Pye Hybrid Colour Sets
By Mike Bennett
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.
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!
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.”
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
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.
D/S-Dual Standard, S/S-Single Standard)
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.
Check RV41(S/S) on the convergence board.
This pot usually falls to pieces if adjustment is attempted.
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.
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.
(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!
(D/S and S/S) Check D53 and VT28 on the CDA Panel.
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.
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.
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.
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
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.
scale highlights change over time:
Change all PCL84 valves on the CDA panel. (And the PL802 whilst you’re at it.)
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.
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.
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
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