Those Pye Hybrid Colour Sets
By Mike Bennett

Of all the early generation colour televisions, one of the most unloved models is the Pye hybrid chassis.  Once they were commonplace in peoples’ garages, in junks shops and at recycling centres, but nobody seemed to want the poor old things and now they’ve faded away into relative obscurity.  But why were they so unloved?  Because they were pretty unreliable at the best of times? Maybe.  Because the plastic veneer on the cabinet chipped off very easily to reveal the nasty chipboard beneath? Perhaps.  Anyway, whatever the reason, if you do choose to get to know one, it’ll be your friend for life and, if you can find one with a good tube, once set up correctly, it will give you a lifetime of easy maintenance and, arguably one of the best pictures of any colour set of the period.

As colour televisions go, there is nothing out of the ordinary about the circuitry in these sets; with a Pye hybrid, to coin a phrase beloved of computer users, what you see is what you get.  Inside, the sets are fairly modularised and, unlike some other TV’s such as the dangerously complicated Philips G6, all the panels are easily removed to swap out or to work on at the bench.   If the decoder fails, you can simply remove it and replace it with a good one.  If you don’t have a good one, then you can still remove it, and the set will carry on working in black and white!

There are several versions of the Pye hybrid chassis, starting with the earliest 25 and 19 inch dual standard CT70 and CT71 sets plus a myriad of other brand name versions such as Invicta, Ekco, Ferranti etc.  These early dual standard sets didn’t have a chassis number, being referred to in the manual merely as the “Dual standard chassis.”

The first single standard sets (“691” chassis), used much of the same circuitry as the original dual standard sets.  The PAL decoder, CDA and frame panels were pretty much identical, and the I.F. panel was also fairly similar except that all 405 line specific components were removed and an intercarrier sound IC was used instead of a discreet audio detector circuit.  The biggest difference was in the line timebase section.  Where previously there was a GY501 EHT rectifier, a PD500 shunt stabiliser and a 25KV line output / EHT transformer there was now a solid state tripler and a much nicer, smaller transformer.  Of course, the line timebase and convergence panels were further simplified by the removal of all unnecessary 405 line specific and switching components too.   The 693 chassis that followed soon afterwards saw the introduction of a varicap tuner, and slightly modified I.F. and decoder panels.  The last incarnation of this chassis was the “697” which used a completely redesigned, printed circuit line timebase panel instead of the chunky metal variety. 

As was noted earlier, the sets were not noted for their reliability, but luckily most faults are generally easy to diagnose and fix.  Even faults in the PAL decoder panel can be fairly easily rectified if a little logic is applied.  As with most restorations, the service manual is essential.  If you don’t have one, most of the service information is available in the “Newnes Radio and Television Servicing” manuals.  Information on the single standard chassis is also available on Paul Stenning’s excellent “Vintage Television Service Data” CDROM.  Failing either of these, there are some stock faults, which occur often, and these are listed below.  Incidentally, if you are faced with a “No colour” fault, the colour-killer can be over-ridden by supplying 2.5V to TP21.  As 2.5V batteries are not commonplace, the same result can be effected by using a 9V battery and a 1K resistor.  This will turn off the colour-killer and then the true fault can be viewed on the screen.  Any colour faults listed below assume that this has been done.

(Note: D/S-Dual Standard, S/S-Single Standard)

Dead set with mains fuse blowing:  Check the mains filter capacitor C318(D/S) or C301(S/S) on the line timebase panel.  This can also cause intermittent fuse blowing and also the on/off switch to burn out.

Bad pincushion distortion:  Check RV41(S/S) on the convergence board.  This pot usually falls to pieces if adjustment is attempted.

Picture fades horizontally from one hue to another:  (D/S and S/S) Depending on the colour, check R390, R391, R392. These are 12K 5 Watt resistors on the CDA panel.

Intermittently incorrect colours (50 percent of the time):  (D/S and S/S) This indicates that the PAL bistable switch is not being phase corrected.  Check D20, L27 (Leads become detached), VT18 on the PAL decoder board.

Horizontal colour “Stripes”:  (D/S and S/S) Check the setting of the ident phase bias control (RV10) as per the manual.  Without the manual, tune in a colour signal and slowly adjust RV10 until a locked colour picture appears.  Continue adjusting the control slowly until the picture locks in quickly after the signal input is momentarily removed and then restored.  Don’t forget that the outer of the aerial connection is probably earthed.  I forgot, and nearly electrocuted myself when I touched the live chassis!

Flyback lines visible:  (D/S and S/S) Check D53 and VT28 on the CDA Panel.

No picture and smoke from CDA panel:  (D/S and S/S) Check that L50 in the anode of the PL802 hasn’t become dislodged and the wires broken.  The coil is very delicate and can snap off extremely easily.

Horrible smell, picture otherwise normal*:  (S/S) This can happen if a hole appears in the tripler unit after prolonged arcing.  The only real cure is to replace the tripler.  *Sometimes the tripler will fail with no picture.

Extremely bad hanover blinds with incorrect colours (Yellow jaundice flesh tones):  (D/S and S/S)  This is caused by the PAL bistable not reversing the phase of the R-Y signal on alternate lines.  Check D26, D27, VT22, VT23 on the PAL decoder board.

Line and / or frame hold controls very sensitive:  Check R35 (D/S and S/S) on the I.F. panel.  The resistor is supposed to be 82K, but often wanders up to 100K or higher.  Also check R38 (D/S and S/S) on the I.F. panel.  The original value of this resistor is 390K, but again it can wander up to 500K or higher.

Grey scale highlights change over time:  Change all PCL84 valves on the CDA panel.  (And the PL802 whilst you’re at it.)

Striations at the left hand side of the picture:  Check Line Linearity damping resistor R215 (D/S) or R228 (S/S).  Also check R355 and all electrolytics (D/S and S/S) on the CDA panel.

Frame collapse (Possibly intermittent):  (D/S and S/S) Check the 2 x 20V and the –20V supplies on the frame panel.  If they are missing, follow the wires back to the line-timebase panel and you will usually find a intermittent contact on the plug / socket arrangement or (on the printed circuit version of the line-timebase board) a crack in the print.

Sound, but no picture - PL509 and / or PY500A overheating: First of all, don’t run the set for any longer than it takes to carry out the checks or the PL509 / PY500A will very quickly pop their clogs.  First check for line pulses at pin 8 of the PL509.  They should be roughly 200V from peak to peak and have a slightly uneven mark / space ratio.  If they are missing or seriously deformed, check the line oscillator circuit.  (As a quick test, replace the PCF802 and see if this cures the problem.  If not, the line oscillator will have to be investigated.)  If there are nice healthy pulses, check C223, C225 and C226 (D/S) or C218 and C219 (S/S).  Also check the PL509 and PY500A by substitution.  If none of these cures the fault, the line output section will have to be investigated. 

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