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Old 2nd Sep 2019, 8:45 pm   #1
dryad1313
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Default A640 chassis help needed.

I have a Bush TV166u set which, having been stored for 44 years, I managed to get up and running until the pitch-coated damp-ridden LOPT failed (see my original thread from 5 years ago here: https://vintage-radio.net/forum/showthread.php?t=106013). Anyway, fast-forward five years and I finally managed to locate a NOS LOPT (non-pitch). For good measure I re-reformed the big electrolytics and they are fine, then fitted the new LOPT but now there's no picture at all. Worried that the new LOPT was faulty I refitted the original failing one, but still exactly the same. There's line whistle present in both cases but nothing else. I've spent many hours going through almost everything, replacing components which were possible suspects or out of spec but to no avail. I've measured various voltages and found that voltages were well down in the vision section, but ok in the sound section. Here's a rundown of measured voltages:

HT1: 179v (should be 220v)
HT2: 164v (should be 195v)
HT3: 231v (should be 230v)
HT4: 173v (should be 205v)

PL504: g1 -27v g2 151v
PFL200: a1 145v (should be 150v), a2 30v (should be 100v)
PY88: a1 180v (should be 220v)
CRT (Mullard A59-11W): cathode 139v (should be 170v), g1 108v (should be 150v), first anode 106v (should be 600v)

Although I have many years' experience repairing and servicing radios and amps, I'm not very experienced at TV repair so please excuse my ignorance. Any guidance as to what to do gratefully received!
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Old 2nd Sep 2019, 8:58 pm   #2
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Default Re: A640 chassis help needed.

Sort out why that first anode on the crt is so low, the other voltages may be near normal after fixing that problem.
3r26 and 3c51, R&TV service book numbers are first to check.
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Old 2nd Sep 2019, 8:59 pm   #3
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Default Re: A640 chassis help needed.

The low 1st anode voltage could be caused by a blue /white .1uF or similar cap.
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Old 2nd Sep 2019, 9:03 pm   #4
dryad1313
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Default Re: A640 chassis help needed.

Thanks for the replies. I've already replaced 3c51 but will check 3r26 shortly. I've also replaced all of the blue/white caps but left the Mullard mustards as they rarely fail.
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Old 2nd Sep 2019, 9:46 pm   #5
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Default Re: A640 chassis help needed.

Just replaced 3r26 but no difference to the above stated voltage readings. It had gone high to 234k (should be 180k) but obviously value is not particularly crucial.
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Old 2nd Sep 2019, 9:51 pm   #6
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Default Re: A640 chassis help needed.

Have checked the boost voltage? When you say you have replaced 3c51, is that recently or 5 years ago?
If 5 years ago worth testing it and also the boost capacitor 3c18.
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Old 2nd Sep 2019, 9:52 pm   #7
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Default Re: A640 chassis help needed.

Is the line stage running? can you draw an arc from the DY802 top cap? (well insulated screw driver). Even so that you can hear the line whisle i think you need to see if any EHT is being produced.
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Old 2nd Sep 2019, 10:00 pm   #8
dryad1313
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Default Re: A640 chassis help needed.

I think so. I haven't tried the screwdriver test yet but noticed that EHt must be present with a glow like this:
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Old 2nd Sep 2019, 10:01 pm   #9
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Default Re: A640 chassis help needed.

Frank - it was 5 years ago. I'll check them.
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Old 2nd Sep 2019, 10:04 pm   #10
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Default Re: A640 chassis help needed.

That glow doesn’t seem right although it could be just how the camera sees. Lift the top cap and see if the boost/A1 voltage rises
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Old 2nd Sep 2019, 11:12 pm   #11
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Default Re: A640 chassis help needed.

Hi!

I don't have handy access to the circuit without rooting thro' all the American Radio History's "Practical Television" archives, but I can say for definite that this is a heavy load on the line output stage, or the line oecillator drive has gone awry, as a result it's producing virtually no boost voltage/e.h.t and loading the h.t. down with it!

The first thing to deal with is the line oscillator stage, which is a cathode–coupled multivibrator using the triode of a PCF80 and half an ECC82 – make sure the anode load resistors (I think one is 10k and the other 33k) haven't wandered up in value, and the PCF80/ECC82 are both OK.

If the line oscillator is ok, check all the high – value resistors in the grid– circuit of the line output valve from the grid to the width/set e.h.t. preset – one going o/c will overload the l.o.p. valve!

