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Old 16th May 2019, 9:44 pm   #1
Mazda Man
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Default Bush TV53 hum bar problems

Evening all, I'm looking for some inspiration to solve a hum bar problem on what is usually a simple Bush tv53 chassis. Having done some work replacing the usual waxies and both the smoothing caps as they would not 're form I was pleased with the results. However when I plugged the set in to raw mains rather than the bench isolation transformer there wad a faint hum bar traversing up the screen. When the bar had nearly reached the top another bar started to appear at the bottom. Sometimes the bar traverses back down the screen and sometimes it just hovers part way up. The verticals are not affected in any way as it moves up the screen and no undue hum on the audio. It's best described as a ghostly hum bar rather than a dark black one. Once 're connected to the isolation TX the effect is all but gone. I've not seen this problem before so I'm hoping someone can provide some inspiration as to what is going on as I can't think of a logical explanation but it's very annoying when watching the set.
Thanks
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Old 16th May 2019, 10:03 pm   #2
Nuvistor
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Default Re: Bush TV53 hum bar problems

Is C36 0.05uf across the mains supply faulty or missing?
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Old 16th May 2019, 10:24 pm   #3
Mazda Man
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Default Re: Bush TV53 hum bar problems

Hi Frank it has a 0.047 fitted. I tried a 0.1 in that position too but it made no difference.
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Old 16th May 2019, 10:30 pm   #4
McMurdo
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Default Re: Bush TV53 hum bar problems

is there any interaction between the tv and the video source perhaps? What are you using for a signal? Is there any earth path in the signal psu? I'm just wondering if it's an earth loop symptom as it suggests mains frequency beating against frame rate
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Old 16th May 2019, 10:44 pm   #5
Chris55000
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Default Re: Bush TV53 hum bar problems

Hi!

This looks like slight heater/cathode leakage in the video amplifier or detector stage or possibly even the CRT, and unfortunately needs something like a Mullard HSVT or a Sussex that will measure the heater/cathode insulation at working voltage with the heater at working temperature!

If you have no valve tester it's a ****** faff to do in series heater lines, but if you have a bench power supply, you can at least rule the CRT out by feeding it's heater at d.c., temporarily disconnecting the original heater leads and short–circuiting them to maintain continuity in the chain, then testing the set again.

If the bar is now cleared, unfortunately the CRT is at fault, but if the fault is still present another valve may be responsible, and it's quite possible the RF stage or mixer/osc could be the culprit, so if the above test rules out the CRT, begin with trying the RF, mixer/osc and video amplifier valves by substitution first, then work thro' the remainder along the signal chain.

Don't forget there may be one or more smaller electrolytics across the h.t. feeding the r.f. strip or video amplifiér stage that may be partially or completely o/c!

I think the TV53 used a metal h.t. rectifier, but I suspect that will have been modernised a long time ago with a diode & surge–limiter, so I wasn't immediately thinking of rectifier problems at this stage!

Chris Williams

PS!

Don't forget any anode or screen–grid decoupling in the frame TB amplifier as well by the way, and if the frame amplifier is a PL84 this has known to be responsible for strange tricks, see "Valves & Their Habits" written by Harold Peters, available from Jan 1965 issue of "Practical Television", A.R.H. site!
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Last edited by Chris55000; 16th May 2019 at 10:52 pm.
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Old 17th May 2019, 8:54 am   #6
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Default Re: Bush TV53 hum bar problems

You may discover that the hum cancelling capacitor has been disconnected due to leakage.
Looks like C31 [Newnes] .5uf [its a BIG fat waxie] J.
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Old 17th May 2019, 10:05 am   #7
Argus25
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Default Re: Bush TV53 hum bar problems

Quote:
Originally Posted by Mazda Man View Post
Once 're connected to the isolation TX the effect is all but gone.
Thanks
If this is the case, that the hum bar you are seeing is gone when the isolation TX is fitted, this suggests there is a 50Hz current flowing in the RF connection to the set and modulating the signal. The problem cannot be residing inside the set, with issues of poor HT filtering, or heater to cathode leakage, or the effect will still be there with the set running from the isolation transformer. It sounds therefore like a classic earth loop or in this case neutral loop issue.

I don't know your set but I assume its a hot chassis type or you would not have deployed the isolation transformer in the first instance.

So I could suggest firstly check, when its running without the isolation tranny, that the chassis is connected to mains neutral and not phase and then if the problem is still there place a 470pF capacitor in series with both the RF connections to the set (the coax earth and the signal feed) to reduce any 50Hz currents in the RF feed.
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Old 17th May 2019, 12:33 pm   #8
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Default Re: Bush TV53 hum bar problems

Thanks to all for your useful feedback. i'm away over the weekend but will get back to it later next week and report on my findings.
Dave
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Old 21st May 2019, 9:12 am   #9
Mazda Man
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Default Re: Bush TV53 hum bar problems

so the Hum bar problem when the Bush set is powered on raw mains is now solved!
As I had just overhauled the set replacing all suspect caps etc I was pretty sure that there were no faulty or missing caps that were causing the hum fault.
First I tied another set on the bench and connected the aurora up to that to see if if it was the set or the signal source. No hum was visible on the substituted Tv so it looked like the signal source was not to blame after all.
Reconnecting the Aurora to the Bush and the hum was clearly visible. even though the aurora / 9v power supply combination worked ok on the other set.
Just to be sure I decided to substitute the 9v Aurora power supply for one of a different brand. these little Ac to Dc power packs are the plug in the wall type and use a little switched mode converter inside them. As they are a high frequency type power supply I did not think a replacement would solve it as the ripple appeared to be low frequency and beating with the frame timebase.
A different branded substitute power supply however reduced the effect by about about 50% with it being much less visible but not gone.
Feeling like I was on to something I next fed the aurora from my variable DC bench power supply which uses a standard linear power supply.
Switched on the Bush and the hum was completely gone, not a trace of it! Bizzare why this Bush set should be sensitive to this effect whilst other sets using the same signal source are not. So now Ive Just got a very intermittent line twitch to sort out!
Thanks to all for their input, particularly Argus25 and McMurdo who got me thinking along the right lines to solve the problem.
Regards, Dave
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Old 23rd May 2019, 10:36 pm   #10
rambo1152
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Default Re: Bush TV53 hum bar problems

I've seen this sort of thing myself. Poor smoothing on an RF distribution amplifier or UHF modulator in a hotel system head-end, only affecting certain models of TV in the hotel.
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Old 24th May 2019, 8:38 am   #11
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Default Re: Bush TV53 hum bar problems

Quote:
Originally Posted by rambo1152 View Post
I've seen this sort of thing myself. Poor smoothing on an RF distribution amplifier or UHF modulator in a hotel system head-end, only affecting certain models of TV in the hotel.
This happened to my son who had a small aerial amp with his TV. On changeover to digital some channels disappeared. It was a dried up cap in the built-in power supply.
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