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Old 16th May 2017, 6:13 pm   #1
matspar
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Default Bush TV75 low brightness

Good evening all,

I've spent most of today working on a Bush TV75 that I picked up over the weekend. Progress has been good. Chassis all nice and clean, lots of wax caps changed, 2 o/c dropper resistors replaced and ran up after the usual checks.

The boost diode (PY81) objected a bit and put on a bit of a firework display so this was replaced and I tried again. Bit more promising this time, had a decent line whistle and a faint bit of screen activity which turned out to be frame collapse. After a few more wax caps and a replacement PCL82 I've got a decent raster. Plenty of width, height and seems to be in focus, but it is extremely dim (only just visible in normal light)

The brightness control is behaving strangely. The original is extremely worn and the brightness won't advance, any attempt causes the dim raster to simply disappear. I disconnected this and hung another pot in temporarily (expecting a nice bright raster) but there is no change. Just a very dim raster which only appears when the control is set to about midway.

The brightness circuit appears to read ok, really just consists of the pot, a 0.01 cap and a pair of resistors on the tube base.
Can anyone point me in the right direction? I'm a bit rusty as I've not been near a 405 set for about 5 years now but its all coming back!

I'm not immediately thinking its a duff tube, as when I switch the set off the brightness improves for about a split second, rotating the line hold also produces some screen activity and with the height reduced it gets noticeably brighter.

Working from Trader sheet 1355/T134 also have the Newnes circuit.

Many thanks in advance
Matt
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Old 16th May 2017, 6:28 pm   #2
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Default Re: Bush TV75 low brightness

Check the ion trap magnet adjustment on the tube neck for maximum brightness. It is a critical setting. If the raster gets bigger on advancing the brightness control, change the EY86 EHT rectifier. John.
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Old 16th May 2017, 6:37 pm   #3
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Default Re: Bush TV75 low brightness

Hi!

This sort of problem can unfortunately be caused by an internal disconnection to the cathode of the CRT or a fault in the first-anode (G2) supply, as well as as (possibly) a partial short-circuit across the heater of the CRT.

The first and second of these faults can be isolated by VERY BRIEFLY shorting the cathode and grid pins of the CRT (pins 2 & 11 if it's a large brown 12-pin base with about half missing) or pins 6 & 7 if you've got the smaller 8-pin base.

This should produce a brilliant peak-white flash of light on the screen if the e.h.t., first-anode supply and the cathode-circuit is intact - note that an internal open-circuit to the grid would produce uncontrollable high brilliance, but the short must be done Very, very briefly to prevent possible damage to the LOPT e.h.t. winding.

If you do get the bright flash there is a fault in the cathode-circuit wiring or the video-output valve is not conducting, indicating a need for a check in this stage.

If there is NO brilliant peak-white flash upon briefly shorting G1-K of the CRT then the A1/G2 supply or EHT is suspect and the next check should be to see if the e.h.t. rectifier (EY86 if I remember correctly) is alight - these rectifiers contain an anti-corona shield in their electrode structure and you can usually see a reflection from the heater/cathode assembly on the shield.

If the rectifier is glowing then e.h.t. is almost certainly being supplied to the final anode of the CRT and the next check is to look at the first anode, pin 10 on the larger base or pin 3 on the 8-pin base - about 450V minimum with respect to chassis should be recorded here - if low or missing check the resistor/capacitor feeding the electrode from the boosted h.t. line.

If the e.h.t. rectifier is alight and the G2 voltage is present, look carefully at the neck to see if there's an ion-trap magnet that might have shifted - if there is one then don't touch it if the dust clearly hasn't been disturbed!

If all the above checks prove OK the final step is to make sure the CRT heater is glowing correctly - hence my earlier remark about a possible partial heater s/c - if it is it's time to have a chat with better equipped Members than me about CRT testing/replacement I'm afraid - I'm only equipped for radio and audio now!

Chris Williams
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Old 16th May 2017, 7:10 pm   #4
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Default Re: Bush TV75 low brightness

Hi Chris,

Thanks for your very detailed reply. I will carry out these checks when I next have a session on the TV. The tube heater and the EHT rectifier are both glowing normally. I'm not getting anything like 450v on the tube base though, it reading around 320v (A1) so that needs investigating. The tube has the larger (12 pin) base. I will also check the ion trap as you suggest. This is a bit loose and someone may well have been fiddling in the past.

Many thanks!
Matt
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Old 16th May 2017, 8:48 pm   #5
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Default Re: Bush TV75 low brightness

Hi!

You mentioned you'd replaced the PY81 boost-diode in your first post so we can reasonably rule that out from the fact you're getting some illumination and that the EY86 is glowing normally.

One other point - is the screen-grid resistor (2k2 or 2k7, w.w. or similar) feeding pin 4 of the PL36 OK and not gone high in value?

If the first-anode (pin 10) is connected to the boosted h.t. line either direct or via a resistor only, the next port of call is the boost-reservoir capacitor - typically 0.1uF or 0.22uF in a 405-line only set - the later types of valved/hybrid TVs often used a white or white/blue plastic-cased Dubilier capacitor notorious for shorting but you'll probably find a waxed-paper type in yours!

