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Old 18th May 2020, 9:12 pm   #61
staticmind
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Default Re: B&O Capri 606 TV (17") restoration

PCL84 measurements - forget my previous post about the close to 0V measurement at pin 3:
Pin 1: 146.3V
Pin 2: 4.16V
Pin 3: 5.72V
Pin 4: 109V
Pin 7: 5.23V
Pin 8: 0.06V
Pin 9: 0.0V.

The Pin 2 (g1) is approximately the same voltage as on the wiper of the contrast pot.

A good thing is that I have a very dark raster. As in very dark with just the slightest amount of snow visible. Brightness control has been set midway for all measurements. Not sure how it should be set as it isn't mentioned anywhere.

Component 63 measures 1.6 ohm, so it is fine presumeably. The RL41 diode in the RF can 971H97 measures about 0.3V in the correct direction... about 1.1V in the other direction. Not sure if that is correct - the meter is a fluke 73, by the way.
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Old 18th May 2020, 10:38 pm   #62
PortugalTV
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Default Re: B&O Capri 606 TV (17") restoration

the paralelled resistor was probably to increase brightness you are right..
if the grid of the tube is at 0v, and 15v is specified at the schematic than i would expect that tube to not conduct due to a lack of bias, that would explain all the symptoms, focus yourself on the dc path from b+ to grid of the tube, i dont see how the tuner can affect that but i will look again at the schematic. only thing possible would be some problem in the agc, IF the AGC acts on the video amp (on some sets it does) i have to look again at the schematic

the cathode of the pcl84 measuring low means either cathode resistor gone high or the tube not conducting enough, most likely second option due to lack of grid bias

explain better to me what you mean by measuring the otput of the tuner, were did you measure and how?

you say this:
The Pin 2 (g1) is approximately the same voltage as on the wiper of the contrast pot.
so the prime suspect is the 1.8m resistor before the pot, check that ASAP


the didoe rl41 that you mention is the video detector, probably a germanium, should read 300mv forward like you said but dont bother with it because from the wiper of the contrast pot to g1 of video amp there is a dc path that doesnt have to go through the diode so BIAS voltage should be there regardless of the diode condition. that dc path is 65 - 66 - 67 and then to the contrast pot

67 should also be checked and the 1n decoulpling cap (68) checked for leakage/short or cutted off temporarily

if all above fails try disconnecting component 69
do all of this and it has to work

Last edited by PortugalTV; 18th May 2020 at 10:58 pm.
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Old 19th May 2020, 8:06 pm   #63
staticmind
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Default Re: B&O Capri 606 TV (17") restoration

Hi again,
It was a mistake that g1 was 0V, it is as stated in my previous post 4.16V. Didn't get to edit the post, when it was possible. I might have swapped the conventional way of counting pins, so the numbers might be in reverse order. Yes, it is supposed to be 15.5V, so I agree the problem must be lack of bias.

As I see it, the AGC involves the video stage, namely it is taken from the plate of the triode section of PCL84. I'm not sure exactly how it works, as it seems to also connect to the LOPTx after the 0.22uF capacitor. As mentioned the LOPTx section has been somewhat modified to accommodate a different (newer) one.
The AGC is then fed back to the tuner and the image IF stage.

67 measures 2.91 kOhm, and the 68 measures 1n. So they seem fine. I measured 37 and 36 too, they are spot on.

Going along those lines, I will measure 47, 48 and 50 Thursday - and the 150pF in the horizontal deflection going to the AGC. I'll also check the 1MOhm resistors before the 2MOhm AGC pot.

The tuner output I measured on the connection going to the Image IF stage, RF circuit 971H96. Multimeter on resistance mode, one probe on the chassis of the tuner, another probe on the (center) conductor/connection to the Image IF stage. This measures 0 ohms, but I am not sure if it is supposed to be that, but as I see it the 820pF capacitor between the two coils should block any DC path to ground. Do you agree?
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Old 19th May 2020, 10:19 pm   #64
PortugalTV
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Default Re: B&O Capri 606 TV (17") restoration

Hi
tube pins are always counted clockwise viewed from the bottom and anti clockwise from the top. counting starts at the place were there is a gap between pins,on some bases there is a notch etc, this is valid for crts also and any pin base you can imagine
i have some tube socket test adapters i made to test tubes from the upper side with the tv turned on,they are very helpfull you could make or buy one, i can post a picture if you want to make one

the agc "comes" from the video amp, simplifying it , it takes black level and the more black the pictue has the more negative it goes, and as you know putting negative voltage on g1 of tubes lowers the gain. they inject that negative voltage on the IF and tuner, sometimes on the video amp. then you have a reservoir cap that provides a "time buffer"so the agc does not jump around fast on different scenes in a movie for example. on your set it does not seem to act on the video amp itself but i might be wrong. you can always disconnect the agc for maximum gain on any set but it often overloads and you loose sync etc

on some sets the reason the agc circuit is tied to the flyback is to obtain some ac, then its chopped to get a negative voltage, that voltage is then regulated by a tube depending on black level and there is your agc
i have a set were they get that negative voltage from the ac on the filament string at a certain point, every maker and model has a different setup


about the tuner, if you have zero ohms from that point to chassis DEFINETELY you have a short there. maybe a coil is shorted against a the tuner chassis or inside 971h96 metal can maybe? maybe
also shielded cables often short on old tv's, had that problem many times but never in the IF/tuner but always on the horizontal stage because the high pulses heat up the wire and it gets brittle, anyway the short has to be checked and cleared

with that short you will definetely loose any signal but you should be able to inject a signal at the video amp and get it dispplayed.i would try removing the detector diode and inject a signal at the grid of the tube via a coupling capacitor. use composite video form a dvd.

