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Old 8th Feb 2016, 1:59 am   #41
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Default Re: Marconi TF144H CT452A RF Generator

Note that I missed seeing the series C129 in the output of the modulation LPFs so as long as this cap is leak free then the others in the A and B LPFs are less important wrt leaking.

How old is this version of the design? I have some genuine HP2800 diodes here that are date stamped 1974 on the bag but I don't know when this diode was first introduced by HP. WME_Bill gave some interesting info in this thread here:

https://www.vintage-radio.net/forum/...d.php?t=123343

Interesting that the first (thermocouple only?) version of this generator dates back as far as 1933. Many years ago I had a Marconi 801 sig gen as my first ever commercial sig gen and this was quite old but not as old as the TF144 appears to be. It cost me about £15 back in the early 1980s.
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Old 8th Feb 2016, 8:27 am   #42
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Default Re: Marconi TF144H CT452A RF Generator

Jeremy. This has turned out to be an interesting discussion. I am very grateful for all the replies.
If you look at my post #35 I have given some examples. I have measured the 1V ALC output with a scope and a X10 probe. Removing the probe does not affect the output. In a much earlier post I mentioned driving the feedback circuit from R107 R171 with a PSU to check the feedback basically worked. Colin
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Old 8th Feb 2016, 6:02 pm   #43
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Default Re: Marconi TF144H CT452A RF Generator

I dug out some 1N5711 Schottky diodes at work today and I've put together a copy of the ALC detector on a piece of scrap PCB. I forced 2.5V at the input to R101 and I included the 1Meg resistor R107 and I used a 1nF to represent the system capacitance at C160. To boost my sig gen output I placed a 2:1 turns ratio TLT transformer at its output.

If I set the sig gen to 1MHz and arrange it such that I see 8Vpkpk (or 4Vpk) at the input to C104 then the detector produces +4.61V DC at C160 when measured with a Keithley 2015 on the 10V range. This meter has an extremely high input impedance on this range. Much higher than 11Meg ohm.

My equation predicts ((4-0.5) * 0.637) + 2.5 = +4.73V. This is quite close to the measured result.

I then changed the level to 5Vpkpk (2.5Vpk) and the detector produced +3.70V. The equation predicts ((2.5-0.5) * 0.637) + 2.5 = +3.77V. Again the result seems to agree with the equation.

If I switch on some AM modulation the ability of this detector to reject the modulation is very impressive indeed. Much better than I imagined. From 0% mod to 40% mod the 3.7V level at the detector output only changes down by 0.01V. By 80% it drops by 0.025V. Because this change is so small there may be some contribution from the quality of the AM modulator in my sig gen. It may not be that great at producing 80% modulation with low distortion.

If I change the RF level by 0.15dB then I get about the same change in detector output so it looks like the 80% modulation only affects the detector performance very slightly. A 1dB change down in RF level drops the detector to 3.54V.

I also did similar tests at 20MHz and got very similar results. So I think the detector behaves as expected. It looks like they chose an ALC detector that would not be affected by the modulation depth.
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Old 8th Feb 2016, 7:56 pm   #44
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Default Re: Marconi TF144H CT452A RF Generator

Quote:
perhaps you can try and explain why they needed two separate systems to monitor the output voltage? They had the thermocouple which only drove the meter.

Then they had the diodes MR101 and MR102 in a sort of bridge arrangement, which is labelled "ALC Monitor", and which feeds back into the ALC amplifier.
I think we are all now getting closer to the truth about the way this generator operates.

If the sig gen operates the way I think it does then the ALC detector simply 'has' to be designed to be immune to modulation depth. This would be a mandatory requirement.

This is because the thermocouple is used to indicate modulation depth. For the thermocouple to (reliably) indicate modulation depth as a function of increased average power then the ALC simply has to keep the average voltage of the RF waveform constant as the modulation depth is changed towards the SET MOD position of the meter. Otherwise you wouldn't know if the increasing average power seen on the meter was due to increasing modulation depth or simply a change in level caused by an underperforming ALC system.

It may well be that the primary function of the ALC is simply to provide this average voltage maintaining feature. So maybe it shouldn't be viewed as a tool to help maintain a level output across frequency. It probably just has to be able to maintain the 'average voltage' of the generator output as the modulation depth is changed at the chosen frequency. This would then mean that the thermocouple readings would be valid for measuring modulation depth.

In other words, the power level calibration and the mod depth calibration comes from the broadband (average power reading) thermocouple and the thermocouple relies on the ALC detector to reliably 'hold' the average voltage of the RF waveform regardless of modulation depth.

