26-08-2026, 12:33 PM
Hi all,
So, I bought one of these from E-Bay, sale or repair a few weeks ago. It's now up and running exactly as expected given its age and this is the story of how that happened.
So, when I received it, it powered up but the output was no greater than 12uW and off frequency by anything between 27 and 200Mhz depending on the thumbwheels.
Anyway, 1st things first as with all equipment of this age. The power supply. Changed nearly all of the electrolytic caps just for safety more than anything. A quick check and tweak and all supply voltages stable and more importantly, ripple free.
Next, check over all the thumbwheels and all outputting the corresponding bcd code when called on. However, the lock light was always off and checking the tuning voltage from the output of the synthesiser, it sat at a constant 22V. Something amiss!
Chased the logic through the synth and ended up replacing all the caps on that board along with the following ICs. 2,3,4,5 and 12. I also removed IC and fitted a socket for it for future repairability. Also cut the tuning voltage track and fitted a removable link. More on that later.
Some success, now locking at 10, 11 12 and 13 Mhz. Nothing higher though.
Onwards to the prescaler. Well, when I took the RF cover off, it looked like a one handed gorilla wearing a boxing glove had been let loose with a welder. In all my years repairing electronic equipment, I've never seen such poor soldering. I'm surprised that the thing didn't go bang when I powered it up, which teaches me at least to make a more thorough inspection before applying power in future to things bought a repair items
Anyway, I again changed all the caps, cleaned up the messy soldering and as per the synth, broke the hard wired link on the board and fitted a removable one.
From there, I replaced all of the rotted co-axial cable that was being used to carry DC with new cable and discovered the output from IC 7 to be wondering around once above 13Mhz. It was also getting red hot. So, replaced both IC6 and 7 as they are used as dividers. Still not locking above 13Mhz, but running cooler with no residual noise.
Had a further deep dive and found that transistor VT1, a bfr90 faulty. Replaced that and great news. Synth now locking across all frequencies.
Now to align it. This is where experience counts as the manual isn't the best. You recall, I cut the tuning voltage track on the synth. This is why. I removed the link and fed the oscillators with an external power supply. Starting on the 10Mhz range, I set the oscillator tuning coil grub 3 1/2
turns in from being level with the top of the coil. Don't ask me how I knew to do that. It's just years of experince working with early synthesisers.
Anyhow, adjusted the external supply to get exactly 10 Mhz and noted voltage which is 2.52V. Then worked though the other oscillators with the supply at that voltage and tuned each coil to its corresponding lowest frequency, or as close as I could get it.
Then repeated the whole exercise again, but this time starting at the maximum frequency for the 10-13.999Mhz range, and rinsing and repeating for the others. Went through this about a dozen times, making slight tweaks as I went along to ensure that everything flowed smoothly. Tuning range voltage between 2.7 and 17.9 approx.
Once I was happy with the oscillators, replaced the link to the synth tuning output and checked to make sure synth locked across the entire range, which it does. So now I had to tackle the low power out.
Anyway, that turned out to be a completely missing link between the amp/modulator board and the step attenuator. Fitted and now have up to 7dBm output across the entire frequency range, stable, clean, well on my spectrum analyser at least and remains so over long periods of time. Even from cold within 50 Hz.
Next job then, check the AM and FM modulation. Well, a nice surprise. Nothing to do on the AM side except a quick alignment as per the service manual. Spot on, 1005Hz and 412Hz, which is close enough for me, with amplitude depth exactly tracking against my Marconi 2955 test set.
FM. Zilch, nada, nought. Onto the FM correction board then. I noticed in the manual that it has a test point that when dropped to earth through a 1.5k resistor starts the latch, at which point I get FM. Not at the level expected, but it's a start. End up replacing IC21, 17. All caps and diodes D7,8,9 and 10. Fantastic. Now have FM. Aligned and mostly where its meant to be. The front panel power meter is a bit slow responding so I need to try and find a suitable replacement. But all in all, fingers crossed, a long term success story.
Just wanted to share in case anyone else has one of these. I still have a little work to do on it. Mostly replaceing rotted cables. But that's now a nice easy 'Saturday potter' day when the Mr's has stopped finding me things to do.
