Marconi 2022A

[david69]

Hello, I would like to know answer for this behavior of my gener Marconi 2022A?
Gener has influance betwen 138-164MHz under 0dB on 50 Ohm termination on the attached picture, otherwise all band 10kHz to 1GHz is OK Do I make something wrong? Or attenuator
is broken? Thanks for any idea.

[George Edmonds]

HI

With the limited information given it is really impossible to help.

It is NOT the course attenuator, this is simply a passive device.

Your quoted frequencies are not related to the frequency generation map.

Do you have the service manual, NOTE it must be the 2022A version to be valid, the RF board has minor/major differences in all versions.

George G6HIG Dover UK



 

[david69]

I don`t have close informations due to my knowledge about this gener.
I have tried replace hybrid amplifier OM360 but without change according to
attached scheme, where are two outputs switched betwen 10kHz-62.5MHz and
RF to 1GHz ?? with OM360 hybrid...??

[George Edmonds]

Hi

I say again without further detailed information it is impossible to give precise help to you, a photograph of a scope without any setting information is of no use at all.  This looks like an AM, vastly over modulated, RF signal with a 10Hz oscillation or a 1GHz oscillation modulation frequency that is being shown, we have no way of telling. DO YOU HAVE AM MODULATION TURNED OFF ?

With the Marconi 2022 series the first thing to check is the power supplies for correct voltage and ripple/noise, the second is to reseat all of the board interconnects, ribbon cables with DIL headers, they go intermittent or open circuit with time (oxidise), this is very important on the E variant for some unknown reason.

I have attached a scan of the 2022A AB1 RF processing board schematic, It MUST be used to fault find a 2022A, the main differences between the original 2022 up to the 2022D are mainly on the AB1 board, the changes were made as the maximum RF output power was increased upwards between variants.  The 2022E was actually a cost cut down 2022C variant for the military, it lacks a high precision 10 MHz standard.

I have also attached a scan the relevant section of the service manual text that describes the operation of the AB1 board.

My best guess without any further detailed information is that you have either a control logic fault or a ALC loop problem, you must follow the signal paths from the IC8, IC9, IC10 signal path splitters through to the RF output amplifier to determine where the problem originates.  For you information OM345’s usually fail totally, very rarely do they oscillate or go noisy, but it is not entirely unheard off.

At this time I can do no more without better information from you.

George G6HIG Dover UK

[George Edmonds]

Hi

Missed this attachment

[david69]

Hi George,

AB1 module board is OK, output level signal is clean in all range...
When I switch AC1 module, I have garbage on output aproximately
about 140-160MHz as attached picture, can I safely open metal box
with attenuator? There should be scheme of antiparalell diodes terminated
100 Ohm for reverse protection, could be there fault?
When I have switched +6dB full voltage about 880-890mV true rms,
output is clean! The output voltage get decrease in HF close 400-1000MHz,
should depend on rf amplifier OM360, the AM mudulation works

[david69]

I would like to add, that atenuator is switched by shift register 40174, where
I suppose to this chip makes pattern for setting power output and this could
show severall mixed signals in output, but it is strange, that garbage is only
140-170MHz, but I have analog scope only up 400MHz, where output is clean

[david69]

Hi George,
You have true, I replaced some electrolytic capacitors in power supply, but
I should replace all. C51 +5VDC is source for 40174 logic.
I will do it.

[George Edmonds]

Hi

I say again, it is virtually impossible for the electro mechanical attenuator to cause the frequency dependant problems that you are seeing, it is a purely passive device.  The attenuator output is sampled by R14 which is a 140 Ohm resistor and then rectified by D1 and D2 to provide a DC voltage for the Reverse Power Protection circuitry.

It would appear that you do not fully understand just how the attenuator system works in a 2022A.  From +6dBm down to approximately -10dBm the electro mechanical attenuator plays no part it is all done within the HF output stage ALC loop for frequencies above 62.5 MHz and similarly in the LF output stage ALC loop for frequencies below 62.5MHz.  The first 10dB stage of electro mechanical attenuator is not used until an RF output level lower than -10dBm is called for from the keypad entry.

