Philips T-Vette

[DoctorWho]

Thanks Steve and Kat for the advice. I am indeed more used to working on valve sets, I'm very familiar with seeing wax condensers and working with HT voltages, I haven't really worked on solid state equipment since leaving college in the late 1980's!

I have measured the heater voltage to the CRT between pin 3 and chassis, and it reads 10.8 volts, so this would seem to indicate that all is well with this set. It certainly performs extremely well on both 405 and 625, suggesting that everything is in good shape.

Thanks again for the advice, if there's anything that you think I have overlooked, or should service as normal procedure for a solid state set of this vintage I would be grateful to know.

All the best,

Peter.

[TNC]

Kat - just wondering how your TV-ette restoration is going? I have acquired one of these intereting sets and whilst the vision side is reasonably OK the sound side is u/s.
I have checked the various sound-related transistor voltages and they are mostly 10-20% too low - do you think that is low enough to suggest component failure or is it more likely the result of cap. leakage. The caps look OK and are mostly Philips yellowish types - these are pretty reliable although of course they are 40 years old! (The rail voltage has been checked and adjusted and is stable at 11.00v)
I was interested in the discussion re the replacement of AF115s...if I go ahead and replace T2006/7/8, I will need to source something suitable...do you have any thought on substitutions? - and where might be sourced?

Thanks, Trevor.

[dazzlevision]

Hello Trevor,

The AF115s in the T-Vette's sound IF stages (T2006 & T2207 - type AF115) are very likely to be u/s and can be replaced by AF125. Don't forget the audio preamp stage (T2008), which (unusually) uses an AF117 (= AF127).

The AF11x range grow tin crystals internally, which cause short-circuits/leakage between the semiconductor junctions and the shield connection.

The AF125 & AF127 should still be readily available from the likes of Cricklewood Electronics or CPC of Preston (a Farnell subsidiary) - to name but two that I use.

The sound output stage tends to fail, damaging the output transistors (OC81 & AC127) and the two 1 Ohm current stabilising resistors (R2033 & R2034).

I have also had trouble with the very small ceramic capacitors fitted inside the IF cans, which can go very leaky. They don't seem to have any protective coating on them - maybe that is why this happens. This can upset transistor biassing, IF gain and IF response curves.

Regards,

Dazzlevision

[TNC]

Hi Dazzlevision - thanks for your very helpful comments.

Tonight I started to dismantle the set and wonder if you have any suggestions as to how best one can improve access without disconnecting much of the wiring! It really is a bit of a bird's-nest!!
I wonder if it would be better to remove the CRT and gain access through that space rather than a massive unsoldering exercise? (I think photographs will also be needed!)

The sound symptom on 405 is complete silence but on 625 there is plenty of useful hiss...but no actual sound. I am therefore wondering whether the o/p trannys may have survived.

By the way you mentioned one o/p transistor as an OC81 but mine has an AC128 and an AC127...so is the OC81 an equivalent for the AC128?

Last point...the manual makes much of the need to use a heat sink when re-fitting transistors...but for the life of me I can't see there is any room to use anything other than a very small pair grips on the lead being soldered...what's you experience of this issue?

Thanks again for your thoughts,

atb Trevor.

[dazzlevision]

Hello Trevor,

I have replied to the points you raised - in CAPITALS, below.

Tonight I started to dismantle the set and wonder if you have any suggestions as to how best one can improve access without disconnecting much of the wiring! It really is a bit of a bird's-nest!! IF I RECALL CORRECTLY, THE LEADS LENGTHS ARE SUFFICIENT TO WORK ON THE MAIN PCB WITHOUT DISCONNECTING ANY WIRING. THERE ARE TWP SCREWS THAT SECURE THE MAIN PCB - AT THE REAR. I THINK I FOUND IT BEST TO HAVE THE SET ON ITS SIDE, IN ORDER TO WORK ON THE MAIN PCB, WHICH SHOULD TILT OUT ENOUGH TO ACCESS MOST COMPONENTS. I'M PRETTY SURE I RAN MINE IN THIS POSITION - WITH SOME INSULATION AROUND THE DY51's ANODE!

