Bush TV22

Have you replaced the capacitor that decouples the CRT A1 connection. They go leaky and could reduce that voltage. Other possibilities are faulty Metrosil, something else loading the  A1 circuit or the pulse at the transformer feeding the Metrosil being low in amplitude.

This is digging back in my memory, if I am wrong someone will correct me.

The Metrosil is a voltage dependant resistor, it will only conduct when the voltage across it reaches a predetermined level. So the pulse supplied by the winding on the LOPTX must be large enough to reach the predetermined level and will charge up the capacitor noted in para 1. It’s acting in a way similar to a diode in this particular configuration.

Yes I have replaced the capacitor. I have been thinking, could I be getting a wrong reading using a digital voltmeter (10Mohm) compared to a AVO which on 1kV range would be 20Mohms?

The circuit I have states voltages taken with an AVO 7 not 8, AVO 7 from memory is 1000 ohms per volt.

It also states that the EHT voltage was taken with an electrostatic meter, I presume they mean just the final anode voltage and not A1.

I restored a TV22 Mk2 (the one with EF80 valves) several years ago. Initially, it went much the same as yours. I could draw a small spark from the CRT anode cap, but no brightness on screen. After applying a signal from the Aurora, I got weak sound from the test tone and a dim horizontal line. I went ahead and replaced all the wax capacitors, including the heater decouplers. After that, there was loud sound and a bright white line - the frame blocking transformer had an open-circuit winding.

For test purposes, I tried the line oscillator transformer from a Thorn 1500 and also the isolating transformer from a shaver socket as a substitute for the original Bush frame blocking transformer. Both of the substitutes worked well enough to obtain a full size picture. The vertical linearity wasn't quite right, but it proved the point. I got the original transformer rewound.

In my case, it was definitely no signal = no brightness on screen even though there was EHT present, so I think it would be worth trying to sort out the faulty RF chassis first.

After having a nice dine out at midday, I decided to have another look at the TV22. I replaced all the waxy capacitors on the RF/IF board, reconnecting and powering up I was rewarded with exactly the same as before ,nothing! I got my signal generator out and thought I would try the sound IF (19.5MHz), Nothing but I got a weak tone on 21.5MHz?  On closer inspection which I should have done before, all the cans have had the dreaded pot fiddler. The first RF coil L3 the slug was practically completely out of the can! Why did I not realise this whilst changing all those waxy capacitors. So this RF/IF will need complete re-alignment.

I decided not to be beaten and got the other RF/IF from the other TV22, Another two hours passed changing all those nasty sticky capacitors.

I then connected this board up to the rest of the TV and when it warmed up I noticed straight away that the board must be working because of noise and static when I  touched the aerial connector.

Connecting the Aurora I was rewarded with the tone and a bright horizontal line which clearly was TCC!

I then went on a search in the shed for a small transformer I could use as a frame blocking Tx for the set. I found a small transformer which had a primary-secondary resistance of 280-170  ohms (should be 540-480ohms for TV22) and connected it up with croc clips see photo. The result was a picture of TCC but clearly lacking height see photo.

In the picture you will see on the RHS next to the tube is where the missing 0.75uF capacitor should go.

It was interesting to note that with no signal the tube is completely void of any illumination.

So I will have to sort out a frame Tx and a 0.75uF.

Excellent progress so far and it shows how a bit of inginuity with the transformer substitution can give results. I wonder if a very small mains isolation transformer would do the trick. You could probably get away with a 1uf cap to replace the 0.75uf.

Just the job, you must be pleased with the results.

Well today I thought I would have a go at rewinding the frame blocking transformer.

I found an old mains to 3vDC power supply, the primary winding resistance was 1k5 ohms. So I dismantled it starting with the laminations. I discarded the secondary windings but keep the bobbin. I then unwound the primary on to the secondary by hand 2200 turns. So the secondary was done. My intention was to take about another 1000 turns off the primary to give me the correct turns ratio.

Unfortunatly disaster had struck the primary start winding had broken off! So I will have to get another transformer and start again. If I can get transformer the same size the secondary bobbin with the windings on will be OK.

Back to the TV22, I decided to reverse the connections from the transformer that I  had fitted its resistance had been 320-170 ohms. So it was now 170-320 ohms. This added an extra inch to the picture!. I did not know how good the ECL80 valve was so I exchanged it for the ECL80 line oscillator which gave me another inch! By chance I turned the front vertical hold control to my amazement it also seems to control the height! I clearly had 7 and a half inches of height and the mask on the screen only allows a display of just over 6 inches. So I secured the transformer to the chassis and soldered the wires into position. The chassis went back into the cabinet and I have been watching it all night.

I have sent off for an equivalent 0.75uF capacitor but I do not know what it must do because the picture and sound look OK to me!

Now its onto the other set. I have had to order some more 0.1uF capacitors for the main frame and sort the LOpT out, the thing that is worrying me is this RF/IF board that seems dead. I have already replaced all the waxies.

