Baird M648 dual standard set

[Heatercathodeshort]

Do not touch the alignment of the tuner. The PC88 and PC86 valves have to be new and 100% before the UHF comes up to near scratch. The valves often cause a good picture to fade to zero after 15mins. Fitting new valves should do the trick but remember a valve UHF tuner will never be as sensitive as the later transistor types.
Saying that the Baird series seemed to produce better results than many of the others. J.

[AC/HL]

Valve UHF tuners always struggled unless the signal was strong. Transistor ones really were a step forward in this case.

[bobbyball]

Hi Folks

I hope I'm not hijacking this thread but I too have now added a Model M.648 to my collection. Same lovely shiny cabinet and all seems complete inside. Lots of poorly looking capacitors to change but I have started to draw up a list.

Quick query. There are several values of capacitor in the line stage that have "non standard" values - ie 0.12, 0.35 and 0.002 uF. These are not "preferred values" and although they could be made up with several parallel items - for example the 0.12 and 0.35 uF ones are shown as +/- 10% on the circuit - would "nearest" values do here or are the values critical?

I would not be asking this question if the set was a colour one of course!!!

[dazzlevision]

Originally Posted by bobbyball

Hi Folks

Quick query. There are several values of capacitor in the line stage that have "non standard" values - ie 0.12, 0.35 and 0.002 uF. These are not "preferred values" and although they could be made up with several parallel items - for example the 0.12 and 0.35 uF ones are shown as +/- 10% on the circuit - would "nearest" values do here or are the values critical?

I would not be asking this question if the set was a colour one of course!!!

Hello Robert,

The 0.12uF and 0.35uF are (IIRC) the "S-correction" capacitors, in series with the line scan coils on 405 or 625 and ideally, you should use the correct values. Otherwise, the horizontal linearity will not be as good as it should.

For my hybrid Baird M662 set, which uses a very similar timebase PCB, I made up the correct values with two parallel Polyester axial caps for each (0.1 + 0.022; 0.33 + 0.022). I also selected on test suitable new caps to have as close to spot-on values as possible.

The 0.002uF, if a high voltage type (i.e. > 600Vdc) is probably LOPT tuning, but I suspect a 0.0022uF will do. You can always select on test a new cap for "closest to original the value".

If you have the correct Baird service manual, full details of each capacitor will be given.

Nice sets these.

Regards,

Dazzlevision

[bobbyball]

Hi and thanks!!

It's a question I "sort of knew" the answer to but I guess I was fishing for opinions!!

I've only the Radio and TV servicing book data which, though it has the circuit and component layouts, doesn't go into any more details. I have noted the values and ratings for each capacitor directly from the ones currently in situ.

Yes the 0.002uF capacitor is rated at 1500V and is on the LOPT itself.

It is a nice looking set and I travelled a "considerable distance" to go and fetch it so I had better get it working properly...

One other thing I noticed when I first looked inside was that the mains filter choke on the top right had side of the chassis seemed to have been overheating at some time as there was a fair "glob" of wax on the underside. I will probably be replacing the main smoothing capacitor (400/400uF) as a matter of course and will check for shorts on the HT line before applying power via a limiter or if I can find it, the variac...!

Cheers again