Cossor ganging oscillator problems - UK Vintage Radio Repair and Restoration Discussion Forum

vinrads · 7th Apr 2012, 12:51 pm

Cossor 343, 1930s vintage.

I tried it the other day, connected it up to my frequency counter the dial frequency is way out it seems to be outputting to high.

Anybody had any experience repairing this model?

David Simpson · 7th Apr 2012, 3:01 pm

Hello vinrads, Yes - I've renovated a 343. What do you mean "outputting too high" ? Possibly the o/p switch is stuck on pos'n 1(high), or the att'n pot is faulty. Sw. pos'n 3 = 5mV, 2 = 50mV, 1 = 500mV, then further graded 0 - 10 via the 100ohm pot.
Is the error fixed over all 5 freq ranges ? It might just be that the dial has slipped on the shaft due to a loose securing nut.

Regards, David

Cobaltblue · 7th Apr 2012, 3:38 pm

Hi

You probably already know that the circuit and handbook are available on Jons site here

Assuming you mean the frequency is too high this has a reactance stage for FM modulation it's possible that it's DC conditions are upset for some reason.

What percentage high is it reading?

Cheers

Mike T

vinrads · 7th Apr 2012, 11:46 pm

Hi sorry for the confusion ,it's the freq that is not right the dial is aligned OK attenuator works OK .

I will check how far the freq is off ,Mick.

Hermit6345 · 8th Apr 2012, 9:00 am

I seem to remember that I found it quite difficult to measure the output frequency using a counter simply because there are other frequencies present in the output which confused the counter. You can measure the 380kHz from the VRV at the input to the mixer and you can also measure the frequency of the MO using loose coupling to avoid pulling the oscillator. I guess that the MO runs at 380kHz above the required output frequency, but it was a long time ago when I did mine and I have a poor memory. If the two oscillators are correct, then at least one component of the output must be correct also. I think I used my BC221 in the end but I wished that I had a spectrum analyser.

Mine was a very early model that differed significantly with my schematic in the VRV, mixer and output tuning arrangements. It didn't have a front panel "deviation" control either. That was inside the box which didn't help matters.

Ian

David Simpson · 8th Apr 2012, 11:29 am

Yes, I'd forgot, Ian is spot-on, the ref osc. runs at 380KHz. The 4THA mixer grid has a band-pass filter L3/R13 to supress harmonics. Best check that.
The triode section forms the variable L.O.
The manual says the HT should be 200V(Using an AVO7 ?). Originally mine was only 161V(AVO8), but after fitting a nice big 100uf smoothing cap - got a nice smooth 186V.
Carefull adjustment of the trig level & attenuator of your counter might be needed. These modern Jappo/Chinko ones dont like it up em. You could make a temp probe of a 120K resistor in series with a 0.05uF capacitor.

Regards, David

vinrads · 8th Apr 2012, 6:28 pm

Hi thanks Ian and Dave for the useful info. Mick.

David Simpson · 8th Apr 2012, 6:53 pm

Mick, I think that there is just a small band of us Forum guys with a passion for Cossor 343 & it's partner - the 339/3339 scope. Please let us know how you get on.
If you need any circuit diagrams or manual copies - let us know.

Regards, David

vinrads · 9th Apr 2012, 7:55 am

Hi David I was just wondering in your experience would the 343 operate with a modern scope ie a Hameg705 or would it have to be a valve scope,Mick.

Hermit6345 · 9th Apr 2012, 9:38 am

Hi Mick, the main problem with using a modern scope is the amount of timebase voltage that the 343 requires in order to sweep a sensible amount. My OS300 for example has a timebase output of about 4 volts whereas the 343 requires, IIRC ,up-wards of 100 volts. You could always build a little valve/transistor amp to overcome this problem though. My 343 worked OK with my little CT52 valve scope. Another problem with many modern scopes is that they don't have a timebase output port. One solution might be to build an external timebase generator with an output to trigger the scope as well as a high level sweep output for the 343. However, I am sure that others will have solved the problem.

Ian

David Simpson · 9th Apr 2012, 2:32 pm

As the Manual says, you'll need an imput at "X" on the 343 of 235V from a 339 or 3339. You could try bypassing R7(2Mohm)(not C49 or C6) with a temp pot, & see if it'll drive V1 with a lower voltage. But I reckon you'll still need a few 10's of volts.
Possibly an amplifier that'll handle non-sinusidal waveforms could be used to amplify your Hameg's o/p.
My Hameg 605 works fine with the sync o/p from a Marconi TF995A/5, but the 339 only likes it's sibling - the 343.

Regards, David

7th Apr 2012, 12:51 pm	#1
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,760	Cossor ganging oscillator problems Cossor 343, 1930s vintage. I tried it the other day, connected it up to my frequency counter the dial frequency is way out it seems to be outputting to high. Anybody had any experience repairing this model? Last edited by vinrads; 7th Apr 2012 at 12:58 pm.

