A Tale of Two Sobells

[HamishBoxer]

So what is that LOPTX from, I ask as I have a new old stock one?

[audiomagpie]

Originally Posted by Heatercathodeshort

By the look of Mikey's picture in post 17 I think a wedding is on the cards..John.

I went to a wedding like that once, lousy reception though!

..............I'll get me coat!

Greg

[Mikey405]

Originally Posted by audiomagpie

I went to a wedding like that once, lousy reception though!

Good grief!

[Duke_Nukem]

Whilst the boys were drawn to a recently arrived colour telly thing like moths to one of them fly-electrocuting traps, the men - man - cracked on with the proper telly

Now EHT was available, time to track down the tri-state brightness control. It's DC resistance had been measured early on and voltage measurements put off as the crt base in insulated in one of those McMurdo connectors. After a bit of pondering - and spotting the deliberate mistake on some of the pin numbers intended to keep us all on our toes - some meters were connected to points in the chassis that lead to CRT grid and cathode to fin that with something like 160+v on the cathode there was either 0.0v on the grid (no picture), circa 150v (=raster), or 185v - at which point the CRT clearly had enough emission to take out the EHT. Enter new brightness pot and (whilst I was at it gain pot) - Mikey's mega box of pots coming up trumps.

The line speed was now clearly wrong (again) but not unexpected as I'd previously stuffed 180pF across the line hold so it was now removed.

Meanwhile, in the front end, T3 primary was o/c and temporarily bridged by 10K relying on stray coupling to get the signal through (one of Tone's good ideas), and after a bit more fettling I now had a locked if uninspiring testcard (image 3).

A few less successful coupling methods were tried but all worse than the 10K bodge. So I decided to wind another primary. It is easy to calculate the number of turns required using an extension of Nukem's 3rd law of electricity. If you need a temporary resistor bodge chances are its 10K. If you need a temporary capacitor bodge it is 10nF. It logically follows that if you want a quicky coil bodge then you need 10 turns. Simples. But not so simple to wind - really didn't want to dislodge anything in the RF section if at all possible so I set to trying to feed 10 turns of wire round the former in place, a job greatly aided by a special tool; I think the tool is for getting springs on/off things, one end has a hook whilst the other has a z-shaped hook. Just the ticket for gradually dragging a bit of wire around an inaccessible core......

[Duke_Nukem]

..... Image 5 just about shows the "new" coil, whilst picture 6 shows the greatly improved picture. I've no idea how this transformer is tuned - must be stray capacitance, valve capacitance, or something. I dare say theres more performance to be had, and Tone was just itching to give T3's core a tweak, but given that the set will only ever get driven by a high signal from a Domino or Aurora I decided to let it be for now.

All along the sound had been humming. The HT cap did have a bit of a white flakey hole in it so the dual 16+16uF HT / screen decoupler for the audio stage was replaced by a top quality dual 20uF item that had been rescued from a scrap Tek scope; there are two 1K resistors between them and the PY82 rectifier so the extra capacitance isn't a problem. Not only did that remove the hum, it definitely seemed to improve the picture still further !

So thats about it really. The frame linearity needs some work - I don't know yet if its a consequence of the incorrect frame transformer, the fact that only a 1/4 of the frame linearity pot actually works, or maybe its just I haven't attacked all the caps. The line sync also isn't all it could be - there is plenty of hold range for the frame circuit but the line hold is a bit more tetchy.Again, maybe a few more cap changes will help.

The final pic is probably my fave; both sets all but working is more that I could of hoped, whilst a suitably impressed-looking Taz - and the TV12 in the background - give them a sense of scale.

TTFN,
Jon

[FERNSEH]

Hi Jon,
The method used in this set to adjust the frame linearity is not the best. If my memory serves me well I believe the frame output valve bias is altered to effect changes to the frame waveform shape. Slight overbiasing the frame output valve results in compression at the top of the picture. It's not an ideal arrangement but it works. Even Murphy employed these technique in some of their sets.

DFWB.

[FERNSEH]

The frame timebase circuit of the Sobell T121. Note the variable resistor P6 in the cathode circuit of the frame output valve. This alters the bias and effects the general frame linearity. Increase the bias and compress the top of the picture, reduce the bias to effect the bottom of the picture.
Might be worthwhile checking C26, the cathode bypass capacitor.

DFWB.

[Heatercathodeshort]

A very common procedure David till around 1952 and as you say it does work surprisingly well. John.

[Tazman1966]

Originally Posted by Duke_Nukem

...Whilst the boys were drawn to a recently arrived colour telly thing like moths to one of them fly-electrocuting traps, the men - man - cracked on with the proper telly ...

Boys? Boys? It was a man sized weird blanking fault on it though I'll let Mike pick up the story in another thread.

As for your Sobell, I'm really pleased that a set that was to be a static non working exhibit at yours is now working with not much more to be done. Nice Handy also of course that you had Mike's to compare with.

[Duke_Nukem]

Taz, yes it was handy for us both to be able to compare sets. Theres no luxuries such as waveforms in the manual.

Pretty much everything from C26 back to C25 on the circuit posted by David has been replaced. P6 only works over part of its range, however there is minor cramping both top _and_ bottom. I might try randomly shoving some capacitance here and there and see what happens (looking at the circuit I wouldn't be at all surprised if thats how Sobell came up with the values in the first place!).

TTFN,
Jon