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Old 20th Dec 2019, 7:14 pm   #121
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Default Re: Marconi TF144H/4S

Thank you. I hope to get back onto this set tonight.
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Old 21st Dec 2019, 5:46 pm   #122
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Default Re: Marconi TF144H/4S

To eliminate any poor contacts, I shorted out all contacts of S100 for range A.

I got different results, not quite what I were expecting though.
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Old 22nd Dec 2019, 12:29 am   #123
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Default Re: Marconi TF144H/4S

Right, this evening I took out every switch wafer for S100. The contacts were very dirty, so I cleaned them up with 1200 grit paper and a fibre glass pen.

Still no real idea on what's going on with the Crystal Calibrator. Maybe there's something I'm not doing right with with this 'couple to V103 with a wire'.

Is it working?

Is it not working?

I need to find another way to scope for the 400k and 2mhz xtal signals rather than just looping around the outside of the valve.
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Old 22nd Dec 2019, 12:24 pm   #124
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Default Re: Marconi TF144H/4S

You could try scoping the osc cathode with an x10 probe to see if it's oscillating, the wire round the valve trick is I would imagine, one end to the probe, the other end free, coupling by stray C.

The osc looks like a Colpitts, judging by the capacitive tap at the cathode.

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Old 22nd Dec 2019, 1:56 pm   #125
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Default Re: Marconi TF144H/4S

Yes, if the pickup wire isn't driving the counter I'd try probing the cathode of V103B with a x10 scope probe connected to a frequency counter. When I used to service ham/CB radios I used to connect the x10 probe to CH1 of a Tek 465 scope and then connect the counter to the CH1 OUT at the back of the scope. This meant that the waveform could be looked at with the scope at the same time as the scope drives the counter.

The extra 12pF introduced by the x10 probe will be across 270pF so it will probably only pull the 2MHz crystal a couple of Hz down in frequency when connected. This tiny error will multiply up by 30 when beating with 60MHz on the top range of the sig gen but it will still be less than a 100Hz error.

Obviously, the idea of the pickup wire is to provide less crystal loading/pulling and easier access for the operator. I'd expect the pickup wire method to work although the counter may need an RC high pass network at its input to filter away any 50Hz mains pickup that might upset the frequency counter.

I'd also try the pickup wire method on ranges G and L rather than A and E as V103A will be powered off across G to L and there will therefore be less energy at the sig gen frequency. This would hopefully mean that the main sig gen RF waveform would be less likely to interfere with the counter on ranges G to L.
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Old 22nd Dec 2019, 8:42 pm   #126
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Default Re: Marconi TF144H/4S

Thanks for the advice.

Well I used a x10 probe and connected to the cathode and I'm certainly getting the 400k and 2mhz signals. When I were coupling to V103 with a wire I were getting nothing but either noise or just picking up the CW frequency.

Adjustment of the C144 and C192 doesn't make a lot of difference, but then again it doesn't appear to really need any adjustment at all.

So, I wonder if there's anything I ought to do to check the crystal oscillator circuit anymore. I wonder why the manual prefers one to couple to the valve with a wire rather than to probe the cathode?
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Old 22nd Dec 2019, 9:15 pm   #127
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Default Re: Marconi TF144H/4S

I think it is for convenience and to reduce the risk of probing near high voltages. However, I suspect that a lot of people will struggle here because there will be lots of stray signals and noise that can be picked up that can upset the counter. I worked out that the loading effect of the x10 probe at the cathode will only pull the 2MHz crystal frequency down about 1Hz or so but it depends on the motional inductance and capacitance of the crystal. I used classic values for a 2MHz crystal in my estimate.

I had another look at the circuit and I think it will be very forgiving as the valves age. The schematic is definitely marked up incorrectly if those test voltages are meant to be valid for every range within (A to F) and (G to L) for V103B.

The voltages are wrong on some ranges because the average current through R216 (on ranges G to L) should always be very similar to the average current through the cathode of V103B on these ranges. Both will typically be a couple of mA or so and both should be about the same.

On range L, 84V at the cathode divided by 33000 ohms = 2.5mA. But the schematic thinks there is 85V at the anode at V103B on ranges G to L. (250V - 85V)/22000 = 7.5mA so this is an obvious violation as the extra 5mA has nowhere to go! V103A anode is disconnected on these ranges and V103B anode connects to 250V DC via the 22k R216.