7th Apr 2012, 3:01 pm	#2
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	Re: Cossor ganging oscillator problems Hello vinrads, Yes - I've renovated a 343. What do you mean "outputting too high" ? Possibly the o/p switch is stuck on pos'n 1(high), or the att'n pot is faulty. Sw. pos'n 3 = 5mV, 2 = 50mV, 1 = 500mV, then further graded 0 - 10 via the 100ohm pot. Is the error fixed over all 5 freq ranges ? It might just be that the dial has slipped on the shaft due to a loose securing nut. Regards, David

7th Apr 2012, 3:38 pm	#3
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,865	Re: Cossor ganging oscillator problems Hi You probably already know that the circuit and handbook are available on Jons site here Assuming you mean the frequency is too high this has a reactance stage for FM modulation it's possible that it's DC conditions are upset for some reason. What percentage high is it reading? Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

7th Apr 2012, 11:46 pm	#4
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,760	Re: Cossor ganging oscillator problems Hi sorry for the confusion ,it's the freq that is not right the dial is aligned OK attenuator works OK . I will check how far the freq is off ,Mick.

8th Apr 2012, 9:00 am	#5
Hermit6345 Rest in Peace Join Date: Oct 2006 Location: North Wiltshire, UK. Posts: 638	Re: Cossor ganging oscillator problems I seem to remember that I found it quite difficult to measure the output frequency using a counter simply because there are other frequencies present in the output which confused the counter. You can measure the 380kHz from the VRV at the input to the mixer and you can also measure the frequency of the MO using loose coupling to avoid pulling the oscillator. I guess that the MO runs at 380kHz above the required output frequency, but it was a long time ago when I did mine and I have a poor memory. If the two oscillators are correct, then at least one component of the output must be correct also. I think I used my BC221 in the end but I wished that I had a spectrum analyser. Mine was a very early model that differed significantly with my schematic in the VRV, mixer and output tuning arrangements. It didn't have a front panel "deviation" control either. That was inside the box which didn't help matters. Ian

8th Apr 2012, 11:29 am	#6
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	Re: Cossor ganging oscillator problems Yes, I'd forgot, Ian is spot-on, the ref osc. runs at 380KHz. The 4THA mixer grid has a band-pass filter L3/R13 to supress harmonics. Best check that. The triode section forms the variable L.O. The manual says the HT should be 200V(Using an AVO7 ?). Originally mine was only 161V(AVO8), but after fitting a nice big 100uf smoothing cap - got a nice smooth 186V. Carefull adjustment of the trig level & attenuator of your counter might be needed. These modern Jappo/Chinko ones dont like it up em. You could make a temp probe of a 120K resistor in series with a 0.05uF capacitor. Regards, David

8th Apr 2012, 6:28 pm	#7
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,760	Re: Cossor ganging oscillator problems Hi thanks Ian and Dave for the useful info. Mick.

8th Apr 2012, 6:53 pm	#8
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	Re: Cossor ganging oscillator problems Mick, I think that there is just a small band of us Forum guys with a passion for Cossor 343 & it's partner - the 339/3339 scope. Please let us know how you get on. If you need any circuit diagrams or manual copies - let us know. Regards, David

9th Apr 2012, 7:55 am	#9
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,760	Re: Cossor ganging oscillator problems Hi David I was just wondering in your experience would the 343 operate with a modern scope ie a Hameg705 or would it have to be a valve scope,Mick.

9th Apr 2012, 9:38 am	#10
Hermit6345 Rest in Peace Join Date: Oct 2006 Location: North Wiltshire, UK. Posts: 638	Re: Cossor ganging oscillator problems Hi Mick, the main problem with using a modern scope is the amount of timebase voltage that the 343 requires in order to sweep a sensible amount. My OS300 for example has a timebase output of about 4 volts whereas the 343 requires, IIRC ,up-wards of 100 volts. You could always build a little valve/transistor amp to overcome this problem though. My 343 worked OK with my little CT52 valve scope. Another problem with many modern scopes is that they don't have a timebase output port. One solution might be to build an external timebase generator with an output to trigger the scope as well as a high level sweep output for the 343. However, I am sure that others will have solved the problem. Ian

9th Apr 2012, 2:32 pm	#11
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	Re: Cossor ganging oscillator problems As the Manual says, you'll need an imput at "X" on the 343 of 235V from a 339 or 3339. You could try bypassing R7(2Mohm)(not C49 or C6) with a temp pot, & see if it'll drive V1 with a lower voltage. But I reckon you'll still need a few 10's of volts. Possibly an amplifier that'll handle non-sinusidal waveforms could be used to amplify your Hameg's o/p. My Hameg 605 works fine with the sync o/p from a Marconi TF995A/5, but the 339 only likes it's sibling - the 343. Regards, David