I wonder if it would be better to remove the CRT and gain access through that space rather than a massive unsoldering exercise? (I think photographs will also be needed!) I DON'T THINK THIS IS NECESSARY.

The sound symptom on 405 is complete silence but on 625 there is plenty of useful hiss...but no actual sound. I am therefore wondering whether the o/p trannys may have survived - YOU ARE PROBABLY CORRECT, BUT I WOULD CHECK THOSE 1 OHM RESISTORS. ON 625, THE 6MHz FM "INTERCARRIER" SOUND IS TAKEN FROM A SEPARATE DIODE DETECTOR IN THE VIDEO DETECTOR STAGE. ON 405, IT IS TAKEN OFF THE VISION IF STRIP EARLIER ON.

By the way you mentioned one o/p transistor as an OC81 but mine has an AC128 and an AC127...so is the OC81 an equivalent for the AC128? YES, BUT WITH A SLIGHTLY DIFFERENT METAL CASE.

Last point...the manual makes much of the need to use a heat sink when re-fitting transistors...but for the life of me I can't see there is any room to use anything other than a very small pair grips on the lead being soldered...what's you experience of this issue? AS LONG AS YOU ARE QUICK WHEN SOLDERING, THERE SHOULDN'T BE A PROBLEM. I HAVE NEVER DAMAGED A GERMANIUM TRANSISTOR THROUGH SOLDERING.

FROM WHAT YOU SAID ABOVE, YOU SEEM TO HAVE THE CORRECT PHILIPS SERVICE MANUAL....? DOESN'T THIS COVER ACCESS FOR SERVICE? MY MANUAL IS IN THE LOFT SOMEWHERE, SO I CAN'T EASILY REFER TO IT! I DO RECALL THE NEED TO DRESS CERTAIN WIRES CORRECTLY WHEN RE-ASSEMBLING, IN ORDER TO AVOID INFERIOR PERFORMANCE.....

I WOULD CERTAINLY REPLACE THE THREE AF11x TRANSISTORS IN THE SOUND STAGES, BEFORE DELVING IN FURTHER.

GOOD LUCK,

DAZZLEVISION

[TNC]

Hi Dazzlevision,

Thanks so much for excellent info.
Yes I do have the Philips service manual but it doesn't say a great deal about dismantlement.
I think you are right in that I can just about get to the subject transistors by gently pulling the i.f. panel whilst easing some of the wiring. Yes - the manual does mention the specific routing of the power supply and the volume control leads.
I have also found it best to have the machine on its l/h side - LOPT
to the bottom. My reason for checking this with you was that you mentioned problems with the small caps. in some of the i.f. cans and I just couldn't believe you could sort those without completely removing the board...perhaps your fingers ar thinner than mine!

Btw I have some BF450 r/a...I wonder if they might serve?

I will update this thread on progress.

Trevor. (maybe see you at the NVCF?)

[dazzlevision]

Hello Trevor,

Same format as before.....

I have also found it best to have the machine on its l/h side - LOPT
to the bottom. My reason for checking this with you was that you mentioned problems with the small caps. in some of the i.f. cans and I just couldn't believe you could sort those without completely removing the board...perhaps your fingers ar thinner than mine! NO, I HAVE BIG HANDS - SO YOU CAN DO IT!

