Author Topic: I'm new, what better way to start with a faulty Marconi 2018 RF generator  (Read 982 times)

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Offline kevzep

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Hi Guys,
So I bought a nice Marconi 2018 0.8-520mhz RF generator recently as I am extending my Audio electronics background into RF (witchcraft as my friends call it). Got it for a good price too, had been used in the Aircraft industry here in New Zealand, no doubt to align Aircraft radios...
So I was running it to align an FM tuner which was going very well.
Near the end of tuning the IF stage, I noticed the signal cutting out, it happened to time perfectly with something I was touching, I thought for a moment there was an issue with the tuner, but no, it was the RF generator signal cutting out, it became more frequent and then it finally "gave up the ghost"...
Panel is still lit, all parameters are adjustable via the digital control panel.
All voltages are present and correct from the PSU....
I have the entire service manual and schematics, I have followed their testing procedure for the unit, and I have established the RF amplifier is good, the control interface is working according to the tests I performed out of the manual.
No signal getting to the RF amplifier, in fact all the boards, modulator, VXCO, Phase detector, frequency standard XO is functioning, fault could possible be on the Frequency divider and FM drive chain, OR the RF Oscillators.
Now the road block I have run into is I can only measure up to 20mhz at the moment with the gear I have, so I cant verify the the RF oscillators (of which there are 4 all running in the hundreds of mhz.....
I have ordered a 1ghz frequency counter which should be here in a few days, so hopefully I will have some joy once this arrives.

I will take any advice going to help further narrow this issue down whilst I wait for my frequency counter to arrive.

Cheers Guys

Oh PS, the only thing I haven't done is followed the testing via the front panel using the binary control commands, I'm not sure I am game enough to give that a go!!

Offline The Radio Shop

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Very interesting fault in this unit. Never been inside one of these but I will see if I can find the service manual and have a look at it.
Could be something as simple as a loose connection? Tested all cables? Sounds like you have verified most of the stages were working.
Buddy kc4umo

Offline kevzep

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Thanks for the reply, yes the service manual and schematics are available online, very easy to find, the 2019 model is exactly the same unit except it has a frequency doubler before the RF amplifier. (goes to 1040mhz)

While I am waiting for the frequency counter, I got in there again to see if I could verify this oscillator is working. The main Oscillator board has 4 oscillators on it, and depending on what frequency range is required they are turned on or off.
So I can see the 4 osc turning on and off when I selecting various frequencies from the front panel, so this is good.

The fault does suggest I have a bad connection, I have checked everything I can get to, I might dig a little deeper and re-check.

I am the kind of person that wont give up, this thing has put the challenge on for me, and I have accepted!! I will find the fault!! Its probably staring me right in the face!!

Thanks for the help...


Offline kevzep

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Well, perseverance certainly pays off.
On the AB2 Divide by 2 and FM drive chain board (for those who have looked at the schematic), I was following back from the output, trying to find where the signal was getting lost, and as I pressed on the Emitter junction of TR4 and TR6, the thing burst into life.
I can't get it to fault now, but I am going to work in this area until I find the intermittent connection, there's bit of a biasing network around this part and a decoupling cap etc etc, most of it surface mount, the BFR90 transistors are surface mount as well, so its all pretty small to be able to try and find the problem, but I will find it....



Offline kevzep

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Well, there it is, all done.

It turned out to be a flaky TR1 (BFR90). I think when I had my scope probe in there measuring, when I touched that junction the impedance of the probe must have pulled the bias of that particular part of the circuit enough to kick start that transistor.
I then reflowed all the solder joints around the junctions and surrounding components because I couldn't find anything out of the ordinary.

Then fired it up and all was well, I was displaying the output on my scope, I thought, why not dial in some modulation, as soon as I thought this, we had some intermittent activity on the output, cutting out, noise, spikes, the works, so I got to with the probe whilst watching the chaos going on and I probed at the oscillator, that was good, then as soon as I touched the probe on the Base of this TR1 (which is the input), it stopped, I could bring it back to life by putting my thumb on it, so I thought ah ha, I hit it with the freeze spray, it cut out, hit it with the heat gun, it started up again....
So there it was, dodgy transistor!!

BFR90 is now obsolete, so just the other day I thought to myself, there is a chance it could have been a transistor, so I found out the modern sub, which is a SOT23 package, so ordered up a bunch, never dealt with something that small before, but it was okay!! So anyway, I got the old TO50 transistor out, and managed to get the BFR92A in there, I have enough magnification to do it so there it was, installed.

The Marconi is now running on the bench, been running at 90mhz for the last 2 hrs, I think its fixed!!

I took a pic so you can see the retro fit....the hole in the board was the size of the other device as I am sure you're all aware, but yeah new to me....Lucky I happened to get some tweezers the other day in anticipation of having to do SMD...