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Old 6th Aug 2022, 7:11 pm   #1
Vectis68
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Default Bush VHF61 - magic eye not closing.

I have no background in electronics and this is my first attempt at restoring a valve radio.

I have a Bush VHF61 that I have recapped (both wax and Hunts capacitors) and I am able to tune stations on both MW and VHF (long wave is quite poor, but that might be due to the lack of a decent aerial at the moment).

The magic eye closes fine on medium wave but not so on VHF, it flatly refuses to change on the ten stations that I can pick up here.

Can I have some pointers please as to what I should be looking for?

Phil.
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Old 6th Aug 2022, 8:40 pm   #2
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Default Re: Bush VHF61 - magic eye not closing.

Could be anything from you being in a weak VHF signal area, to a weak ecc85, to a resistor affiliated with the em81 going high, or a leaky hunts you've missed. Have you changed the "upside down" FM discriminator electrolytic (positive to chassis)?

A very nice set
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Old 6th Aug 2022, 9:23 pm   #3
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Default Re: Bush VHF61 - magic eye not closing.

Quote:
Originally Posted by Vectis68 View Post
I have no background in electronics and this is my first attempt at restoring a valve radio.

I have a Bush VHF61 that I have recapped (both wax and Hunts capacitors) and I am able to tune stations on both MW and VHF (long wave is quite poor, but that might be due to the lack of a decent aerial at the moment).

The magic eye closes fine on medium wave but not so on VHF, it flatly refuses to change on the ten stations that I can pick up here.

Can I have some pointers please as to what I should be looking for?

Phil.
Hi Phil, I'm assuming you have the circuit? Going by the Trader Sheet 1301 the grid of the magic eye (shown as TI...Tuning Indicator) is driven from the AGC rail on AM via S18 and from the discriminator on FM via S19. It would be worth cleaning the switches first which are part of the wavechange switch (if you haven't done so already). As suggested already, check that electrolytic C56 which is in the DC reservoir circuit. If this component is faulty (quite likely) there will be insufficient DC developed and the eye will not operate correctly or at all. Usually if this component has completely failed it will cause distortion on FM. Don't forget this capacitor goes positive to chassis. It's shown as a 5uF. The modern replacement will be 4.7uF. Choose a working voltage the same or higher than the one fitted.

If you can receive 10 stations then the ECC85 seems pretty healthy but still worth trying a new one.
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Old 7th Aug 2022, 8:34 pm   #4
Vectis68
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Default Re: Bush VHF61 - magic eye not closing.

Thank you for the replies/suggestions.

I've gone over S18 and S19 contacts with some 800# wet/dry paper, and I've also tried gently pushing the contacts together when the set is on. No improvement unfortunately.

The only other ECC85 that I have is in another VHF61 set (currently non working) I've given it a try, but the results are the same.

I have already changed C56 and the polarity is correct (positive to chassis)

The set sounds fine on VHF with the stations it pulls in, there is little background noise and it doesn't need much volume. The modern radio I have in the kitchen doesn't pick up some of the stations I get on the valve set.

I've noticed that the EM81 gets slightly brighter when a station is tuned in, but it just doesn't "fan out"
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Old 8th Aug 2022, 8:09 am   #5
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Default Re: Bush VHF61 - magic eye not closing.

So what voltage do you get on the control grid of the EM81 when tuned in to an FM station? Compare that to the voltage you get when tuned in to an AM station. In fact see if any of the voltages differ around the EM81 between FM and AM.
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Old 8th Aug 2022, 8:38 am   #6
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Default Re: Bush VHF61 - magic eye not closing.

The set has three IF stages so should be reasonably sensitive, however when these sets were made, unless it was used in a strong signal area they were expected to use an external VHF aerial.
What aerial are you using and are you in a good reception area?
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Old 8th Aug 2022, 10:22 am   #7
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Default Re: Bush VHF61 - magic eye not closing.

The VHF61 FM is normally very good, it is AM that is noted for being weaker than expected given that it has an extra IF stage. You are receiving 10 (strong?) stations so I do not think it is sensitivity that is the problem. Have you tried swapping over the EF89's? Do you have another EABC80 you could try?
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Old 8th Aug 2022, 9:25 pm   #8
Vectis68
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Default Re: Bush VHF61 - magic eye not closing.

