Eddystone 730/4 BFO not working
 

Hi,

Recently acquired an excellent Eddystone 730/4 and almost everything seems to be OK ... except the BFO.

When I move the knob from the off position to past the detent and into the "on" zone, there's just the slightest of clicks from the speaker, as if something were happening ... but no beat tone.

I've replaced the associated valve (V12), but no change.

Has anyone else had this fault, and/or can anyone suggest a fault-finding strategy? I don't own a signal generator or oscilloscope, so am limited to what can be tested using a meter.

Despite having held a full licence for 40 years, I am not the world's greatest technician, so all suggestions and inspiration will be gratefully received!

73
Steve
G4YXU

Re: Eddystone 730/4 BFO not working
 

Check voltages around the valve and look for any out of spec resistors for starters.

Re: Eddystone 730/4 BFO not working
 

Service Manual and Restoration notes on the Eddystone User Group website, well worth a visit.

https://eddystoneusergroup.org.uk/

Mike

Re: Eddystone 730/4 BFO not working
 

With this era of comms receivers, the BFO is arranged to whisper gently into the envelope detector to generate CW sidetone and is operated at very low power with associated high value anode feed resistors- Eddystone tended to use rather "average" quality carbon composition resistors and the high value ones are prone to going rather higher in value or even open circuit with age. The associated paper dielectric decoupling capacitors tend to go electrically leaky, further hobbling associated circuitry.

Re: Eddystone 730/4 BFO not working
 

Thanks for that.

I'll have a look at the resistors, but the only voltage info I have is EMER 742 Part 2, Fig. 2501b (p. 34 of the attached PDF), which indicates 90V at pin 5 of V12 and originally had 20V at pin 6, with this crossed out by hand and something illegible added instead (!). Do you have any other data?

Regards,

Steve

Re: Eddystone 730/4 BFO not working
 

Thanks, already downloaded their manuals -- indeed, I obtained the set via an ad on the EUG site!

Worryingly, the manual on there gives 1 V for the anode, whereas the EMER gives 90V.


Thanks. Will look into the resistors and caps.

Re: Eddystone 730/4 BFO not working
 

It's ages since I delved into a 730/4.

My #1 question would be "is the BFO not oscillating at all, or is it oscillating but doing-so on a frequency that's significantly 'off' from where it needs to be?"

Do you have any sort of frequency-counter you can use to 'sniff' for proof-of-oscillation (even if it's on the wrong frequency)? A length of insulated wire poked into the BFO-coil can should pick up enough oscillation - if it's present - to drive a digital frequency-meter.

If no oscillation, check for failed components.

If the oscillation's there but significantly off-frequency that narrows the problem down to the tuned-circuit components.

Re: Eddystone 730/4 BFO not working
 

Sadly own nothing more sophisticated than a multimeter, so I'm just going to have to test the resistors. If replacing any dodgy ones doesn't fix it, I may just replace the caps as well.

Re: Eddystone 730/4 BFO not working
 

What sort of multimeter? Is it a high-impedance modern digital-type or an older moving-coil-and-pointer analog one?

Looking at the circuit, if your meter is a modrn high-impedance type I'd suggest you connect one probe to the radio's chassis, and the other - through a resistor of at least 100KOhms - to the junction of C113, R63 and the valve's grid-terminal.

Set the meter to a low-volts range [3 or 10V] and then see if the voltage you measure varies significantly between the BFO being switched on and switched off.

If the BFO is oscillating [but on the wrong frequency] you will see a significant voltage-change - a few volts - between BFO-on and BFO-off, because of grid-rectification in the valve.
If it's not oscillating you won't see much of a voltage-change.

[In the above example, the inclusion of the resistor between the test-probe and the circuit is to stop the meter and its connecting-leads providing enough of a radio-frequency load to prevent the BFO from oscillating. When taking any measurements always remember the comment of Werner Heisenberg, the famous physicist, who said 'To Observe is to Interfere' - don't let your measurements introduce extra uncertainties into what you're trying to measure!]

