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Gi4CZW Cliff 13th Feb 2021 9:32 am

MK.123 spyset BFO fault.
1 Attachment(s)
Hello and I hope some-one can help. Iíve had a number of qsoís on this Tx/Rx until the bfo started to delay oscillating from mains-power-on ( as different to itís own on/off switch ) now itís no osc. all the time. DC Voltages donít change osc/no-osc but Vp-p only show when working. Component values with green dot checked, R22-23 look reversed values on component listing and changing C42 no improvement. C42 is a Dubillier type 400 ( resistor looking ) with orange, black, brown bands.
I used scope with X10 probe for voltages, avo for resistors and 1KHz capacitor tester. Lovely set, component access a problem and I hope to get it going again.Cliff.

Station X 13th Feb 2021 9:51 am

Re: MK.123 spyset bfo fault.
I have one of these sets, but have never applied power to it.

I know we've been here before, but C42 is specified as 300pF which would appear to match the colour code. You measured 1200pF out of circuit. Has it been replaced?

Gi4CZW Cliff 13th Feb 2021 12:51 pm

Re: MK.123 spyset bfo fault.
Hello Graham, that's a good one. I had prepared the text and then done a copy-paste, so part of it has not come across ( could be operator error ).
Yes, C42 changed and no effect.

Gi4CZW Cliff 21st Feb 2021 10:39 pm

Re: MK.123 spyset bfo fault.
Sadly I've been unable to progress on this problem. Valve seems to be passing current and there is a brief burst of 465KHz as the set warms up but after this the bfo is dormant. I'm sure I've missed a clue. Help appreciated. Cliff.

G0HZU_JMR 23rd Feb 2021 8:25 pm

Re: MK.123 spyset bfo fault.
I'm afraid I know next to nothing about valves (especially pentodes!) but it might be worth trying to work out what the DC startup conditions should be for your CV466 based BFO. I can try and kick this analysis off but I'm really a novice with valves.

At startup I presume the heater is already hot and the oscillator circuit is selected via the relay connection to the HT. This will instantly connect 100V to the top of R22 (150k) and R23 (220k). I presume that the R22 connection goes to the g2 connection (screen grid?) and this has a startup time constant of 150k + 1nF (C48) which means it will take over 150us for the screen grid to rise to a reasonable voltage.

During the start of this rise time at the screen grid I think that little anode current will flow and so I think there should be 100V at the anode of the pentode for maybe the first 20us after the relay clicks on. Then I'd expect to see the anode current begin to flow so the anode voltage will start to rapidly fall as the screen grid voltage at C48 starts to rise higher. As the anode current starts to increase then the DC voltage seen at the cathode and the grid should also start to climb because of the increasing anode current through R30 (10k).

I think the next bit is important: At some point during this risetime process the valve will cross through a region where it will have adequate anode voltage and current to generate a reasonable transconductance (gm) and this will be sufficient to start up the oscillation. I suspect that the time constant of 150k and 1nF at the screen grid will deliberately permit the anode current to rise fairly gracefully over the first 200us after startup. Hopefully this will allow sufficient time to begin oscillation at the sweet spot bias condition. However, if the 1nF cap C48 were to fail then the anode current would rise very quickly at power on and the valve might latch into a 'hard switched on' condition where it might not start as easily.

So I would check the 1nF cap C48. If you have a 2 channel scope you could watch the screen grid voltage and anode voltage during the first 200us after startup although I think you would need a storage scope or a modern digital scope to do this. Looking at your test point readings I'm not sure why the grid voltage (18V) is 5V higher than the cathode voltage (13V) in your diagram. Does this mean there is a (50uA) leakage path from the screen grid or the anode? Is it normal to see a 50uA leakage from the screen grid to the grid like this? It could be that the valve has a problem or maybe this is what happens when the valve latches into the on state with a very low anode voltage and no oscillation. I'm really just guessing.

You could try giving it a 'super graceful' power on condition by increasing C48 to maybe 3.3nF as this would allow the valve more time to cross the sweet spot zone where it can generate enough transconductance to start up. Once it starts it probably goes into a large signal condition where the valve will virtually switch on and off the anode current at 466kHz.

All the above is guesswork as I know very little about pentodes but it might kick off some conversation and something helpful might result from it...