Would this make sense to anyone who has used this generator? It's beginning to make more sense to me now so maybe someone who has lots of experience with these generators can comment.
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Old 8th Feb 2016, 8:49 pm   #45
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Default Re: Marconi TF144H CT452A RF Generator

Obviously, the other important thing that the ALC will hope to provide will be a fairly constant RF carrier level over a fairly long time period at the chosen frequency. i.e. it will help offset any drifting in generator output level due to temperature changes inside the generator.
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Old 8th Feb 2016, 8:55 pm   #46
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Default Re: Marconi TF144H CT452A RF Generator

I've had a look at that detector, too, and there seem to be two different timeconstants at work. There is one of roughly 500us and a slower one of about 500ms.

The ALC in the generator (I used to have one as my only sig gen at home for several years) doesn't do a very good job of holding the level steady, it varies as you tune across a band and can jump a lot as you change band, so you have to set your frequency with CW selected, trim the carrier level to the mark on the meter, switch in AM and trim the modulation to move the meter up to the wanted modulation percentage mark.

The detector charges up the two 0.1 uf capacitors (the DC block in the RF feed to it) via the effective RF source Z and the two 1k resistors. They can discharge when the RF goes away via the two 1k resistors. This gives a fast-ish timeconstant OK for 400Hz, but useless for 20kHz external modulation. The 1 meg + 100k resistors to the 200pF capacitor aren't too bad, but it's the capacitance of the feed from the modulation source which slows things down.

The averaging is done by having a relatively quick detector whose output is averaged by a slower timeconstant. The detector needs to be able to discharge itself so it can reasonably follow the whole of the modulation envelope. If the detector is too slow, it records the peak area only and is in slow free-fall through the trough of the modulation envelope, and thus it overestimates the level of the signal and reads higher than the average. With the speeds I've seen, this circuit should as Jeremy says respond well to the average, but though OK at 400 and 1000 Hz, I think the ALC will not be good with 20kHz applied externally. Still, turning the mod off and setting the carrier level to nominal manually, then applying mod to move the pointer to the wanted level will still work.

These are very labour-intensive sig gens!

David

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Old 9th Feb 2016, 1:49 am   #47
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Default Re: Marconi TF144H CT452A RF Generator

I'm still not really following what you are saying about this detector David. You seem to be suggesting that the core of the detector is a fast peak detector.

I think it is simply an averaging detector circuit but Marconi have opted to very lightly side tap the output from D102 cathode via an external RC integrator. This will ultimately give a higher DC output level from the averaging detector but because D102 cathode has a raw RF half cycle on it the detector will need an extra RC integrator using R107 etc to get this boosted DC output and I think that R107 needs to be big (compared to the loading resistor R101 at 100k) or I think it will upset the detector action.

Therefore, I think the averaging performance of the detector should be immune to corruption from mod rates up at 20kHz if I assume the RF carrier frequency is well up somewhere in the LW/SW/HF/VHF bands.
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Old 9th Feb 2016, 2:11 am   #48
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Default Re: Marconi TF144H CT452A RF Generator

To detect the average of an AM signal, you must take the average over the whole of the modulation cycle, or else you get an offset result.

If I make a slow detector with diodes charging large capacitors and large value discharge resistors, the diodes and the source impedance of the RF are low-impedance and can charge the capacitors quickly, so the detector can follow the modulation envelope up to the peak. But if the discharge rate is too slow to follow the modulation envelope and the capacitor voltage stays too high and for too long. The 'DC' output will not be the average, but will have been moved upwards because the positive peak got followed while the negative one was not.

To perform an average detection, the RF waveform must be rectified fast enough to switch properly at the RF frequency, and then the resulting rectified voltage must be averaged in such a way that the stored average voltage does not influence the function of the rectifiers. Essentially you have to isolate the averaging function from the rectification function, and in this circuit, the 1 meg resistor does this.

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Old 9th Feb 2016, 2:56 am   #49
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Default Re: Marconi TF144H CT452A RF Generator

I think you are trying to analyse this as a classic envelope detector followed by an RC integrator. This circuit doesn't work like this. Forget the side tap circuit comprising R107 and C160 and look instead at C105 as the detector output.

I think that for this to act as an averaging detector the MR102 diode MUST always conduct for the whole of the forward half cycle of the RF carrier and the MR101 diode MUST always conduct for the whole of the negative half cycle of the RF carrier. I think the circuit is configured to ensure this happens. This is therefore not an envelope detector circuit.

