Baird M648 dual standard set - UK Vintage Radio Repair and Restoration Discussion Forum

Hunts smoothing bomb · 5th Apr 2011, 8:43 am

Hi everyone,

I'm working on the above set and I have changed all the unreliable wax caps including the Hunts types in moulded Bakelite or whatever plastic it is.

The set powers up but I have a strange problem that I am struggling to pinpoint.

I have measured the boost voltage from the slider of the width control to chassis and have 960vdc. I spoke with Tas and he says it sounds high, the other problem is that I have far too much width and after measuring EHT with my EHT probe I only have 13kv.
This set has a 23” CRT and I would expect to find around 20kv?

Does anyone have any info on what the boost voltage should be as in the trader sheet and red books it does not mention it.

This set also uses a PY88 boost diode which is rated for higher insulation between cathode and heaters than the PY800/801 so I’m wondering if it is supposed to have a higher than normal boost voltage?

As to low EHT I am a bit stumped, I’ve tried changing the DY87 and no difference, surely I can’t have a shorted turn in the LOPT as I would have no EHT right?

Any help will gratefully be appreciated.

Cheers

Mike Phelan · 5th Apr 2011, 8:51 am

That is too high, Lee.

Is it the same on 625 and 405? I ask because the system switches on these sometimes don't quite 'go home' properly because of the rather crude rocking bar across the top of the chassis that is operated by the cam on the tuner.

High boost and low EHT could be a harmonic tuning cap problem on the LOPT. A single shorted turn on the overwind will not kill the EHT completely, but wouldn't make the boost 200 volts too high.

Hunts smoothing bomb · 5th Apr 2011, 11:52 am

Hi Mike,
I have replaced the harmonic tuning cap as it was a hunts bakolite/plastic moulded thing, it was rated at 0.0022uf at 1.5kv I replaced it with two paralell connected 0.001uf 1250v caps so overall 0.002uf. Could 0.0002uf difference give me the problem I'm experiencing

?

I will check the system switches and on 625 tonight then report back here.
Cheers

gezza123 · 5th Apr 2011, 12:13 pm

HI.Lee,best I can offer the VDR1 -part no.RE4413 gives two much width.
Found circuit in attic in a bit of a state.
here is part of the line circuit.400 series.+ faults list.
The boostshould be about 800v and final anode 20kv.

the last scan 005 is of a baird 660 whitch is the same circuit with a write up of how it works,the reference to the components are different if you want I can scan the circuit of the 660 line to help you.

good luck Gezza

gezza123 · 5th Apr 2011, 12:32 pm

HI Lee. here is the baird 660 line stage.
gezza

Hunts smoothing bomb · 5th Apr 2011, 2:58 pm

Hi Gezza thanks for this,

I have noticed that it is in the info for the 660 that it states 800v for the boost voltage and that the boost diode in that set is a PY800, mine is a PY88, does that mean that the boost voltage will be higher on my model or not?

Cheers

Tazman1966 · 5th Apr 2011, 5:59 pm

Hi Lee.

According to an article by the late Les Lawry-Johns on servicing these sets, the boost line is shown as 730V measurable at the junction of C129, C131 and the top end of R101, the width control. Hope that helps.

Hunts smoothing bomb · 5th Apr 2011, 7:28 pm

Hi chaps,

I have just measured again after switching between standards a few times (set off!) and I now have a boost voltage of 910vdc on 405 and 780vdc on 625.

I haven't measured the EHT on both standards yet, that's coming next.

Cheers

Hunts smoothing bomb · 5th Apr 2011, 7:54 pm

Right,

EHT on 625 is 15kv and 13kv on 405, can I bridge high value resistors across the VDR in the width circuit so I can see what happens with boost and EHT voltages?