It's connected from the top of the line output transformer primary winding to the receiver's main h.t.+ line, and very likely as leaky as a sieve!!!

Use a Vishay-Rodestein MKT1613 type or similar of the right value rated at 630V or preferably 1kV DC to replace it - other Members might be able to offer something suitable!

This a very hard-worked capacitor and one of the most crucial to the correct operation of a thermionic-valve line-output stage!

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Old 20th May 2017, 8:36 pm   #6
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Default Re: Bush TV75 low brightness

Hi,

Time for an update!

I've spent nearly a full day on the TV75 and have made fine progress. Started off by changing a load more wax caps which I'd been waiting for all week. Also went through most of the resistors and found two which had gone a bit high.

Fired her up with the aurora connected and after playing with the holds and linearity managed to get test card C on the screen.

Still have a few issues however. The low brightness has improved a great deal, but I think it could still be better. Now when advancing the brightness there seems to be a delay before it has an effect on the picture. It darkens briefly and then the brightness creeps up.

When advancing the brightness it also causes a negative image, actually behaves a lot like how a vision interference limiter does. It does settle down after a few seconds but the definition fades off a bit. I have adjusted the ion trap for max brightness, it was very slightly off.

The picture itself looks ok and is quite stable, just low. The focus seems fine.

I fitted a brand new EY86 eht rectifier which improved it a little more. Also changed C60 in the lopt box (0.25uf)

I think the set seems to be being under run a bit. Somebody (not me) has replaced the selenium (HT) rectifier with a BY diode. looks like this was done a long time ago.

I'm getting 270v on the rectified side of the diode, HT from the main smoothing can is 265, 200, and 195v. I can't find a voltage figure quoted for the HT on either of the 2 circuit diagrams I have, but am wondering if this (and the rectifier) is a bit low?

The smoothing can (2x 200uf & 1x 100uf) does also have a visible bulge, so also wondering if this is dragging the supply down a bit? Still only getting 360v A1 voltage which should be 430v according to the circuit. Voltage checks on various valve bases are all quite close to the circuit.

I still have a handful of 0.01uf wax caps to replace in the i.f. chassis. Not yet arrived

Any ideas would be gratefully recieved!! Thanks to you all for your advice so far. Here's a couple of pics to give you an idea.

Matt
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Old 21st May 2017, 9:02 am   #7
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Default Re: Bush TV75 low brightness

Hi!

Those voltage-figures you quote are roughly about correct - a selenium rectifier connected direct to 230VAC in a TV with just a surge-limiter between the rectifier cathode and the first (reservoir) electrolytic would give about 260 - 270V nominal which after the first resistor, typically 200 - 210V at the main h.t. line!!

Have a look at thel lop-stage screen-grid feed resistor and check the anode-resistors in the oscillator preceding it - most Bush sets of that vintage used a multivibrator type if my memory serves me correctly - inadequate or distorted drive can cause low o/p from the circuit as well as potentially over-running the LOP valve!!!

Chris Williams
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Old 21st May 2017, 5:23 pm   #8
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Default Re: Bush TV75 low brightness

Hi Chris,

Thanks again for your assistance. I've been through all the resistors in this area again and nothing is seriously adrift. I have changed some of the electrolytics and the A1 voltage has perked up a bit now to around 415v.

The line stage I think is fine. No overheating lop valve and there is no lack of width so I would suggest the line drive and EHT is adequate. I have put a signal on the set today and it is terrible. The picture is certainly a negative image which gets worse and disappears as the brightness is increased. The contrast control also doesn't appear to do anything. I'm still wondering if the crt has a partial short across the heater as you mentioned originally.

I've found a couple of other threads where somebody had the same problem and have read a bit about the Mullard AW43-80 where its been mentioned that they can suffer from this.

Alternatively I'm thinking valve trouble or the few remaining wax caps. I don't have any spares here at the moment to substitute. What are your thoughts on the crt?

I do have a good CME1703 tube somewhere but haven't looked into the specs or even if its possible to fit this!
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Old 21st May 2017, 5:57 pm   #9
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Default Re: Bush TV75 low brightness

An expanding picture that disappears is down to lack of EHT.

Picture going negative on increasing brightness 'could' be the tube.

The second important thing you should have done right at the start after checking EHT voltage should have been to check the voltage across the tube heater against what's specified in the data for that particular tube. If you haven't already done this then do it now and report back.
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Old 21st May 2017, 6:15 pm   #10
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Default Re: Bush TV75 low brightness

Yes I have just been reading about this!

Its a 6.3v heater and my meter is showing initially 5v ac on the heater pins and then dropping to around 3v. Looking like the culprit. Also thinking about running the crt heater independently to see if there's any improvement. Somebody also mentioned on a thread about turning the crt upside down to see if the heater 'sags' in the opposite direction.

Is there any other cheats or tricks I could try?!

