AVO 9 Mk2 removal from case

[Richard m0dsu]

Just acquired this model, No. 5732.466 according to scale inscription, so presume built April 1966. After replacing batteries all ranges tested work ok including DC and AC volts, even 3kV range although haven't checked current yet. Ohms ÷ 100 pot doesn't seem to do much but near enough to 0Ω on scale when checked.

Apologies for this very basic question but it must be 40 years since I last used one whilst working in BBC London OB Comms, and can't remember taking one apart even then. It appears that I remove the six case screws and gently prise apart. I believe there are two long hex pins with tapered ends that locate in back of case and together with 3kV terminal pins, form the contacts for same to main instrument. What of battery connections though as presumably battery case remains within main case?

There is also on r/h side of case well below the central case screw another hole revealing recessed metal part that fully plugs the hole with signs of red paint. Is this to be ignored when removing case?

Finally I have a copy of user manual including circuit schematic but is there a fuller service manual available for this model?

Apologies for all the questions

Regards, Richard

[The Philpott]

The 'seventh screw' on the case DOES need to be removed (after it's tamper seal has been dug out.) The female thread is likely to be bakelite not a brass insert- so may strip out when reinserted if too much torque applied. (not a major issue, a dab of medium loctite would secure it)

Don't worry about the terminals coming from the battery box to the main instrument, as they are spring leaves which press on the hex pillars in the main instrument....and note also that the resistors for the high voltage 3kV range are attached to the battery box (again, contact is via spring tongues/leaves.)

The Ohms (divide by 100) pot. is often now tarnished and makes poor contact..come back to us when you get to it as it's quite easy to polish it up (and you do not necessarily have to remove the meter movement to service it)

Dave

[The Philpott]

With this model of AVO it should also be shaken up and down a little with switch panel uppermost, whilst you listen for a clink-clank noise. Both mine had loose screws on the front of the movement. (I have a photo which identifies these screws)

Hopefully yours won't suffer from this.

Dave

[Richard m0dsu]

Many thanks for that invaluable help Dave. Having removed most of the red paint it was easy to get the security pin moving but too tight in case recess to pick it out. Then had a moment, used a powerful magnet and it shot out. Just a plain pin BTW.

Apart from one slightly bent screw it came apart easily. Very clean inside with just a tiny amount of oily residue in bottom of case - photo's attached.

The Ω÷100 pot I can see partially with wirewound track clean. Would a squirt of switch cleaner be worthwhile?

Will check for loose screws.

Richard

[The Philpott]

Advise against switch cleaner- quite likely to be dull brass on the part of the knob that bears against the leaf spring. (picture)
*If the knob is removed it's also possible to bend the leaf over slightly if contact pressure is weak.

About the casing: Interesting that a plain pin was below the red paint- normally it's just a short machine screw..

(*Above all else don't allow ferrous tools to get pulled against the movement magnet)

Dave

[Richard m0dsu]

Thanks again Dave. Track of the Ω÷100 pot looked OK so cleaned spring contacts and re-assembled. Also cleaned all battery terminals (new batteries). Made direct short lead for terminals and checked resistance ranges.

All calibrated OK but Ω÷100 pot does nothing, however the Ω pot does vary deflection when on the former range. It appears I'm not getting a switch between these two ranges. Having looked at circuit diagram now suspect the hj contacts are remaining open when they should be closed for Ω÷100 selection, this being the only electrical change between these two ranges. Do you think this a fair assumption?

Richard

[The Philpott]

Getting there Richard! The leaf contact stacks are certainly a problem in their old age- i would say it's the most common 'moving part' fault to occur in this generation of meter. (I haven't got the circuit diagram in front of me but i think you've isolated the issue)

The switch leaf contacts occasionally go black, if so they can be polished with a strip of cardboard cut from the inside of a loo roll- this is just abrasive enough. (sometimes i use a drop of isopropyl alcohol on the cardboard, to finish)

(Use of normal abrasive paper ruins the tolerances, obviously.)

Dave

[Richard m0dsu]

Many thanks Dave. I'll try not to bother you again

[pmmunro]

Richard,

You're in good hands with Dave's guidance.