This behaviour happens in snapshots at the RF frequency. All the modulation does is dictate how much (average) current gets conducted (in and out of C105) for each RF half cycle because at modulation crests each individual RF cycle is big in amplitude and at troughs the amplitude is small.

So this detector is best analysed on an RF cycle by cycle basis. Think of each complete RF cycle as a separate +ve current injection and -ve current extraction event that can do its bit towards the long term average voltage at C105. The C105 capacitor acts as a reservoir that will (long term) store the average of all this.

For each RF cycle there will be a continuous half cycle of positive current charging C105 and for each negative cycle there will be current being sucked out of C105 with the time profile of a half sine wave. Over time you get a DC voltage at C105 that is proportional to the average RF voltage. I think that that is how this type of detector operates. Marconi complicate it a bit by doing their side tap trick with R107 and C160 etc.

See below for a basic version of this detector without the 2.5V DC pedestal at R101 and without R107 etc. Note that if you swapped the R101 load and C157 over to the right hand side the detector output voltage at C105 would be negative rather than positive.

Does this help? (note that it's late and I'm tired so I'm probably not explaining this very well)
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Old 9th Feb 2016, 6:35 pm   #50
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Default Re: Marconi TF144H CT452A RF Generator

To simplify the circuit still further, you could remove R101 and C157 completely as in the circuit below and also increase the value of C105 so it is very large with a very low reactance at the RF frequency.

This way the detector becomes 'balanced' and when an RF carrier is fed into it the positive half cycle will inject charge into C105 and the negative half cycle will remove exactly the same amount of charge.

So the long term average dc voltage at C105 should remain at zero in theory at least.To the circut driving it the whole detector will just look very similar to a 1k resistive load because C105 will appear as a virtual short circuit. R103 will look like the 1K resistive load on positive half cycles and R102 will look like the 1K load on the negative half cycles. Obviously, you need a fairly big driving signal in terms of Vpkpk for this to be the case because of the diode action.

If you then reconnect the 100k load resistor R101 and C157 to the left hand side, the circuit will go out of balance and it will reachieve equilibrium when C105 has charged up to a certain DC voltage. This voltage will be proportional to the average RF voltage and at this sweetspot detector voltage the net charge in and out of C105 will average to zero. i.e. the same amount of charge injected into C105 in the positive half cycle will be sucked out again in the negative half cycle. This means the long term average detector voltage will stay constant at this dc voltage.
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Old 10th Feb 2016, 12:04 am   #51
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Default Re: Marconi TF144H CT452A RF Generator

I have now performed a similar test to Jeremy but with no modulation, as my fault is apparent without mod.
I removed the turret and lifted the top end of C104 to reduce the load on a borrowed genny. I kept the input (connected to the lifted end of C104) to 4V p to p, the max I could get. I then monitored the output at the junction of R107 & R171 with a scope and a X10 probe. I stepped the input from 1Mhz to 40Mhz. The output stayed at between .8 to .95V DC which I thought was acceptable. The output reduced smothly to zero if I lowered the Input volts as expected. (No power applied to the genny)

In the fault condition the output seems stuck at around 1 volt when on the 4 upper frequency ranges. I was wondering if its something to do with the pot down R195 and R192. I do get around 2.5V here when powered with out the turret which is about right. When on range K I think this point is ment to be held low through the turret contacts? More investigation needed here perhaps tomorrow! Colin.
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Old 10th Feb 2016, 6:30 pm   #52
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Default Re: Marconi TF144H CT452A RF Generator

Reading Colin's last post may have given me a bit of a Eureka moment...

Quote:
David: Jeremy is definitely right that this is a slow loop,
I'm now not so sure I am right. I think I'm both wrong and right at the same time about how the detector and ALC system operates.

But I think (hope) I can move us one step closer to how this system works. Read on...

I now think David is correct in that the system has a fast loop for modulation (call this detector MODE 2) and I also think the loop has a slow feedback response as well for the RF level (call this detector MODE 1).

The core of the detector is definitely an averaging detector with a reservoir at
C105. I think in the absence of modulation it does act in a slow loop because we have the R107 and large capacitance at C160 and the DC voltage at C160 will be a function of the average voltage of the RF waveform.

But when modulation is fed into the system it is fed in at C160 which is the output of the detector. I'm now wondering if the system takes on a different dynamic when modulation is fed in here...

If you drew the modulation system as a negative feedback system then I think the C160 node becomes a virtual ground when the MODE 2 loop is closed. So the large capacitance at C160 doesn't influence the loop response to modulation. It is effectively gone as far as the modulation is concerned.

