19th Feb 2015, 6:05 pm | #41 |
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Re: Marconiphone VRC74DA
There is a quick and easy method to determine if the frame output stage is OK. Inject 50 cycle AC into the control grid of the KT33C. The AC voltage can be sourced from low down on the heater chain, in fact the voltage across the CRT heater will be fine. To be on the safe side insert a 0.047mfd capacitor is series with the test voltage. If there is some vertical deflection then you know the frame output stage is OK.
DFWB. |
19th Feb 2015, 6:49 pm | #42 |
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Re: Marconiphone VRC74DA
Hi David,
Just tried your method, CRT heater to pin 5 on the KT33C via 0.05 cap. Not a sausage I am afraid, I cannot hear any frame buzz either, I have tried another valve but no difference. Mark |
19th Feb 2015, 6:57 pm | #43 |
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Re: Marconiphone VRC74DA
Just a quick thought. The scan appears to have moved downwards. Early versions of the 1807 series did not employ an output transformer. The coupling was via an electrolytic to high impedance scan coils. If transformer fed, check the voltage to pins 3 [anode] of KT33C. If absent check transformer for O/C promary. Pin 4 is screen grid and should have roughly HT. Check cathode voltage pin 8 to see if the valve is conducting. John.
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19th Feb 2015, 7:06 pm | #44 |
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Re: Marconiphone VRC74DA
Is the HT voltage correct from the old smoothing caps and rectifier?Think I have replied to an earlier post.
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19th Feb 2015, 7:12 pm | #45 |
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Re: Marconiphone VRC74DA
Hi John, It is transformer fed, I just checked the KT33C anode. No volts on pin 3, 230v on pin 4. Looks like the FOP has an open primary
Mark |
19th Feb 2015, 7:55 pm | #46 |
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Re: Marconiphone VRC74DA
Sadly I can confirm the primary is open
If anyone has one, or something that would do the job, please let me know. Mark |
20th Feb 2015, 10:45 am | #47 |
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Re: Marconiphone VRC74DA
I have just had a look in my spares dept, I have a frame output tx from an LV30.
I wonder how critical the impedance is? The primary is 1340 ohms, secondary is 1.5 ohms. The original tx primary is 1000 ohms, secondary is 1.5 ohms. Mark |
20th Feb 2015, 12:45 pm | #48 |
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Re: Marconiphone VRC74DA
That would be an acceptable substitute, you may need to adjust the resistances on the height control.
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20th Feb 2015, 1:14 pm | #49 |
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Re: Marconiphone VRC74DA
Yes, it seemed close enough to me. So I have lashed it in and now have frame scan
Observations are that my new height and frame hold pots are doing what they should be, however I am getting no picture whatsoever. Just a raster with flyback lines, sound is there but very obviously suffering from vision on sound. The focus pot is jumpy, so I will attend to that. Likewise the linearity pot needs a clean, but it is also working At least it is progress! Mark |
20th Feb 2015, 1:31 pm | #50 |
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Re: Marconiphone VRC74DA
whats that black/red diode doing, not got a circuit to hand.
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20th Feb 2015, 1:45 pm | #51 |
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Re: Marconiphone VRC74DA
Not too sure, I can't find it on the circuit diagram.
Mark |
20th Feb 2015, 6:34 pm | #52 |
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Re: Marconiphone VRC74DA
I have replaced all the low Z77's in the RF deck with good EF91's, but am still not getting anywhere close to getting a picture
I still can't find any reference to the Diode on the main deck, this may be part of the problem. Any suggestions of what to check through would be much appreciated. Bear in mind that this is a Birmingham set, so the RF deck is a superhet circuit unlike it's London brother. Mark |
20th Feb 2015, 8:14 pm | #53 |
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Re: Marconiphone VRC74DA
Do you have a signal generator ?, its easy to work from the video back through the stages to find the fault.