One point I would add though is that it's not a good idea to spray switch cleaner inside these meters, just use it on some card or similar. It has a tendency to migrate to the coils of the hairsprings in the movement where it bridges the turns sticking them together.

PMM

[The Philpott]

No problem at all Richard, i am simply popping in and out of the forum in idle moments. Quite a few meters have been fixed this way from Cologne to the Canaries, with input from all the people who've delved previously.

It is usually possible to carefully bend a leaf to alter a contact clearance. The factory specified duck-bill pliers to do this (which i do not have so have made do with snipe nose pliers or obstruction pliers) As Peter says, (he's been delving a lot longer than I!) sprays shouldn't be used inside.

Dave

[The Philpott]

I have remembered the other glitch with my Model 9 MkII- the 10mA AC current range was dead. I think that there is a leaf switch contact dedicated to this range and it was not quite closing.
Functionality easily tested with a resistor and a neon (usual electric shock precautions)

Dave

[Richard m0dsu]

Tweaking the hj contacts solved Ω÷100 issue. Easy to find being bottom of the leaf contacts on right, reverse when unit inverted. Bent longer leaf slightly and cleaned contacts using Dave's suggestion with that range selected. Calibration pot for that range now works as intended.

Bad news was that I found the cast aluminium scale support bracket fractured in both legs and it eventually broke away completely. Guessing it was previous transit damage since I've been very careful. Managed to coat all edges of breaks with epoxy glue and wedged it in place to dry. Seems to be holding up but unsure whether its a realistic permanent repair. One leg of this bracket also supports the swamp resistor bobbin.

[The Philpott]

Ouch! Not quite sure whether they are Aluminium or a Zamak/Mazak zinc alloy. If it was down to 'zinc pest' in Mazak there would be crumbly bits produced, so if there's no debris it could be a bad aluminium casting i suppose. (Hard to see how impact would kill it unless it was already flawed!) If it re-breaks i may have another one, but changing it would need care as the needle/pointer is prone to getting bent. (being as it is effectively invisible from certain angles)

Dave

[pmmunro]

Richard,
Fracture of the aluminium bracket is by no means unknown and is probably caused by very heavy handling.

If your repair is not successful, it may be possible to find a salvaged replacement.

PMM

[Richard m0dsu]

Thanks for that PMM. Repair seems to be holding up but obviously far less robust now. Have investigated further and that bracket seems to be well integrated with the basic meter movement having seen some close up pictures, see one attached. As such is it feasible to replace without, for example, disturbing movement pivots?

I would add that I bought an original mark AVO 8 yesterday, built March 1954 as per scale marking, sold as spares and paying just over £10. Interesting provenance as leather case marked BBC and nearby seller knew many of my ex colleagues from the period in the 70's when I worked in BBC OB Comms, London. Scruffy state and minus battery cover and leads, but shows all the signs it may be a working unit (indicated DC volts correctly) so will tidy it up, clean corrosion from battery compartment and fabricate cover, unless anyone knows of suitable one. Seems to be same style flat metal plate as my AVO 9 Mk2.

[pmmunro]

The Avo movement of this era is designed so that the 'A' frame, scaleplate and moving coil assembly can be removed as a unit from the magnet assembly. If you were to replace the frame, you would need to dismantle the moving coil assembly and rebuild it onto the replacement frame. The degree of skill and techniques are similar to clock repairing.

Afterwards the jewelled bearings would need to be adjusted and the movement would need to be balanced. It is delicate work but by no means impossible.

If you feel you want to change the frame, please let me know and I will send you detailed instructions on how it can be done. (There are several important points to be taken into account and it is best to be aware of these before starting).

PMM

[Richard m0dsu]

Many thanks for the advice PMM. It looks like the best I could hope for given my limited skills is replacing the complete movement, so if needed will work towards that goal.

[The Philpott]

Yes, looks like i misled you there, the A-frame swap is a job i would not tackle either. (Such is the problem of replying to a query without bothering to go to the workshop and get eyeball on a similar assembly!)

Should the glue give way in future (and depending on where the break is) a splint could be used to reinforce. Having said that modern glues are pretty impressive.

Dave