Btw, I have some BF450 r/a...I wonder if they might serve? I SAW ON A WEBSITE ONLY YESTERDAY THAT THIS SILICON TYPE CAN, IN SOME APPLICATIONS REPLACE GERMANIUM AF1xx TYPES, BUT I WOULD SOURCE AF125 & AF127 TYPES IF I WERE DOING IT. THE SOUND IF STAGES OPERATE AT 6 & 38.15MHz AND DEVICE CAPACITANCES CAN HAVE AN EFFECT ON TUNED CIRCUITS, STAGE GAIN & STABILITY, ETC. IN ADDITION, THE BF450 WILL HAVE BASE-EMITTER VOLTAGE OF TYPICALLY 0.7V, WHILST GERMANIUM TYPES IS 0.3V, SO THE BIASSING MAY WELL BE UPSET. YOU ARE VERY UNLIKELY TO HAVE THESE PROBLEMS WITH THE AF125 & AF127 AS REPLACEMENTS FOR THE ORIGINAL TYPES FITTED.

Trevor. (maybe see you at the NVCF?) SADLY NOT, AS I HAVE OTHER FAMILY COMMITMENTS.

BEST WISHES,

DAZZLEVISION

P.S. I HAVE SENT YOU A P.M.

[TNC]

I must say, as someone who is more familiar with valve technology, the world of transistors seems much more difficult - esp. wrt diagnosis.
The TV-ette is also suffering with a power fault. After approx. 5 mins. running, the HT1 volts fall away - first to around 9.5v and thereafter to about 3-4v. The entire process takes about 30 seconds.
I have checked voltages in to T1513 from the rectifier - around 12.5v, and out from T1513 via R4049 to its connection with T4012 where it is 14.75v (on UHF setting) - about .25v too low.
When the voltage starts to fall, the input to T1513 varies between 13v and 12v - but settles to a fluctuating 13v (+/- .5v)(by that time the screen is completely dead) and the voltage going into T4012 similarly varies but again settles at about 13v.
I have looked at the input voltage to the zenner X4006 and that is very steady at 1.77 until the problem begins - it then falls away to .5v.
I have noticed that the problem is initiated ("announced") with a very slight very high-pitched oscillation - something like linebase but hardly audible...but clearly something is suffering!
Turning off for 2/3 minutes seems to reset the circuit but its probably quicker to the collapse - say 2/3 minutes...so heat may be an issue? Is it safe to cold-spray some components in that area?
My logic (probably flawed) tells me that if the voltages around T1513 and T4012 stay above 11ish then presumably the problem lies further in-perhaps with T4014 or X4006. It seems difficult to pin it down exactly and I so I am daunted at the thought of changing a stack of components, all but one of which will be fine!!
Grateful for any thoughts on how I might diagnose more exactly?

Trevor.

[Steve_P]

Right, well first of all, I would clean S1 - All pins. Both fuses and holders. Soldering - especially F2 to Deck. Then, try it on 12 Volts and see what it does. If this is OK then try it again on mains.

If it's OK on 12 Volts, but iffy on Mains, go for the rectifier stack.

Note the different voltages on VHF and UHF.

Cheers,

Steve P

[renato_enca]

I have noticed that the problem is initiated ("announced") with a very slight very high-pitched oscillation - something like linebase but hardly audible...but clearly something is suffering!

That seems the power supply is being overloaded, or some thermal runaway on the switching transistors. I don't know the set, but the sympthoms you describe make me think that may be the problem. Today's power supplies have integral overload protection which shuts down the outputs if an overload is detected. As that set is somewhat old, it may not have in-circuit overload protection, as that role is played by fuses. If you can, try to disconnect (unsolder the pin) the power supply's DC "HT1" output, and place a 470R load resistor on the same output (SMPU's should NEVER be operated without a load). Measure the "HT1" voltage and tell if it has dropped after some time. The idea is to isolate the rest of the circuit so all the power is given to the resistor. If the voltage has dropped, the fault is in the supply. If no, the fault is outside the supply, so I would see the line, frame and sound output stages - there may be a tired transistor loading up too much the supply after warming up: thermal runaway.

