Marconi TF144H/4S

[Sinewave]

Hi Bill, that's interesting, thanks for that, something for us to think over.

RMS certainly does seem more likely in that case.

At the moment with the output set to full, RMS output with 'set carrier' fully clockwise is 2.4v RMS, that's measured by a scope which will have a high impedance output. If I short across the scope with a 50ohm terminator it'll drop to 1.17v RMS.

'Set carrier' fully anti clockwise is 7.2v RMS no load and 2.9v RMS with a 50 ohm across it.

Of course the output is EMF, which will be a voltage with no current flowing, so perhaps that is somewhat right.

I wonder if adding the thermocouple changes those readings?

[Sinewave]

What is the actual consensus on what would blow the thermocouple?

High HT? Or high output (poor attenuation?)?

[WME_bill]

TF144H thermocouple meter.
I have always heard 100% overload will risk burning out a thermocouple.
Now in this circuit, the meter measures before the attenuator, at the high output level. So attentuator settings will have no risk.
High HT will, as it will increase the output voltage applied.
But the ALC (automatic level control) should control the output, so if that is operating correctly, the voltage is restricted.
Only other thing is the Set Meter preset RV103. Keep turned to min (CCW), as that will effectively short the thermocouple to chassis. Then turn up slowly to calibrate the meter. Also as described in the manual the SetCarrier RV102 and Set ALC RV101 pots.
I have not yet worked out the purpose of MR103/MR104. High speed germanium diodes, which are reverse biased 6.5V from the heater supply, but AC coupled across the thermocouple. Perhaps it shunts away excess RF/AC output voltage, and so acts top protect the thermocouple.
What views from others?
wme_bill

[Sinewave]

Hi Bill.

I'm happy with the HT level, so where previously HT rose to a level which blew the 500mA HT fuse extensive work has been completed to remedy HT issues.

So the condition of my attenuators will make no difference what so ever in the preservation of thermocouples - so I can forget worrying any further about those.

In that scenario, my R100, R186 and R198 are fine. I need to check C195 and make sure voltage across the TC doesn't exceed 6.5v and the manual says that's peak to peak.

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So today, with a wire soldered onto the tag strip at the junction of R186 and R198 where TC102 would be connected, with an oscilloscope I measure with (main operator panel controls)

Set Carrier fully clockwise, 6.4V Pk-Pk

Set Carrier counter clockwise, 17.6V Pk-Pk

Notably, this is with attenuated output on max, any other range below max and the highest voltage measured at that network is 14.4V Pk-Pk.

Hmmmm, I wonder if this is the effect of running without the TC, if the TC provides enough volt drop at max when connected and if the removal of the TC is affecting the ALC?

[Sinewave]

I've now set RV103 fully CCW. I now measure 5.8V pk-pk at the R186/R198 junction with Set Carrier fully CCW.

I also note that the RF output voltage has reduced to 2.23V RMS with attenuators set to max output.

When I rotate Set Carrier clockwise, the RF signal and measured junction signal disappears just before Set Carrier reaches halfway around.

I just need to be sure that all sounds right before fitting the thermocouple.

[Sinewave]

I made a little mistake, for some reason I touched RV101, which moved the ALC to about 100V. It wasn't until I was checking the ALC circuit that I thought something isn't right. So I've set RV101 back to where it should be, by the previous waveforms taken and measured the voltage at V102 to make sure it's back to 48V on pin three.

Now, the one I should have touched, RV103. Fully CCW, voltage at the resistor junction is 4V pk-pk with Set Carrier CCW and 1.03V when fully clock wise - and this time the waveforms/output remain!

RF output level at 5.87V RMS and 1.47V RMS respectively (EMF).

And when the RF stage wakes up shortly after power up, that 'peak' has gone away.

Now, things are much better.

But I'm still too chicken to fit the thermocouple, despite all measurements taken suggest that it's likely to be absolutely fine to fit the TC.

[Sinewave]

I went for it and fitted the thermocouple.

It does affect the pk-pk readings, I make lower readings now. Set carrier CCW is 1.4v pk-pk and fully CW is 344mV.

Output of the generator appears good, seems to be working well. I'm able to set carrier to the meter.

I know that since I've turned RV103 fully CCW, I will need to somehow calibrate the meter, however procedure of doing so by injecting DC into the unit is something which earlier in this thread has been strongly advised against.

I'm sure we'll work out another way some how.

[Sinewave]

Originally Posted by G4XWDJim

One of the tests and setting up procedures in the Marconi manual tells you to feed 2 volts into the output socket to calibrate the output meter. DON'T!!!!!