Cheers

Heatercathodeshort · 5th Apr 2011, 8:14 pm

The PY88 boost diode is just an uprated version of the PY800. The big difference is the heater voltage. 19v PY800. 30v PY88. I think the correct valve would be a PY800. I don't think the PY88 was available untill the late 60's. The fitting of a PY88 will reduce the voltage available in the heater chain and this could give reduced performance with an aged receiver. It will have no effect on boost voltage. J.

gezza123 · 5th Apr 2011, 8:24 pm

HI: No,

you can't bridge the VDR1 as it supossed to stablize the voltage going back to the grid if you get two high a volts then the EHT could
rocket,so please be very careful.I think the boost is about right ,but someone may pull me up on it,as I have not looked at a tv for 20 or so years my brain may be a bit dim.

A PY88 heater is 300 mA at 30volts,with a P.I.V.max=7.5Kv.vh-k(pk)max cathode positive =6.6Kv
A PY800 is 300mA at19v heater and has a P.I.V.max = 6.6Kv.vh-k(pk)max cathode positive = 6.0Kv
The line is fed to the grid of V10 PL500, and its anode is fed to the LOPTX
then the Boost diode DY88 V11,this charges the boost diode C129 during the
flyback around 800v to 1Kv and is generated at the top cap of PL500 V10,and is then boosted to aprox 15Kv to 25Kv depending on the LOPTX /CRT size.
IE ,AW47-91 = 15.5Kv
AW59-91 = 16 -18Kv
If the EHT is low you will get a wide picture,
I put the 660 model on to give you a guide of how the set worked it is allmost identical but using different valves
hope this helps.Gezza.

Hunts smoothing bomb · 5th Apr 2011, 9:32 pm

Hello chaps,

Well the time has come for me to Er...... eat a rather large slice of humble pie!

I have been racking my brains and going through the circuit and all the component changes I have done, (many were a month or so ago) and then I spotted it,

Capacitor 130 is a 0.002uf 1.5kv device and connects between a section of the primary on the LOPT via the system switch and two capacitors for scan correction on both standards and I had replaced it with a 0.02uf type! only a factor of 10 out then!

I am so annoyed that I produced this fault but at the same time pleased I have found it and rectified it.

The set now is performing well with a testcard of the correct aspect ratio

Many thanks to everyone for their help, there will be screen shots to come but not for a couple of days as I am busy the next few evenings.

Cheers everyone

gezza123 · 6th Apr 2011, 3:37 pm

Well done lee.

.Gezza

Tazman1966 · 6th Apr 2011, 5:43 pm

Don't beat yourself up too much! We've all done it one time or another

Heatercathodeshort · 6th Apr 2011, 6:48 pm

Hello Chris! Join the club..We ALL make mistakes like this and I still do. It is very easy and that is why I suggest that newcomers to servicing replace a single component at a time. Mistakes can then be very easily back tracked. I know your not new to this and it is comforting to know that others make similar mistakes to me. At least you admitted to it rather than just letting the thread die away with no further comment or explanation. Very annoying that. Cheers, John.

Hunts smoothing bomb · 9th Apr 2011, 10:07 am

Morning all,

Here are some pics as promised, Doesn't look too bad considering that the line linearity sleeve is missing and the fact that there are no picture centring magnets fitted as they broke when I was trying to adjust them

Never mind I have a linearity sleeve and magnets on the way thanks to other forum members, they know who they are

Cheers

Hunts smoothing bomb · 9th Apr 2011, 10:45 am

And a few more...

Mike Phelan · 9th Apr 2011, 5:01 pm

Looking good, Lee. Sorry for being late in replying to your question as not been around much this week.

Focus Diode · 9th Apr 2011, 5:08 pm

Lovely looking set. Well done on getting it going so well.

It looks like a console version of a Baird d/s table set we had at school I saw working in 1978. Gave a superb picture.

Brian

Hunts smoothing bomb · 10th Apr 2011, 4:50 pm

Hi all,

I have another problem on this set, on 625 lines it is unwatchable, the picture is so faint in the background of noise and also the sound does not come i at the same tuning point as the picture

I took the cans off the UHF tuner valves to find the PC88 up to air, it had a cracked base so I fitted a used replacement which I had.

The results are marginally better, I think a realignment of the tuner is probably called for, what do others think?

