Stella ST1093A epic saga, or 5 minute fix?

[Sean Williams]

A couple of piccys - No progress over the past couple of days - Have been on call at work.....

[Sean Williams]

Ah, a little spare time this afternoon.

Have fed 41.5 and 45mhz in the band 1 tuner, and sure enough 405 sound and very poor weak bars on the screen, so there appears to be a shortage of gain somewhere.

625 and UHF is a different story - feeding in 38mhz directly to the IF point results in a big fat nothing!

I have proven to a point that the 6mhz sound circuit is working after a fashion (it will demodulate FM sound if I lob lots (3V) of RF into the IF port)

Im Working off trader sheet 1679 - seems to be an excat replica of what this set has to offer.

Other observation - the Contrast control seems to do nothing - I assume this works as a "Gain" control for the IF stages?

[Sean Williams]

Have found a couple of O/C resistors in the contrast feed line - 100k and 10M - have replaced these, but have hit a stumbling block - the circuits I am working from differ around the contrast control....

Looks like hard work from here in....

[valveman49]

Hello
If you have the BVWS CD’s of the W.E.T. circuit diagrams on CD ROM, The Stella ST1039A is covered on Trader Service Sheet 1588.
Regards Stan

[Sean Williams]

Er thanks Stan, However, mine is the 1093A, not the 1039A.......

Onwards and downwards.....

Sussed the way the contrast circuit works, or in this case doesn't...

150-160v on the track of the contrast pot, the wiper taps this and feeds to a 2.2M decoupling resistor, this then disappears off to the gain control line.

On the other end of the 2.2M resistor (opposite end to the pot) there is a diode connected to ground, as far as I can see there is no short to ground, however, the 150V maximum on the control gets lost at the 2.2m resistor - the resistor reads ok.

Checking the grids of the IF valves, shows a -3v reading, but this appears to only vary by around .25V - I would expect a much bigger swing here.

Open to suggestions.....

Sean

[Sean Williams]

Seems most likeley now that there is an issue in the IF department.

On 405 I can get a decent sound signal through, and a very weak vision signal.

As far as I can se it there is a problem with the bias line on the IF stages, but the trader sheet is not helpful in this area.

Could do with a hand here!

I have tried adjusting the system switch, to see if that was overshooting its position, and it appears fine.

Otherwise I am at a loss, it seems this must be quite a rare set - there is no mention of the set in any manual I have....

Open to suggestions from our other dual standard afficionados

Sean

[murphyv310]

Hi Sean.
I vaguely remember these sets at the start of my apprenticeship and amongst my junk in the loft I think I might just have the Philips manual for it (I was a Clepto). I am going to venture into the loft tomorrow armed with goggles and my nuclear fallout suit and will have a look, if successful I'll let you know tomorrow sometime.

[Sean Williams]

Thanks Trevor - might well come in handy!

More news!

405 "picture" seems fixed - the vision detector diode was o/c - only had a OA91 to hand, but I am now getting somewhere near decent sensitvity on both S and V for 405.....

625 is a new challenge.... the screen brightness increases dramatically, but no vision signal coming through...... time to check this circuit methinks.

[Sean Williams]

Ok, flummoxed.....

405 simple, AM reciever for sound, and a similar thing for vision.....

625 not so simple.....

FM sound, squashed in the carrier by some form of witchcraft, and inverted polarity vision from 405?

So, question.....

405 line, no signal = no screen illumination

625 line, no signal - white screen?

Is this right?

Or have I got my theory arse about face....

This thing will not beat me!

[Steve_P]

Partly right.

405 Line is AM Vision with Positive Modulation and AM Sound, which is 3.5 Mhz below vision.
625 Line is AM Vision with Negative Modulation and FM Sound, which is 6 Mhz above vision.

So, yes, 625 Line Vision is opposite to 405 in a way. And although it does not affect you, the 625 colour subcarrier is 4.43 Mhz above vision.

Always worth trying valves on the IF stages, and check anything that is 625 Lines only. C230, C229, C239, C240. And was the set a rental one. Some (not all, I hasten to add) rental engineers were not against tweaking to get a set to work.

Cheers,

Steve P.

[murphyv310]

Hi Sean.
Managed to get in the loft but, the manuals I had were the 210, 230 & 320 series sets, I could have sworn blind though I had one. Do we know if the set is in the Newnes books as I have the full set?

[Sean Williams]

Been away from the set for a while - work gets in the way.

Thanks fo that Steve, will get my head around the theory eventually

Trevor,

Not sure if it appears in Newnes - I got shot of mine - didnt think I needed them.....

Sean