[TNC]

Thanks all for your helpful feedback.
I checked the switching and made sure of the power connections and then powered up with a slightly dirty 12v (car charger) supply. Apart from the effect of the supply - hum and screen distortion - the system held up for over 20 minutes and there seemed to be no obvious stress so it looks as if the rectifier is suspect.
I am assuming the original rectifers are not now available so I will try an integrated bridge to check that it provides the necessary quality. Am I right to assume that operating at around 12-13v. will not affect the choice of component - other than its power rating?
On another front, having spent an "interesting" couple of hours replacing three of the sound stage transistors (not the output pair), I was rewarded with plenty of volume with no distortion. (Thanks vm Dazzlevision). Threading the 4 wires into their correct holes was just about the most fiddly job I have come across!! My vocabulary was sorely tested!!
The final issue is a noisy picture but I think that is all in the VHF tuner. As it acts as an amp. for the UHF system, I think poor performance on VHF will also affect UHF - and wobbling the plug-in trannys on the VHF tuner created absolute mayhem on the screen so its there I will have to look.

Trevor.

[dazzlevision]

Hello Trevor,

Glad to hear you are making some real progress!

The original STC selenium rectifier is probably NLA and any that you may find may well be u/s!

You can use either a single silicon bridge rectifier, rated at 50V and 3Amps (minimum) and probably bolted to the chassis, to act as a heat sink. Alternatively, four individual silicon diodes, such as the 1N5402 (50V, 3A) should suffice.

Regards,

Dazzlevision

[TNC]

Back again with a progress report!
Now with the sound stage working correctly, I have spent sometime replacing the STC rectifiers with a silicon bridge unit - bolted to the chassis, it gets mildly warm but seems very happy, consumption now just over 1.25 amps and the voltage nicely stabilised at ~10.95v - 11.05v. FTB runs happily and there is plenty of contrast/brightness.

The remaining problem is fairly severe picture instability on both VHF and UHF. At first I thought the VHF stage was responsible as tapping the plug-in transistors certainly causes interference but looking at it closely, the problem is not interference/weak signal (snow/shadowing/etc)...but could almost be too strong a signal/noise on vision effect.
Anyway, since the UHF tuner feeds via the VHF tuner I bridged out the latter and took the signal direct from the UHF tuner to the pcb...and the problem is still there so I think that eliminates both tuners...so I am wondering how I can narrow down the remaining field of possibilities.
Dazzlevision has suggested problems with the small caps in the IF cans...but where to start?
Grateful for further thoughts!

Trevor.

[dazzlevision]

Hello Trevor,

Try attenuating the RF going into the aerial sockets (VHF & UHF), until the vision AGC action stops. If the AGC isn't working correctly, you should get a stable but snowy picture when the RF is very low but the symtoms of overloading when you go up again, beyond the AGC intervention threshold. If you attenuate the RF too much, the picture will disappear into the noise.

These sets are quite sensitive - when working correctly. So, with no RF signal, the screen should be pretty full of noise/snow on UHF and, to a lesser extent, on VHF.

If that doesn't work, have you tried adjusting the AGC preset, R2080 and the video bias preset, R2076? Procedures are in the Philips service manual.

I'd get these checks done before you look for leaky ceramics in the IF stages.

Regards,

Dazzlevision

[TNC]

Thanks Dazzlevision...helpful advice which seems to be moving the repair in the right direction.
My attenuators total 24db but amazingly that wasn't sufficient to kill the AGC - or even make the picture visibly snowy so I turned to R2080 and found that approx. an 1/8th turn virtually stabilises the picture - but at the expense of a much lower contrast level. Watchable in low light!
The manual states that video bias should be 11v but I found the picture (and sound) began to disappear at around 10.3v and in any case that is near the end of the adjustment range. In the end I left it at 10v.
Time to check voltages around the AGC and particularly on T2018 and I find that its b and e are 9v and 9.2v respectively - near the spec. of 10v - but that its collector is at 4.8v - so I think that points to the need for its replacement. (The resistor values around that area seem about right - but the collector leads back to the tuners - could a fault in one or other of those caps cause that low voltage?)
Hopefully that's progress...and perhaps the IF can caps are OK?? I'll report back when I have sourced an OC75 (is there an easier to obtain equivalent?)
Thanks,

Trevor.

