R1155: Removing/replacing three gang tuning condenser

[R1155-novice]

Can anyone please supply step-by-step instructions for removing (and replacing..) the three-gang tuning condenser (C82, C83, C84) on an R1155?

Resistance checks reveal shorts on C83 and C84 when tuning vanes are fully closed to three quarters open (C82 reads 'open-circuit' over full range of vane movement). The shorts mean signals are only received on all bands when vanes of C83 and C84 are three-quarters to fully open.

Attempts to clean vanes of C83 and C84 'in position' have failed to clear the shorts - there is no visual indication or 'feel' when moving tuning dial that the vanes are obstructed or touching - so removal for bench examination seems the only option.

Any advice gratefully received.

Many thanks.

(R1155-novice)

[RobinBirch]

This is without having the manual or a set infront of me. It's a bit of a ****** to do this as there is a lot to come off.

You have to remove the tuning knob and reduction gear. This is all done from the front.

Remove the moon shaped piece of aluminimum that the window is mounted in.

Remove the tuning needle from the capacitor shaft.

Un solder the wires from the capacitor and undo the three bolts that hold the whole thing in place.

You can probably strip the condensor down. Use stiff card to get the large bits out and a feather to get all of the debris out. Feathers are slightly sticky and won't leave stuff behind in the way paper will..

Hope this helps.

Robin

[R1155-novice]

Hi Robin,

Yes, this is all helpful - thank you.

I note there is a small hole leading to what looks like a screw in the fine outer tuning knob for removing it from the capacitor shaft (not that I can budge the screw...if that's what it is ), but how is the larger coarse tuning knob removed?

The set is fitted with the early version of reduction gear and knobs which were replaced in later versions (larger knobs, different gearing ratio) .

And have you any tips re "removing the reduction gear"?

Is the gear a 'push-fit' affair with no screws, bolts or anything else holding it together which I need to undo?

And how is the tuning needle connected to the variable condenser shaft? A push-fit?

Best wishes,

David

[XTC]

It's a long time since I've done this, and I rewired and recapped the whole enclosed section (the tuning heart) under the tuning cap, so it wasn't really the same problem.

Looking at a scrap set, what Robin says is right, but there's a piece of paxolin/tagboard attached to the back of the tuning cap with a screw. To access the screw would you have to push a screwdriver through a maze of wires in the tuning heart, and attempting to do that could easily damage a component or crumble the insulation on a wire.

Possibly, when you've done as Robin suggested, there will be enough give to ease the tuning cap forwards and access the screw with an offset screwdriver, without damaging anything.

Of course, when you've got the tuning cap out and sorted out the short, you've then got to put it back without damaging or disturbing anything.

I think you would be better off having another go at getting rid of the short without removing the cap. Sometimes you can spot touching vanes or dirt bridging the vanes if you use a torchbulb on leads so that you can look through the vanes.

HTH

Pete.

[XTC]

Originally Posted by R1155-novice

The set is fitted with the early version of reduction gear and knobs which were replaced in later versions (larger knobs, different gearing ratio) .

And have you any tips re "removing the reduction gear"?

Is the gear a 'push-fit' affair with no screws, bolts or anything else holding it together which I need to undo?

And how is the tuning needle connected to the variable condenser shaft? A push-fit?

I'm not familiar with the earlier reduction gear, but I believe the later one was a field replacement. With the later ones, I think you can undo two screws and it pulls off intact. There's a fork which engages with a shaft on the tuning needle. The needle attaches to the shaft of the cap with a fork clamp which is tightened with a screw. It may be a good idea to mark where the needle was mounted before taking it off.

Pete.

[R1155-novice]

Hi Pete,

Thanks for the helpful points - this Forum is a fantastic resource....

On my R1155, the small can housing C2 and C39 is missing (leaving just a hole). Leaving aside why a previous owner removed them (although I would like to know what functions the caps performed..), could you tell from your scrap set whether cutting out the hole to the edge of the chassis 'shelf' on which the can was mounted would give easier access to the screw holding the paxolin tagboard to the back of the tuning cap? (Albeit at 90 degrees...)

(As you say, access to the screw via the tuning heart would be next-to-impossible and could cause damage.)

Also, Pete, could I check my resistance theory with you?

I assume that a variable tuning condenser without shorts should read 'open circuit' when the vanes are moved with one probe of a digital multi-meter set to low resistance on the 'hot' tag of a tuning vane, with the other probe to earth (chassis)?

(I made the measurements after disconnecting connections to the bottom three 'hot' tags of C82, C83 and C84).

A short is present (I assume) when the vanes are moved and a resistance reading is obtained?.

Open-circuit (fig 1 on a digital multimeter) was obtained on C82 throughout the full movement of its vanes. Suggesting no shorts.