Great Result ? 

Posted by: abctelevision

I have sent off for an equivalent 0.75uF capacitor but I do not know what it must do because the picture and sound look OK to me!

Its non critical, its just a hum cancellation cap, I just snipped it out and left it off mine.

Posted by: abctelevision

Now its onto the other set. I have had to order some more 0.1uF capacitors for the main frame and sort the LOpT out, the thing that is worrying me is this RF/IF board that seems dead. I have already replaced all the waxies.

Ah but if it was just simply down to cap changing wouldn't this hobby be boring, where's the fun in that? Look at the great results you got from your ingenuity with the blocking tx. Now the real fun with this next RF/IF deck starts in earnest, time to inject a signal, break out the scope and follow its path. Good hunting ? 

Well I have put the back on this TV22. I will not be using it to fault find with the faulty RF/IF board. I already blew the mains fuse with that panel when investigating the fault. I will try and get the other main board up and running on the other TV22. I need to "stick" the LOpT back together and sort out another frame blocking transformer. I have just come back from the car boot with a suitable looking mains transformer for 50p. I will have to crack it open and get those laminations out!

I have now started on the second Bush TV22. The capacitors date are Nov 52 so it is probably an early 1953 set which makes it about a year younger than the other. I have used epoxy resin to fit the broken part of the former base and soldered the broken wires to the LOpT. I have also made a frame blocking  transformer . (see pic)

I fitted the LOpT back in the chassis and replaced all the waxy capacitors. Whilst doing this I refitted the 25uF capacitor that had fallen out. I then noticed someone had cut out the CZ1 thermistor. I found a suitable replacement.

Then via a variac I checked the valve circuit at 240v the current was exactly 300mA , so the replacement thermistor was OK.

I then connected the red wire back to the dropper circuit so the DC supply could be tested. At 240V it came in at 190V.! Unfortunately no sound or first light! I connected tube cathode to brightness grid for I knew this would give a bright line on the tube.if it was working.

I know by substitution in my other Bush TV22 that the PL33, PZ30, ECL80 and PL38 valves are all good.

I measured the voltages on the base of the tube. Zero volts on brightness grid! 1st anode only 75 volts (my working Bush 150V). Tube heaters only 5V? I can draw a arc from the EHT overwinding though! 

I thought I would investigate the brightness circuit. The cover for the brightness pot was missing?  On measurement I found the pot appeared to be OC (should be 50k). On removal I found the wiper was working but it was just the HT side which was O/C. By using a small piece of laminate steel I was able to make the pot functional again (see pic) of course now its 47k ohm!  The brightness voltage is now OK but still a blank screen! I found the 2uF capacitor going to the line scan coils was short circuit, replacing that still no picture. I measured the boost DC voltage and that was only 180V (280V), so I replaced C32, that made no difference! So I have had enough for today.

Should I be worried about only 5V on the tube heaters? The heater is glowing at what appears to be the correct brightness. Any suggestions will be gladly received.

Something at the back of my mind remembers Mullard heaters from this date can partially short out. If there is 300ma flowing through it and the voltage is low this could be a possible reason for 5v.

Still really good progress with the restoration.

Is there any EHT at the CRT anode? It could be the EHT rectifier valve is faulty if no EHT.

I was also thinking that if the CRT was soft, ie a poor vacuum, then no image may be possible. This could explain the low heater voltage, but I might be wrong. If it were me, I would like to test the CRT to see if it is capable of functioning.

The good thing is that you have another working set for reference ? 

" A small piece of laminate steel?"

The classic bodge if a replacement pot wasn't to hand was a small piece of ali foil from a fag packet wedged behind the resistive element!

Never did that myself but we were Bush dealers, so always went prepared. They failed quite frequently.

Yes I can draw an arc, but it may be low. I have just read LL Johns article. He says the 470 pF across the LOpT could be leaky. I will try this tomorrow. He also suggests removing the EHT supply to the tube to see if the metrosil voltage rises, if it does the tube is probably faulty.

Just done an accurate DC check on the tube heaters. at 300mA heater voltage 5.1V.

At 6.3V heater current 344mA.

I have just had a search on the UKVRRR forum about shorted Mullard heaters, suggestions were to use an isolation transformer on the heater to supply 6.3 volts, obviously this will overrun the heater but may give it more life.

I suggest you accept the low voltage for now and see what the picture is like when the set is working, if it warrants a transformer then try that, the transformers are special low interwinding capacitance types.

Posted by: Nuvistor

... the transformers are special low interwinding capacitance types.

Only necessary in the case of a heater/cathode short to reduce the capacitance between the cathode and earth to keep shunting of the video signal to earth as low as possible.

In this case we are talking about an internal short entirely within the heater assembly, one side of which is already tied to earth, so the capacitance is immaterial.

Very true Terry, my mind just automatically thinks of HKS in CRT’s. ?