VIDOR CN429 - IF alignment excellent but RF issue - UK Vintage Radio Repair and Restoration Discussion Forum

6SN7WGTB · 14th Jan 2022, 9:03 pm

I'm just restoring a CN429.

I've replaced all the tubular waxies, and the elect.

Using two bench PSUs, one set to 1.4V for filaments, turned on first, and the other I then switch on at 50-ish and bring up to 90V.

Injecting a (400Hz) modulated 470kHz to the VC as per service instructions on MW, with a couple of iterations around the four IF slugs it peaked well and I have a very strong and undistorted, hiss- and crackle-free audio output.

However, putting 600 kHz in direct to the MW aerial connection I can get a poor and pretty distorted audio with the VC at 550+m - so not at the 500m.

Adjusting the coil slug does nothing (both coils test OK for continuity - ca. 2Ω and 5Ω). This is the coil with only one slug (L6 in VIDOR data, L3 in Trader). Trimmer no effect at 1500kHz.

Trying 1500kHz at 200m point - no output at all and the trimmer has no effect.

LW no apparent output at all - trimmer no effect.

So far as I can tell:
(a) The aerial are connected and show appropriate continuity and no shorts
(b) The LW/MW range switch appears to work
(c) I tried an alternative set of valves
(d) Anode voltages seem OK
(e) No VC shorts
(f) The two (mica?) C2 and C6 (VIDOR) caps are within 10%
(g) The 532p cap appears close to value, as does the 470p

I assume I have no oscillation?

This is my first valve radio restoration, so I'd value any pointers to avoid me doing a complete engine-out if there is something obvious I should be looking at.

BTW my plan is to use 10 x PP3 and 3 x 1.4V AA NiMH (kinder to filaments) in parallel once sorted to make it 'portable' and free from any mains interference.

Thanks.

Herald1360 · 14th Jan 2022, 9:50 pm

Tired DK96?

Have you tried listening for any LO signal using another set alongside the 429?

ronbryan · 14th Jan 2022, 10:04 pm

Have you tried a loose coupling between the signal generator and the frame aerial? You may be damping the tuned circuits or overloading the input by connecting direct to the MW aerial coil. The Trader sheet suggests dangling the generator output leads near the frame aerial or connecting just the generator live lead to the chassis frame if the first method does not give enough coupling.

Ron

6SN7WGTB · 15th Jan 2022, 10:30 am

Good ideas re the loose(r) coupling. I realise that I've overlooked that - and yes of course I can see the potential damping issue.

Will report back.

kalee20 · 15th Jan 2022, 10:44 am

Loose coupling will prevent you setting the RF trimmer properly. But you should still be able to hear an output and set the oscillator trimmers.

I'd say the DK96 is perhaps not oscillating,same as Herald1360.

6SN7WGTB · 15th Jan 2022, 1:26 pm

Quote:

Originally Posted by kalee20

I'd say the DK96 is perhaps not oscillating,same as Herald1360.

I did try a different DK96.

I'm not seeing any local resistors out of spec.

I've measured voltages around the DK96 but they're completely out:

Pin 2: a 83.6V (per spec)
Pin 3: g2 83V (vs. 25V) (but measured with DMM, not 500Ω/V per service sheet
Pin 5: g4 68V (vs. 47.7V) (ditto)

The pin 3 being high also suggests lack of oscillation? Am I right in assuming that if this WAS oscillating, then the 0.047µ cap to ground would pull the voltage down to a much lower value?

kalee20 · 15th Jan 2022, 1:55 pm

Pin 5 you could put down to using a higher resistance meter, but not pin 3 - it's just too far out.

Are the bypass capacitors to ground, OK or O/C?

Listening with another radio placed beside it, tuned to the higher frequency (shorter wavelength) end of MW, as you twiddle the radio-under-test around the lower frequency end of MW should elicit some squeals from the other radio, again as Herald says.

ms660 · 15th Jan 2022, 1:59 pm

The mixer anode and screen voltages look about right with the meter you're using, the oscillator's anode (g2) is at B+ potential which suggests that it might not be connected, eg: valve socket to valve pin if the valve itself is OK.

For RF alignment, loose coupling (as loose as you can get away with) is the key.

EDIT: Post crossed.

Lawrence.

6SN7WGTB · 15th Jan 2022, 2:27 pm

Well, well, found a broken valve socket pin...

Can't be sure that is it, but clearly a potential.

Funny how this ONE socket on the CN429 is bolted in...

@kalee20 both bypass caps are new, and tested before putting in.

Sideband · 15th Jan 2022, 4:14 pm

Quote:

Originally Posted by 6SN7WGTB

Well, well, found a broken valve socket pin...

Can't be sure that is it, but clearly a potential.

Very common with battery valve sets. You might be able to use an unused pin from another valveholder if it's the same type. I would be inclined to check the other valveholders as well.

Herald1360 · 15th Jan 2022, 5:01 pm

You've probably just found what relegated the set to someone's loft or shed back in the day. They probably did the sums on repair plus future running costs vs new set and splashed out on a newfangled transistor set!

Heatercathodeshort · 15th Jan 2022, 5:03 pm

Yes, very common fault with the excellent McMurdo valve holders. Had it a number of times especially if batteries have been left in it for years and the chemical fumes have attacked the metal pin, they just become brittle and break but look ok visually. As SB and Laurence mentioned you can often rob a pin from an unused position or a good spare MM valveholder. John..

6SN7WGTB · 15th Jan 2022, 8:06 pm

Will check them all.

There are signs of the 'grey mist' although it's very light, but/so hardening of the contacts is likely.

Overall, the very clear and strong IF response gives me hope.

6SN7WGTB · 2nd Feb 2022, 9:50 pm

Thanks to all.

Pleased to report that with new oscillator valve base all is now working.