Based on the 12AU7 datasheet curves, the anode voltage will be about 190V on range L as this means the average anode current will be much closer to the 2.5mA average current in the cathode at 84V on this range.
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Old 22nd Dec 2019, 9:35 pm   #128
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Default Re: Marconi TF144H/4S

Quote:
Adjustment of the C144 and C192 doesn't make a lot of difference, but then again it doesn't appear to really need any adjustment at all.
Based on a classic 2MHz crystal model and a couple of pF for the grid capacitance, I estimated that the 2-19pF trimmer could adjust the 2MHz crystal over a 500Hz range. But this assumes the oscillator can start up OK as the trimmer cap is tested across its full range. Presumably Marconi specified this 2MHz crystal to be resonant with about a 10-15pF load capacitance. The 400kHz crystal was probably specified for a 30pF load capacitance.
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Old 22nd Dec 2019, 10:40 pm   #129
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Default Re: Marconi TF144H/4S

Quote:
So, I wonder if there's anything I ought to do to check the crystal oscillator circuit anymore.
Because the DC operating point of V103B changes across the various ranges the startup conditions for the oscillator will be different across the ranges. So best to double check it starts up OK on every range across A to L.

The real test will be to listen to the heterodynes in table 2.1 with a set of headphones. Hopefully all of them will be audible. However, I'm not sure how many people ever make use of the calibrator function on this sig gen. It will be useful as a crude confidence check that the frequency dial is roughly right but that's about it. So I'm not sure it's even worth trying out all the heterodynes in table 2.1. I guess it depends if you ever plan to use this feature?
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Old 22nd Dec 2019, 11:33 pm   #130
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Default Re: Marconi TF144H/4S

Yes hopefully I'll try that tomorrow.

With regards to using that feature, I had thought I probably won't, but it would be nice to have run through everything the set has to offer.

So after checking the crystal frequencies, I moved onto the next step which is to check the voltages across the crystals.

Both XT101 and 102 should be between 2.5 and 16v. I measure 19v for 101 and 29v for 102. This may or may not be a problem, or indeed may be an error in the drawings again.
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Old 22nd Dec 2019, 11:44 pm   #131
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Default Re: Marconi TF144H/4S

Back in post #95 you measured 55.6V DC at the cathode at range G. With 33k x2 in parallel the cathode resistance will be 16500 ohms and so the average cathode current will be about 3.4mA in your case.

You also measured 168V DC at the anode on range G. So with 250V HT and a 22k resistor R216 the average anode current will be (250-168)/22000 = 3.7mA. This agrees quite closely with the cathode current above. Some of the difference will be component tolerances so I think you made good measurements here because the anode and cathode average currents are very close

Based on the datasheet and the schematic I predicted a 5.7mA startup current for V103B on range G but this will typically fall by a factor of about 0.75 once the oscillator has settled because the valve will be in a low duty cycle pulsed mode once the oscillator settles. So I think the current should drop to 3.7mA with a new valve on range G. Your valve is probably quite healthy if it still manages 3.4mA here.

So my prediction for range G was:

0.0037 * 16500 = 61V at the cathode with a settled oscillator. (you measured about 55.6V)

250- (0.0037 * 22000) = 168.6V at the anode with a settled oscillator.(you measured about 168V)

This agrees very well with your earlier measurements and hopefully this will give you confidence that your circuit is probably OK and that the test voltages on the schematic are not really valid for all ranges.
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Old 22nd Dec 2019, 11:52 pm   #132
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Default Re: Marconi TF144H/4S

Quote:
Both XT101 and 102 should be between 2.5 and 16v. I measure 19v for 101 and 29v for 102. This may or may not be a problem, or indeed may be an error in the drawings again.
Can I ask, what are you using to measure this RF voltage? It must be something quite decent with a high impedance because I would expect this part of the circuit to be very sensitive. I think a lot of probes will drag the oscillator out of oscillation when probing here. I would have tried using a cheapo classic diode probe here. I'm sure you know the one I mean with the 4.7Meg resistor feeding to a DVM. VDC on the DVM = Vrms at RF. However, I'm not totally sure if this probe will be OK to use here. Are you using a valve voltmeter here? It must be quite a good one in terms of having high impedance...
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Old 23rd Dec 2019, 12:48 am   #133
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Default Re: Marconi TF144H/4S

Because of dealing with RF, I was using a high impedance scope probe and a scope. I didn't really get much different results using a Fluke 87V surprisingly. I've got an adapter to get that probe to fit a DVM.

