Marconi 2955A Switchpad problem - UK Vintage Radio Repair and Restoration Discussion Forum

PETERVU2PJP · 5th Aug 2014, 7:24 pm

Split from this thread: https://www.vintage-radio.net/forum/...ad.php?t=96747

I am new to this forum and this is my first post. I have one Marconi 2955A. We got it repaired by the authorised service centre few years back, replacing the FBT (LOT) etc.

The equipment was kept in the shop rack for couple of years and when we recently opened the front flip cover, all the front panel switches were gone bad and on removing the front panel some kind of liquid was all over the switches and the black top portions of most of the switches came off. The contacts and the shorting plate on the switches were tarnished and corroded. Removed all the black tops from the switches, cleaned the pcb with solvent and dried, refitted into the front panel.

On powering up the green text display in the CRT is having a fold over in Right side. (Photo attached) Using a meter probe to short the switch contacts we could enter the self test except couple of the tests all are Failed.

I want the equipment back to my Service table. I need help from the expert members on where to start and what to do first.

Biggles · 6th Aug 2014, 6:44 pm

Hi Peter and welcome to the forum. my personal experience is only on the 2955 but there may be similarities with the 2955A. I believe the A had an off-air receiver of some kind in it? It may be that a switch is stuck or leaky upsetting the power up sequence. Also I wonder what the liquid was inside the keypad. It may be that some fine PCB tracks have corroded through and gone open circuit. If you have found corrosion inside I would check all the plugs and sockets for correct connection and if there are ICs in sockets (particularly the eproms), check that the pins are making good contact with the sockets. A check of power supply voltages would be a good idea and the foldover on the display may be something like a faulty capacitor on the LFB board. If you decide to have the unit powered up with the covers off, be aware of the presence of dangerous voltages inside. The common problems found on the 2955 are OM345 chips failing, RF input attenuators going intermittent, and the line output transformer (LFBT) failing, although there was a modification available for this. Hope this helps.
Alan.

PETERVU2PJP · 6th Aug 2014, 7:08 pm

Also I wonder what the liquid was inside the keypad. It may be that some fine PCB tracks have corroded through and gone open circuit. If you have found corrosion inside I would check all the plugs and sockets for correct connection
Thank you for the suggestions. Let me check the PSU and the LFB board.
The liquid on the switches are something came out of the plastic used to make the top of the switches!

Will post the findings. I ordered the switches-60 nos, it is available in element 14(USD1.5 approx). Let it come will start the repair.

5th Aug 2014, 7:24 pm	#1
PETERVU2PJP Retired Dormant Member Join Date: Aug 2014 Location: Palakkad-Kerala, India Posts: 5	Marconi 2955A Switchpad problem Split from this thread: https://www.vintage-radio.net/forum/...ad.php?t=96747 I am new to this forum and this is my first post. I have one Marconi 2955A. We got it repaired by the authorised service centre few years back, replacing the FBT (LOT) etc. The equipment was kept in the shop rack for couple of years and when we recently opened the front flip cover, all the front panel switches were gone bad and on removing the front panel some kind of liquid was all over the switches and the black top portions of most of the switches came off. The contacts and the shorting plate on the switches were tarnished and corroded. Removed all the black tops from the switches, cleaned the pcb with solvent and dried, refitted into the front panel. On powering up the green text display in the CRT is having a fold over in Right side. (Photo attached) Using a meter probe to short the switch contacts we could enter the self test except couple of the tests all are Failed. I want the equipment back to my Service table. I need help from the expert members on where to start and what to do first. Attached Thumbnails Last edited by AC/HL; 7th Aug 2014 at 12:23 am. Reason: Thread split

6th Aug 2014, 6:44 pm	#2
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Marconi 2022 2955 OM345 OM360 Replacement Hi Peter and welcome to the forum. my personal experience is only on the 2955 but there may be similarities with the 2955A. I believe the A had an off-air receiver of some kind in it? It may be that a switch is stuck or leaky upsetting the power up sequence. Also I wonder what the liquid was inside the keypad. It may be that some fine PCB tracks have corroded through and gone open circuit. If you have found corrosion inside I would check all the plugs and sockets for correct connection and if there are ICs in sockets (particularly the eproms), check that the pins are making good contact with the sockets. A check of power supply voltages would be a good idea and the foldover on the display may be something like a faulty capacitor on the LFB board. If you decide to have the unit powered up with the covers off, be aware of the presence of dangerous voltages inside. The common problems found on the 2955 are OM345 chips failing, RF input attenuators going intermittent, and the line output transformer (LFBT) failing, although there was a modification available for this. Hope this helps. Alan.

6th Aug 2014, 7:08 pm	#3
PETERVU2PJP Retired Dormant Member Join Date: Aug 2014 Location: Palakkad-Kerala, India Posts: 5	Re: Marconi 2022 2955 OM345 OM360 Replacement Also I wonder what the liquid was inside the keypad. It may be that some fine PCB tracks have corroded through and gone open circuit. If you have found corrosion inside I would check all the plugs and sockets for correct connection Thank you for the suggestions. Let me check the PSU and the LFB board. The liquid on the switches are something came out of the plastic used to make the top of the switches! Will post the findings. I ordered the switches-60 nos, it is available in element 14(USD1.5 approx). Let it come will start the repair.