As I mentioned, open-circuit (figure 1) was only present on C83, C84 when the vanes were between three-quarters and fully open on each gang. Resistance readings were obtained for both when vanes were between three quarters open and fully closed.

I agree that removing the tuning condenser has to be a last resort - although I've tried everything from high pressure air blasting to teasing thin card between the C83, C84 vanes without eliminating the shorts.

The gangs appear clean, uncorroded and move smoothly without detectable contact between fixed and moving vanes.....making bench examination appear the only option.

Best wishes,

David

PS what does HTH stand for?

[Brian R Pateman]

David,

Hope

This

Helps

!

Regards,

[R1155-novice]

Hi Brian,

Do you mean does your quote help?

("Always going forward 'cos we can't find reverse")

Er, not really.....but thanks for the thought.

D

[R1155-novice]

Ah, Brian - apologies.....I now realise you were telling me what HTH stands for.....

Thanks.

Best wishes,

David

[GMB]

Do the moving vanes look correctly centred? I have encountered tuning capacitors where there is an adjuster for the centring (I haven't checked to see if R1155 has one) on the end of the shaft. If this comes loose then the vanes start to short out and it's trivial to fix. Also check that there is no end float on the shaft due to warn bearings.

By the way, you are better off with a traditional meter for looking for shorts like this. DVMs integrate over a time period and so miss anything but solid problems, whereas an old meter will at least twitch its pointer.

[R1155-novice]

Hi GMB,

Good points - thank you.

Can anyone tell me if the R1155 tuning condenser has an adjuster for the moving vanes? (I can't see one...)

And do you think a bottom-of-the-range analogue multimeter from Maplins eg model with order code UZ82D on www.maplins.co.uk site might do the job?

Best wishes,

David

[ppppenguin]

A battery and buzzer or bleeper is even better. You can hear the slightest contact without having to look at the meter. Many meters have a "bleep" function.

[Skywave]

Do the end moving vanes of the cap. have radial splits in them? (Fine adjustment of tracking). One of these sub-vanes may be excessively bent, causing the short.

Al / Skywave.

[GMB]

I have now opened my R1155 up and yes it does have an adjuster screw on the end of the shaft.

[XTC]

Originally Posted by R1155-novice

On my R1155, the small can housing C2 and C39 is missing (leaving just a hole). Leaving aside why a previous owner removed them (although I would like to know what functions the caps performed..), could you tell from your scrap set whether cutting out the hole to the edge of the chassis 'shelf' on which the can was mounted would give easier access to the screw holding the paxolin tagboard to the back of the tuning cap? (Albeit at 90 degrees...)

C39 (0.1uF) is in parallel with C95 (0.5uF) and they both decouple the screen of the RF amp.
I can't quite see why, but they probably had problems with resonance in one of the caps or maybe C39 was a spare section in the can.

C2 is two 0.1uF sections of the same can, and it looks as if it decouples the RF AGC line which disappears into the direction finding section. If your set's been modified, there probably isn't a DF section and there will be a manual RF gain control added.

So, you could probably omit them those caps.

It might be a bit easier to reach the screw by cutting the chassis but it seems a shame. I suppose it depends what state the set is already in.

Originally Posted by R1155-novice

Also, Pete, could I check my resistance theory with you?

I assume that a variable tuning condenser without shorts should read 'open circuit' when the vanes are moved with one probe of a digital multi-meter set to low resistance on the 'hot' tag of a tuning vane, with the other probe to earth (chassis)?

(I made the measurements after disconnecting connections to the bottom three 'hot' tags of C82, C83 and C84).

A short is present (I assume) when the vanes are moved and a resistance reading is obtained?.

Open-circuit (fig 1 on a digital multimeter) was obtained on C82 throughout the full movement of its vanes. Suggesting no shorts.

As I mentioned, open-circuit (figure 1) was only present on C83, C84 when the vanes were between three-quarters and fully open on each gang. Resistance readings were obtained for both when vanes were between three quarters open and fully closed.

If you have a tuning cap with no connections, it should be open circuit across the whole range.

Originally Posted by R1155-novice

I agree that removing the tuning condenser has to be a last resort - although I've tried everything from high pressure air blasting to teasing thin card between the C83, C84 vanes without eliminating the shorts.

The gangs appear clean, uncorroded and move smoothly without detectable contact between fixed and moving vanes.....making bench examination appear the only option.

The US equivalent of the R1155 was the BC348. The manual for the BC348 specifically says that using an air line to clean the variable cap is discouraged as it can distort the vanes.

You don't need much of a bit of conductive material to cause enough of a short to cause a variable cap to play up. What's the resistance when it sorts out? Is it a dead short or a few hundred ohms?

