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Bush TV135LU

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PYE625
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I agree, the Bush is looking just that little more upmarket. The trim on the Murphy front seems cheap in comparison. Not only that, the Bush tuning is much easier to operate than the Murphy. The Murphy ones seem cumbersome and are very difficult to tune in.

Finger-killers I think I'll call them. ? 

I'll say one thing though, the 405 picture on the KB is much better than either of the above. 625 is about the same.

https://www.radios-tv.co.uk/community/black-white-tvs/kolster-brandes-19-inch-dual-standard/

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 18/08/2019 9:24 pm
Nuvistor
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The Murphy was provided with a tuning tool that clipped to the cabinet back, if it was missing I used a box spanner , 0 or 2 BA (forgotten which) with a handle like a screwdriver.

I agree though the Bush cabinet is nicer.

 

Frank

 
Posted : 18/08/2019 10:38 pm
neil1974
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That's a very good result there Andrew ? .  I've got some sets with this style chassis in them that need some work but I've nowhere to work on them as the bench is full just like everywhere else ? .

Cheers

Neil.

 
Posted : 18/08/2019 11:54 pm
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PYE625
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After giving this set a run this evening, another fault has become apparent. Nothing to do with the line stage, but at the signals side of things this time.

Perfectly ok on 625 in every respect, but 405 has what seems like excessive uncontrolable sound gain in the IF stages. You need to de-tune to fade out the sound to a degree otherwise there is really bad sound on vision. This in turn results in a poor definition picture of course. The problem is also intermittant with quite large variations in sound together with crackling and popping. Removing the audio input at the modulator and the crackling and popping can still be heard. Reducing contrast (gain in this case) eases the problem as does an attenuated RF input signal.  Time to investigate.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 26/09/2019 9:15 pm
Nuvistor
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A fault in the bridge T 38.15Mhz sound rejector circuit would cause bad sound on vision and tuning away from the sound carrier would reduce vision definition and sound on vision. Reducing the gain would also reduce the SOV. Not sure about the crackling sound though.

 

Frank

 
Posted : 27/09/2019 8:09 am
PYE625
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This afternoon, things took a dramatic turn for the worse.

I switched on with the set at 625 and all was well. Shortly after, I switched over to 405. I was greeted by a tremendous crashing sound from the speaker, disturbed picture, then as I looked around the back I saw flashing within an EF80 followed by a resistor burning up. Not only that, the system switch was arcing away on the IF panel. I could smell a mix of burning paxolin plus a burning resistor as I quickly switched off.

I think we can conclude from this the set has a serious fault. ? 

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/09/2019 3:32 pm
PYE625
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With reference to the Bush TV125 data in the library, the problem would seem to centre around V25, the EF80 sound IF valve. R237 a 220 ohm HT feed to that valve has burned out. Physically opposite S21E contact slider on the standards switch is another switch at HT potential. The switch was arcing from its contact slider to the S21E slider. They are close to each other, at probably a mm or so, on the paxolin switch slider itself.

Now, S21E switches the audio IF signal feed (405/625) to the grid of V25 plus an agc voltage on 405. When on 625, S21E is earthy. On 405, the impedance is quite high so this would explain why the original symptom was confined to 405. That symptom must have been caused by a higher positive voltage leaking onto the switch, hence the high gain (agc being affected) and the crackling noises on sound. I briefly powered the set on 625 and the arcing of the switch was much more severe as a nice track to ground has been formed by the HT leakage.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/09/2019 3:53 pm
PYE625
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My next plan is to remove all connections to S21E, wire them up temporarily as they would be in the 405 selection, replace R237 and the EF80 as it now has the grid shorted to the screen. I will then see if I can operate the set and see if any futher damage has occured.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/09/2019 4:52 pm
PYE625
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Having done the above, I am very relieved that the set is working fine. It is still ok on 625 but on 405 the sound is normal with no crackling and the tuning of sound/vision is ok too with none of the afore mentioned problems. The sound AGC is working with a variation of roughly about ov to -5v on 405 according to gain setting. All voltages around the EF80 are normal.

