Sobell ST195

Hello all.

This write up features a set that was just looking at me at the last Wooten Bassett event. It’s a Sobell ST195 19” semi convertible 405/625 line set. I say convertible but the only bits that are 625 line ready in the ST195 are a 625 line horizontal hold control, wiring from the timebase panel and LOPT to where a system switch can be added and a tempting (at the time anyway!) UHF position on the channel selector. There are also holes ready to accept knobs for the system switch and the (rotary) UHF tuner. Even the IF strip is still single standard system A. Anyway, I parted with some cash and the set was mine.

I examined the set the very next day after I got it home and it looked fairly much untouched although there were one or two bits of evidence of repairs in the past. A newer PCL805 frame timebase valve and associated cathode bias electrolytic and resistor for example. Rather more unusual was some resoldering to some of the fly leads to the tagstrip where the fully dual standard version would have had the system switch. Additionally some wiring to the LOPT appeared to have been modified. There was also a rather unusual mod to the IF panel in that the EB91 vision detector diode was missing replaced by a glass diode on the back of the empty valve holder.

Rather more mundane were the many “Hunts” and “TCC” waxies that would need replacing for sure. The boost cap had actually melted!

I replaced some of the more critical caps in the timebases and the mains filter and decided to power up. The HT is supplied via a PY33 rectifier so it would not be a quick blast to the sleepy smoothing/reservoir electrolytic block so with a deep breath and a hand on the switch, I plugged it in. After what seemed an age, there was the glow of heaters in the valves and shortly after, a line whistle although it didn’t sound like 10.125kHz to me! After a while longer I heard the rustle of EHT but alas no raster was forthcoming. The sound of line whistle could be made to vary when adjusting the fine tuner so I was assuming that there were signals of some sort present. I switched off and a brief flash of raster occurred as it shrunk into the centre followed by a very bright switch off spot. I had another look at the circuit and spotted 0.1uF decoupler in the brightness control circuit and measured it finding that it was all but short circuit. On replacing that I now had a raster and a picture of sorts but it was negative, completely the wrong line speed (as I had suspected) and there was no sound to boot! Plenty to be getting on with then!

I discovered that the 405 line horizontal hold control was inoperative but the 625 one was working. On winding that right down with a little tweak to the preset hold pot, a picture could now be discerned. I was beginning to get an idea of what was going on. The set had at some point I suspect in the late 70s perhaps early 80s been modified for operation as a DX tv for receiving transmissions from the continent with negative going video and 625 line VHF reception.

The first modification was to remove the glass diode and re-instate the valve vision detector. With this done I now had a normal picture and good sync. The linearity was poor, the width was too wide and there was still no sound though. I was worried that perhaps the sound section had been extensively modified to FM sound but thankfully the solution was easier than that. The EF80 sound IF amplifier had a bad connection on the valve socket. Sound now burst forth.

At this point, the set having been on for 10 mins or so, the width started closing in. Oh dear – or words to that extent! Undeterred, I set about reversing all the modifications to 625 as I felt that that on a 625 line setting being artificially wound down to 405 lines may be mucking up the drive to the output stage.

With the wiring back to original, the 405 line hold was now the operational control, the correct scan coil coupling components for 405 line operation were now in circuit as were the correct caps etc around the line output transformer. The initial excessive width was now more normal and I had more height too. I ran the set like this noting the rubbish frame linearity after about 20 mins the width started to close in. I tried a new pair of line output/boost diode bottles but to no improvement. It was then that I noticed that the primary of the LOPT looked very sweaty, I placed my finger on it (with the power off of course!) and recoiled as it was very hot indeed. The transformer was possibly duff. I did have another one to try but first, getting my priorities wrong as usual set to work on sorting out the frame stage.

I had already replaced the capacitors in the linearity network and now set about checking the resistors. They were all out of tolerance but replacing them one at a time to note improvements did not bring about the dramatic changes that I’d hoped for. As an aside at this point, I noticed that the set seemed to last for longer and longer before the width came in and the LOPT got hot. Eventually I replaced the 70s replaced cathode bias electrolytic and got a reasonable amount improvement. There was still some cramping at the bottom though in spite of another new PCL805 so with everything replaced, I tried a larger value electrolytic and with a 1000uF (instead of 250uF) the linearity was good. The frame lin control was still at one end though. I wonder what on earth it could be? All valve voltages are spot on by the way.

I turned the set on the following day to give it another run and this time, even after a couple of hours, the width was stable. It seemed that all the dampness in the transformer had been driven out. I was relieved because even though I had another transformer to try, the EHT overwind had a big crack in the insulation.

The only fault I had left now was some corona type interference on the picture and a faint hissing or sizzling from somewhere in the line output box. With all the lights off and brightness turned down leaving the workshop pitch black I could see a faint purple spark from one of the tags on the transformer housing. I suspected that it was a duff connection but the reality was easier than that. One of the Hunts caps that had fallen to bits when I replaced it had shed one of its legs and that had become lodged on the transformer end of the connection to the line output anode. It wasn’t connecting to anything but was nice and pointy and near the housing. Problem solved.

After a good clean of the chassis but not overly so it was time to tackle the cabinet. This was in really good condition so all that was needed was a clean and polish. I was a bit too vigorous with the “Brasso” though and managed to partially remove the Sobell Nineteen legend. Not quite sure what to do about that now.I like this era of sets and I know that a few of you on here have some examples from this era which I understand was an uncertain one in the trade.

I'll upload some pics soon too.

All the best,

Tas