[dazzlevision]

Hello Trevor,

Since the video bias preset may affect the AGC preset adjustment (they are both in the T2019 video pre-amp stage), try setting the video bias as per the manual (first) and then adjust the AGC preset.

Don't forget to turn up the "user" contrast control to 3/4 of max when adjusting the AGC preset. If this produces a good picture, you can then advance the user contrast control further. The "user" contrast control shouldn't affect the syncs or AGC.

My transistor equivalents book suggests an AC125 as a replacement for the OC75. (Can't help you with an OC75, I'm afraid).

Have you checked HT4 (86V)? This is used in the AGC and video output stages. If it is low, check the reservoir capacitor, C2114, 20uF.

Do you have a 'scope? This may be the quickest way to solve the video distortion problems. Follow the video from the vision detectors (Can "A") through to the video output stage and look for crushing of the syncs and/or peak white. A staircase pattern from a pattern generator is best for doing this. Failing that, a staionary picture/pattern.

Regards,

Dazzlevision

[TNC]

Back again - but this time not so much progress!

Taking your advice Dazzlevision, I readjusted the video bias as per the manual - however, I could only wind it up to 10.3v. - after that the vision and sound just fade out so its just on the limit at 10.3v. Adjusting the AGC after that was little different from before- lowering it brings a fair degree of stability but at the expense of a lot of contrast.

At this point I replaced the OC75 with an AC126 (looks to be a fair equivalent) - this resulted in very little difference to the AGC action and no change to the instability.
I also checked HT4 and that is steady at 83.3v.

After this I tried a few experiments and voltage measurements. The first interesting reading was the voltage on the collector of AGC T2108 which I had previously noted was low - with the new component it was still only 5.3v (spec. 10v) - but that was with a signal...remove the signal input and it rises to 9.3v.
I then removed the tuner feed to the IF panel - that results in clean blank raster- no instability whatever but contrast control has no effect. Partially re-connecting the plug produces a low-contrast picture with no instability...replacing firmly/fully and the picture contrast returns with the instability.
I have a Gould 30mhz scope - on the tuner output the signal appears and is unstable in time with the CRT display - however I feel this may be coming up from the IF board rather than from either of the tuners - although I am not sure I can really justify that thought.
Placing the scope further into the circuit around T2017 (Aurora/Test Card C) gives a pretty clear view of the waveform - no crushing but the instability takes the form of narrow blocks of the waveform appearing to jump up and then back down again randomly/rapidly in time with the CRT instability - I am assuming that's caused by a variation in signal voltage. These results have made me re-assess the instability with I now think must be a random fluctuation in the contrast level. Should I be looking at any particular set of components - I am assuming a leaky cap. must be the culprit?
I have resistance checked C2114 - it reads 26.1k/27.5k depending on the polarity of the test leads - but without removing one leg I am not sure I should draw any firm conclusion from that. Could a faulty cap. affect the signal along its entire path from tuner to crt?
I wonder how I can narrow down the faulty component without having to unsolder many pcb components - which is something I try to avoid. Any thoughts on this or the process so far would be much appreciated.

Thanks again,
Trevor.

[dazzlevision]

Hello Trevor,

I find the symptoms you describe very difficult to visualise and therefore suggest a likely cause! Are the symptoms the same on 405 and 625?

One supply I overlooked is HT3 (22V – derived from the LOPT), which feeds the T2019 stage (also T2017 and T2008). It is smoothed by C2062 (10uF).

I’d use your scope to check for ripple on HT3 & HT4 (should be no more than 1% (pk-pk) of the dc supply voltage.

