1st Jan 2017, 3:02 pm | #21 |
Hexode
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Re: Bush TV22 Mk 1
Another update on the Mk 1:
I've replaced most of the perished rubber wiring with silicone, and many of the wax capacitors: enough I hope to power it up safely. The dual smoothing electrolytics measure okay for capacitance, so I'm giving them the benefit of the doubt for now. I haven't checked the 16 uF under the chassis yet however. The switched pot seems to be non-original as it measures 220 k rather than 1 Meg end to end. The switch is also a single pole, with the wires of the missing pole soldered straight together. Unfortunately the switch measures open circuit so it will have to be dismantled for cleaning. Unfortunately I managed to nick the track of the wire-wound brightness pot while opening it, but I have some conductive zinc paint which will hopefully bridge the gap. I should really have left this alone. I managed to grind down the top pip of the PL38 as suggested and to attach a wire pigtail. Just need to find some cheap epoxy as per Marc's suggestion for attaching the top cap. The electrolytics so far seem to be dated from Jan-May 1950, so I suppose this could make the set quite an early model? Does anyone know how to dechiper the serial numbers? Mine is 83/00895 on the rear of the main deck. Valve filaments and wound components so far test okay except the frame transformer noted earlier. Carbon resistors have also been kind to me so far and are mostly within tolerance. Hopefully I should be powering up relatively soon. The main things I'm waiting for are replacements for the big 1000 V waxy and the mains r.f. bypass - although I could probably skip the latter for now. I've attached a couple of pictures of the unrestored set below. Cheers Liam Last edited by Voxophone; 1st Jan 2017 at 3:15 pm. |
9th Jan 2017, 1:18 pm | #22 |
Hexode
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Re: Bush TV22 Mk 1
Very close to first power up now! All perished wiring replaced, including r.f. deck connector. Just one or two minor things to attend to and I will be applying the mains.
Since this is my first TV restoration, what should I be looking for in particular on a first test? Hopefully I'll get a raster and hear the LOPT sound, but is there anything else obvious to look out for? I also need to make some sort of set-up for discharging the EHT. Advice much appreciated. Cheers Liam |
9th Jan 2017, 2:04 pm | #23 | |
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Re: Bush TV22 Mk 1
Quote:
I'm not sure where on the track that the brightness pot got damaged. Lets say the wiper crosses that point, wherever it is, leaving an open circuit between the wiper and ground. The CRT grid will then assume the full HT voltage, making it significantly more positive than the CRT cathode voltage (unhealthy for the CRT as there can be a high grid current and possibly damage to the emitting surface of the cathode). The bottom line is the that the CRT's control grid should never be allowed to go more positive than the cathode for a CRT. I'm not sure what parts you have on hand, but one temporary precaution is to put a 1N4007 diode from the grid to the cathode of the CRT with the stripe (cathode) connected to the CRT cathode and the diode anode to the CRT grid. At least that will stop the crt grid going more than 0.7V positive with respect to the cathode. The presence of the diode won't stop the set from working, it might slightly lower the video high frequency response (due to the diode's capacitance) and cause a tad of loss of picture HF detail. But it is better than risking any CRT damage on account of a dodgy brightness pot, until you can replace that pot. In the case of this circuit too, with the diode I suggested, if the brightness pot opens it could also then pull the cathode to a more positive potential, which could then threaten the cathode heater insulation, so it might be better not to risk it and replace the pot before you attempt to power the set. Last edited by Argus25; 9th Jan 2017 at 2:23 pm. |
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9th Jan 2017, 3:34 pm | #24 |
Hexode
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Re: Bush TV22 Mk 1
Thanks for the advice. I didn't realise it was such a serious issue! Pleased to know about it beforehand.
The break is part way down the track, dividing the 50k into two parts of about 15k and 35k. I dismantled the pot to try and fix it, but ended up doing more harm than good I fear. Bridging the gap with tinfoil restores 50k end to end, and I have some conductive zinc paint which I might try as a permanent solution. Failing that, R29, the line hold pot, is also a 50k wirewound and tests fine. Would it be safer to swap this with the brightness one and place the (hopefully) fixed original in the line hold role? |
9th Jan 2017, 4:51 pm | #25 |
Hexode
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Re: Bush TV22 Mk 1
I think for testing purposes, to be safe, I may just replace the brightness pot with a temporary fixed resistor.