Really hoping to get this one running nicely as its a fantastic looking telly

Matt
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Old 21st May 2017, 6:18 pm   #11
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Default Re: Bush TV75 low brightness

Tapping the tube neck with a screwdriver handle sometimes clears the fault, but it'll probably return and you also run the risk of making it worse.
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Old 21st May 2017, 6:32 pm   #12
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Default Re: Bush TV75 low brightness

Along with many other Mullard CRTs The AW43-80 CRT is prone to a partially short circuited heater. As mentioned in a previous post try tapping the neck.
The AW43-80 was one of the best CRTs to be made by Mullard.

DFWB.
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Old 21st May 2017, 6:39 pm   #13
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Default Re: Bush TV75 low brightness

You can try all the various tricks you've read about and see what happens, but you will probably end up having to power the tube heater from a separate transformer.

It's a shame you didn't do the tube heater voltage check right at the start before you did so much other work on the set, but you're certainly not the first to have a set with a faulty tube after a whole lot of work has already been carried out.
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Old 21st May 2017, 6:52 pm   #14
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Default Re: Bush TV75 low brightness

I will try all of these things when I next have a session and see what happens. Never thought about checking the heater supply until now but you do live and learn with hobbies such as ours!
Hoping that a replacement tube may turn up eventually. If anyone reading this happens to have a good Mullard AW43-80 or suitable equivalent they'd like to sell me (even a complete/scrap set with suitable crt) and you're not a million miles away do let me know. I am in Portsmouth.

Thanks everyone
Matt
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Old 24th May 2017, 8:21 pm   #15
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Default Re: Bush TV75 low brightness

Ok, update time. I had another play with the Bush tonight, turned it upside down and fired it up and guess what? I was rewarded with quite an acceptable picture. Put the meter across the crt heater pins and its showing a healthy 6.3v and is glowing visibly brighter than before. Still has a totally dead contrast control however but I'll look into that over the weekend.

I'm now proposing to flip the tube 180 degrees. Will just need to extend the final anode connection.
Another thing I've noticed it the aerial connection. With the aerial pushed firmly home the picture still goes a bit negative. Withdrawing the plug a bit returns the picture to normal so I'm thinking the signal from my aurora may be a bit strong, although I've not had this with any of my other sets.

Does anyone use an attenuator with theirs on particular sets and what sort of value if so?

Matt

I will post some pics over the weekend if everything goes to plan (fingers firmly crossed!)
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Old 25th May 2017, 7:05 am   #16
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Default Re: Bush TV75 low brightness

Hi Matt.

Turning the CRT 180 degrees will certainly help your partial heater short problem but unfortunately it's unlikely to last very long in my experience. Your best bet is to look out for a scrap set with a donor tube. The AW43-80 was an otherwise excellent tube, and was fairly widely used by many setmakers. The downside is that you are not guaranteed to get a good one! All part of the fun......!

Regarding your contrast problem, check the continuity of the contrast controls track, I've had one go open circuit before, - it's easy to overlook this when your mindset is looking for leaky capacitors and out of spec resistors!

Good luck with this, it's well worth all the effort in the end!


Cheers. SimonT.
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Old 25th May 2017, 11:17 am   #17
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Default Re: Bush TV75 low brightness

Hi Matt,

The Aurora's output can be way too strong for some sets. I use a variable attenuator like the one in the photo, available from the usual auction site for around £5 or £6 ish.

Marc.
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Old 26th May 2017, 10:23 pm   #18
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Default Re: Bush TV75 low brightness

Thanks Marc and Simon, Well, this evenings job has been a total success and I'm very pleased. The tube was persuaded out of the chassis and the scan coils gently slackened off and rotated half a turn. Then it was all cleaned up properly, much easier to do with the crt on the bench. It all went back together, everything reconnected, and the final anode connection extended and sleeved with the lead and rubber connector from an old tripler. Then the big switch on. All nice and bright and a health 6.85v on the crt heater. I then had to line the set up with the picture shift as the positioning was all messed up from being disturbed.

I also picked up a 20db variable attenuator from Maplin earlier. With this wound right up I have now got a pretty decent picture. Still had a bit of negativity so I swapped both the EF80s to see if this made any difference and it seems to have cured it! The gain seems incredible high on this set. It has now been running for the last 3 hours and seems quite happy so I suppose now the time has come to think about the cabinet resto! I'll put some pics up when its all done and back together. The pics taken tonight don't do it any justice, looks a lot better by eye
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Old 29th May 2017, 12:46 pm   #19
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Default Re: Bush TV75 low brightness

All done, and back in the cabinet for anybody still reading! I ended up respraying the gold finish on the bakelite front panel as it was faded and worn off in places. The cabinet itself wasn't too far gone and a good once over with some scratch cover and a waxing has transformed it! so, a day of Norman Wisdom films it is then!!!!
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Old 29th May 2017, 5:47 pm   #20
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Default Re: Bush TV75 low brightness

Great job! This has motivated me to get on with my KB TV. Saving up for an Aurora again!
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