So, I bought one of these from E-Bay, sale or repair a few weeks ago. It's now up and running exactly as expected given its age and this is the story of how that happened.
So, when I received it, it powered up but the output was no greater than 12uW and off frequency by anything between 27 and 200Mhz depending on the thumbwheels.
Anyway, 1st things first as with all equipment of this age. The power supply. Changed nearly all of the electrolytic caps just for safety more than anything. A quick check and tweak and all supply voltages stable and more importantly, ripple free.
Next, check over all the thumbwheels and all outputting the corresponding bcd code when called on. However, the lock light was always off and checking the tuning voltage from the output of the synthesiser, it sat at a constant 22V. Something amiss!
Chased the logic through the synth and ended up replacing all the caps on that board along with the following ICs. 2,3,4,5 and 12. I also removed IC and fitted a socket for it for future repairability. Also cut the tuning voltage track and fitted a removable link. More on that later.
Some success, now locking at 10, 11 12 and 13 Mhz. Nothing higher though.
Onwards to the prescaler. Well, when I took the RF cover off, it looked like a one handed gorilla wearing a boxing glove had been let loose with a welder. In all my years repairing electronic equipment, I've never seen such poor soldering. I'm surprised that the thing didn't go bang when I powered it up, which teaches me at least to make a more thorough inspection before applying power in future to things bought a repair items
Anyway, I again changed all the caps, cleaned up the messy soldering and as per the synth, broke the hard wired link on the board and fitted a removable one.
From there, I replaced all of the rotted co-axial cable that was being used to carry DC with new cable and discovered the output from IC 7 to be wondering around once above 13Mhz. It was also getting red hot. So, replaced both IC6 and 7 as they are used as dividers. Still not locking above 13Mhz, but running cooler with no residual noise.
Had a further deep dive and found that transistor VT1, a bfr90 faulty. Replaced that and great news. Synth now locking across all frequencies.
Now to align it. This is where experience counts as the manual isn't the best. You recall, I cut the tuning voltage track on the synth. This is why. I removed the link and fed the oscillators with an external power supply. Starting on the 10Mhz range, I set the oscillator tuning coil grub 3 1/2
turns in from being level with the top of the coil. Don't ask me how I knew to do that. It's just years of experince working with early synthesisers.
Anyhow, adjusted the external supply to get exactly 10 Mhz and noted voltage which is 2.52V. Then worked though the other oscillators with the supply at that voltage and tuned each coil to its corresponding lowest frequency, or as close as I could get it.
Then repeated the whole exercise again, but this time starting at the maximum frequency for the 10-13.999Mhz range, and rinsing and repeating for the others. Went through this about a dozen times, making slight tweaks as I went along to ensure that everything flowed smoothly. Tuning range voltage between 2.7 and 17.9 approx.
Once I was happy with the oscillators, replaced the link to the synth tuning output and checked to make sure synth locked across the entire range, which it does. So now I had to tackle the low power out.
Anyway, that turned out to be a completely missing link between the amp/modulator board and the step attenuator. Fitted and now have up to 7dBm output across the entire frequency range, stable, clean, well on my spectrum analyser at least and remains so over long periods of time. Even from cold within 50 Hz.
Next job then, check the AM and FM modulation. Well, a nice surprise. Nothing to do on the AM side except a quick alignment as per the service manual. Spot on, 1005Hz and 412Hz, which is close enough for me, with amplitude depth exactly tracking against my Marconi 2955 test set.
FM. Zilch, nada, nought. Onto the FM correction board then. I noticed in the manual that it has a test point that when dropped to earth through a 1.5k resistor starts the latch, at which point I get FM. Not at the level expected, but it's a start. End up replacing IC21, 17. All caps and diodes D7,8,9 and 10. Fantastic. Now have FM. Aligned and mostly where its meant to be. The front panel power meter is a bit slow responding so I need to try and find a suitable replacement. But all in all, fingers crossed, a long term success story.
Just wanted to share in case anyone else has one of these. I still have a little work to do on it. Mostly replaceing rotted cables. But that's now a nice easy 'Saturday potter' day when the Mr's has stopped finding me things to do.
