Marconi 2019A no RF output

[jack2006]

Purchased this generator as faulty back in the 1990's from Anchor surplus and only just got around to having a look at it as works gone a bit quite at the moment, but guess better late than never!

Carried out quite a few searches and read several posts on this web site which I found very useful so thought I would share what I have managed to sort out.

After switching on code 03 appeared in the centre window, so guess a self test and then changing to 06 in left hand window. This error is stated as being in Eprom for calibration values so thought perhaps reason for no RF?

Started with simple checks to confirm voltage levels were all correct at the PSU and they were fine.

Next check was the 10MHz ref oscillator. It looked like someone had been at this already judging by the solder splash! The 10MHz signal was ok so just cleaned up the mess and moved onto the 4 section oscillator which is accessed by lifting up top cassette and removing outer and inner shields.

The 4 oscillators were checked by selecting various frequencies from front panel with referance to service manual. It was found that as each one was selected output was stable and within tolerance so moved onto the AM PCB.

It was decided to stick with a test frequency of 100MHz so front panel controls were used to set this. The filter PCB was disconnected from the AM pcb and signal was traced into and out of the AM board.

Filter PCB was reconnected and signal checked through the filter but found to stop just after the first buffer stage which is where diode switching is used controlled by 74LS logic.

Checks on the 74LS273 IC's that control the diodes showed approx 0V which was wrong because 100MHz had been selected and filter section with range within this frequency should have been approx 3.6V - 5V.

So checked the data lines D0-D7 using scope and these were active when a new frequency was selected and accepted by the 'MHz' button. There will be 'traffic' on these lines when button is pressed etc because they are shared by other circuits but the specific check is when the MHz button is pressed which would mean data being moved to the respective filter diode switch.

But to make sure only that data was moved via the 74LS273 flip flop to select correct filter required correct address which is by the 74LS138.

Checks were carried out on this IC which is supposed to decode the address for filters and select the 74LS273. It was found that the address lines changed as expected and the out enable but nothing changed on the select for the 74LS273.

So the 74LS273, 74LS138 and the 7407 were replaced using turn pin sockets in case I had it all wrong and needed to do some hard wire fault finding!!

Switched back on and still had the 06 error but success! RF output across the range up to 1.2GHz, although this did look a bit low at the higher end using my 100MHz probe on 2.9 GHz spectrum analyser!

Still need to sort the 06 error and also check the AM and FM side in case they are faulty but decided to call it a day while 'in front'!

Hope this might be of use to somenone and if anyone needs any more detail please let me know.

John G7EJK