Bush DAC-10 - defective dropper section

[6SN7WGTB]

After spending many hours rewiring a VGC DAC-10, and testing (almost) everything, I now find that the very bottom section of the dropper (the 950Ω section to the heaters) is open circuit...

I have removed it to be sure and indeed it is. The other two sections are both 150Ω as expected.

Aside trying to find a replacement (I will post a wanted thread) what is considered best practice here? Is this particularly common (not that it matters now...)?

I estimate the dissipation of that section to be ca. 10W, and the two 150Ω sections to be ca. 4-5W each - does this sound right?

Thanks.

EDIT: looking closely at the 'band' at the top of the 950Ω has revealed a tiny crack - and scraping some green coating off shows that band to be rusty. Not evident in rest of set but corrosion may have caused the issue. Odd as rest of radio is near mint.

[paulsherwin]

As I've posted in your wanted thread, all the 4+R radios using these valves use very similar droppers, so you should find it easy to adapt something from a scrap set.

Alternatively you could modify the radio to use a capacitor or diode dropper. Search the forum for more info on this.

[G6Tanuki]

Faced with an unavailable dropper [or a client wanting their radio back ASAP rather than waiting weeks for the right part to be posted to you] the historically-correct serviceman technique would involve one or more RS 'Polo Mint' ceramic-encapsulated dropper-segments and some form of additional bracket/bit of threaded rod to mount them.

The modern equivalent being a bit of aluminium as a heatsink, and one of the gold-coloured aluminium-clad power resistors.

[Leon Crampin]

No heatsink is needed for the Al clad resistors - the chassis is fine. In my view, using a separate resistor screwed to the chassis is the best possible fix. It keeps the circuit original and spreads the heat, avoiding cracked cabinets on white sets such as the DAC90A.

Leon.

[6SN7WGTB]

As am doing this for someone else, I will probably resort to replacement of some kind.

For my own use, I'd probably do something more innovative.

[David G4EBT]

Originally Posted by Leon Crampin

No heatsink is needed for the Al clad resistors - the chassis is fine. In my view, using a separate resistor screwed to the chassis is the best possible fix. It keeps the circuit original and spreads the heat, avoiding cracked cabinets on white sets such as the DAC90A.

Leon.

Whilst it's true that you can use metalclad resistors without a heatsink, you have to derate their wattage to do so.

To quote from ESR Electronics who sell them:

"Wirewound resistors housed in an aluminium case which ensures high dissipation at a relatively low surface temperature.
Full rating can only be dissipated when mounted on a heatsink".

Unquote.

https://www.esr.co.uk/components/pro...-resistors.htm

They sell 25-Watt ones, which are quite compact (29 x 28 x 15mm) and would I guess be more than adequate in free air, but as Leon says, better still if it could be bolted to the chassis. Order Code 942-210 is for a 1K one which is just 5% over, which is neither here nor there.

[6SN7WGTB]

I've used power wirewound for some high duty purposes and another way of supporting David's comment is to note the derating for 'free air' use, which is generally down to 30% of the rated value (which is only achieved on the required heatsink).

Orientation becomes important as no part of the resistor should exceed 200˚C.

For the ca. 1kΩ heater dropper you'd need a 50W W/W in free air, and that's a big device - typ. 75mm long x 30mm wide over lugs.

A smaller 15W device screwed to the chassis, more like 35mm x 20mm footprint - the 'rated heatsink' being ca. 20cm x 20cm (i.e. the chassis would do).

[Sideband]

Originally Posted by paulsherwin

Alternatively you could modify the radio to use a capacitor or diode dropper. Search the forum for more info on this.

That would be my way forward with the capacitor as favourite. No heat which is always an advantage. I have several radios and a TV running with cap droppers.

[Techman]

When it comes to watty dropper resistors you always need to at least double up on any calculated wattage rating. I'd fit an additional dropper in place of the original o/c section and over rate the power rating for cooler running. Obviously make sure that the original dropper section is either completely disconnected or has had its winding cut, as it's not completely unknown for them to 'repair' themselves over time and be paralleled with the replacement dropper resistor with disastrous results.

[Leon Crampin]

The wattage rating of a resistor has no bearing on the heat produced for a given electrical energy input. A physically larger component will run cooler due to the reduced energy density, but the total heat generated is the same.

I use a 15W metal clad resistor bolted to the chassis. No part then runs hotter than hand warm.

Leon.

[Techman]

Originally Posted by Leon Crampin

The wattage rating of a resistor has no bearing on the heat produced for a given electrical energy input. A physically larger component will run cooler due to the reduced energy density, but the total heat generated is the same.

Obviously. What I should have said is that it will be cooler per square inch.

I must try to remember to give more in depth explanations in my posts!