Based on what you've said above, I'll choose a lower value and keep the brightness on the low side, since with the o/c frame transformer I won't be expecting a full raster. Maybe 5-10k would be ok to keep the brightness low but still establish whether the tube is ok? |
9th Jan 2017, 8:59 pm | #26 |
Dekatron
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Re: Bush TV22 Mk 1
Hi Liam, if you are stuck I may have a used 50K pot here.
Ed |
10th Jan 2017, 11:09 am | #27 |
Hexode
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Re: Bush TV22 Mk 1
Thanks Ed. That would be useful. I’ll send a PM.
So I finally powered up the Mk 1 last night. No flames or smoke, all valves lit up, faint speaker hum, LOPT whistle sounds healthy to me. Did not get a picture however, and I think the problem lies with the voltages at the CRT anodes. The first anode is meant to read 300V but I’m only getting 8-14V depending on the position of the horizontal hold pot. The anode is fed from the Metrosil, which reads 250V on its other side, so a high voltage is present but is not getting through. There is a wax capacitor (Trader C28) between the anode and chassis which has not been replaced. Could a leak here be responsible or is the Metrosil not conducting? I have so say I’m not really sure what the Metrosil/varistor actually does in this circuit. The second anode appears to be getting no EHT. I am unable to draw a spark from the CRT or PL38 top cap after switch off and there is no static build-up on the CRT screen. The LOPT whistles however, and the PL38 has a blue glow. I’m not sure how effective my top cap repair has been, or whether the vacuum has been broken in the valve. A couple of other observations: •CRT grid reads 24V and CRT cathode reads 79V versus a design value of 75V. The brightness pot is replaced by a 10k resistor between grid and chassis, representing a ‘low’ brightness setting. •The HT measures 173V at connector 1 (Trader) between decks. •The LOPT whistle changes frequency as the horizontal hold pot is turned. I’m not sure how correct this is as I always thought the frequency was fixed. Advice much appreciated as I'm new to fault finding on TV sets. Cheers Liam Last edited by Voxophone; 10th Jan 2017 at 11:23 am. |
10th Jan 2017, 12:06 pm | #28 |
Octode
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Re: Bush TV22 Mk 1
Your remaining 0.1uF capacitor C28 would be a very likely reason for the low boost voltage, hopefully you have replaced the 2uF boost capacitor C29 on the trader sheet.
If the PL38 was down to air it would have a milky appearance where the gettering is or was. The horizontal hold will change the frequency, that is normal. Your HT voltage is a little on the low side at 170 volts, this should be around 190 volts. It be down to the mains input setting or perhaps the PZ30 rectifier is tired.
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Stephen _________"It`s only an old telly" ___ |
10th Jan 2017, 12:10 pm | #29 |
Hexode
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Re: Bush TV22 Mk 1
Another thing I forgot to mention: I'm not 100% sure whether the small diode (V9) on the side of the LOPT is working or not. I think I can see the filament glowing, but it's almost completely covered with pitch.
Edit in response to Freya - C29 has been replaced with a 2uF film capacitor rather than an electrolytic. Mains tapping is as found, corresponding to 250V according to the Trader sheet. When you say a 'milky' appearance, I'm assuming you mean white? I can see the orange filament glow and a light blue glow around the middle of the valve, only visible in darkness. Looks a bit like a mini plasma globe. |
10th Jan 2017, 12:13 pm | #30 |
Octode
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Re: Bush TV22 Mk 1
This will not be working if you are unable to draw a spark from the PL38, it is driven from the line output transformer, as soon as you have EHT then this will be working, hopefully.
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10th Jan 2017, 12:24 pm | #31 |
Hexode
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Re: Bush TV22 Mk 1
Does the presence of the whistle indicate that the LOPT is ok or could there still be a problem with it?
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10th Jan 2017, 12:52 pm | #32 |
Octode
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Re: Bush TV22 Mk 1
What you are hearing is magnetostriction within the transformer, I would say its trying to work but being strangled by the potential shorted capacitor on the output, are you out of stock on the 0.1 uF ?
