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Old 21st Sep 2016, 6:15 am   #201
sobell1980
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Default Re: HMV 653 Broadcaster all wave.

Thanks, Lawrence. I shall try this tonight after work.
I think I get the math. I shall read through it several more times throughout today and put it into practice after work. Thanks for your reply.
Dave.
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Old 21st Sep 2016, 8:53 am   #202
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Default Re: HMV 653 Broadcaster all wave.

Basically the oscillator frequency should be above the RF tuned frequency (the signal) by an amount that equals the IF.

In some SW receivers the oscillator frequency is below the signal frequency on the high band such as 10 meters, but it's not a common practice.

The oscillator frequency and the signal frequency must track together to maintain the difference frequency which is the IF.

That's why it's important when attempting any alignment to check that the IF tuned circuits are set to the specified IF before attempting the oscillator and RF alignment, many have got into mess trying to do it the other way round.

Lawrence.
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Old 21st Sep 2016, 12:17 pm   #203
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Default Re: HMV 653 Broadcaster all wave.

These posts seem to be contradictory:
#195 you say that SW and MW are working OK but LW is not oscillating. Did you re-align SW and MW now you have changed the micas for ceramics?
#197 you say that the SW and LW waveband switch positions are reversed?

A rough check with your scope on the oscillator anode (x10 probe) should show if the oscillator is working in all 3 positions and confirm if the switch position is incorrect as SW will be a much higher frequency.

With your comment in #197, is it the SW band that is not working?
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Old 21st Sep 2016, 7:50 pm   #204
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Default Re: HMV 653 Broadcaster all wave.

Hi Lawrence.
I've scoped the set. It was set to LW at 1500 metres.
Please see the pics I took of the trace and settings.
Foolishly I had the time base mag. Set at x5. You can see it was selected (pushed in) . This will effect the math.
Would you kindly be able to run through the math with me with my settings as they are on my scope? I think the x5 will alter it from the previous example you gave me to follow. I can write it down in my notebook for future workings out then. Appreciate your time and help.
Many thanks and hope I'm not being to much of a pain and troublesome. I do appreciate all your help and time as I do everyone else on here.
Dave.
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Old 21st Sep 2016, 8:32 pm   #205
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Default Re: HMV 653 Broadcaster all wave.

PJL, Sorry for any confusion. Yes it would appear the LW switch contacts are being made in the SW position. In the SW position, contacts s1,s5 ,s8 and s11 are being made. And in LW position the SW contacts are being made. I've proved this by putting the switch in the SW position and measuring continuity from the bottom of c12 through either end of L10 I have continuity. If I switch the wavechange switch from SW to any other waveband it goes open circuit.so S11 is being switched when set to SW.
I have changed the mica caps but omitted the extra one across C33 as it wasn't shown in the diagram. This is where my problem lies with aligning LW I think.
I have no oscillation on SW which is now the LW position. However I'm not sure if my scope will cover this which maybe why I can't see it. I have aligned IF first as per service data, then MW then LW. SW is last. I'm receiving strong stations on MW, Absolute radio and talk sport etc. However no Radio 4 on LW. I'm sure it's to do with this extra cap across C33. I measured the old one at 22 pF.
But rather than guess work and fitting caps randomly I wanted to use my scope to find out why it's not there at 1500 metres and exactly what frequency I do have at 1500 metres so I can diagnose where the fAult lies and learn something at the same time. It may even indicate what size cap I need to fit across C33 to tune LW circuit correctly. Many thanks.
Dave.
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Old 21st Sep 2016, 8:37 pm   #206
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Default Re: HMV 653 Broadcaster all wave.

My scope reading was taken from pin g2, oscillator anode.
Dave.
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Old 21st Sep 2016, 8:57 pm   #207
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Default Re: HMV 653 Broadcaster all wave.