Re: Eddystone 730/4 BFO not working
 

Sounds like a good scheme. Thanks! My meter is indeed a modern digital type (though I feel I morally ought to be using a big old AVO on this set!), so I'll measure the voltage through a 100 kOhm resistor and see if it changes as I switch the BFO on and off.

Re: Eddystone 730/4 BFO not working
 

Some of us keep old AVOs for exactly that reason :-)

Some DVMs read wrongly if there is RF involved.

Having a spare general coverage receiver in your workshop can be useful. If you have one, you can put a piece of wire into its antenna connector and place the other end near an oscillator you're investigating, then tune around to find the oscillator's signal.

David

Re: Eddystone 730/4 BFO not working
 

Using a second receiver as a crude frequency counter sounds like a good idea. Thanks!

Re: Eddystone 730/4 BFO not working
 

Hi Steve, as I read it the Eddystone manual indicates 90v (J-) anode and 20v (D-) grid 2 of the BFO valve V12. These are for the higher impedance voltmeter so more likely to mirror what you will measure with a DVM. The EMER circuit also shows 90v anode but there is an amendment that changes the grid 2 resistor from 68k to 330k and a commensurate drop of the grid 2 potential from 20v to 7v. Can you post what you have actually measured please? If the valve has tolerable HT voltage then the listening test on an adjacent receiver is a good idea in theory. However, The BFO is intended to run at a frequency either side of the receiver's IF which is 450kHz. The snag is you are unlikely to have a receiver that will receive signals at 450kHz (=666 metres). However, you might be able to pick up the second harmonic at 900kHz. The whistle should come and go and change note as you adjust the BFO control. If it does then it is likely that the BFO has drifted and you should be able to tune it back by very carefully adjusting the core in the BFO coil (T7). If it is an iron dust core be careful not to split it with a screwdriver. A plastic toothpick or knitting needle filed with a screwdriver flat is recommended. If the dust core is too tight to move, then put a small drop of white spirit on the top and leave to soak. Don't use methylated spirit as it can dissolve the outer plastic coil former. If the BFO isn't oscillating, and you have good HT then there are some paper capacitors nearby that might be leaky and need replacing such as C123 and C116 (0.01uF / 10nF). Also worth checking C114 and C115 (0.5uF / 500nF) paper. Good luck, Jerry

Re: Eddystone 730/4 BFO not working
 

Hi Steve,
Have you tried backing off the RF gain control. The BFO injection on these receivers is pretty low level - you may not hear a beat note if the signal is strong.

Re: Eddystone 730/4 BFO not working
 

The problem of a missing BFO is fairly common here Steve and often means the frequency is miles out. If you don't have an oscilloscope and the BFO frequency is supposed to be say 450KHz, then look for an unmodulated carrier (on the same receiver).The third harmonic at 1350KHz should be present, but if the frequency is say 10KHz out this might be 1320 to 1380KHz.
You may also have a wrong IF that someone mis-tuned by twiddling IFT tuning.
For example the IF might be 465KHz with the BFO sitting on say 450KHz.
73 Allan G3PIY

Re: Eddystone 730/4 BFO not working
 

Hi Steve, I had the same problem. See if you used the right tube. There has been an upgrade (EMER E749) to use a CV4009 instead of a 6BA6. In that case R64 of 68K was replaced by a 330K resistor. In my 730/4 that was the case and I replaced R64 by 68K (and 6BA6). Also C116 C123 were very bad (0.01 uF). I also replaced the other resistors and capacitors because the BFO unit was open anyway. Works well again now.

73
Kees , PA0KME

Re: Eddystone 730/4 BFO not working
 

If you are realigning the IFTs you may need to be remember that it will not be exactly 450K/cs but aligned to the crystal filter which will be several k/cs above or below - the same goes for the BFO.

If you only have a simple emetr then it should be possible to check the operation of the BFO by usinf a simple diode probe of a diode connected to a length of wire wrapped around the BFO valve with the other end of the diode connected to the meter shunted by a capacitor i.e. a simple diode detector