Marconi TF-2015 ALC circuit - UK Vintage Radio Repair and Restoration Discussion Forum

orbanp1 · 1st Apr 2021, 6:28 pm

Hello Everyone,

Just wondering if there was a happy resolution to the discussion here on the TF-2015 ALC circuit:
https://www.vintage-radio.net/forum/...d.php?t=135372

The reason I am wondering is that I got a non-working TF-2015, and after fixing the PS in it, it exhibits the same distorted RF output as mentioned in the above thread.
The signal is distorted in every band.

So far I tested in the lowest band that the oscillator output from the A12 board is undistorted, while the signal on the A16 amplifier board at TR4 transistor, that drives the modulator DBM and the counter output, the signal is already distorted.

I also quickly scanned through the Marconi groupsio group where they mentioned that this was known problem with the 2015s, though no detail was given.

I guess I will have to dig into this sooner or later.

Thanks, Peter

orbanp1 · 10th Apr 2021, 9:33 pm

Hello Everyone,

I finished repairing the generator, it is working fine now.
The core problem was that one of the oscillator modules was shorted, and it also shorted out the secondary power supply.
The generator has individual oscillators for each band, and the bands-witching, among others, switches the supply voltage to the individual oscillators.
That short interfered with the output of the first ALC circuit on the other bands, resulting in over driving the following stages and distorted output.
The first ALC rectifier and amplifier itself was working fine, the simulations matched the measurements.
I did not see what was shorted in the oscillator module. Once I removed that module, it checked out fine, no shorts. Similarly on the "mother board", that houses the oscillator modules, could not find any shorts there.
When reassembling, I was soldering back the connections one by one, and measuring for shorts after every connection, could not find anything wrong.
I guess my original statement that "the output is distorted on every band" was not totally true, once switched to the band with the short, there was no output, and it took out the secondary power supply pass transistor again.
Interesting and a convoluted series of faults because of that short...

Thanks, Peter

Cruisin Marine · 10th Apr 2021, 10:46 pm

Well done! I have a soft spot for the 2015, I had one in my workshop for many years and it earned me a lot of money. I later fitted the synchroniser and I made a sweep unit for it. It never let me down.
Had many Marconi gennie's, all have been superb in their own way.

1st Apr 2021, 6:28 pm	#1
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Marconi TF-2015 ALC circuit Hello Everyone, Just wondering if there was a happy resolution to the discussion here on the TF-2015 ALC circuit: https://www.vintage-radio.net/forum/...d.php?t=135372 The reason I am wondering is that I got a non-working TF-2015, and after fixing the PS in it, it exhibits the same distorted RF output as mentioned in the above thread. The signal is distorted in every band. So far I tested in the lowest band that the oscillator output from the A12 board is undistorted, while the signal on the A16 amplifier board at TR4 transistor, that drives the modulator DBM and the counter output, the signal is already distorted. I also quickly scanned through the Marconi groupsio group where they mentioned that this was known problem with the 2015s, though no detail was given. I guess I will have to dig into this sooner or later. Thanks, Peter

10th Apr 2021, 9:33 pm	#2
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Marconi TF-2015 ALC circuit Hello Everyone, I finished repairing the generator, it is working fine now. The core problem was that one of the oscillator modules was shorted, and it also shorted out the secondary power supply. The generator has individual oscillators for each band, and the bands-witching, among others, switches the supply voltage to the individual oscillators. That short interfered with the output of the first ALC circuit on the other bands, resulting in over driving the following stages and distorted output. The first ALC rectifier and amplifier itself was working fine, the simulations matched the measurements. I did not see what was shorted in the oscillator module. Once I removed that module, it checked out fine, no shorts. Similarly on the "mother board", that houses the oscillator modules, could not find any shorts there. When reassembling, I was soldering back the connections one by one, and measuring for shorts after every connection, could not find anything wrong. I guess my original statement that "the output is distorted on every band" was not totally true, once switched to the band with the short, there was no output, and it took out the secondary power supply pass transistor again. Interesting and a convoluted series of faults because of that short... Thanks, Peter

10th Apr 2021, 10:46 pm	#3
Cruisin Marine Heptode Join Date: Jul 2018 Location: Worthing, West Sussex, UK. Posts: 986	Re: Marconi TF-2015 ALC circuit Well done! I have a soft spot for the 2015, I had one in my workshop for many years and it earned me a lot of money. I later fitted the synchroniser and I made a sweep unit for it. It never let me down. Had many Marconi gennie's, all have been superb in their own way.