Re: Post#204....Maths isn't my strong point either but here goes (only done to four decimal points):

If that's the oscillator waveform then looking at the 'scope screen starting with the second +ve peak from the right hand side gives 13 complete time periods over 7 divisions, that equals 1.8571 periods per division (13/7)

That's with x5 magnification, so at x1 magnification it would equal 9.2855 time periods per division (5*1.8571)

Continuing with the x1 deduction figure:

The 'scopes timebase is set to 10us per division, so that means there's 9.2855 time periods per 10us, that means a single time period is 1.0769us (10/9.2855)

The frequency in mHz equals 1/t (t = the time period in microseconds) therefore the frequency equals 1/1.0769 which equals .9285 mHz which is 928.5 kHz.

Assuming the IF's are in proper alignment the IF will be 465 kHz, so the RF would need to be tuned to 928.5 kHz minus 465 kHz which is 463.5 kHz.

So based on the above, the oscillator frequency is too high for reception on 1500 meters.

If I've got that right, see what the rest of the gang think.

This post might cross.

Lawrence.

Last edited by ms660; 21st Sep 2016 at 9:12 pm. Reason: correction and addition
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Old 21st Sep 2016, 9:25 pm   #208
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Default Re: HMV 653 Broadcaster all wave.

If that's right it strongly suggests that more C is required across the LW oscillator coil to get the frequency down to 198+465=663kHz. If the gang's at about half way, it'll have about 250pF across the coil. If that's giving 928.5kHz, then L will be about 120uH. To get that down to 663kHz will need a total of 493pF, so about an extra 240pF. Making reasonable assumptions about strays suggests that the extra C should 220pF.

Hmmmm, 220.....22....?

Dodgy cap or old mistake? Who knows, but try 220pF and see!
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Old 23rd Sep 2016, 12:06 pm   #209
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Default Re: HMV 653 Broadcaster all wave.

Dave, here's a link to a video showing frequency measurement with a 'scope, you'll see the same kind of approach as I described to you earlier, it should help clarify etc:

https://www.youtube.com/watch?v=OQMnjaMSKn8

By the way, the bloke who does those videos is well respected and he has more videos covering electronics in general.

Lawrence.
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Old 25th Sep 2016, 5:58 pm   #210
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Default Re: HMV 653 Broadcaster all wave.

Lawrence,
Thanks for the video link. Well worth watching to anyone who thought that working out this is tough. I have made notes and run through some examples on my own scope and gone over and over it till its sunk in and I can do it without really thinking about it. Getting the right amount of decimal places so you got the correct frequency at the end. Your example, Lawrence combined with the video has made things very clear. All part my learning curve.

So, after this I tackled the problem of LW on this set being wildly out of tune due to not enough capacitance across C33. I used my scope and the math I learned, and guess what?
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Old 4th Oct 2016, 4:59 am   #211
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Default Re: HMV 653 Broadcaster all wave.

The set is now working well on all wavebands, even though SW position and LW position are swopped around in regards to the wavechange switch. Perhaps it is another wavechange knob from another set but certainly matches the others so not quite sure what went on here.
I'm awaiting on a braided mains cable to install then the set chassis can be installed and put on test for several hours. I shall look forwards to writing my success story.
Many thanks to everyone who has helped, given advice and followed this throughout this lengthy repair.
Many thanks.
Dave.
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Old 4th Oct 2016, 12:53 pm   #212
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Default Re: HMV 653 Broadcaster all wave.

Err what exactly fixed it? More C?
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Old 30th Oct 2016, 8:23 pm   #213
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Default Re: HMV 653 Broadcaster all wave.

Yes more capacitance Chris solved it as you rightly said before in an earlier post. 220pF solved. It was good fun trying different values and seeing the results differ on my scope, using a bit of math to achieve the correct frequency and trace on my scope. This set is now finished. It is on lengthy testing before being given to a friend as a Christmas present who requested a vintage radio from me a while ago. He has no idea and thinks I've forgotten I guess.
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