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Old 20th Aug 2019, 8:52 pm   #41
FERNSEH
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Default Re: Stella ST1007U

Quote:
Originally Posted by davyrocket2 View Post
hello David in the photo is that a yellow capacitor i can see which looks like one of its leads is loose
Hi David,
It's C72 (0.047uF) and is a decoupling capacitor connected across the vision and sound circuits HT supply. It was moved away from the tag strip to make access to the bad solder connection.

DFWB.
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Old 21st Aug 2019, 8:38 am   #42
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Default Re: Stella ST1007U

Yes David, the 1768 series always did produce Tom cat sounds when it warmed up presumably due to the pre amp anode load supply being from obtained from the boost rail via that 2.7m resistor.
It's a long time ago but something jolts my memory that the cathode by pass capacitor could be O/C causing this. We are going back to 1964 here! The O/C capacitor had little effect on the sound other than a slight lack of bass. I will dig the manual out when I have a quiet moment. John.
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Old 22nd Aug 2019, 3:51 pm   #43
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Default Re: Stella ST1007U

Hi John,
This set fixes itself! No more funny sounds during the warm-up period.
Friend Kevin has made a new springy blue steel band for the CRT.

DFWB.
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Old 22nd Aug 2019, 4:54 pm   #44
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Default Re: Stella ST1007U

In an attempt to improve the picture brightness a 10Kohm wirewound resistor has been connected between the mains input and pin 1 of the CRT. This action has given the tube a 15% boost.

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Old 22nd Aug 2019, 9:12 pm   #45
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Default Re: Stella ST1007U

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Originally Posted by FERNSEH View Post
Hi John,
This set fixes itself! No more funny sounds during the warm-up period.
DFWB.
Are you sure Jimmy's cat was not hiding behind the sofa? J.

[I reckon that bias cap healed itself]
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Old 23rd Aug 2019, 8:07 am   #46
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Default Re: Stella ST1007U

A nice set David, I've not seen one of these "lefties" before, also interesting reading through all the trials and tribulations that it has presented to you.

Cheers
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Old 23rd Aug 2019, 9:13 am   #47
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Default Re: Stella ST1007U

Quote:
Originally Posted by Heatercathodeshort View Post
Quote:
Originally Posted by FERNSEH View Post
Hi John,
This set fixes itself! No more funny sounds during the warm-up period.
DFWB.
Are you sure Jimmy's cat was not hiding behind the sofa? J.

[I reckon that bias cap healed itself]
It was probably both of Kev's Mainecoons .... whilst he [a mechanical genius] played the Jewish Harp on the new CRT band!!!
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Old 24th Aug 2019, 11:02 pm   #48
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Default Re: Stella ST1007U

The restoration job was signed off today. The final task was to sort out the video "soft" button. The soft setting is actually an alternative interference limiter control adjustment. When the "soft" button Sw4 is depressed the rear control is switched out and the under chassis slider pre-set control R41 can be adjusted to set the desired amount of peak white clipping. The pre-set control had gone open circuit and was causing almost complete loss of picture. It has been replaced by three resistors which can provide three soft picture settings.

We're on the lookout for a good AW43-80 CRT.

DFWB.
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Old 25th Aug 2019, 9:09 am   #49
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Default Re: Stella ST1007U

Nice job David. Did you end up reconstructing a new cabinet? It looks much better with the controls on the right.
Those sliders were known to go high/OC. The one across the boost rail [focus] used to drastically reduce in value but the lack of brightness went unnoticed by the customer. It was only when the boost volts were dragged down to such a degree that the height dropped to half that I received a service call! J.
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Old 25th Aug 2019, 12:33 pm   #50
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Default Re: Stella ST1007U

Hi John,
The only attention the cabinet required was to re-glue the cabinet floor parts and after that a good polish.
It's a chunky looking cabinet but in fact it is constructed with thin plywood.

When one Considers the simple circuits employed in these receivers it's often remarked how Philips were able to produce such high quality 405 pictures. But there quite a few circuit design subtleties to take into consideration. The 1768U series have a mean level vision AGC system. To compensate for variations in the AGC voltage due to picture content a clever and cheap auxiliary AGC system is employed.
It's explained in the 1960/61 R & T servicing book.
Video Amplifier Circuit
" V8 cathode is coupled via R48 to V7 cathode, providing a form of mean-level AGC. An increase in mean picture-level produces a greater D.C. positive voltage on the video grid, and raises the cathode voltage. This rise is appied to V7 cathode and reduces the gain of this stage. The reverse occurs when V8 cathode falls less positive. C42 is included to prevent video degeration, allowing only slow voltage changes to be fed back."
There is a direct connection between the video amplifier anode to the cathode of the picture tube, thus the video DC component is maintained without the need for DC restoration.
These circuits were carried forward into the 17TG100U and ST1017 110 degree CRT models.

