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Old 22nd May 2020, 3:09 pm   #1
Bigpimp347
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Default Marconi 2955 and 2952 help (again)

Hi all new member first post (thanks admin)
reading through the forum and find a few post's regarding the 295..
ok here's my issues..

2955A fails self test on 11, 25 and 27.
11 is a frequency issue although the 10Mhz reference seems bang on.
25 and 27 are related to RF generator output reading low.
now watching John Hawkes 2955 video on YouTube he had the same fault and went to the OM345 and cured it.

here's my problem.
VHF and UHF are fine, output nice and smooth on the sig gen.
HF up to 50Mhz steps up as the ATT pads switch in and out.
on the OM345 i have 13v in,
signal in on pin 1, increased signal out on pin 5 so according to my maths they're both working ??
on the test point i get a nice smooth trace from the VR as per johns video so the control is fine (if it wasn't VHF and UHF would be bad as well)
so do i buy two copy OM345 and change them to find they don't fix the issue, or are they the issue ??i have output and it steps in and out but no variation as you scale up or down ??


second issue. 2952.
are you all still here ?

bought this as a project for £45.
lights up, smells of burning dust and heat.
now the RF power meter works..
that's it..
ok maybe more works but the frequency counter reads randomly
but example 145.500 shows 00.500 but with number fluctuating.
can't seem to get much else working so maybe a few bad caps, dirty contacts, but what i require is a service manual..
Mauritron don't reply to messages regards their 'information'
i also require some of the flick switches four or five are snapped maybe a reason things don't work.
also four or five of the switch button covers are missing.
love to get this old beast restored for future use..

test gear i have.
10Mhz reference source.
26 to 28Mhz signal generator.
poorly 2955,
Atten 1Ghz analyser
Micronat scope,
Hameg scope.
plenty of radios for RF and such like.

anyone ever tackled a 2952 ?
you thoughts on it, worth restoring or scrapping ??

thanks..
Mark..
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Old 24th May 2020, 11:22 am   #2
Bigpimp347
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Default Re: Marconi 2955 and 2952 help (again)

ok..
well tested the OM345 with my toy scope and see an increase from source to output so there is amplification.
got given a Hamgen scope the other day so will hook that up and try again as it has more fiddly bits will also try the other OM345 and see what happens..
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Old 24th May 2020, 11:52 am   #3
Dickie
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Default Re: Marconi 2955 and 2952 help (again)

First thing to say is that the self test loops the sig.gen to the reciever, so a fail doesn't pin the problem down very well. You need to test the sig.gen output on its own for correct frequency and level. If its OK at HF and above the problem is unlikely to be the OM345s. More likely to be a dud attenuator.
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Old 24th May 2020, 4:08 pm   #4
Bigpimp347
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Default Re: Marconi 2955 and 2952 help (again)

well here's an update.
VHF and UHF in to my Chinese analyser shows a nice steady up and down signal even the signal level matches up roughly with the tin pot £80 analyser.
so proving the 2955 signal generator is fine on VHF and UHF.
on HF as in 60Mhz or below is where it is notching.
there is no smooth movement.
i did the test on the test point on my scope and can see the VR is fine and moving freely (if it didn't VHF/UHF wouldn't work, but hey tested it)
so it seems HF is where it's at.
i can't measure HF signal level but as i remember it was low in to my HF set and CB (what i bought it to repair)
once my new analyser comes next week i'll have a better look at the spectrum and the signal levels.

but as you say possibly a dodgy amp,
the OM370 i thought was at fault as it gets hot, but VHF and UHF show fine.

all i can describe it as is not linear, just steps up as the ATT pads kick in and out, but all of them in steps up and up and up, and down and down and down, well you get he idea
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Old 24th May 2020, 5:28 pm   #5
Dickie
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Default Re: Marconi 2955 and 2952 help (again)

So we are looking at a level issue below 60MHz. If you put in a level increment of 10dB and then step the level up and down using the increment key does the level do what you expect? If you wind the level up and down over a 10dB range using the knob, what happens?
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Old 25th May 2020, 3:28 pm   #6
Bigpimp347
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Default Re: Marconi 2955 and 2952 help (again)

i didn't think to try the buttons to be honest,
as the variable control works ok on VHF and UHF i assumed it would work on HF (below 60mhz) ok.
when i get time i'll tyr it again.

i have some goodies coming today so can try it out in more depth and let you know..
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Old 25th May 2020, 3:31 pm   #7
Dickie
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Default Re: Marconi 2955 and 2952 help (again)

I don't think I understand the problem, then. You said it is "notching" and "non-linear" below 60MHz. Can you be more precise?
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Old 25th May 2020, 9:26 pm   #8
Bigpimp347
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Default Re: Marconi 2955 and 2952 help (again)

ok when you adjust the variable control in RF (signal) generator mode it should goes up in linear and as in nice and smoothly,
but it doesn't it only goes up each time an attenuater pad is switched in or out.
yet once above 60mhz mostly VHF and UHF it moves up smoothly..so i assume VHF and UHF are fine..

also testing today putting my 10Mhz standard in to it.
on 1Mhz counter reads 1Mhz power reads 2.25mw on N socket and BNC.
on 5Mhz N type shows 5Mha 22.4mw BNX shows 5Mhz 23.4mw
on 10Mhz N type shows 10Mhz 19mw yet BNC shows 10Mhz and still 20.5mw
so higher i go the power readings differ showing less in the N type socket than the BNC,
maybe this is because the N is a higher RF socket than the BNC ??