If all ok, check the small high–voltage disc ceramic capacitors dotted around the primary and secondary windings of the LOPT for leaks/shorts, if nothing found then you're looking at a duff new LOPT – unfortunately the non–pitch type with plastic bobbins, etc., were not as robustly insulated between windings/turns of each windings as the original pitch type were, and failures of these were very common – when I did the later s.s. A774s, (same transformer in a simplified 625–line only circuit) I frequently had to send virtually brand–new ones back for early replacement countless times!

Chris Williams
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Old 2nd Sep 2019, 11:33 pm   #12
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Default Re: A640 chassis help needed.

PS!

You shouldn't have corona like that in the base of the DY802 e.h.t. rectifier either – if you've tried another valve and get the same symptoms with a similar glow, the heater winding insulation on the LOPT could be defective – if you have another "parts only" TV, you can try making a new heater winding out of it's e.h.t. cable – one full turn is normally enough for a DY802 – which has a 1.4V 550mA heater, with (usually) a small piece of resistance–wire looped around it's holder in series with the heater to drop any surplus voltage.

It's not easy to measure the heater–voltage directly and it's NOT safe to attempt it in a working set producing any e.h.t. as you'll simply flashover the meter's insulation – however if you copy the exact number of turns round the core in same place as the original heater winding you'll not be too far out – it's rare for valve e.h.t. rectifiers to be seriously overdriven in most sets – only the Ferguson 306T in a limited serial no. range had this fault & Mullard produced a special over–rated valve called the "TY86F" with a 7.4V 77mA heater for the sets affected!

Anyway please do check your DY802 and it's heater–circuit – you don't normally get a load of haze/corona like that in one – that's why they have an internal anti–corona shield fitted to the electrode assembly!

Chris Williams

PPS!

You could probably get away with using a Mazda U26 at a pinch – this has a 2V 350mA heater and the lower heater current would compensate for the lower nominal voltage – I can send you a U26 for postage–cost only if's any use, but regrettably my LOPTs got dumped twenty years ago!
.
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Last edited by Chris55000; 2nd Sep 2019 at 11:55 pm.
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Old 3rd Sep 2019, 2:13 am   #13
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Default Re: A640 chassis help needed.

purple glow on eht rectifier means air inside of the tube...change asap it and you are good to go. dont run it like that because you can stress the flyback... if you have a selenium stick high voltage diode from say a 70's tv you can use it to test, just remove the tube and stick the diode in...done it many times while waiting for eht rectifier tubes on order...if it doesnt fit tight it doesnt matter as long as you keep it away from anything that it can arc into... preety much any eht rectifier tube can be made to fit atleast temporarily in there as long as you have the appropriate socket and the appropriate number of turns on the heater coil on the lopt...

dc voltage on g1 of the picture tube varies as you adjust brightness ,dc voltage at the cathode usually comes from the plate of the video amp and may vary as you adjust contrast...so as long as there are voltages present and within ballpark you are good to go, doesnt have to be the voltage specified on the schematic. the difference between those 2 voltages is what creates the bias for the crt... hope i made sense, if not let me know and i can explain better. but all voltages seem fine to me.
the first anode doesnt really matter, it is used for focus i believe, just ignore it for now.

change the eht rectifier tube and you will get a raster.

you then will need to apply a signal and tweak the horiz frequency (most sets have a coil for that) in order for the picture to lock, because you changed so many components......, after that adjust the width of the picture (with signal applied) so it barely fills the screen and then check the boost voltage, if it is within spec or a bit lower then everything is fine with your horizontal stage.
If not, then check components and or replace the horizontal output tube and or damper tube because lets say if the horizontal output tube is weak, you have to turn the width way up, stressing the flyback and creating a higher than normal boost voltage... boost voltage and picture width are good simple indicators of horizontal stage health...dont forget that without a signal the raster will always be too wide, so do the testing with a signal applied


be aware that this is generic advice, i did not look up the schematic for your particular set, but this applies to most vacuum tube sets

if you want to measure your eht you can draw a spark with an insulated screwdriver and eyeball the distance, the longer distance you can hold an arc will tell you the "exact" value of your eht....use any plastic handle screwdriver to the cap of the high voltage rectifier, with the cap off. hold the tool by the tip of the handle, make sure metal doesnt protrude into the handle like on a chisel for example....you will not die dont worry. 0,5cm of arc at the very least will give you enough juice to have light on the screen but in normal conditions 15kv will allow you to draw a 1cm arc. dont ground the screwdriver or anything, just hold it and approach slowly the point you want to check, as soon as it starts to arc just start go get back slowly till it stops. simple

regards
hugo

Last edited by PortugalTV; 3rd Sep 2019 at 2:36 am.
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Old 3rd Sep 2019, 2:50 am   #14
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Default Re: A640 chassis help needed.