I did and it blew the thermocouple. There's a thread on here about it somewhere.

I've been giving this some thought.

There is no way I can calibrate the meter unless I do the above procedure.

Given all of the components I've replaced and the voltage which would be placed across TC102 under normal operating conditions, I couldn't see from my calculations how I'd damage the TC.

Following the procedure very carefully, I set the range control between two ranges, connected to the Direct Output with its switch on Normal and very carefully used a voltage calibrator to bring 2V into the unit. I then slowly adjusted RV103 until the meter sat on the 'set carrier' mark.

I have now successfully calibrated the level meter to read the 'set carrier' correctly.

I've just now got to sort the modulation out. Once I've done that, the level monitoring is sorted and then it's onto the rest of the set up procedure.

[Diabolical Artificer]

Good to see you making progress and no thermocouples were harmed in the process : )

Andy.

[Sinewave]

I wonder if anyone who has completed the set up process can help me with the next stage I'm on.

I'm perhaps not understanding the terminology, but here goes!

3 Crystal Calibrator

3A (d): couple to crystal calibrator by looping wire round V103.


Ok, I assume literally loop a wire around V103, but (other end of the wire?) where am I coupling to?

[Sinewave]

I'm obviously doing something wrong. I'm assuming taking the screened cover off V103, loop a wire around the valve, scope probe on the other end, but all I pick up is a noisy version of the RF out.

[Sinewave]

Tonight I skipped the above and took a look at the crystal voltages. If I measure XT101 from the junction of C144 and XT101 to ground I get 28V. I get the same using the same method to measure across for XT102.

I'm wondering if V103 is not functioning correctly.

[WME_bill]

TF144H.
Crystal calibration. Para3A. Measure frequency of crystal oscillator by looping wire around the valve, with screening can off, and feed that signal to your frequency counter/oscilloscope.
Or put your scope on the cathode of the oscillator (pin8) to see if it is working.
Para3B. Voltage (AC, so use scope) across crystal seems sharply different from that given in this para. Suggests that oscillator valve and buffer (V103) are not working. Lots of switching here, so check the Range switch at all settings (S100A).
The manual gives DC voltages around that valve. Have you checked them. You can solve many problems by checking the operating DC voltages.
wme_bill

[Sinewave]

Hi Bill, I checked some voltages last night.

I'll check the voltages on the drawing around pin 1 and 6 tonight, but last night I recorded the voltages at the C143 and C192/144 junction.

The drawing states 22V (A-F) and 84V (G-L) ranges.

I recorded the following

• A 20.76V
• B 20.91V
• C 22.05V
• D 22.68V
• E 35.35V
• F 34.6V
• G 55.64V
• H 55.64V
• I 83V
• J 83V
• K 83V
• L 83V

[WME_bill]

TF144H
Ranges E & F look a little high. Ranges G & H look a little low. But none seem disasterously wrong. I assume these are all DC. What does your scope show on the cathode of the oscillator valve. Try over all ranges. There is so much switching, that one dirty contact on S100A could be your problem.
wme_bill

[Sinewave]

Hi Bill,

I had to solder test wires into the pins as access is almost impossible, but here's my findings.

Tuning scale set to mid-range. Set carrier control affecting readings on some ranges.

Measuring pin 1 voltages of V103

Ranges

A 102.2V
B 105.2
C 111.2
D 114.5
E 118.4
F 114.2

G - L should be zero volts, but I see minus a few hundred milivolts to little over minus 1 volt on some ranges.

Measuring pin 6 voltages of V103

Ranges

A 72.6V
B 74.6V
C 78.7V
D 81V
E 91.9V
F 88.7V
G 168.5V Set carrier control doesn’t affect readings
H 168.5V Set carrier control doesn’t affect readings
I 189.4V Set carrier control doesn’t affect readings
J 189.4V Set carrier control doesn’t affect readings
K 189.3V Set carrier control doesn’t affect readings
L 189.3V Set carrier control doesn’t affect readings


Some of those are cause for concern.

[Sinewave]

Re: Cathode, pin 8. Using my scope I get 4.6V pk-pk.

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Re: Cathode Pin 8 (ignoring the message above)

Cathode of V103 (pin 8)

With set carrier. All readings pk-pk measured with an oscilloscope

Ranges

A 5.8V
B 3.88V
C 3.6V
D 2.56
E 1.8V
F 1.22V
G 744mV
H 520mV
I 460mV
J 600mV
K 470mV
L 200mV

By the time it got to millivolts, the waveform was getting a little noisy.

[Sinewave]

I just had the valve out to check the pin condition and put it back and now am not getting the same test results. The socket looks ok though.