Cheers

5th Apr 2011, 8:43 am	#1
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Baird M648 dual standard set Hi everyone, I'm working on the above set and I have changed all the unreliable wax caps including the Hunts types in moulded Bakelite or whatever plastic it is. The set powers up but I have a strange problem that I am struggling to pinpoint. I have measured the boost voltage from the slider of the width control to chassis and have 960vdc. I spoke with Tas and he says it sounds high, the other problem is that I have far too much width and after measuring EHT with my EHT probe I only have 13kv. This set has a 23” CRT and I would expect to find around 20kv? Does anyone have any info on what the boost voltage should be as in the trader sheet and red books it does not mention it. This set also uses a PY88 boost diode which is rated for higher insulation between cathode and heaters than the PY800/801 so I’m wondering if it is supposed to have a higher than normal boost voltage? As to low EHT I am a bit stumped, I’ve tried changing the DY87 and no difference, surely I can’t have a shorted turn in the LOPT as I would have no EHT right? Any help will gratefully be appreciated. Cheers __________________ Lee

5th Apr 2011, 8:51 am	#2
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Baird M648 dual standard set That is too high, Lee. Is it the same on 625 and 405? I ask because the system switches on these sometimes don't quite 'go home' properly because of the rather crude rocking bar across the top of the chassis that is operated by the cam on the tuner. High boost and low EHT could be a harmonic tuning cap problem on the LOPT. A single shorted turn on the overwind will not kill the EHT completely, but wouldn't make the boost 200 volts too high. __________________ Mike.

5th Apr 2011, 11:52 am	#3
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Baird M648 dual standard set Hi Mike, I have replaced the harmonic tuning cap as it was a hunts bakolite/plastic moulded thing, it was rated at 0.0022uf at 1.5kv I replaced it with two paralell connected 0.001uf 1250v caps so overall 0.002uf. Could 0.0002uf difference give me the problem I'm experiencing? I will check the system switches and on 625 tonight then report back here. Cheers __________________ Lee

5th Apr 2011, 2:58 pm	#6
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Baird M648 dual standard set Hi Gezza thanks for this, I have noticed that it is in the info for the 660 that it states 800v for the boost voltage and that the boost diode in that set is a PY800, mine is a PY88, does that mean that the boost voltage will be higher on my model or not? Cheers __________________ Lee

5th Apr 2011, 5:59 pm	#7
Tazman1966 Nonode Join Date: May 2006 Location: St Albans, Herts, UK. Posts: 2,193	Re: Baird M648 dual standard set Hi Lee. According to an article by the late Les Lawry-Johns on servicing these sets, the boost line is shown as 730V measurable at the junction of C129, C131 and the top end of R101, the width control. Hope that helps. __________________ All the very best, Tas

5th Apr 2011, 7:28 pm	#8
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Baird M648 dual standard set Hi chaps, I have just measured again after switching between standards a few times (set off!) and I now have a boost voltage of 910vdc on 405 and 780vdc on 625. I haven't measured the EHT on both standards yet, that's coming next. Cheers __________________ Lee

5th Apr 2011, 7:54 pm	#9
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Baird M648 dual standard set Right, EHT on 625 is 15kv and 13kv on 405, can I bridge high value resistors across the VDR in the width circuit so I can see what happens with boost and EHT voltages? Cheers __________________ Lee

5th Apr 2011, 8:14 pm	#10
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Baird M648 dual standard set The PY88 boost diode is just an uprated version of the PY800. The big difference is the heater voltage. 19v PY800. 30v PY88. I think the correct valve would be a PY800. I don't think the PY88 was available untill the late 60's. The fitting of a PY88 will reduce the voltage available in the heater chain and this could give reduced performance with an aged receiver. It will have no effect on boost voltage. J.

