Eddystone 840C tuning indicator. - UK Vintage Radio Repair and Restoration Discussion Forum

Lengman_rich · 28th Dec 2014, 8:10 pm

Dear Colleagues!
I have appeared in the collection of beautiful and in excellent condition Eddystone 840C. Unfortunately it did not work on the tuning indicator DM71 (V4) - it is always in the same state - lights all the bars. What is the reason for this failure, I can not define itself. You tell me what to do?
Valery.

g4aaw pete · 28th Dec 2014, 10:03 pm

Hello Valery

The tuning indicator on the Eddystone 840 series is well known for being very insensitive.
It needs a very strong signal to give an indication.

Some circuit changes are necessary for it to become useful as an indicator.

You may be able to find some information on the Eddystone user group site:

http://www.eddystoneusergroup.org.uk/

Good luck
Pete

JHGibson · 28th Dec 2014, 10:34 pm

I had the luck to find a pristine 840C, but it had the usual problem of an insensitive magic eye.
I downloaded the spec sheet for the DM71 and it gave the circuit diagram that Eddystone uses but that particular circuit required 15 volts or more on the AGC line to extinguish the luminous bar while the 840 only has about 4 volts AGC with a strong station tuned in.
Looking some more at the spec sheet I saw that if the DM71 plate voltage is reduced to about 60, then a AGC voltage of 7 will extinguish the eye.
To obtain a stiff 60 volts for the DM71 plate, I jumpered a 10K resistor across the existing 470K plate resistor and connected another 10K resistor from the plate of the DM71 to chassis.
This modification made the magic eye more responsive to tuning.
John.

Lengman_rich · 30th Dec 2014, 9:42 pm

Yes, thank you for your replies friends. Unfortunately the state of adjustment indicator does not depend on the signal strength. This is a problem. Once I watched this receiver - it worked correctly. Then I bought it already with these shortcomings.
John, what do you think could be causing the problem capacitance C56?

P.S. I thank the moderator for the creation of a separate topic on my question.

JHGibson · 31st Dec 2014, 5:00 pm

If you are tuned to a strong station there will be several negative volts across C56. You will need a high impedance voltmeter to do that. If no volts across C56 then it could be a short circuit. Try substituting a good capacitor for C56 in that case.
John.

Lengman_rich · 31st Dec 2014, 6:54 pm

I understand you correctly: C56 may be the reason?!
Valery.

JHGibson · 1st Jan 2015, 4:04 pm

Rich, what is the voltage across C56 when the receiver is tuned to a strong station?
John.

Lengman_rich · 1st Jan 2015, 6:33 pm

John's question I can answer only 2 weeks. By the way, a better measure the voltage with a voltmeter or a tube can be ordinary?

Herald1360 · 2nd Jan 2015, 11:13 am

I think the answer to your question is that either a high impedance (10 megohm) digital voltmeter, or an older vacuum tube voltmeter was suggested as suitable.

You could also use an oscilloscope with X10 probe to do the same measurement.

Are you using online translation, perhaps?

Lengman_rich · 2nd Jan 2015, 8:48 pm

Yes, it's faster and easier to write messages.

Herald1360 · 3rd Jan 2015, 3:27 pm

Hopefully our replies make sense, then. Machines are not good at anything idiomatic or technical!

28th Dec 2014, 8:10 pm	#1
Lengman_rich Retired Dormant Member Join Date: Dec 2014 Location: Moscow, Russia Posts: 5	Eddystone 840C tuning indicator. Dear Colleagues! I have appeared in the collection of beautiful and in excellent condition Eddystone 840C. Unfortunately it did not work on the tuning indicator DM71 (V4) - it is always in the same state - lights all the bars. What is the reason for this failure, I can not define itself. You tell me what to do? Valery.