maybe the short creates a black picture and the agc blocks the gain of the video amp? have to study better the schematic, i only glanced at it. with that short i would expect a blank raster but that depends how they designed the circuitry. it would be great if other people also gave their insight, they might more skilled than i.

if your tuner uses a rotating drum they dont be afraid its very easy to take apart and service. just dont move any components around or you will mess up the alignment just by changing the component position. the shortshould be easy to find. often there are some ceramic tube insulators and a wire is run on the inside between different sections on the tuner. its not uncommon for the ceramic to break and thewire to short to gnd when wiggling the tubes around too hard

Last edited by PortugalTV; 19th May 2020 at 10:46 pm.
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Old 19th May 2020, 10:50 pm   #65
PortugalTV
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Default Re: B&O Capri 606 TV (17") restoration

after clearing the short try disconnecting capacitor 69. its the only way i can see for the agc to act on the contrast/video amp gain

i was writing as i was thinking so ignore any repeated things etc

Last edited by PortugalTV; 19th May 2020 at 10:56 pm.
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Old 20th May 2020, 10:30 pm   #66
staticmind
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Default Re: B&O Capri 606 TV (17") restoration

As always, thank you very much for the insight on this. The AGC functionality makes sense.

I stumbled upon this site with an unbelievable amount of information Philips tuners:
https://www.maximus-randd.com/tv-tuner-history-pt1.html

My TV has an Philips AT7432 tuner. The schematic of it is also shown on the site as: https://www.maximus-randd.com/upload...uit_1_orig.jpg

Here it shows that there's a coil at the IF output going to ground. But in the TV's schematics I don't see that the coil is connected to ground - it is in series with the output, or am I interpreting this wrong? If there's a coil going to ground, it seems correct that I'm measuring 0 ohms, provided the coil has few turns.

The tuner is a rotating drum type with coils etched on multiple (10) PCBs. It was easy to take apart and reassemble.
The only thing I changed in the tuner was a few resistors that measured more than 15% out of tolerance, and then one crusty 820p cap. I still have the old components, in case it messes with the alignment. There are a couple of screws on the tuner chassis with loctite or paint. I know they are for alignment, so didn't touch anything here at all. The replacement resistors follow the same path, completely.

You're also right about the wire going between tuner sections. Thinnest layer of enameled wire I've ever seen, will investigate if there are any shorts here as well. I'll also get to check the shielded cable and 971H96.
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Old 20th May 2020, 11:04 pm   #67
staticmind
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Default Re: B&O Capri 606 TV (17") restoration

Correction: I have an AT7632 tuner.
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Old 21st May 2020, 12:33 am   #68
PortugalTV
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Default Re: B&O Capri 606 TV (17") restoration

i am printing that website about tuners as bedside reading

definetely keep the old cap! a tv with tuner or IF out of alignment is useless. on your schematic there is no dc path to ground,could be a different revision of the tuner or a misprint, investigate that and let us know.

good luck
Hugo
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Old 21st May 2020, 9:15 pm   #69
staticmind
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Default Re: B&O Capri 606 TV (17") restoration

Thanks Hugo. If you have trouble falling asleep, I'm sure that it will help reading

My tuner matches that of the website, not the B&O schematic.
There is no damage or shorts to be identified inside the tuner. I did not change back the few resistors I replaced yet, but will remember to do that, for now that is not the main issue.

Trying my (borrowed) Fluke 175 instead, I measure 0.5 ohms to ground at the IF output of the tuner.

I measured the components that I mentioned in my previous post and found no problems - my Fluke 175 does not measure caps below 1n, so for the 150p and similar small caps I just measured it wasn't shorted.
Everything is fine inside the 971H96 tin as well.

Before experimenting with disconnecting the AGC impact on the video section as you suggested, my mind is still bugged by the fact that the b+ voltage is as low as it is. So I'm thinking I have damaged the selenium recitifier by for a split second having overloaded/shorted the output by tampering with the tubes as I did. So my next plan is to use a high current rating diode and a 20W dropper/power resistor (15 ohm should do), and ensure that b+ is as it should be. I will be quick to turn off the set if the cathode voltage becomes excessive.
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Old 22nd May 2020, 10:20 pm   #70
staticmind
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Default Re: B&O Capri 606 TV (17") restoration

Ok with an 1N4007 and a 15 Ohm dropper, I have about 180V on b+, but it didn't solve anything, as you also suggested much earlier.

Disconnecting C69 did nothing to the voltages. It might be my hearing, but when it is disconnected I don't hear the line whistle.

I did measure out voltages of the 3x EF80 in the image IF section, they seem about right. As I have no signal integrity to worry about, the most important measurements would be from the last EF80 (no 3.). On the anode, I have about 185V.

I have spent some hours analysing the "output stage" of the image IF, and how it affects g1 of PCL84.

In the 971H97, only AC is coupled by a coil/transformer, so the anode voltage passes through the coil and 10 kOhm resistor to g2, so I actually don't see how it can affect g1 of PCL84.

As I see it, the ONLY way to affect g1 of PCL84 is by voltage rail +a, going through the 1.8M resistor to the pot, then through resistor 67, the combined resistor/coil 66, then to can 971H77, where it goes to resistor/coil 65 and then to g1 of PCL84.

However, in 971H77, it also passes to resistor/coil 63 and down into 971H97 and forwarded through the video detector diode RL41.

When I measured RL41 last time, I did detect the correct forward voltage, but also a higher voltage when measured in the opposite direction. What bugs me about this is that if the diode conducts in the other direction to, DC can pass through "backwards" though the circuit if you know what I mean.
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