Marconi do specify a valve volt meter for these areas, but I don't have one currently serviceable.
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Old 23rd Dec 2019, 11:18 am   #134
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Default Re: Marconi TF144H/4S

Yes, I think you are supposed to use an RF VTVM for this measurement. The meter must not have any DC leakage or it will alter the DC bias of the grid and pull it negative wrt the cathode and this would cause the oscillator to lose a lot of loop gain. I also think that the VTVM would need to have low input capacitance, especially for probing the 2MHz crystal.

I think they are looking for an RF measurement in terms of AC Vrms at 400kHz and 2MHz rather than a DC measurement. I'd expect the meter to be AC coupled with an internal series blocking capacitor and I'd expect it to have a really high input impedance at 2MHz.

I think it will require a 2MHz crystal with a relatively low ESR to get several Vrms at this point on range E. I'm not sure it would be possible to get 16Vrms here on range E. It might be possible on range L.
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Old 23rd Dec 2019, 12:33 pm   #135
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Default Re: Marconi TF144H/4S

A simple and cheap homebrew diode detector probe like the one below would probably be adequate for sniffing at the crystal but I can't be certain.

http://n5ese.com/rfprobe1.htm

Note that I would recommend using a 1N5711 Schottky diode if you want something with fairly high input impedance across all signal levels. It won't be quite as sensitive as the 1N34A at low RF voltages but it should load the circuit less (especially for small signals) and it will only have a probe capacitance of a couple of pF.

I think a suitable probe needs to be <2pF or it will begin to eat away at the gain margin on startup when on range E (2MHz crystal).

The above diode probe will be a bit deaf at RF voltages below about 3Vrms. At 2MHz at 2Vrms it might read 1.75Vrms for example.

At 1Vrms it will probably indicate just under 0.8Vrms. At 3Vrms it should read about 2.7Vrms. Close enough and I think it should have a very high impedance when detecting these RF voltages.

Looking in the TF144H manual they mention the TF2600 voltmeter but this wouldn't be suitable for this measurement as the internal capacitance is way too high at 16pF.
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Old 23rd Dec 2019, 9:59 pm   #136
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Default Re: Marconi TF144H/4S

I wonder if I could borrow the probe from one of my 'not working properly' Marconi valve volt meters and connect it to a DVM?

Trying to decipher my scribbles on the paperwork I last night gave you the voltages measured with my DVM in DC with a regular probe but I didn't actually give you the scope readings, which at the time I thought I had.

My scope probe is 16pf when set to x10 so may well be affecting the voltage reading a little.

So looking at my scribbles and thinking it looks as though I haven't remembered something, I made the measurement again with a x10 probe on my scope. XT101 measured 1.6v pk-pk and XT102 2.96v pk-pk.

I think this DVM is supposed to be ok around RF to a certain degree, though probably not as high as 2mhz.

I'm not too concerned with any of these voltages at the moment as they may well read differently with the specified valve volt meter.
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Old 23rd Dec 2019, 11:09 pm   #137
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Default Re: Marconi TF144H/4S

I think a lot depends on the ESR of the 2MHz crystal. In my old notes from 25 years ago, a crystal in the 1-2MHz range can have an ESR of about 250 ohms. However, the only 2MHz crystal I have here measures over 500 ohms. This is a cheapo HC49 crystal designed to be used with a microcontroller.

Adding shunt capacitance across the crystal node will eat into the negative resistance produced by the 12AU7. With a crystal with a 500 ohm ESR this would probably be enough to stop it oscillating if I assume a startup gm of <1mmho for the tube and a 16pF scope probe across the crystal.

However, if the crystal ESR is as low as 250 ohms then the oscillator would probably still run OK with the 16pF across it. However, the oscillation level would begin to drop if there wasn't much margin here. If there is plenty of margin then the oscillation level would still be quite strong even with the probe attached.

You are only seeing 1.6Vpkpk at 2MHz which seems a bit low. However, I'm not sure how this circuit can produce 16Vrms at 2MHz on range E as per the upper limit in the manual.