I would strongly recommend that you do not touch the electro mechanical attenuator at all until there is absolute proof that it is the cause of the problems, you are nowhere near proving that at the present time. Your chances of obtaining a spare 2022 electro mechanical attenuator are minuscule to zero.  It may be worth checking that there is no damage to the SMA connector or the PTFE coax used, on a number of times I have seen this connector/coax outer damaged by careless handling.

I would suggest that the electro mechanical attenuator be disconnected from the AB1 board at the SMA connector and a 50 Ohm terminator be placed on the AB1 SMA connector.  Monitor the output of the AB1 board with the 2022A set for a frequency in the range that you are having problems with and the RF output level set initially at +6dBm.  Then reduce the output level from +6dBm downwards by 1dB steps to -10dBm constantly checking to see if the fault does becomes present.  AM modulation must be set off to do this test.

If it does become present the problem is with the HF RF output stage or ALC control.  Both scope waveforms that you have posted are typical of a stage self oscillating.


George G6HIG Dover UK

PS I wrote bulk of this email earlier, with the 2022 series it is always a good idea to replace the power supply electrolytic capacitors and check for the correct voltage/ripple.  The waveforms that you posted can easily be the result of one of the monolithic voltage regulators self oscillating, I frequently put 10uf capacitors from the common to both the inputs and outputs on the actual regulator device themselves.  The situation is not helped by the longish lengths of wire used to connect some of the voltage regulators and the +5VDC pass transistor to the power supply PCB.  Incidentally, the HF RF output stages use the + and – 12VDC supplies in the ALC loop.

[david69]

Hi George,
All Elyt`s are refurbished, self oscillation continues on 150MHz +-10MHz
The RF unit is broken on this band, when load output 50 Ohm termination.
David

[david69]

Hi George,

The HF ALC has superponed AC voltage 0,5V RMS changed by synthetizer unit
400-450kHz, otherwise the test pin is +10VDC, AC voltage ocures mess on output.
I have changed TL072 without improvement

David

[david69]

The broken signal is on cathode of D50 and SMD0805 33pF C115 ?has bad value?
I will try replace it

[George Edmonds]

Hi David

My opinion from the very limited information that you have provided is still that there is some instability in the high frequency signal path, ALC loop, or possibly the high frequency output stage itself.  It is impossible from your original scope photographs to even determine the oscillation frequency of the instability.

Have you checked/replaced the decoupling capacitors around IC14 (C109, C142)?  I have attached a magnified portion of the RF tray schematic so we are both using the same schematic.

I believe that you have a fast scope, if so look at the junction of C102/R93.  Is the RF signal clean at this point for all defective frequencies, bearing in mind the frequency limitations of the scope in use.

Do the same thing as the above at pin1 of IC14.

With AM modulation turned off and at a fixed RF output level there should be a stable DC voltage present at ALC HF test point which is at the junction of R112 and R114.

On the attached scan I have marked a signal line with an X.  This is the RF fine attenuator control and AM modulator signal, with the AM modulation turned OFF is this signal a clean DC voltage at all defective signal generator frequencies, which will vary smoothly between Zero to +10dB attenuator settings?  For example at RF output levels of +10dBm down to 0dBm.  The electromechanical 10dB steps attenuator should not operate between the above RF output levels if operating correctly

With regard to the ALC control loop which is provided by the TL07.

The problem is the word loop, as it is a loop the cause of the problem could be anywhere in the loop.  Such faults can be extremely difficult to find.  You may have to break the loop to fault find it, sometimes the careful application of a finger on a component will stop the unwanted oscillation.

I ask again, are the +5VDC, +12VDC and -12VDC supplies to the HF output stage free of noise and at the correct voltage?

73 George.G6HIG Dover UK

PS Regarding your lasting postings.  You may be correct suspecting C115 33pF which is an RF output sniffer, BUT it is part of an ALC control loop so it may not be the problem.  I will try to work out a method of disabling the ALC loop.

[david69]

Hi George,
It was heavy to remove feedback C115, but I would like to know, how it will change
behavior of output amplifier IC14? The OA TL072 is clean, nothing modulated by AC voltage,
and output from IC14 OM360 is not oscillationed but not sinusoidal.
Would I try right capacity instead of C115 ??
David

[david69]

The IC14 is OK, the noise which source output amplifier IC14 diffuse from diodes
D23 D31 BA482 on input IC14. The frequence is changing in range 3 kHz.
The power +5V,+12V,-12V is right.