I've tried changing the EF89(x2) and the EABC80 valves - no improvement (purchased as NOS tested)

I'm not using the sets internal aerial as I have it out of it's cabinet. I don't have access to a proper external aerial at the moment, so I am use two pieces of enamelled wire, approximately 76cms each for the aerial.
I don't believe I am in a poor reception area, I'm in the city of Plymouth and the nearest transmitter is in Plympton (about 5 - 7 miles away)

I've taken some voltage readings from the EM81, as follows:

Pin 1 (grid) when tuned to a station

VHF = -0.5mv
MW = -2.2v

Pin 2 (cathode)

VHF and MW both at -0.2mv

Pin 7 when tuned to a station

VHF = 4.8v
MW = 75v

Pin 9

VHF 177v
MW 185v

Phil.
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Old 10th Aug 2022, 9:33 am   #9
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Default Re: Bush VHF61 - magic eye not closing.

I guess you're using the radio with the internal VHF aerial.

The signal level might seem to be more than adequate, but as the EM81 magic eye works fine of AM and not on FM, the signal level from the internal FM aerial may be insufficient to operate the eye, which requires several volts negative on the grid to appreciably change the display. I'd suggest that you check the negative volts on the grid (Pin 1) of the EM81 with a station tuned in on MW strong enough to operate the eye, then tune in a VHF station at seemingly good strength and check the negative Volts on the grid.

I think you'll find that whether on or off tune, the voltage won't change much on FM.

I say this because some years ago I restored a Pye Fenman 1 which worked fine on AM and FM, apart from the eye not opening on FM. I checked all the voltages and resistors associated with the eye, and all seem to be as it should be. As the Russian equivalent of the (EM80) eye were cheap at the time, I bought a replacement, but to no avail. I made a little tester on which I could vary the negative volts of the grid, which proved that the old and new eyes worked fine.

Then it dawned on me that I should try an external FM aerial. Ironically, I had a ribbon FM aerial in the workshop 'hiding in plain sight'. I plugged it in, and the eye then worked fine on FM. I probably spent more time faffing around until the penny dropped than I did to restore the radio.

Likewise, I restored a little Unitra 'Figaro Special' (AM only) which worked fine on its internal Ferrite Rod aerial with stations at good strength, but the DM70 eye didn't alter much. I checked the voltage on the grid, both on and off tune. Off tune it was 0.45V negative. On tune, it was 0.75V negative. With a short external aerial plugged in 2 Metres of wire), it was quite a different story: 4.5V negative off tune, and 9V negative on tune, when the DM70 'exclamation mark' eye closed completely. The volume didn't change greatly, with or without the external aerial - the AGC did what it's supposed to do and controlled the gain.

Might be worth a try with a simple FM aerial - just a length of twin speaker wire to mimic an FM ribbon antenna would do. They're cheap enough to buy if you don't have one:

https://www.ebay.co.uk/itm/251486153...IAAOSwKClidqZr

Hope that might help.
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Old 10th Aug 2022, 3:12 pm   #10
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Default Re: Bush VHF61 - magic eye not closing.

While the EM81 target voltages (pin 9) seem fine, the anode voltage (pin 7) of 4.8v on FM seems much too low. Check the anode load resistor (usually 1M to 2M). Jerry
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Old 10th Aug 2022, 6:13 pm   #11
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Default Re: Bush VHF61 - magic eye not closing.

Yes pin 7 is odd..low on VHF but why is it OK on AM? That 470K on the anode is definitely worth a check but I still can't work out why the voltage on the anode when on VHF is so low since the same resistor is used fed from the same point on the HT. Change that 470K anyway....you never know....!
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Old 10th Aug 2022, 6:48 pm   #12
Vectis68
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Default Re: Bush VHF61 - magic eye not closing.

Thanks everybody for your helpful suggestions.

I've checked that resistor before, it comes in at 559K. I'll get some ordered this evening and change it.

I'll also see what I can do about getting a better aerial setup.

I'll not have much time for radios for the next 3 days as I've got my 4yr old Granddaughter staying with me.

I'll report back next week.

Phil.
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Old 10th Aug 2022, 7:06 pm   #13
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Default Re: Bush VHF61 - magic eye not closing.

The value of that resistor isn't critical. 559k is a bit high, but won't be responsible for the symptoms you're seeing.
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Old 10th Aug 2022, 8:35 pm   #14
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Default Re: Bush VHF61 - magic eye not closing.

Apart from the low voltage on pin 7 when on VHF, when you state that Pin 1 (grid) when tuned to a station on VHF = -0.5mv and on MW = -2.2v, that confirms that for whatever reason, there is virtually no negative voltage swing when a station is tuned in on VHF. You will only see a display on VHF similar to that when a station is tuned in on MW if you have a similar level of negative voltage on the EM81 grid.

Attached below is a clip from the (ERT) data regarding the operation of the magic eye on both AM and FM.
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Old 10th Aug 2022, 8:57 pm   #15
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Default Re: Bush VHF61 - magic eye not closing.