The fact that the video detector is dc coupled (on 405) all the way through to the AGC transistor (T2018) makes pin-pointing a particular component difficult. The fact you cannot set the video bias pot for 11V is suspicious and may be due to a change in the dc conditions here.

However, it still seems to me the vision AGC is suspect. How about substituting a battery and pot for the AGC source? I suggest a 5k pot across a battery supply of 9V; connecting one end of the pot to chassis and the other to the AGC line at the (probable) wire that links T2018 collector to the bases of the RF stages in the VHF & UHF tuners. Disconnect the AGC feed by removing this wire at the T2018 end.

If you can then control the vision IF gain and therefore contrast, it would seem the AGC section is suspect. Keep the scope on the emitter of T2019 to see if the video signal is distorted.

Alternatively, those tiny ceramic capacitors in the IF cans, might be causing an unwanted voltage to couple into the AGC rail or the video emitter follower (T2019) base bias. If you desolder one in the IF can (desolder braid is best), you can check for dc leakage with an Ohmeter (analogue is best for this, I suggest) - C2041, C2059. I would also remove the vision detector can “A” and test C2063, C2072, C2079, C2070, C2071 & C2072. Whilst the can is out, test the forward and reverse resistance of the two vision detector diodes.

It might also be worth disconnecting one end of R2044, R2070, R2071 and R2075, to see if they have gone high. I’d also test by substitution C2074 & C2081 in the AGC amp stage.

Are there any relevant fault symptoms in that Philips workshops list earlier on in this thread?

I fear you are going to have to disconnect and test quite a few componets, as it is quite likely more than one is faulty.

Finally, are the braids on the coax leads connected securely? The two way "Pressac" connector on the IF coax should have its two female contacts tightened up with pliers.

Dazzlevision

[Studio263]

I've probably come to this a little late in the day but my one of these had similar problems.
In my case the HT faded away becasue the line scan coupling capacitor was leaky, it's one of those yellow "block" ones and is hidden right down at the front behind the LOPT. The one in mine had started to melt, I should have looked earlier (but when you see where it is maybe you will understand).
I'd also suspect the AD149 series regulator and the sound output transistors (especially the NPN one of course) if the HT falls with use. The sound stage is fragile, apparently the positioning of the wires to the loudspeaker is critical but I can't recall the details (it's in R+TV S, 1968 > 69 I think, the first one with colour sets in anyway).
The correct HT (and hence tube heater) voltage for all these sets is 10.8V, this allows it to run from a car battery with enough left over for the regulator to chew on.
I can't help liking this model but technically they are rubbish compared to the Sonys. Mind you, can you imagine the hash BRC would have made of it if they'd tried something similar?

[TNC]

Have finally managed to spend sometime on diagnosis...and I think the results clear the AGC circuit.

As suggested, I substituted a 9v battery + pot (actually a 50k seemed to be work best) in the AGC feed to the tuners.

On VHF - without AGC, just a trace of a signal displayed - obviously floating around - no sound, no vertical locking.
With 5+ volts of agc, as before with good contrast and sound and continual, severe instability - the scope reflecting this in varying signal strength - but
no detectable distortion of the video.
Below 5v, obviously contrast and audio fading away but much greater stability - not perfect but almost watchable.

On UHF a somewhat different reaction. With the AGC line disconnected from c of T2018,
the picture/vision still appeared quite strong - but considerable instability.
On connecting battery power (5v), contrast and sound up further but video signal much distorted and screen display unintelligible.
Disconnected battery but coupled agc line to chassis via .002mf and 100k in parallel and vision/sound restored but still quite unstable - as reflecetd on the scope.
I also took the UHF feed direct (rather than via the VHF tuner) to the pcb and got broadly similar results.

I think all of this confirms that the problem lies between the tuners and the final vision output but with the agc in the clear...the instability worsens as the signal strength rises which looks like leaking of some sort - so I will now turn my attention to Can A!
Trevor