The Philips 1757U is a high performance 405 line TV receiver and was introduced in 1956. The 1757U is presented in a console cabinet, other models fitted with a similar chassis were the 1756U table set and the 2157U console cabinet model.
I'm restoring the set for a client. The chassis was removed from the cabinet and the taken to the workshop so all the activities of the chassis have been monitored by the oscilloscope.
Just like the Stella ST1007 the chassis is full of black tar capacitors and of course a good number needed replacement. There is evidence that the set was in service right up to the end of the sixties. The tuner valves are dated July 1964 and a Mazda EF80 is a late sixties type. That valve has been taken out and replaced with a Mullard one, can't have Mazda valves in a Philips set!
The 1757A uses a sync cancelled vision AGC system. Works extremely well after replacing the decoupling capacitors on the AGC line. Frame timebase required capacitor replacements to get it to work properly, at least the scope shows it does, we'll find if that is so when the chassis is delivered to it's owner later today.
The line timebase employs a flywheel sync controlled oscillator. The pentode section of an ECL80 functions as a Hartley oscillator and the triode as a reactance valve controlled by a two diode discriminator supplied from a phase splitter transformer, see attached circuit. It was the line oscillator that gave the most problems, line drive was present at the grid of the PL81 but the repetition rate was 80microseconds, should be 98.8 for 405 line TV. Changing parts improved matters not much. The oscillator transformer is actually fitted on the side control panel and it turned out it has an adjustable core, readjustment corrected the line TB frequency. The flywheel sync is working and the pull-in range monitored on the 'scope. Again, we'll only know if all is well when the set is delivered today.
There is up to 50 volts P - P video at the cathode of the CRT base connector pin 11. Good quality sound. Got a good feeling about this set but we'll only know how good this set is when the chassis is delivered to it's owner today in Gateshead.
Hi Till, I have the table model 1756U, a set which I had been searching for over a long period. It is working well with just a handful of replacement components, It has 90% of the Goudron tar capacitors still fitted and gives a decent picture and even though it does have a partial short on the CRT heater.
Is that smoothing can original, mine is fitted vertically near the sound output transformer. Does yours also have the sensetivity jumper at the rear of the signals panel? My grandparents set didn't have this so may have been an early version.
Hi John, it is indeed the original smoothing capacitor. Didn't notice any local-distant sensitivity jumper wire but will take another look at the chassis next week.
The chassis was delivered to my customer and installed into the cabinet. After connecting up everything the set didn't work and this turned out to be the fuse in the mains neutral making bad contact with the holder clips. That was put right and after allowing the set to warm up we were gratified with results, a blank but synchronised raster. The Freebox will need retuning. Before that was done I simply disconnected the video to the Aurora converter and we were greeted with the test tone and a Test Card C image.
The set works! The rebuilt CRT is excellent displaying really bright pictures with sharp focus. The CRT rebuild company was Liver Tubes, anyone remember them? Firm used to advertise in the Practical Television magazine.
After retuning the Freeview box we are watching pictures.
Just few niggles to sort out, the picture is cramped at the bottom and the line and frame sync is a bit touchy. The chassis has been taken out again and is now back in the workshop. Nevertheless, I reckon this set has all makings of being a really good one.
More progress. The frame linearity is improved but there is only just enough height, same goes for the width. The attached pictures might look good but on certain scenes there are black level variations which in this set shouldn't be possible because the DC component is maintained from the detector through to the CRT cathode. Also, the oscilloscope reveals that the sync pulses are compressed. The most likely cause for this condition is the video amplifier cathode bypass capacitor and the 10uF screen grid decoupling capacitor. Like the Stella ST1007 which was the subject of another topic in this forum the Philips 1757U has a feedback system between the cathodes of the video amplifier and the IF amplifier. One could consider this as a form of AGC within the main vision AGC system. The function of this arrangement is to compensate for the short comings of simple vision AGC systems.
The oscilloscope trace shows the waveform present at the grid of the frame output valve.
I'm sure you will have already checked it but with my 1756U which showed similar frame sync symptoms R122 was virtually o/c. Looking for the cause of bottom cramping where the cathode bypass was blameless on replacing C83 gave me a considerable increase in height with bags of overscan. The set also has more than ample width but it can vary at times, setting it for correct scan in the morning it will be underscanned in the evening probably due to mains variation.
I also had trouble in the video stage with excess contrast and the control having no effect, this was traced to C52 in the cathode of the video output stage being short.
Circuit diagram of the Frame timebase. C83 with R67 form the first frame sync integrator. V15A is the frame sync pulse inverter. Further integration of the frame sync by C112 and the anode resistance of V15A. After the frame oscillator is locked to the sync the negative going flyback pulse from the output transformer is applied to the grid of V15A to block any noise pulses.
Sorry Till that was a typo. It was C85 in the feedback network that greatly increased the amplitude.
Both C85 and C86 needed replacement in the set I'm servicing. The sawtooth waveform at the anode of V16B and on the top plate of C87 is perfectly linear but the waveform at the grid of V15B is quite different. Obviously it's predistortion to compensate for non linearity in the output transformer.