I now think that the ALC detector is more complex than I first imagined because I think it can do TWO important jobs rather than just one.

It absolutely NEEDS to behave as an averaging detector in order to produce a DC level at C160 that keeps the RF carrier level constant (with or without modulation) because this DC voltage should represent the average voltage of the RF waveform.

But it can also 'rectify' the whole RF signal and feed this raw waveform into R107 along with the average DC voltage taken from C105 plus the 2.5V DC pedestal. Although it hasn't been filtered down to just audio as in a regular envelope detector I'm now wondering if this complex signal is able to 'null' the modulation at C160 when the system is healthy and in closed loop?

This would provide a kind of MODE 2 ALC where MODE 1 ALC uses the averaging feature of the detector and MODE 2 is there for the case of modulation. Because C160 is a virtual ground there wouldn't be a rolloff in ALC performance up at high modulation frequencies.

I think there is just 1 Meg feeding into a virtual ground. So the MODE 2 loop should support a wide range of AF frequencies without rolloff .

Does this make any sense? There may still be quite a few mistakes in the above but I think it is at least worthy of debate...
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Old 10th Feb 2016, 8:19 pm   #53
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Default Re: Marconi TF144H CT452A RF Generator

Sorry for the double post, can a mod remove post #52 please?

If the 2.5V pedestal is not used on this range then, if 4vpkpk (2Vpk) is fed into the detector then I think the output would be (2 - 0.5) * 0.637 = 0.955V so I agree that the detector looks OK with your latest tests Colin.

You could also look at C105 as a double check of detector health because I think this should give about 0.47V here because this 'average' should be half the voltage seen at C160 if there is no 2.5V pedestal.

Hope nobody minds my hit/miss attempts to unravel how this all works but one curious aspect of the latest MODE 2 theory would be that the modulation drive level would have to be very puny in order for the ALC detector to close the loop because of the huge size of R107.

If you visualise a classic inverting opamp circuit as the model for the negative feedback system, the input modulation would be the 4.7K series resistor R187? feeding into the virtual ground of the opamp and the R107 resistor would be the feedback from the opamp output. The opamp output would be the input to R107 from the ALC detector.

So if we have several volts of AF coming back from the ALC detector, this would correspond to a modulation input voltage at R187 of just a few millivolts if the input resistor for the modulation was 4.7k ohm for example. This is because the opamp would have an inverting gain of (1000,000/4700) which is over 200.

This seems a really small waveform? I'm definitely confused about this one...
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Old 10th Feb 2016, 10:10 pm   #54
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Default Re: Marconi TF144H CT452A RF Generator

It's the sort of circuit you just want to file under A-for-avoid!

There are definitely two things going on at once in it and we each saw only one at first... and we also saw a different one each.

That amplifier pentode-triode also has positive feedback applied to the pentode cathode to sparkle it up a bit... then there are timeconstant bodges all over the place to try to keep it stable.

Taking a step back, screen grid modulation is never very linear and a sig gen needs to be good, so closed loop modulation even just enough bandwidth to cover its internal 400Hz and 1kHz tones, is going to be needed.

The detector isn't a fast peak detector. It's a quick enough detector to follow the envelope discharging quickly enough to follow the modulation troughs so that when averaged in later bandwidth reduction, the average taken over the full modulation cycle comes out, so it does control the carrier.

On top of it all, it relies on the user fixing the failings of this system by manually having to twiddle carrier and modulation levels using the thermocouple meter!

I had one, but I didn't keep it.

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Old 10th Feb 2016, 10:46 pm   #55
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Default Re: Marconi TF144H CT452A RF Generator

Quote:
There are definitely two things going on at once in it and we each saw only one at first... and we also saw a different one each.
Agreed.

I saw the average voltage detector with the two diodes and it makes sense why they need this averaging feature and it does look like the loop is 'slow' when setting and holding the carrier level.

But the clever trick they have done is to side tap it and it then acts as an average voltage 'doubler' using the R107 C160 timeconstant. But (hidden to me at first) this also allows them to feed a raw rectified half of the RF waveform back into the loop along with a doubling of the DC voltage that represents the average. So this detector is definitely a two trick pony

Quote:
Taking a step back, screen grid modulation is never very linear and a sig gen needs to be good, so closed loop modulation even just enough bandwidth to cover its internal 400Hz and 1kHz tones, is going to be needed.
I don't have your knowledge about valves to understand the limitations of the oscillator but I guess it would be difficult to make it modulate cleanly in open loop.