The two oscillator mica caps are 15-20% out so the alignment is a bit 'end of adjustment' on MW and at the extent of padder TC it's 100m out on LW.

Sound and selectivity strong though, so worth me replacing those two caps when I have a moment. Unless anyone sees something else I should be doing.

14th Jan 2022, 9:03 pm	#1
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	VIDOR CN429 - IF alignment excellent but RF issue I'm just restoring a CN429. I've replaced all the tubular waxies, and the elect. Using two bench PSUs, one set to 1.4V for filaments, turned on first, and the other I then switch on at 50-ish and bring up to 90V. Injecting a (400Hz) modulated 470kHz to the VC as per service instructions on MW, with a couple of iterations around the four IF slugs it peaked well and I have a very strong and undistorted, hiss- and crackle-free audio output. However, putting 600 kHz in direct to the MW aerial connection I can get a poor and pretty distorted audio with the VC at 550+m - so not at the 500m. Adjusting the coil slug does nothing (both coils test OK for continuity - ca. 2Ω and 5Ω). This is the coil with only one slug (L6 in VIDOR data, L3 in Trader). Trimmer no effect at 1500kHz. Trying 1500kHz at 200m point - no output at all and the trimmer has no effect. LW no apparent output at all - trimmer no effect. So far as I can tell: (a) The aerial are connected and show appropriate continuity and no shorts (b) The LW/MW range switch appears to work (c) I tried an alternative set of valves (d) Anode voltages seem OK (e) No VC shorts (f) The two (mica?) C2 and C6 (VIDOR) caps are within 10% (g) The 532p cap appears close to value, as does the 470p I assume I have no oscillation? This is my first valve radio restoration, so I'd value any pointers to avoid me doing a complete engine-out if there is something obvious I should be looking at. BTW my plan is to use 10 x PP3 and 3 x 1.4V AA NiMH (kinder to filaments) in parallel once sorted to make it 'portable' and free from any mains interference. Thanks.

14th Jan 2022, 9:50 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: VIDOR CN429 - IF alignment excellent but RF issue Tired DK96? Have you tried listening for any LO signal using another set alongside the 429? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

14th Jan 2022, 10:04 pm	#3
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: VIDOR CN429 - IF alignment excellent but RF issue Have you tried a loose coupling between the signal generator and the frame aerial? You may be damping the tuned circuits or overloading the input by connecting direct to the MW aerial coil. The Trader sheet suggests dangling the generator output leads near the frame aerial or connecting just the generator live lead to the chassis frame if the first method does not give enough coupling. Ron

15th Jan 2022, 10:30 am	#4
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	Re: VIDOR CN429 - IF alignment excellent but RF issue Good ideas re the loose(r) coupling. I realise that I've overlooked that - and yes of course I can see the potential damping issue. Will report back.

15th Jan 2022, 10:44 am	#5
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: VIDOR CN429 - IF alignment excellent but RF issue Loose coupling will prevent you setting the RF trimmer properly. But you should still be able to hear an output and set the oscillator trimmers. I'd say the DK96 is perhaps not oscillating,same as Herald1360.

15th Jan 2022, 1:55 pm	#7
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: VIDOR CN429 - IF alignment excellent but RF issue Pin 5 you could put down to using a higher resistance meter, but not pin 3 - it's just too far out. Are the bypass capacitors to ground, OK or O/C? Listening with another radio placed beside it, tuned to the higher frequency (shorter wavelength) end of MW, as you twiddle the radio-under-test around the lower frequency end of MW should elicit some squeals from the other radio, again as Herald says.

15th Jan 2022, 1:59 pm	#8
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: VIDOR CN429 - IF alignment excellent but RF issue The mixer anode and screen voltages look about right with the meter you're using, the oscillator's anode (g2) is at B+ potential which suggests that it might not be connected, eg: valve socket to valve pin if the valve itself is OK. For RF alignment, loose coupling (as loose as you can get away with) is the key. EDIT: Post crossed. Lawrence. Last edited by ms660; 15th Jan 2022 at 2:13 pm.

15th Jan 2022, 2:27 pm	#9
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	Re: VIDOR CN429 - IF alignment excellent but RF issue Well, well, found a broken valve socket pin... Can't be sure that is it, but clearly a potential. Funny how this ONE socket on the CN429 is bolted in... @kalee20 both bypass caps are new, and tested before putting in.

15th Jan 2022, 5:01 pm	#11
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: VIDOR CN429 - IF alignment excellent but RF issue You've probably just found what relegated the set to someone's loft or shed back in the day. They probably did the sums on repair plus future running costs vs new set and splashed out on a newfangled transistor set! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

15th Jan 2022, 5:03 pm	#12
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: VIDOR CN429 - IF alignment excellent but RF issue Yes, very common fault with the excellent McMurdo valve holders. Had it a number of times especially if batteries have been left in it for years and the chemical fumes have attacked the metal pin, they just become brittle and break but look ok visually. As SB and Laurence mentioned you can often rob a pin from an unused position or a good spare MM valveholder. John..

15th Jan 2022, 8:06 pm	#13
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	Re: VIDOR CN429 - IF alignment excellent but RF issue Will check them all. There are signs of the 'grey mist' although it's very light, but/so hardening of the contacts is likely. Overall, the very clear and strong IF response gives me hope.

2nd Feb 2022, 9:50 pm	#14
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	Re: VIDOR CN429 - IF alignment excellent but RF issue Thanks to all. Pleased to report that with new oscillator valve base all is now working. The two oscillator mica caps are 15-20% out so the alignment is a bit 'end of adjustment' on MW and at the extent of padder TC it's 100m out on LW. Sound and selectivity strong though, so worth me replacing those two caps when I have a moment. Unless anyone sees something else I should be doing.