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Stephen _________"It`s only an old telly" ___ Last edited by Freya; 10th Jan 2017 at 1:00 pm. |
10th Jan 2017, 1:02 pm | #33 |
Hexode
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Re: Bush TV22 Mk 1
I have plenty. I didn't replace all the wax caps as I wanted to test it with minimal disturbance if possible (less opportunity for going wrong somewhere).
Good to hear that it might be responsible for both the missing anode voltages. I'll try and replace it tonight. |
10th Jan 2017, 1:13 pm | #34 |
Octode
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Re: Bush TV22 Mk 1
It might be advisable to have a replacement EY51 diode to hand as they are often no good after being sat around for many years. Their thin wire leads have a habit of failing at the glass pinch.
You will only know though once the EHT is up.
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10th Jan 2017, 5:19 pm | #35 |
Nonode
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Re: Bush TV22 Mk 1
The first anode supply has a high source impedance so any loading will effectively kill the 300V supply. The service manual says this supply should be measured with an electrostatic meter. C28 will almost certainly be leaky and should be replaced.
The metrosil is a voltage dependant resistor and is being used a rectifier. This works because of the waveform at pin A of the LOPT. The mean voltage would be the 250V you measured but the peak voltage will be much higher. Once you have EHT if there is nothing visible on the screen briefly connect the CRT cathode to ground via a resistor of around 10k to 47k. If all is well the screen should light up. Sometimes nothing will be seen on the screen at any setting of the brightness control until a signal is connected. This happened on my TV22. Keith |
10th Jan 2017, 10:27 pm | #36 |
Hexode
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Re: Bush TV22 Mk 1
I’ve just given the set another run.
After replacing C28 the first anode now reads a much healthier 180Vdc. This is still low compared with the Trader value but the DMM seems to drag it down from an initial value of 200V or more. Using Keith’s method of applying a resistor to the cathode, I now have a raster! (See pictures). No raster is present without this resistor however. There appears to be a shadow at the centre of the screen, which unfortunately I’m guessing may be a feature of the CRT. Not sure how it could end up with that particular shape however. EHT is still giving problems as I still can’t draw a spark. I wouldn’t have thought the raster would appear with EHT completely dead however? Regarding the o/c frame transformer: I’m confused as to how I’m obtaining a full rectangular raster with this fault in place. Does this indicate the transformer must be ok or am I misunderstanding the role it plays? I’m unsure where to go from here as I don’t have a 405 line signal source. Are there any other tests I could try before I get my hands on an Aurora? Thanks everyone for your advice so far. Cheers Liam |
10th Jan 2017, 10:49 pm | #37 |
Octode
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Re: Bush TV22 Mk 1
Hi Liam,
I don't know if you have misinterpreted the connections on the frame osc transformer when taking out your measurements during testing but according to your raster the transformer is definitely working without a doubt. (you would have a thin bright horizontal line if it was open circuit) The dark patch in the centre of the CRT that you refer to is ion burn, nothing to worry about I reckon as your CRT looks fairly bright at the moment and I'm thinking that it will only get better with use and so the ion burn will not be as noticeable. Cheers
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11th Jan 2017, 9:37 am | #38 |
Octode
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Re: Bush TV22 Mk 1
That's looking ok, until you get the Aurora there is not much you can do.
If you have an AM signal generator with modulation it can be used to get some interaction in the form of horizontal bars. It will also produce a tone.
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Stephen _________"It`s only an old telly" ___ Last edited by Freya; 11th Jan 2017 at 9:53 am. |
11th Jan 2017, 10:55 am | #39 |
Hexode
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Re: Bush TV22 Mk 1
I have a 40MHz digital sig-gen with AM and FM modulation. I'm not sure if the frequency will be high enough for TV but it did come with software that allows you to 'construct' signals on a PC and may permit a higher frequency. When I know more about TV signals I might try building up a 405 line waveform on this software.
Going back to what you said about AM, what specifically did you have in mind? I'm keen to try different things as much of the TV circuitry is still unfamiliar to me. |
11th Jan 2017, 2:20 pm | #40 |
Octode
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Re: Bush TV22 Mk 1
Most old school service generators had a fixed AM setting of 400Hz at 30% modulation depth. Turn this on and sweep freq around 45MHz and you should see something on the screen.
Sweep around 41.5MHz and you should hear the tone. Note the above frequencies assume your TV22 is set to London Ally Pally frequencies. |