Quote:
Originally Posted by David G4EBT View Post
I guess you're using the radio with the internal VHF aerial.

The signal level might seem to be more than adequate, but as the EM81 magic eye works fine of AM and not on FM, the signal level from the internal FM aerial may be insufficient to operate the eye, which requires several volts negative on the grid to appreciably change the display. I'd suggest that you check the negative volts on the grid (Pin 1) of the EM81 with a station tuned in on MW strong enough to operate the eye, then tune in a VHF station at seemingly good strength and check the negative Volts on the grid.

I think you'll find that whether on or off tune, the voltage won't change much on FM.

I say this because some years ago I restored a Pye Fenman 1 which worked fine on AM and FM, apart from the eye not opening on FM. I checked all the voltages and resistors associated with the eye, and all seem to be as it should be. As the Russian equivalent of the (EM80) eye were cheap at the time, I bought a replacement, but to no avail. I made a little tester on which I could vary the negative volts of the grid, which proved that the old and new eyes worked fine.

Then it dawned on me that I should try an external FM aerial. Ironically, I had a ribbon FM aerial in the workshop 'hiding in plain sight'. I plugged it in, and the eye then worked fine on FM. I probably spent more time faffing around until the penny dropped than I did to restore the radio.

Likewise, I restored a little Unitra 'Figaro Special' (AM only) which worked fine on its internal Ferrite Rod aerial with stations at good strength, but the DM70 eye didn't alter much. I checked the voltage on the grid, both on and off tune. Off tune it was 0.45V negative. On tune, it was 0.75V negative. With a short external aerial plugged in 2 Metres of wire), it was quite a different story: 4.5V negative off tune, and 9V negative on tune, when the DM70 'exclamation mark' eye closed completely. The volume didn't change greatly, with or without the external aerial - the AGC did what it's supposed to do and controlled the gain.

Might be worth a try with a simple FM aerial - just a length of twin speaker wire to mimic an FM ribbon antenna would do. They're cheap enough to buy if you don't have one:

https://www.ebay.co.uk/itm/251486153...IAAOSwKClidqZr

Hope that might help.
This definitely agrees with what I've found and measured in the past- without the AGC backing gain off to a significant extent, even the weak signal from a short aerial can sound pretty strong and clean without producing much response from the magic eye, it can really take a bit more of a "push" from a decent aerial to bump up the AGC bias and consequently produce marked eye deflection,

Colin
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Old 11th Aug 2022, 8:55 am   #16
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Default Re: Bush VHF61 - magic eye not closing.

Pin 7 of EM81 low voltage on FM....V = I*R, no mystery (Ia):

http://www.r-type.org/pdfs/em81.pdf

Lawrence.

Last edited by ms660; 11th Aug 2022 at 9:00 am. Reason: link added
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Old 11th Aug 2022, 9:58 am   #17
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Default Re: Bush VHF61 - magic eye not closing.

A simple test would be to measure pin7 voltage on AM when not tuned into a station. Personally I think the EM81 and resistor are fine, but happy to be corrected.

Could you maybe try the set outdoors or in a different location? Also, the FM aerial in this set, if I remember correctly,is a simple ribbon attached to the back panel. The "in cabinet" aerial is for AM. You don't need the cabinet to plug the FM aerial in, just the back panel. You ought to try it with its own FM aerial
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Old 11th Aug 2022, 11:22 am   #18
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Default Re: Bush VHF61 - magic eye not closing.

My experience with the VHF61 is the VHF deflects the magic eye better than AM but as I recall I did replace both EF89.

Is there any sign of the IF transformers having been adjusted? It may need realignment?
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Old 15th Aug 2022, 9:49 pm   #19
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Default Re: Bush VHF61 - magic eye not closing.

I managed to get to B&Q yesterday and bought the necessary parts to make an end fed dipole aerial. I haven't attempted to fit it outside yet, at the moment it is stood upright by the window.

I've checked the voltages again and there is a small improvement, not enough though to make the tuning indicator work.

The grid voltage has increased from 0.5mv to 2.1mv when tuned to a station.

The cathode voltage has increased slightly from 4.8v to 5.0v when tuned.

Phil.
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Old 15th Aug 2022, 10:00 pm   #20
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Default Re: Bush VHF61 - magic eye not closing.

I can tell you for sure that the magic eye on the VHF61 always works far more efficiently on VHF/FM than it does on AM, so if yours is working the other way round then I'd suspect a fault within the set that still needs tracking down. However, now you're going to make up an aerial you'll have to see how it goes with that.

Poor long wave is usually down to an ageing ECH81 valve.
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