Marconi Radio communication Test set 2955A
 

I have a Marconi Radio Communication test set 2955A.
The attenuator block which comprises three three relays works but the switching(poor contacts probably) in of one specific attenuator is intermittent. The relay operates and some times the signal is provided but usually the signal disappears. The service manual does not recommend taking the unit apart. Locating a spare is difficult.
So my question is has anyone any experience of dismantling the U2632 attenuator block on these test sets? The 2955R uses the same one so buying an old 2955R being sold for parts might solve the problem but ideally I would like to get the original attenuator block working again reliably.
Any advice would be appreciated.

Many thanks.

Re: Marconi Radio communication Test set 2955A
 

I know it's a fairly common problem with the 2955, and unfortunately both mine have intermittent attenuators. We used to send them back for repair to Marconi with this fault as it required re-calibrating after so I have never taken the attenuator block apart. Exercising the relays by winding the signal level up and down a few times seems to help sometimes. One day I may be brave enough and find the time to dismantle mine. I'll bet the relays are expensive too. I suppose you could use a fixed external attenuator on the output to slug the real level output to a range that is working but this would be very much a last resort, and also creates the risk of blowing up your attenuator if you happened to transmit into it by mistake.
Alan.

Re: Marconi Radio communication Test set 2955A
 

That's readily believable. :-) In my experience, 99% of Marconi test equipment seems to have been deliberately designed to make maintenance as difficult as possible! >((
And, FWIW, I derive absolutely no pleasure whatsoever in stating that.

Al.

Re: Marconi Radio communication Test set 2955A
 

The first thing to ascertain is whether the solenoids are operating correctly. This is best done by observing them moving when stepping the sig. gen. output level in 10 dB steps over its full range. To see this you have to remove the attenuator by unscrewing the two SMA semi-rigid pipes and the two screws fixing it to the side rail and then take off its thin ally dust cover. Just listening for the clicks is not very diagnostic. If the solenoids are not operating correctly you can do some straightforward fault finding to find the problem. If they are working fine 100% of the time then you will need to dismantle the assembly.
Taking them apart is not recommended because you need a steady hand and a clean environment. The internals are small, delicate, and easily damaged. The switch contact areas are minute. Having said that, they are perfectly straightforward, and if you have ever serviced a french clock movement should present no problems.
With the attenuator removed from the instrument remove the two end plates which are held in place by 4 hex cap-head screws each. Then slowly remove each of the screws holding the two halves of the attenuator body together.
When all screws are removed it should be possible to gently lever the two halves apart. Do this very gently, keeping the two halves aligned and parallel to each other.
The bottom half of the attenuator will have the alumina resitive pads clipped into the body. The gold pads of these can be VERY CAREFULLY wiped with a soft cloth or other similar non-abrasive material. The top half will comprise the drive solenoids, the push rods, and the RF line ribbon connectors. These latter are very delicate and should only be cleaned if necessary. If required the solenoids can be removed for attention, but the push rods and ribbons cannot be removed without damaging them. Re-assembly is the reversal of the above procedure, as they say, and make sure everything goes back in the same place and orientation it came from. Keep everything scrupulously clean, as the contact areas are small and the slightest scrap of dust could result in an open circuit. Tighten the two halves progressively, cylinder head fashion. Use of a contact cleaner is not to be recommended.

A point of detail - The 2955R uses a different attenuator, although the technology is the same. It will provide some spare parts but who's to say they won't have the same problems?

Good luck

Re: Marconi Radio communication Test set 2955A
 

Dear Douglas,

At my last job (before retirement) half a dozen 2955 series units were acquired, all faulty.

I repaired all but one; the boss did not want to spend £10 on parts to repair a logic fault, so it was broken down into spare parts.

According to the list, he has a 44429-034J ADO attenuator module in his stock (but this was nearly a year ago.

It could be worth speaking to him. PM me if you want his contact details

Kevin

Re: Marconi Radio communication Test set 2955A
 

Gentlemen,
Many thanks for these responses. They are very informative.

Best Regards to you all