[david69]

Can I look at range control logic ? The diodes D23, D31 are switched IC6 74LS26 PINC,
There should be capacitors in power eliminate switching logic ? I will try look into this
logic

[TheDefpom]

Diodes can get noisy if they start breaking down, I would be checking those too.

[david69]

The high voltage gate 74LS26 switches PIN C and PIN D,
PIN C has voltage 12V > 0.3V
PIN D has voltage 11V > 0.2V
If I switch PINC and inductor L18 to ground then output 150MHz is clean!
There must be something wrong in circuit IC13 ?

[david69]

diodes switching voltage >
D20 0.698V BA482
D21 0.624V PIN diode
D22 0.666V PIN diode
D23 0.711V BA482
D24 0.365V HP diode
D25 0.608V
D26 0.678V
D31 0.715V BA482
D49 0.584V
D50 0.585V

[david69]

R80=47Ohm but I measure 133Ohm, D25 partly conductive in both polarity ?
Change it?

[George Edmonds]

Hi David

You are wasting both your own and everybody else’s time with your method of fault finding.  Most of the diodes that you are measuring the voltages from are no more than signal path switchers.

As with all items of electronics you do a binary chop of the signal path first once, the signal path is understood, in the case of a Marconi 2022 you start off at looking at the output of IC9, if your scope is capable of doing so, if the output of IC9 is clean you then follow the signal path for the problem frequencies  through IC11a and IC11b to the output amplifier as detailed in Paragraph 80 and 81 of the attached scans.  The trick is to find the earliest point in the signal path that the problem shows and start looking just backwards from there.  I would not be surprised if the problem is with IC11a/b as these are active divide by two stages.  A quick short cut would be to look at the signal on R16, which is a 10R, with a scope whilst in the problem frequency range.

As your problem is related to the set output frequency, but NOT the signal frequency path change over points, I seriously doubt that the problem is with the Range Control Logic, but as these control signals are DC voltages they are simple to check for noise.

The attached scans are taken from a 2022D manual for readability reasons, so the output stage details may not be correct, but the signal path is.

Stop floundering around and apply simple logic and good fault finding practices.

George G6HIG Dover UK.

[TheDefpom]

Hi George,
Do yo have an electronic version for the 2022D service manual ?

I haven't been able to find that version myself, would be nice to get it as I have a 2022D.

[George Edmonds]

Hi

Sorry I don't have a PDF of the 2022D service manual, likewise I have never been able to find one.

I only have a paper copy, this is about as rare as hens teeth, at present I have no way of scanning it other than a few A4 pages.

With a few small exceptions you should find that a 2022C manual is identical.  The 2022D is a 2022C with a membrane keypad not the individual switches of a 2022C and a modified final PA stage, but the frequency generation schema is identical

George G6HIG Dover UK

[david69]

Hi,
The output IC13 OM360 failures, output rezistors 47 Ohm both OM360 changed value, and I replaced PIN diode on output IC13. IC13 realy produce noise.
Thank You for help and understanding David

[david69]

Ahoj George,
I replaced J510 transistor, both OM360, blocking capacitors on supply,
OM`s output resistors,
The 160Mhz is clean, but the range 10kHz-60Mhz is bad,,, the RF board
is complex in bad condition
Thanks for Your help
David

[david69]

Ahoj George,
I have replaced some actives and passives components, so it looks like better
One small bug is small self oscillation about 150MHz, but all range 10kHz to
1000MHz is clean and output voltage is adjustable in all range
The gener was really in bad condition, and still have unreadable display.

[david69]

Ahoj Georgu,
HF ALC amplitude modulation produce noise in range 150MHz, there is mixed signal 350kHz
which destroy output of AB1 board. Modulation is switched off.

[david69]

Level Control C5 is oscilating, DAC with CA3046 not work properly

[david69]

add resistor 22 Ohm to divider against self oscillation in band 125Mhz - 250Mhz

[david69]

CA3046 replaced by two BC546 tranzistors and add 15pF cap between outb and refb pins of DAC.
Gener is OK, no oscilation, correct voltage level in all band