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20th Feb 2015, 9:05 pm | #54 |
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Re: Marconiphone VRC74DA
Hi Mark,
I'm not absolutely sure, I believe that the IFs in these receivers are almost near the later BREMA figures of 35mc/s vision and 38Mc/s sound. I hope the X78 frequency changer valve has not failed. It's the funny one with the common cathode and heater pin. When I return to the shop I'll swot up the circuit of the superhet version of the 1807 and report back with some ideas. DFWB. |
21st Feb 2015, 12:16 am | #55 | |
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Re: Marconiphone VRC74DA
Quote:
David, I have tested the X78 on the Taylor and it is ok, I have also borrowed one from My HMV 2811, but no difference. The only positive change by swapping out the Z77's for EB91's has been to clear the vision on sound problem. With no signal input, I can tell the frame and line hold controls are working, although when (if) I can get some video on screen I may have to change some resistors to get the presets in the middle of their travel. I have the manufacturers 'provisional service info' but it does not cover the Birmingham superhet receiver. The Pye FOPT has now been properly fitted to the chassis, and does not look out of place. luckily there was a mounting point on top of the TX to fit the tagstrip to. Mark |
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21st Feb 2015, 9:48 am | #56 |
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Re: Marconiphone VRC74DA
Might be worth checking the valve sockets for broken pins, I tend to use a leg of a 5 watt resistor, inserting into each pin checking for correct physical resistance so to speak.
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21st Feb 2015, 10:18 am | #57 |
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Re: Marconiphone VRC74DA
The valve sockets are pretty good, I did clean them when I had the RF unit on the bench. Mind you, knowing how low quality these are, I will double check the sockets. The only intermittent socket was on the CRT, this is a particularly low quality item.
I carefully cleaned and re tensioned the contacts at the start of the restoration, as it had intermittent heater connections. Mark |
21st Feb 2015, 12:14 pm | #58 |
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Re: Marconiphone VRC74DA
The diode [Westector] is fitted in later models. It is the frame interlace filter but will only cause weak frame lock if it is O/C. Try all the Band 1 channels from the AURORO. The chassis may have been retuned at some time. Check the X78 frequency changer to see if it is oscillating. Fed the output from your signal generator to the anode of the X78 and swing it's modulated output around IF frequency. You should be able to see a strong modulated bar on the picture and the sound tone when you are around the correct IF frequencies and will confirm the IF strip is working.
You raster looks cramped at the top due to transformer miss match. You should be able to bring this into line by reducing the value of the KT33C bias resistor. Do not go too low. John. |
21st Feb 2015, 12:36 pm | #59 |
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Re: Marconiphone VRC74DA
Circuit diagrams of the RF unit in the "Birmingham" Marconi VT73A. The IFs are 34mc/s vision and 37.5Mc/s sound.
To establish if the local oscillator is working connect a test meter to the grid of the triode section of the X78, pin7. You can't connect the meter lead direct, this will simply stop the oscillator, so a resistor of say 100K must be inserted in series with the lead. With a working oscillator there should be about 4 volts negative present of the triode grid. There quite a few awful features about the design of these sets, plenty has been written about the subject. Just look at the where the contrast control is, the cathode of the RF amplifier. That should be the sensitivity control and the contrast control should be in the vision IF amplifier. DFWB. |
21st Feb 2015, 3:47 pm | #60 | |
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Re: Marconiphone VRC74DA
Hi John, thanks for the info on the diode, I think it may be O/C so a replacement will have to be found.
I have gone through all the channels on the Aurora, the sound only comes through on channel 4. I will dig out my signal generator and do as you suggest and report my findings. The raster is cramped at the top, not helped by the scan coils having slipped down the neck of the CRT. I cannot believe that HMV state that the scan coils should be held in place by rubber bands I will attend to the cramping and the many other issues that will be apparent when I finally get a picture on the screen. David, thanks for posting the circuits. It will be a great help, as none of the circuits I have quite match the RF unit. I will do as you suggest to see if the X78 is oscillating, if it is I will then get in with the signal generator as John suggests. Quote:
I think the best quality item is the cabinet! The dropper is of good quality though, as are the octal valve holders. shame about the rest of it If only Plessey had made the TV chassis as well as the radio, this would have been an easy restoration! One thing that has been proved, is the valves from it's HMV brother are all in good order. So after a suitable break to restore a more worthy set, I will have another go at getting that to work. Mark |
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