5th Apr 2011, 8:24 pm	#11
gezza123 Heptode Join Date: Mar 2010 Location: Eastham, Wirral, Merseyside, UK. Posts: 788	Re: Baird M648 dual standard set HI: No, you can't bridge the VDR1 as it supossed to stablize the voltage going back to the grid if you get two high a volts then the EHT could rocket,so please be very careful.I think the boost is about right ,but someone may pull me up on it,as I have not looked at a tv for 20 or so years my brain may be a bit dim. A PY88 heater is 300 mA at 30volts,with a P.I.V.max=7.5Kv.vh-k(pk)max cathode positive =6.6Kv A PY800 is 300mA at19v heater and has a P.I.V.max = 6.6Kv.vh-k(pk)max cathode positive = 6.0Kv The line is fed to the grid of V10 PL500, and its anode is fed to the LOPTX then the Boost diode DY88 V11,this charges the boost diode C129 during the flyback around 800v to 1Kv and is generated at the top cap of PL500 V10,and is then boosted to aprox 15Kv to 25Kv depending on the LOPTX /CRT size. IE ,AW47-91 = 15.5Kv AW59-91 = 16 -18Kv If the EHT is low you will get a wide picture, I put the 660 model on to give you a guide of how the set worked it is allmost identical but using different valves hope this helps.Gezza.

5th Apr 2011, 9:32 pm	#12
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Baird M648 dual standard set Hello chaps, Well the time has come for me to Er...... eat a rather large slice of humble pie! I have been racking my brains and going through the circuit and all the component changes I have done, (many were a month or so ago) and then I spotted it, Capacitor 130 is a 0.002uf 1.5kv device and connects between a section of the primary on the LOPT via the system switch and two capacitors for scan correction on both standards and I had replaced it with a 0.02uf type! only a factor of 10 out then! I am so annoyed that I produced this fault but at the same time pleased I have found it and rectified it. The set now is performing well with a testcard of the correct aspect ratio Many thanks to everyone for their help, there will be screen shots to come but not for a couple of days as I am busy the next few evenings. Cheers everyone __________________ Lee

6th Apr 2011, 3:37 pm	#13
gezza123 Heptode Join Date: Mar 2010 Location: Eastham, Wirral, Merseyside, UK. Posts: 788	Re: Baird M648 dual standard set Well done lee..Gezza

6th Apr 2011, 5:43 pm	#14
Tazman1966 Nonode Join Date: May 2006 Location: St Albans, Herts, UK. Posts: 2,193	Re: Baird M648 dual standard set Don't beat yourself up too much! We've all done it one time or another __________________ All the very best, Tas

6th Apr 2011, 6:48 pm	#15
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Baird M648 dual standard set Hello Chris! Join the club..We ALL make mistakes like this and I still do. It is very easy and that is why I suggest that newcomers to servicing replace a single component at a time. Mistakes can then be very easily back tracked. I know your not new to this and it is comforting to know that others make similar mistakes to me. At least you admitted to it rather than just letting the thread die away with no further comment or explanation. Very annoying that. Cheers, John.

9th Apr 2011, 10:07 am	#16
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Baird M648 dual standard set Morning all, Here are some pics as promised, Doesn't look too bad considering that the line linearity sleeve is missing and the fact that there are no picture centring magnets fitted as they broke when I was trying to adjust them Never mind I have a linearity sleeve and magnets on the way thanks to other forum members, they know who they are Cheers Attached Thumbnails __________________ Lee

9th Apr 2011, 10:45 am	#17
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Baird M648 dual standard set And a few more... Attached Thumbnails __________________ Lee

9th Apr 2011, 5:01 pm	#18
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Baird M648 dual standard set Looking good, Lee. Sorry for being late in replying to your question as not been around much this week. __________________ Mike.

9th Apr 2011, 5:08 pm	#19
Focus Diode Octode Join Date: Jan 2006 Location: Borough of Gateshead, UK. Posts: 1,420	Re: Baird M648 dual standard set Lovely looking set. Well done on getting it going so well. It looks like a console version of a Baird d/s table set we had at school I saw working in 1978. Gave a superb picture. Brian

10th Apr 2011, 4:50 pm	#20
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Baird M648 dual standard set Hi all, I have another problem on this set, on 625 lines it is unwatchable, the picture is so faint in the background of noise and also the sound does not come i at the same tuning point as the picture I took the cans off the UHF tuner valves to find the PC88 up to air, it had a cracked base so I fitted a used replacement which I had. The results are marginally better, I think a realignment of the tuner is probably called for, what do others think? Cheers __________________ Lee