28th Dec 2014, 10:03 pm	#2
g4aaw pete Heptode Join Date: Jan 2008 Location: Birchington Kent, UK. Posts: 596	Re: Eddystone 840C tuning indicator. Hello Valery The tuning indicator on the Eddystone 840 series is well known for being very insensitive. It needs a very strong signal to give an indication. Some circuit changes are necessary for it to become useful as an indicator. You may be able to find some information on the Eddystone user group site: http://www.eddystoneusergroup.org.uk/ Good luck Pete

28th Dec 2014, 10:34 pm	#3
JHGibson Retired Dormant Member Join Date: Apr 2003 Location: Salt Spring Island, BC, Canada Posts: 368	Re: Eddystone 840C tuning indicator. I had the luck to find a pristine 840C, but it had the usual problem of an insensitive magic eye. I downloaded the spec sheet for the DM71 and it gave the circuit diagram that Eddystone uses but that particular circuit required 15 volts or more on the AGC line to extinguish the luminous bar while the 840 only has about 4 volts AGC with a strong station tuned in. Looking some more at the spec sheet I saw that if the DM71 plate voltage is reduced to about 60, then a AGC voltage of 7 will extinguish the eye. To obtain a stiff 60 volts for the DM71 plate, I jumpered a 10K resistor across the existing 470K plate resistor and connected another 10K resistor from the plate of the DM71 to chassis. This modification made the magic eye more responsive to tuning. John. Last edited by JHGibson; 28th Dec 2014 at 10:37 pm. Reason: Typo

30th Dec 2014, 9:42 pm	#4
Lengman_rich Retired Dormant Member Join Date: Dec 2014 Location: Moscow, Russia Posts: 5	Re: Eddystone 840C tuning indicator. Yes, thank you for your replies friends. Unfortunately the state of adjustment indicator does not depend on the signal strength. This is a problem. Once I watched this receiver - it worked correctly. Then I bought it already with these shortcomings. John, what do you think could be causing the problem capacitance C56? P.S. I thank the moderator for the creation of a separate topic on my question.

31st Dec 2014, 5:00 pm	#5
JHGibson Retired Dormant Member Join Date: Apr 2003 Location: Salt Spring Island, BC, Canada Posts: 368	Re: Eddystone 840C tuning indicator. If you are tuned to a strong station there will be several negative volts across C56. You will need a high impedance voltmeter to do that. If no volts across C56 then it could be a short circuit. Try substituting a good capacitor for C56 in that case. John.

31st Dec 2014, 6:54 pm	#6
Lengman_rich Retired Dormant Member Join Date: Dec 2014 Location: Moscow, Russia Posts: 5	Re: Eddystone 840C tuning indicator. I understand you correctly: C56 may be the reason?! Valery.

1st Jan 2015, 4:04 pm	#7
JHGibson Retired Dormant Member Join Date: Apr 2003 Location: Salt Spring Island, BC, Canada Posts: 368	Re: Eddystone 840C tuning indicator. Rich, what is the voltage across C56 when the receiver is tuned to a strong station? John.

1st Jan 2015, 6:33 pm	#8
Lengman_rich Retired Dormant Member Join Date: Dec 2014 Location: Moscow, Russia Posts: 5	Re: Eddystone 840C tuning indicator. John's question I can answer only 2 weeks. By the way, a better measure the voltage with a voltmeter or a tube can be ordinary?

2nd Jan 2015, 11:13 am	#9
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Eddystone 840C tuning indicator. I think the answer to your question is that either a high impedance (10 megohm) digital voltmeter, or an older vacuum tube voltmeter was suggested as suitable. You could also use an oscilloscope with X10 probe to do the same measurement. Are you using online translation, perhaps? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

2nd Jan 2015, 8:48 pm	#10
Lengman_rich Retired Dormant Member Join Date: Dec 2014 Location: Moscow, Russia Posts: 5	Re: Eddystone 840C tuning indicator. Yes, it's faster and easier to write messages.

3rd Jan 2015, 3:27 pm	#11
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Eddystone 840C tuning indicator. Hopefully our replies make sense, then. Machines are not good at anything idiomatic or technical! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O