tried the audio output and it's fine.
frequency counter is fine,
VHF and UHF is fine.

so i suspect i have an amplifier failed ??
or a short, bad cap in the HF section ??
can't inject any RF in to it as have no signal generator for HF apart from a 5w HF radio
so will try that for a start to check power meter but i suspect is ok..
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Old 25th May 2020, 9:59 pm   #9
Dickie
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Default Re: Marconi 2955 and 2952 help (again)

To generate signals below 60MHz, the instrument mixes its 165-260MHz VCO with a fixed 200MHz LO on the AA3 board. It would be worth checking the signal between the AA3 and AA4 output amplifier and observe what happens above and below 60MHz. If the signal is too low below 60MHz its possible that the levelling loop on AA4 may stop working correctly. There is a buffer amp (an OM345) on the output of AA3 but since it is common to all frequencies is probably working fine.
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Old 26th May 2020, 9:17 pm   #10
Bigpimp347
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Default Re: Marconi 2955 and 2952 help (again)

strange thin as all OM345 show signal increase on the scope so i assume all are amping up, as they all read the same.
a mates mentioned a couple of amplifier transistors on one of the boards (not sure the number i just got a picture )
SC945 ?? so may check those.
i'll get my scope and counter thing out and try what you suggest to see what signals i get and report back.

thanks again for your help so far..
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Old 1st Jun 2020, 2:00 pm   #11
Lodekka
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Smile Re: Marconi 2955 and 2952 help (again)

Have you tried the Marconi Heritage collection on the groups.io page to see if there is a 2952 manual available?

There is a large collection of Marconi information available although note that the 2952 was actually a 'factored product' sourced from Neuwirth in Germany, as was the original 2950.

But they might have a manual...


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Old 1st Jun 2020, 8:07 pm   #12
Bigpimp347
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Default Re: Marconi 2955 and 2952 help (again)

Thanks for that, will search that now, thank you...
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Old 1st Jun 2020, 8:10 pm   #13
Bigpimp347
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Default Re: Marconi 2955 and 2952 help (again)

Further on the 2955.
as i keep banging on about the OM345
i have fitted the 4.7k resistor hack to each and all OM345 and still the same fault..

on my mates advice i check the transistors he referred to and were good, but i did find a cap showing resistance, so replaced that, but still the faults are there.

so i have bought two OM345 but won't fit them as i suspect the fault isn't the OM345 so will start checking caps and see what happens..

it was cheap for a reason....now i know why..!!!
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Old 1st Jun 2020, 9:50 pm   #14
Dickie
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Default Re: Marconi 2955 and 2952 help (again)

Did you get anywhere checking the AA3 to AA4 signal levels?
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Old 2nd Jun 2020, 5:13 pm   #15
Bigpimp347
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Default Re: Marconi 2955 and 2952 help (again)

ok with help from various other sites and videos and such,
here's what i did today.
was told to put sig gen on 20Mhz and check i for accuracy into a counter.
so using my counter and frequency standard
i was able to get the difference from 20Mhz and the 10Mhz clock (and also 5Mhz) down to 00.000.008hz

all this messing back and fourth and such i decided to run a self test and it passed..!!
yup it passed..

so i adjusted the counter,(blue potentiometer near the 10Mhz clock) and 2955 to all show 10Mhz and 5Mhz.
10.000.000 and 5.000.000 and apart from the difference being 00.000.001 they're pretty close,
did another self test, it passed,
since then iv'e done half a dozen test's and all pass.
so is it a case of let it warm up for fifteen to half an hour.
or maybe because i have it flat on the bench and not upside down or on it's end ??

so iv'e bought two OM345 for nothing it seems.
my mate said if his was on it's end it would fail some test's due to a small foil type shield moving and touching something.
so i need to ask him where his is..

also i have tried it on sig gen in to my HF set and it's linear, all adjustments nice and smooth, so really not sure what the issue was, or is, or what iv'e done.

so now i'm going to get the 2952 out and sort that..

thanks for your help, i'll try it again next week and let you know if it fails and which skip it's gone in
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Old 2nd Jun 2020, 5:19 pm   #16
Dickie
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Default Re: Marconi 2955 and 2952 help (again)

That's good. The trouble is it will probably go wrong again at the most inconvenient time when you need it to work!
And I know you are only joking but don't put it in a skip. If it's got a good CRT and CRT driver board people will be queueing up to buy it!
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Old 3rd Jun 2020, 12:20 pm   #17
Bigpimp347
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Default Re: Marconi 2955 and 2952 help (again)

Quote:
Originally Posted by Dickie View Post
That's good. The trouble is it will probably go wrong again at the most inconvenient time when you need it to work!
And I know you are only joking but don't put it in a skip. If it's got a good CRT and CRT driver board people will be queueing up to buy it!
i know where there's a guy got two 2955A and R and a whole heap of boards for £200
as soon as you ask about postage he goes shy...
he's in Kent an i'd snap his hand off as the two are part units but he says complete and enough spares, boards and dummy load and such to maybe repair one if not both..
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Old 3rd Jun 2020, 12:23 pm   #18
Bigpimp347
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Default Re: Marconi 2955 and 2952 help (again)

Further on the 2952

that's going in the skip..!!
changed two switches, pulled a wire off,
found a lot of caps leaked, and a couple of switches i'll never be able to change without causing damage or pulling wires off and not knowing where they go..

so that's ready for parts donation if anyone needs a 2952 for parts..
it's not free but doesn't owe me much so can let it go cheap.

damn things, nothing is easy in life..
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