GMB made an interesting point with regard to play in the bearings. They are adjustable via a screw at the back, and presumably, if there's play, you'd notice a difference if you wiggled the shaft when you find the shorted position. Anyway, don't just fiddle with the adjustment screw, as that could really cause you grief.

The last time I had a problem like this, with another set entirely, one of the tweakable bits of a tweakable vane, (the ones with slots) was contacting the other vane, but it left no track in the fixed vane and it only had to be tweak back a couple of thou to be OK. After it was tweaked back, there was next to no noticeable difference in the spacing.

It's hard to know what to recommend, but I'd certainly be running bits of cotton etc, between those vanes before trying to get the cap out.

1155s just weren't designed to be maintained, and they can be pigs to work on, partly because they're cramped and partly because the rubber insulation was often such rubbish. So, start to mess with them and fixing one thing can cause another problem and so on, leading to rebuilding the whole thing.

Pete.

[R1155-novice]

Thanks, GMB.

Is it the large screw on the back-plate of the tuning condenser unit on the side nearest V4?

How does it change the centering of the moving vanes?

Will turning the screw adjust the 'gap' between each vane of each gang by the same amount?

And should I try turning the screw by small amounts while taking resistance readings?

D

[GMB]

XTC correctly makes the point that you can really mess them up if you are not careful. I pointed out the screw because it was locked with paint and so could have come loose (it has a locking nut too). I have encountered this problem in the past so it does happen but I have also found it very hard to get the adjustment correct again.

Do the vanes look correctly centred? You should do your upmost to see why it is shorting out before diving in.

[R1155-novice]

Hi Pete,

The set was destined for a skip - and in a poor state with several dodgy mods (internal mains psu and 6V6 output stage added with internal speaker fitted behind gap left by ripped out Jones socket etc.....df section has been removed and - as you correctly deduced - a manual rf pot added.

While I've been replacing fixed caps, re-doing dry joints, re-wiring where rubber insulation has perished, reducing HT to a tolerable 218 volts, substituting NOS valves in place of originals with low emission etc ....and getting reasonable performance from a quarter of each band, the tuning probs are a real headache.....

The set has been 'battered and abused' so much that cutting an access gap for the rear retaining screw would make little difference appearance-wise. I'm just aiming to return the set to safe working over the full range of each band....

Thanks for explaining the functions of C2 and C39.

I can also report that the small cans containing C29, C30 and C31; C32, 33 and 36 and C95 are missing - leaving just empty holes in the chassis.

You mentioned C95, but what do the other missing caps listed in the previous par do?

I've only squirted a bit of air from a car-tyre foot pump through the vanes - not enough (I hope...) to distort them...

The resistance readings on my digital multi meter set at the 2K (lowest) range show (once the meter 'settles down' ) readings of 10 or 12 on C83 and C84 between vanes fully closed and three quarters open. With 'open circuit' between three quarters and fully open. I plan to obtain an analogue meter to compare readings.

Thanks again, Pete - my understading of the set is growing by leaps and bounds thanks to the Forum....

David

[jim_beacon]

I would be looking for a short on one of the vanes to start with - the fact that only two sctions are faulty suggests that the fine tracking adjustment (the slotted vanes - you bend a small part of the vane to correct tracking over part of the range) have been played with, and badly bent, though you would normally feel the rubbing through the tuning drive.

I wouldn't suspect the screw - it isn't really a centering adjustment. The moving vanes have a ball race at the drive end, the outer track being formed from a depression the capacitor frame, and the inner being turned on the capacitor spindle. The non-drive end normally has a cup turne in the end of the spindle, and a cup in the srew, with a single ball bearing between them. The screw adjusts the bearing preload, so that the apacitor moves freely, but has no end float in the bearings. In a new capacitor, the limit of adjustment is much less than the clearance between the vanes (practically, there should be about 0.001 inches of difference between "sloppy" and "jammed solid"). It may be worth dissassembling the capacitor, cleaning the bearings (petrol or parrafin will dissolve the old grease), checking the balls for damage or wear, and reassembling using a very light grease.

Jim.

[Skywave]

Originally Posted by jim_beacon

I would be looking for a short on one of the vanes to start with - the fact that only two sctions are faulty suggests that the fine tracking adjustment (the slotted vanes - you bend a small part of the vane to correct tracking over part of the range) have been played with, and badly bent, though you would normally feel the rubbing through the tuning drive.
Jim.

Quite - which is what I was thinking of in my earlier Post, #13, q.v.

The fact that we now know that this set has been very substantially "got at" includes the possibility that someone has "had a go" at the RF alignment - and made a hash of it

Al / Skywave.