Bush VHF81 Heat Deflector

[stitch1]

Hello,
I’ve just been dismantling my VHF81 and the service sheet says ‘slide the Presspahn heat deflector outwards from the top inside of the cabinet’ but there isn’t one instead there’s a foam pad glued to the top.

I’ve looked at photos on the net but can’t see any others like this so is this original or a later modification, anyone seen this before?

Does anyone know what this is made of, it is like a fine cotton wool, is it likely to be anything nasty like asbestos?

Thanks for looking.

John

[Boater Sam]

Form? What are the forms for, ordering pizzas?
Sorry, realise its a typo, couldn't resist.
What sort of foam, colour, texture? Original or added? Could it be the asbestos substitute that someone has fitted?
I have some black foam that is used in prison mattresses and is totally fireproof and insulating, I mean like blowlamp proof.
Never seen any foam in any '81 I've had in.

[Keith]

The Presspahn company still seem to exist (in Bradford) making various paper based products. I wondered if there was originally a layer of foil on top of that pad (foil to reflect the heat/pad to insulate)?

[stitch1]

Yes a typo, and somehow I managed to post it twice!

It’s white (or was), texture like cotton wool but finer. I can’t say if it’s original or not but it seems original and the radio doesn’t appear to have ever been repaired in any way.

John

[Nuvistor]

Could it be fibreglass?

[stitch1]

Originally Posted by Keith

The Presspahn company still seem to exist (in Bradford) making various paper based products. I wondered if there was originally a layer of foil on top of that pad (foil to reflect the heat/pad to insulate)?

Always nice to see an old name still going.

Originally Posted by Nuvistor

Could it be fibreglass?

Yes I think it could be.

John

[paulsherwin]

It looks like fibreglass to me.

[stitch1]

So probably fibreglass, may not be original ... any issues with this or just leave it in?

John

[stitch1]

Originally Posted by Keith

The Presspahn company still seem to exist (in Bradford) making various paper based products. I wondered if there was originally a layer of foil on top of that pad (foil to reflect the heat/pad to insulate)?

Out of interest ...
Presspahn
We are a privately owned business that was founded in 1925 as specialists in the supply of paper insulation used in the manufacture of power transformers.

John

[davebms]

hello all you have a part missing !!. bellow the felt is/was a heat resistant board 14" long 3.5" wide with 90deg lip,it fits under the felt above the ferrite rod ,the lip is curved the held in with the screws that hold the back on
its a pig to get back in

[stitch1]

Thanks Dave, I’ve seen a picture on the net showing this in situ. I could look for an alternative or maybe I’ll try this https://secure.presspahn.com/Cart/in...2g8li0aihgugg0 cheap enough and no min order but unfortunately £7.20 postage.

John

[davebms]

hello all, yes that's the stuff, mine does not get very hot, so poss a different heat resistant sheet ?. £7.50 a bit much for a cheap product

dave

[G6Tanuki]

My VHF81 has the white fibrous pad as shown, but there's no kind of shield or anything over it.

The top of the case doesn't get hot to any sort of extent that would cause me concern, even when the radio's on for 10 hours at a time.

[davebms]

Originally Posted by G6Tanuki

My VHF81 has the white fibrous pad as shown, but there's no kind of shield or anything over it.

The top of the case doesn't get hot to any sort of extent that would cause me concern, even when the radio's on for 10 hours at a time.

possibly , some did not have them ?,, but pics in the radio museum show it and there not easy to get back in so possibly got binned

dave

[stitch1]

Originally Posted by G6Tanuki

My VHF81 has the white fibrous pad as shown, but there's no kind of shield or anything over it.

The top of the case doesn't get hot to any sort of extent that would cause me concern, even when the radio's on for 10 hours at a time.

That's interesting, so it could be original perhaps on a later version, unfortunately there's no sign of a date anywhere.

It's not clear from the photo on RMorg if there's a pad and card but if others have both then it's likely to be missing.

John

[stitch1]

There's one on eBay at the moment and it just has the pad but it's smaller and not even above the dropper, which perhaps suggests the card is missing.

John

[Half a Mullard]

Is this the part in question? On my set there is a small fibre glass type pad and what I assume is the heat deflector slides underneath and is held in position by the screws securing the back.

Although the set has 7 valves it doesn't seem to run unduly hot, possibly because the dropper has less to do with more valves in the chain?? (compared with say a DAC90A)

[stitch1]

Hi Howard, yes that's it, thanks for the pics I can see how it fits over the chassis now.

As you say it doesn't run very hot but I may see if I can get some Presspahn (or a cheaper alternative) I'll drop them an email and see if they have any cheap off cuts

John

[Half a Mullard]

Hi John
That sounds like a plan, a small piece would do. Good luck with it!

[julie_m]

Originally Posted by Half a Mullard

Although the set has 7 valves it doesn't seem to run unduly hot, possibly because the dropper has less to do with more valves in the chain?? (compared with say a DAC90A)

The valve heaters and dropper together will still be dissipating 230V * 0.1A = 23W, so a set with 7 valves will be putting out exactly the same amount of heat as a set with just 4 or 5 valves. What has changed is the dispostion in space of the heat sources. A set with FM and a tuning indicator of additional RF stage will be emitting heat in eight different places (seven valves and the dropper) versus the usual six (or five if a UCL8_ is used instead of a UBC% and a UL%). The improved distrubution of heat dissipation means individual components will run cooler, but the exhaust air will have the same temperature. Slightly higher if anything, thanks to the anode dissipations of the additional valves; but this is negligible compared to the UY% and the UL% valves, which are responsible for the lion's share of this contribution to cabinet heating.