Ideally, this oscillator just needs to produce enough drive to the grid to make the valve act like a low duty cycle 'electron gun' at 2MHz.
On the positive peak of the sinewave at the grid I think the valve will be just over the Vg=0 curve on the datasheet and this will turn it on strongly. But on the negative half of the sinewave I'd expect the grid to turn the valve off like a switch so there will be zero anode current for a fair bit of the RF cycle.

I'm guessing that it will be possible to estimate the peak anode current by looking at the Vg=0 curve in the datasheet and then look at the average voltage across the 12AU7. If there is 40V across anode and cathode of V103B then the VG=0 curve shows just under 5mA. So if the circuit is working normally, the anode current would be gating on and off with 5mA pulses of current at a 2MHz rate.

On range L where V103B acts as the mixer, this pulsed current would probably be 12mA peak as there is about 100V DC across anode and cathode. So the anode would act like a switching local oscillator that would switch on and off any leaked signal to v103B from the main sig gen from V103A and this will produce mixing in V103B. I think this is how it is able to act like a mixer to produce audible beat notes in the headphones.

I'd expect this to work quite well even with a fairly wide variation in RF voltage across the crystal. This is because V103B is already biased at Vg=0 so it will be easy to drive it into the non linear 'pulsed' anode current mode where the V103B triode will then act as an RF switching mixer at 2MHz on ranges G to L.

On ranges A to F the RF waveform at V103B will leak across to V103A and the mixing takes place in V103A across ranges A to F. Presumably, V103A gets driven into a non linear state by the main RF signal generator signal and V103A is also biased at Vg=0 to make this easier.
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Old 24th Dec 2019, 12:24 am   #138
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Default Re: Marconi TF144H/4S

If it helps I think the various V103B waveforms will be as the drawing below on range E. Sorry the diagram is a bit hasty and wobbly but I think this is what you might see on range E with the 2MHz crystal. It might manage 4Vrms at the grid with a lowish ESR crystal for example.

The 27V DC level is just a guess as it will depend on several things but I'd expect to see something in this ballpark. You can see that the grid waveform in green should roughly match the voltage seen at the cathode at the sinewave peaks. This is when the triode will be at Vg=0 and the anode current will peak at maybe 5mA according to the datasheet. On the negative dips of the grid waveform the grid voltage is much lower than the cathode voltage so the triode is going to be effectively pinched off so there will be close to zero anode current for a fair chunk of the RF cycle.
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Old 24th Dec 2019, 10:31 am   #139
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Default Re: Marconi TF144H/4S

Yes the waveform looked very similar. I seem to recall when coupling the scope to DC, I did measure such levels for the 2mhz xtal.
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Old 24th Dec 2019, 2:31 pm   #140
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Default Re: Marconi TF144H/4S

That sounds good thanks.

One thing to bear in mind is that I think a classic x10 scope probe or a DMM will present a 10Meg leakage path to ground so I assume that the DC voltage measured at the grid will get dragged down slightly and will appear at about 10/11 of the true value because there is a 1Meg resistor between the cathode and the grid and this will form a potential divider with the 10Meg external scope or meter. This will also cause the valve to be slightly biased off so it will affect the starting gain a bit. Also the capacitance of the probe will cause issues here as well. So I think the scope will only show an approximation of the true (unloaded) voltages. However, I think the scope waveforms will be close enough assuming the crystal is a low ESR type and the circuit can still oscillate strongly with the probe attached.

I had a rush of blood and phoned up Hardings in Cheltenham to see if they still had any of these old valves in stock so I could have a play here.

The owner said no but he said they cost a lot of money nowadays although it depends on the manufacturer.

Looking on Ebay the prices are scary... This was a big shock. It seems these valves are favoured by HiFi enthusiasts and guitarists so the prices have rocketed. I think the valve base/holder is the same as the one used for the old Denco MW/SW coils and I know I still have a box of Denco coils and bases in the loft somewhere. So this makes it all the more tempting to have a play.

I guess the chances of buying a 'new old stock' 12AU7 are slim these days.

Presumably a used and slightly tired version will have lower electron emissions at Vg=0 vs anode voltage so the DC biasing will change for one thing. So I'm not sure I'd learn much from a used example if I wanted to compare against the datasheet.

The other interesting thing is that these valves often get used by audio enthusiasts at HT levels as low as 12V.
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