An odd Marconi 2955a problem - Page 1
Forum section: electronics / repair

author: jonathanlemoine
author_rank: Contributor
timestamp: April 16, 2020, 12:43:51 pm
content: Hi guys. A few months ago I blew the front end in my Marconi 2955. I was aligning a tube radio and dropped my test lead and it came across about 300v DC on the 1watt BNC input/output jack. Needless to say it didn't like that. I decided to get around to fixing it and found it damaged a relay in the switching section. So I managed to fix that but I also can't switch the modulation. I mean to say the buttons on the keyboard to choose the modulation type are not responsive but only in RX (reciever test) mode. In TX (transmitter test) mode the buttons respond fine. I checked the logic level keyboard interrupt on the AB4 board and the microprocessor sees the button push. Also if I select a preselection where I had different modulations programmed it switches. This should really narrow down the problem but I'll be damned if I know where I have narrowed the problem down to. I'm going to keep working on it and if I figure out out I'll let everyone know what I found. Any help would be great!

author: George Edmonds
author_rank: Frequent Contributor
timestamp: April 16, 2020, 04:11:37 pm
content: HiI frequently repair 2955's, from what you have said I would expect the damage to be in the AC0/AD0 modules or on the AA4 board, which relay did you change?Although I will probably get complaints about this comment may I say that Marconi service manuals contain many mistakes or failures to update.A lot of the keyboard decoding logic is on the AB1/1 board, please be aware that the 2955 and 2955A AB1 boards are different from each other.There is a support group for Marconi Instruments, it is.https://groups.io/g/Marconi-Test-InstrumentsGeorge G6HIG

author: jonathanlemoine
author_rank: Contributor
timestamp: April 17, 2020, 01:13:07 pm
content: I had to change the RLA relay on the AC2 board. Expensive little bastard. Thanks for the extra info about the differences between the 2955 and the 2955A. I actually have the 2955 but I could only find the manual for the 2955A so I was just hoping they were the similar enough to get me by. I'll pick at it some more later. If I can't figure it out I may just leave it because it's still useable. It passes all self tests now. (it failed all of them when I blew the AC2.)

author: jonathanlemoine
author_rank: Contributor
timestamp: April 20, 2020, 06:30:16 pm
content: Quote from: George Edmonds on April 16, 2020, 04:11:37 pmThanks to you George I now have a manual for the 2955. I've been running around like a chicken with it's head cut off. I haven't found the problem yet. My knowledge is very limited when it comes to digital logic circuits and how buffers and latches work. As thorough as the manual is it doesn't seem to be getting me anywhere but I think that's just my lack of understanding. Following the schematics I had a few ideas what it could be but not sure. I looked at the keyboard logic on the AB1 board and from what I could tell the interrupts appeared to be the same for every button on the keypad so I was thinking that the button push is being acknowledged. Also I figured it would be strange for the buttons to work on TX but not RX. So I thought maybe the processors is not communicating with the AB6 board to tell it which modulation to use. That brought me to IC8 because that seems to output stuff to the AB6 board from the processor and memory. But when I scoped the connection there wasn't anything there. But I don't know if there is supposed to be anything because there isn't anything in the manual about it. Maybe there was but I've been back and forth through so many things I don't know what to think. Maybe there was and I forgot. To make things difficult I don't have an extender card for the AB boards. I've been running wires. Needless to say I'm frustrated.

author: jonathanlemoine
author_rank: Contributor
timestamp: April 21, 2020, 02:32:48 pm
content: I finally found a problem. Checking the decoder IC8. It says to look for negative going pulses on AB1 SKC 10b. I found no negative going pulses, only a high logic level. I find this a little odd because it uses the rotary encoder on the front and you change the frequency generator frequency to test. It's odd because the encoder works and changes the frequency without an issue. But at least it's something and it was an IC that I suspected anyway. Now to figure out how to tell if the of is getting an input signal.... This is where my knowledge falters. A address bus is going to the IC and a rd/wr signal going through 2 and gates (IC33). Also something on pin 6 from another IC (IC25 which is fed by IC5). I guess it would be cheap enough to change all of the ICs related to that circuit but I'd like to narrow it down.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: April 21, 2020, 08:00:15 pm
content: Hi,I have read your posting a number of times,You were very lucky that RLA acted as a sort of lightening arrestor, I have seen 2955’s that have been so badly damaged by the accidental overload of the BNC input that the attenuator had been damaged and the AA4 board largely reduced to carbon. Very surprising is that the BNC input overload bridge rectifier circuitry survived.I have a tremendous advantage over you as I have a known working set of the cage boards to enable a fault to first be isolated to board level.In order to help me would you please carry out the following.(a) In self test mode do all of the yellow panel LED’s illuminate ?(b) In RX mode set up the signal generator for the following:-N port selectedPress Green “RF GEN”Key in Frequency 50MHzKey in Level -20dBmPress Green “Set Mod”Press Orange “Level”Key in 50 on key padPress Orange “AM key”Does the CRT now display the level as 50% in the green highlighted CRT boxDoes the Orange AM LED illuminate?Please say at what point the above fails.The interaction between the AB4 processor board and the AB6 AF board in complex, over the long production life of the 2955 range a number of engineering modifications have been made to both boards such that it is important to keep the instruments original boards together as a set.George G6HIG

author: jonathanlemoine
author_rank: Contributor
timestamp: April 22, 2020, 01:04:25 pm
content: thanks George. I am such a fucking idiot. Apparently I don't know how to use the equipment or I had forgotten along the way. There is nothing wrong with the damn thing. I forgot that you had to change the level before changing the modulation. I was just hitting the modulation buttons but of course that wouldn't work because 3.5khz modulation doesn't work on AM. What a dumbass. Oh well. Yeah. All is good now George. I was chasing a problem I didn't have. FFS. Thank you so much. If you didn't tell me that I would be probably looking for boards to change..

author: jonathanlemoine
author_rank: Contributor
timestamp: April 22, 2020, 01:07:40 pm
content: I mean I could still have a problem that I don't know about... Like I said the test in the manual for IC8 didn't work but you did say the manual had mistakes so until I see something that isn't working I'm not going to chase it.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: April 22, 2020, 06:55:40 pm
content: All is well that ends well.I thought that it was finger trouble so I set you a test routine to show you.You are not the first to make this mistake and no doubt will not be the last as setting the modulation mode on a 2955 is not intuitive.Always glad to help, more easily found on the Marconi groups.io support group.Best wishes.George.