There is a healthy full HT sitting upon the now vacant S21E switch connections. This now of course presents me with the problem of what to do about the standards switch S21E and how to select between 405/625 sound.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/09/2019 5:23 pm
Nuvistor
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Is it possible to scrape away the conductive section of the Paxolin or is it too far gone. Any spare sections on the switch?

 

Frank

 
Posted : 27/09/2019 5:45 pm
PYE625
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Hi Frank, the proximity of the contacts and lack of access makes that impossible. The switch is so designed that the slider cannot be withdrawn for examination either. There is a possible switch above the faulty section that could be used however. This S21G switch alters the voltage on the screen of V26a PCF 80, IF and limiter valve. It might be possible to get away with leaving this set in one connection, as in my set, only a 22k resistor is switched to ground on 625 lines. I might experiment and see.

IMG 5233 50

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/09/2019 6:50 pm
Nuvistor
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I never had to change one on this model of set but changed a few on the Pye 11U chassis, both the receiver section and the time base section.

 

Frank

 
Posted : 27/09/2019 7:27 pm
PYE625
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I have tried doing away with the switching of the 22k (R241) from the screen of V26a and leaving it permanently connected to ground. This has the effect of reducing the voltage applied to the grid on 405 lines. The result is only a very slight reduction of sound on 405, 625 being unaffected of course. I can live with that. Now this frees up S21G completely.

So I have transfered the connections from S21E to S21G. I have also completely removed the now unused contacts from the switch S21E. The switch opposite S21G is operating at a low voltage so there is no danger of leakage here. Normal operation so far has been attained and all is fine on both standards. It has been quite fiddly and involved some lateral thinking, but I believe I have found a satisfactory compromise that cures the fault.

First picture is of the complete switch, quite a complex article to replace as a whole even if I had a new one. The second picture shows the removed contacts where S21E used to be (centre of pic), and the connections transfered to the section S21G directly above it. Last picture is of R241 the 22k now permanently connected to ground.

IMG 5237 50
IMG 5238 50
IMG 5239 50

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/09/2019 8:21 pm
PYE625
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S21E is the only section of the standards switch where a high and low potential exists directly opposite each slider. I can easily see why the insulation breaks down, especially when the set is first switched on as the HT approaches 320vdc with no current being drawn before the valves warm up.

320vdc across this small central gap between sliders..... naughty naughty. But then having said that, it can hardly be a common problem with this chassis otherwise it would have come to light before now.

IMG 5233 50

 

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/09/2019 8:53 pm
Nuvistor
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Nice job. ? 

Frank

 
Posted : 27/09/2019 9:08 pm
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PYE625
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Posted by: @pye625

This has the effect of reducing the voltage applied to the grid on 405 lines.

Not the grid, but the screen of the PCF80 of course. ?️ 

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/09/2019 9:11 pm
PYE625
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I also had a quick check with the Murphy set using the same chassis and this is fine.

Using that set as an example, the first picture shows S21E and the second picture shows S21G with the 22k that is switched to ground on 625. This can be moved to connect permantly to ground, and the three connections to S21E moved up to S21G. Hopefully, no-one will ever need to do this, but it serves as a useful hint to get around such a problem as I had.

IMG 5240 50
IMG 5241 50

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 28/09/2019 5:54 pm
acj1980
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what a beautiful "set", it seems to be in very god condition, the emission for the picture tube is ok, but could be better, do you want to re-gen it? 

when i have a tube with low emission, i let the tube be connected to my tester for 12-24 hours, often i get better emission if the tube is god, it is a funny thing, its like the tube is "sleeping" after serval years without a picture on the screen.

 

Alex 🙂 

 
Posted : 29/09/2019 6:12 pm
PYE625
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Thanks Alex ? 

The tube is so bright I had complaints from drivers of passing cars that they were being dazzled. I now need to close the curtains in the event of using this set in the evening.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 29/09/2019 6:23 pm
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PYE625
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I am starting to really quite like this chassis. The more ups and downs, the better it gets. Boring if it all worked fine with no faults. I got a bit tired with the LOPT issues I must admit, but that is all part of the fun as they say.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 29/09/2019 8:33 pm
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