G0HZU_JMR 23rd Feb 2021 9:41 pm

Re: MK.123 spyset bfo fault.
It's probably too soon to start analysing the rest of the BFO circuit (as an oscillator) but I did put the tank circuit into a simulator using a simple gm model for the valve. As long as the various capacitors and the inductor L10 are healthy then it looks like the valve only needs to produce a gm of -0.3mA/V to be able to generate enough negative resistance to start up the BFO at 465kHz. This assumes all caps are healthy and the inductor L10 (237uH?) has a Q of about 100. Even if the Q of L10 is only 45 the circuit should just start up with a gm of -0.3mA/V.

This circuit does look like it should be keen to start up but the initial power up conditions need to be such that the valve is able to produce that minimum gm for long enough for a complete/full startup to occur.

I think it might latch into the hard switched on state if it can't start and maybe it can get stuck like this if something in the circuit is no longer as it should be.

G0HZU_JMR 23rd Feb 2021 11:13 pm

Re: MK.123 spyset bfo fault.
One other thing to consider. If there is 13V DC at R30 (10k) then this means 1.3mA cathode current in theory. With 150k at the screen grid and 220k at the anode and only 100V HT I'm not sure how this adds up. What DC voltages do you see at the anode pin and the screen grid when you see 13V at the cathode when it is not oscillating? Is R30 definitely 10k ohm? (green dot says it is)

Gi4CZW Cliff 23rd Feb 2021 11:41 pm

Re: MK.123 spyset bfo fault.
Hello Jeremy, gosh what a lot of detail. Just to let you know, I will need to spend most of tomorrow getting the answers. Part of the problem is, R22, R23, C48 and the wire to grid-2 are hidden behind an inner panel. Looks like a busy day coming. Thank you for the insights. Will be in touch. Cliff.

G0HZU_JMR 23rd Feb 2021 11:54 pm

Re: MK.123 spyset bfo fault.
Hi Cliff, I'm really just guessing at it all and hopefully someone with more experience can advise about the various DC voltages at power on.

When biased normally for amplifier service it looks like this valve can produce a typical gm of -6mA/V so the startup requirement of maybe only -0.3mA/V might at first seem easy for this valve. However, it looks like this circuit will force the valve into a semi-saturated state with low anode voltage if it fails to start up. I think the gm will fall to a very low level in this state. I'm not sure how low though.

G0HZU_JMR 24th Feb 2021 8:32 am

Re: MK.123 spyset bfo fault.
Don't spend ages on it today if accessibility is poor. It might be better to wait for another forum member to advise.

This is because at the moment I can only offer you questions rather than answers. I'm wondering if there is a metering/loading issue when you measure the DC voltages. Otherwise I can't explain 13V at the cathode because this implies 1.3mA current here and the HT is only 100V.

If 85V was dropped across the anode resistance of 220k and 85V was dropped across the screen grid resistance of 150k this only provides 0.39mA + 0.57mA = 0.96mA total current going in via the HT.

I'm also a bit confused how there is 18V at the grid and 13V at the cathode. This means the Vgk is at +5V which seems highly unusual to me at least. Could this be another meter loading issue? One possibility for the strange DC readings is that the tube is preferring to go unstable and oscillate at a much higher frequency than your test gear can display although I think this is unlikely.

I did manage to find some data and curves for the equivalent EF73 valve in pdf format and I've uploaded them to here:

Maybe this will help someone advise better about the circuit. This datasheet does suggest that the valve can still generate a gm of 1mA/V even with just a few volts at the anode.

Keith 24th Feb 2021 10:48 am

Re: MK.123 spyset bfo fault.
Could it just be a tired valve - have you tried replacing?

G0HZU_JMR 24th Feb 2021 12:15 pm

Re: MK.123 spyset bfo fault.
Yes, that was what I was wondering too. It looks like the DC startup conditions put the valve in a low anode voltage condition where gm is quite low. Maybe the valve is just tired enough such that there isn't enough starting gain when the anode voltage is just a few volts. The same valve might still work fine in a regular amplifier circuit but maybe this oscillator circuit is more critical?