Note that I do still think the ALC detector (when tested on its own) has the design integrity to work well as an averaging detector even at mod AF frequencies up at 20kHz and at 80% mod depth.

I tested my copy of it here using a Marconi 2024. This is the +25dBm version of the generator and it can ouput really good AM up to about 12dBm. The detector rejected the modulation (as in the DC level representing the average didn't change at C160) right up to 20kHz mod frequencies and gave out an accurate DC detection voltage for the average. I even tested it with 100% OOK to see if it could give out an average DC voltage at C160 and it could do it easily even at 20kHz OOK with 50% on/off ratio.

By contrast the detector was able to spot deficiencies in two of my other sig gens. I have a couple of newer HP/Agilent ESGD sig gens here and the AM modulator didn't perform as well into the detector even though it looked OK on a scope.

I found out the reason by looking on a spectrum analyser. The high power Marconi sig gen maintained a solid carrier power level up at 80% mod but the other two showed sagging of the carrier at high mod levels. So when they were tested the detector voltage sagged as the mod went up. I concluded that I was running these generators too close to their upper limit for accurate AM modulation. The older ESGD 4000A had rolloff in its modulation at 20kHz as well when looked on a scope so it wasn't really the right choice for this task.
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Old 10th Feb 2016, 11:35 pm   #56
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Default Re: Marconi TF144H CT452A RF Generator

In case anone is puzzled about my sketchy description of the negative feedback system I knocked up this simple AC sweep of an inverting opamp.

The 4k7 resistor R2 represents the series mod resistor R187 and R1 represents the 1Meg resistor R107 in the TF144. C2 represents the 200pF shunt capacitance at C160. I think the op amp output represents the output node at MR102 in the ALC detector. So overall this is an ultra crude model of a negative feedback system.

Normally, an RC of 1Meg and a 200pF cap would rolloff a lot by 10kHz but in this case, when the loop is closed you can see that the bandwidth of this system is flat way past 20kHz.

I've put in 10mV AC at the input to the 4k7 resistor and it feeds back about 2V via the 1 Meg resistor.

So the 200pF capacitance doesn't cause rolloff because it is at a virtual ground in the feedback system and the AC level here is cancelled/zero due to the feedback action.

Hope this helps more than it confuses but some people might still be wondering how the MODE 2 modulation loop can be 'fast' with that 1Meg resistor R107 and the high capacitance at C160.

Note that this still might all be wrong but I think that together, we might be getting closer to how this strange old sig gen operates
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Old 11th Feb 2016, 12:29 am   #57
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Default Re: Marconi TF144H CT452A RF Generator

The opamp you chose is helping you out. That 200pf capacitor puts an extra pole in the loop transfer function, and although it doesn't directly lose closed-loop gain, it robs some of the stability margin by increasing phase shift. Quite a bit of the sticking plaster work in this system must be trying to compensate for this.

There's been an immense amount written on frequency determination and stability in oscillators, and studying the huge variety of the things, but very little written on the amplitude-determining mechanisms. Clarke & Hess; Communications Circuits Analysis and Design is one of the few. Modulating an oscillator is particularly difficult. The other thing you risk is large amounts of FM as a by-product.

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Old 11th Feb 2016, 11:12 pm   #58
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Default Re: Marconi TF144H CT452A RF Generator

Yes, the opamp demo is really just meant as a crude model or concept demonstrator of the negative feedback system to help visualise it.

I'm still very confused about the very small modulation drive level required. The TF144 manual suggests the system needs many volts pkpk for the external modulation yet the model suggests that because R107 is so big, the modulation at the 4k7 resistor R187 needs to be just a few mV to close the loop. There are some front panel user pot controls to turn down and set the (external) level but an ultimate mod level of just 10mV at R187 seems a bit unrealistic.

I found a SPICE model of the 1N5711 today and I did some SPICE simulations of the averaging detector using squarewave modulation up to 20kHz. The resuts agree very closely with my lashup copy on my workbench.

I'll post them up when I've finished playing.

Hope Colin makes good progress with the troubleshooting and repair this weekend
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Old 11th Feb 2016, 11:30 pm   #59
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Default Re: Marconi TF144H CT452A RF Generator

We may just have put him off that signal generator...

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Old 12th Feb 2016, 11:29 am   #60
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Location: Rainham, Kent, UK.
Posts: 523
Default Re: Marconi TF144H CT452A RF Generator

Not giving up yet! I had previously been used the genny for a few months, aligning my B40's and such. I found that once I had set the CW level it remained fairly constant over all bands. Hopefully have some interesting fault finding notes after the weekend. Colin
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