Quote:
Originally Posted by PortugalTV View Post
purple glow on eht rectifier means air inside of the tube

change the eht rectifier tube and you will get a raster.


hugo
Exactly what I was thinking. With the gas heavily ionized in the tube it;

1) conducts in both directions and..

2) has a very low resistance. Effectively, from the AC perspective, shorting the capacitance of the CRT bulb across the EHT winding and heavily loading the transformer.

(another) Hugo.
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Old 3rd Sep 2019, 10:25 am   #15
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Default Re: A640 chassis help needed.

As others have said that DY802 looks like it has air in it and is probably the reason why the Line Output Stage is being loaded.
A nice simple test before you order a replacement. Just lift the Top Cap off the DY802 then run the set and check the HT voltage to the HT stage, and the Boost Voltage. If they all rise then a new DY802 should be all you need to clear the fault.
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Old 3rd Sep 2019, 12:19 pm   #16
dryad1313
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Default Re: A640 chassis help needed.

Thanks for the replies. Now I think about it I did see a small amount of flashover inside the DY802 when I first switched it on. Also, I can't see any orange glow from the heater, and it shows up as a dead short when trying to measure continuity. Could the flashover have caused the heater filment insulation to fail? Anyway, I've just ordered a replacement and will follow up with the results as soon as it arrives.
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Old 3rd Sep 2019, 1:06 pm   #17
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Default Re: A640 chassis help needed.

Quote:
Originally Posted by Argus25 View Post
Quote:
Originally Posted by PortugalTV View Post
purple glow on eht rectifier means air inside of the tube

change the eht rectifier tube and you will get a raster.


hugo
Exactly what I was thinking. With the gas heavily ionized in the tube it;

1) conducts in both directions and..

2) has a very low resistance. Effectively, from the AC perspective, shorting the capacitance of the CRT bulb across the EHT winding and heavily loading the transformer.

(another) Hugo.

My thoughts also, I had exactly this on my Sony KV1320, swapped out the 3AT2 rectifier with a new one and normal service resumed.

Cheers
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Old 3rd Sep 2019, 2:32 pm   #18
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Default Re: A640 chassis help needed.

You started off in post #1 by telling us that all the HT rails are ~40V low. It is not surprising that all the other voltages which have been quoted are low also.

Notwithstanding the DY802 diversion, has any attempt been made to prove that the line output stage is responsible for pulling the HT rails down? I haven't noted one.

Given the current needed to pull the HT down that low, I would have expected further evidence, such as a red hot output valve anode to have been noted.

Are the smoothing caps running cool? Have you tried shunting them with a suitable high value electrolytic in case the capacity is low? Has someone replaced the 14 ohm section of 3R57 with a higher value?

I'm going away for a few days, so won't be able to catch up with this until next Tuesday. I will be interested to see what transpires in the interim!
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Old 3rd Sep 2019, 2:50 pm   #19
dryad1313
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Default Re: A640 chassis help needed.

No red hot anodes anywhere. The smoothing caps are running as cool on as they are off, and if the heater chain is broken then the HT takes a good fifteen minutes after switching off to reduce to a safe voltage. 3R57 (part of the mains dropper) has been replaced by me with an RS NOS one of the correct type, and has not drifted from their specified values. I am now quite convinced it's the DY802. The only reason I didn't before is because I had replaced it with a NOS one last time I worked on the set.
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Old 3rd Sep 2019, 7:51 pm   #20
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Default Re: A640 chassis help needed.

Either the DY802 has an internal short, gassy or there is a short to chassis on the heater winding [unlikely]. Definitely change the DY802. Removing the top cap anode of the faulty DY802 should spring everything to life. As mentioned a U26 can be used as a test replacement or a DY86/7 will work fine.

Best dual standard chassis or certainly one of the few with the best performance. J.
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