VT FUSE 24th Feb 2021 4:58 pm

Re: MK.123 spyset bfo fault.
Hello Cliff
I just took a peep into my spares box and find 4 x NOS CV466 in boxes,my future needs will not require all of them and wonder if you would like an unused one to try? ISTR that Mk.123 has weeny B8D valve holders thus an easy matter to unplug the valve once it is accessed-the leadout wires obviously need shortening to fit.
Are you subject to varying mains voltage,this could fess with the Oscillations somewhat!
PM me if you would like one in the mail tomorrow foc.

VT FUSE 24th Feb 2021 6:39 pm

Re: MK.123 spyset bfo fault.
Circuit is part of a whole manual at Crypto Museum,easily found-not sure about putting circuits etc on this forum hence chose not to supply a link.
The BFO architecture is discussed and also mention of the operator controls.
The receiver is fed by a gas regulator valve so earlier comment on mains fluctuation is a red herring on my part (not the 1st time one gone u/s however)
May even be down to oxidised wire lead outs on the cv466,the diddy B8D sockets comprise very basic metal type and thus could be suspect after many years-could be worth a drizzle of contact cleaner on a cotton tip maybe? surprised that they are not silver plated.
The voltages around cv466 do look very unusual!

Gi4CZW Cliff 24th Feb 2021 6:48 pm

Re: MK.123 spyset bfo fault.
Hello Keith, Yes good suggestion, just a bit tricky in this set. Thanks. Cliff.

Gi4CZW Cliff 24th Feb 2021 6:50 pm

Re: MK.123 spyset bfo fault.
Hello Mike, very interesting. PM sent. Thank you. Cliff.

Gi4CZW Cliff 24th Feb 2021 7:14 pm

Re: MK.123 spyset bfo fault.
1 Attachment(s)
Hello Jeremy, can't get at C48 but found screen grid G2 tag. Capacitance test at 1KHz including the resistors G2 to earth showed very little C, so put extra 1nF g2 to earth and getting 465KHz now. It's ok for CW but not SSB as it's saw-tooth in shape. I do know C42 has aged from 300pF to 1200pF would that sort it? Sadly cant remember listening to SSB and not sure if that is the way it used to be. Photo included. Cliff

G0HZU_JMR 24th Feb 2021 8:14 pm

Re: MK.123 spyset BFO fault.
Hi Cliff, looks like there's some signs of life from the BFO now.

I misunderstood earlier because Graham already mentioned replacing C42 (1200pF but should be 300pF) and asked if it had been replaced. I think it would be a good idea to fit a suitable 300pF cap here.

I think that the value of C42 will impact the risetime of the gate bias voltage with respect to the cathode bias voltage just after the BFO is powered up. Any lag in the way the gate risetime tracks the cathode risetime just after the BFO is powered on will cause the gm to reduce (because the gate to cathode voltage could lag negative by a few volts) and I expect that this will impact the starting gain for the oscillator. An ageing change from 300pF to 1200pF for C42 might slug the gate risetime enough to make things marginal in terms of gm just after power up.

Once the oscillator is running and settled I'd expect to see the anode current arriving in pulses at 465kHz so I'd expect to see a pulsed waveform at the anode on a scope. However, there is also a 30pF cap here (C41) and this might be able to clean it up to look more like a sine wave. I'd expect to see some distortion though. It might be normal to see a sawtooth here. I have to repeat that I'm guessing at all of this because I'm not even in short trousers when it comes to tinkering with real pentodes in a real receiver.

G0HZU_JMR 24th Feb 2021 8:57 pm

Re: MK.123 spyset BFO fault.
One other thing to note is is that if C48 has failed o/c then I think the circuit would have lost some benefits of being a pentode. I think the screen grid g2 needs to be decoupled by a capacitor at RF because otherwise the valve will begin to take on some of the less attractive features of a triode. This includes an increase in the effective feedback capacitance to the grid if the (non-decoupled) screen grid can also swing at RF in a similar way to the anode. This increase in feedback would make Miller Effect more significant and this could degrade the starting gain as well. This might be partly why it starts up better with your new 1nF padding cap at g2.

Gi4CZW Cliff 25th Feb 2021 2:10 pm

Re: MK.123 spyset BFO fault.
I'm releved to make progress and delighted this little set now on the mend. So great big thank you to Jeremy, for all the input and pushing me towards C48. Also, armed with EF73 data, I expect to trim components for a sign wave output.
Again, thanks to Jeremy, Mike and Keith.

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