus side you have got a 6.3v tapping for the dial lamps cheaper and brighter Ed Dinning on this forum makes these transformers kind regards Bob

[Ed_Dinning]

Hi Gents, it is possible to dispense with the dropper and use an autotransformer as in the AC90 (?). A good few of these have been used by owners of white/ cream versions of the set to stop the cracking that is much more obvious in sets of this colour.

This transformer is configured for both the 90 an the 90a and also incorporates a tap for the dial light (remove shunt resistor) so a much better illumination is achieved without risk of surge damage to the lamps.
The dropper is removed and the transformer installed in its place.

Note that it is still a live chassis set and it also cannot be used on DC mains

Ed

[bobhowe]

I second that about using the autotransformer on cream versions I am lucky that my DAC90A is perfect with no cracks in the cabinet and I have also used this transformer on a Black DAC90 as well. Both sets work faultlessly.

Regards, Bob.

[Techman]

I agree that an autotransformer is the best way to go regarding a modification. The diode/resistor and also the capacitor mods are the cheap, but more risky ways to do it, unless you add in some protection, as has been suggested.

[bobhowe]

If there had been no DC mains would the DAC90 would have been AC90 ? or the DAC90A would have been AC90A ? what are your thoughts kind regards Bob

[Heatercathodeshort]

Just 90 or 90A. No DC so no AC required. J.

[bobhowe]

Thanks John for reply kind regards Bob

[kalee20]

Originally Posted by Heatercathodeshort

Just 90 or 90A. No DC so no AC required. J.

But Bush made battery sets too, there'd need to be some sort of identifier to distinguish between a mains set and a battery set!

[ms660]

Originally Posted by kalee20

But Bush made battery sets too, there'd need to be some sort of identifier to distinguish between a mains set and a battery set!

The NOTB90

Then there's backward Britain trying to export to the civilized world.... The DAC90.



Lawrence.

[murphyv310]

Cap dropper all the way. A wattless dropper, no heat produced and a reduction in overall electricity consumption. A motor run cap will last and last as its very lightly run in a DAC90 or 90A. The reduction in heat is well worth it.

Use Paul's calculator that is available here to work out the value of cap required.

https://www.vintage-radio.com/repair...per-calcs.html

[Heatercathodeshort]

OK B90/90A for the battery models. Other makers identified battery models by just adding B.
Personally I think the autotransformer is the way to go. It has been proven with other Bush designs that have employed it.
You still have a far more dangerous situation with the live chassis. The dropper is mounted so close to the card back that even if in good condition is accessible to an inquisitive child that can gain access to it with anything metal, knife fork toy etc. The man in the street can feel the heat but may not be aware of the silent hidden danger.
I pulled this example from a box of DAC90/90A receivers and all looks so harmless from the front. Turn it round and you see the hidden horror. It's a difficult problem to solve other than a small isolating transformer but probably easier and cheaper to fit a small transistor radio chassis inside such as one from a scrap Roberts Rambler.
That would take care of the safety and heat problems. We don't want any accidents introducing legislation demanding the destruction of all electrical equipment over 30 years of age. Regards, John.

[murphyv310]

A bit too much paranoia there John.
If we think the AC/DC set police task force with battering rams are heading for the front door is ever likely to happen, we are not being realistic.
In reality even sets like the common DAC90A are actually not that common. A cheap Chinese Wall wart or their plugs with dummy fuses are much more dangerous than a few DAC90A radios. As collectors and engineers we are more aware of safety and will do our level best to make the sets as safe as possible. A wired in RCD is helpful as is heat resistant mesh, hard wired mains cable and like I say a cap dropper or suitable auto TX. OK there is still the live chassis issue but a hard wired mains lead to a hard wired pluggable RCD goes some way to improve safety.

[bobhowe]

Hi i agree with John and Trevor the autotransformer or the capacitive dropper is the way to go and Trevor is right about cheap Chinese goods are more dangerous than a few DAC90's kind regards Bob

[paulsherwin]

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))

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.

[ms660]

For practical purpose the calcs. for a diode dropper are straight forward, Vout = Vrms in*0.707 or for the same difference Vout = Vpeak in/2

Therefore for R subtract the heater string voltage from the above and divide the result by the heater current.

In terms of reducing power dissipation by using a diode dropper, they are best suited to circuits where the rectifiers surge limiter is connected directly to the switched mains.

Lawrence.

[bobhowe]

Hi done the autotransformer from Ed Dinning & the capacitive dropper by Murphyv310 an both work really well with no issues not sure about the diode dropper as they can fail . Autotransformers wont fail & a motor run capacitor would go open circuit as long as you use a branded type