DFWB.
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Old 26th Aug 2019, 1:53 am   #51
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Default Re: Stella ST1007U

i think i can find you a good aw43-80 but i would have to ship it since i am located in Portugal. I recently i went on a trip to our capital,Lisbon and i met a guy who had bought a building were there was a tv repair shop deactivated since 1975. it contained about 100 1950's tvs, all of them destroyed my mice and moisture, but i tested about 10 crt's and of all that i tested i only found a bad one...i bought 4 tv's to the guy to gut for the crt's. one of them was a nordmende diplomat 59 and the crt tests like new (aw43-88) . i will be needing that crt for me, but there were more, many more.. i told the guy to hold on to the tv's for a couple of months and the ones he cannot sell i will buy them all but i told him i would offer him a very low price for each set due to their condition...i told him to try and sell the ones he can and i will buy the reamining ones once he wats to clear the shop.... if all goes well nobody will buy any of them because they are filled with rat feces and so i can grab them for crt's and lopts etc.
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Old 26th Aug 2019, 9:35 pm   #52
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Default Re: Stella ST1007U

Hi Hugo,
I've sent you a PM.

DFWB.
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Old 26th Aug 2019, 10:42 pm   #53
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Default Re: Stella ST1007U

Just a quick line. I too am restoring a Stella with the PCC89 in the tuner. Having replaced everything that you have said, I have low slightly distorted sound with the PCL 83 running far too hot. Also I have no picture just a blank illuminated screen. I am now struggling to locate the problem. I have double checked that all the caps changed are in the right place and correct value. I have scoped the sound from the detector and it seems to be sitting on on the negative side of a sine wave which I thought was a hum bar. Any Thoughts PLEASE
David
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Old 26th Aug 2019, 11:59 pm   #54
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Default Re: Stella ST1007U

i dont know about the circuit on the stella and i dont know if you are a beginner on tv's or not but a blank raster together with faint distorted sound at the same time means you probably have tuner issues, IF issues or AGC issues. either a bad tuner tube, low gain on the IF due to bad tube also, perhaps an open coil on one of the if cans, open resistor that feed the plate of an IF tube , low or missing agc voltage killing the IF and tuner activity, etc

first thing i would check is the easiest stuff, tube swapping, then mesure AGC voltage,if its absent or wrong you can use batteries or a power supply to simulate it, just put the positive to chassis and the negative to the agc test point, then check dc voltages on plates of the IF tubes.this is general advice and not related to your particular tv. if you have a tuner substitute or a device like a sencore that can output IF you can use it to substitute the tuner on the tv.

you also might have two separate faults...one on the video amp for example and other on the audio section but i thing its unlikely.

other simple thing to check is making sure your modulator or whatever you are using to feed video is actually working well and that the output is suitable for a 300 ohm or 240 ohm antenna input, in my case it expects 75ohm so i have to use a balun plug for my vintage tvs

also note that audio amp tubes always run very very hot, WAY hotter than say an IF or tuner tube

also to rule out the video amp you can (generally) feed direct composite video into the grid of the video amp tube and it wil display on the screen, also generally you can feed audio into the volume pot to check both outputs

Last edited by PortugalTV; 27th Aug 2019 at 12:13 am.
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Old 27th Aug 2019, 1:05 am   #55
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Default Re: Stella ST1007U

It is certainly possible there are separate faults in the sound and vision stages.
However, on a number of occasions instability in the sound IF amplifier has had an adverse effect on the vision IF amplifier. The fact there is a high negative voltage on the grid of the PCL83 would indicate one of the sound IF amplifiers is oscillating. The attachment shows the sound IF, detector and AF amplifier stages in the Stella ST1007. A similar circuit was used in many other Philips 405 line receivers.

DFWB.
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Old 27th Aug 2019, 12:15 pm   #56
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Default Re: Stella ST1007U

Quote:
Originally Posted by davidbi View Post
Just a quick line. I too am restoring a Stella with the PCC89 in the tuner. Having replaced everything that you have said, I have low slightly distorted sound with the PCL 83 running far too hot. Also I have no picture just a blank illuminated screen. I am now struggling to locate the problem. I have double checked that all the caps changed are in the right place and correct value. I have scoped the sound from the detector and it seems to be sitting on on the negative side of a sine wave which I thought was a hum bar. Any Thoughts PLEASE
David
Hi David,
link to the Radios-TV website: https://www.radios-tv.co.uk/communit...ns-battery-tv/

In the Ekco TMB272 the sound an vision faults were caused by faulty decoupling capacitors in the vision and sound IF amplifiers.

DFWB.
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Old 27th Aug 2019, 3:10 pm   #57
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Default Re: Stella ST1007U

Thanks for all that, it will be a week before I get a chance to investigate further, but one other thing I forgot to mention is that when it first starts the sound output is loud and then drops down low, as if the AGC is pulling it right down.
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Old 27th Aug 2019, 5:10 pm   #58
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Default Re: Stella ST1007U

Reference to the circuit diagram on post No. 50. With a signal supplied to the receiver check for a positive going voltage or 'scope waveform across the vision detector load resistor. If there is there any instability or oscillations in the IF amplifier this show up as a voltage developed across the load resistor.

DFWB.
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Old 27th Aug 2019, 7:06 pm   #59
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Default Re: Stella ST1007U

I have this old Stella here in Norwich if any good to you David? 43-80 is showing emmision on my tester. Problem is i am in Norwich. Cheers David
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Old 27th Aug 2019, 7:46 pm   #60
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Default Re: Stella ST1007U

Hi David,
It looks like an use only for spares set and the CRT would certainly solve the dim picture problem in the Stella ST1007.
But I'm located in the North-East 250 miles from Norwich.

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