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Old 24th Dec 2019, 7:18 pm   #141
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Default Re: Marconi TF144H/4S

If I get a chance later, I'll be having a bit of lab time, I'll probe it again and take a photo and show you.

The other thing I want to do for the sake of it is the last crystal calibrator check. From the manual on page 38 under Crystal Calibrator checks, 3C. It says I need to connect an audio oscillator to pin one of the RF box tag strip. Now, there are a few tag strips in the RF box, the monitor tag strip has two strips numbered 1 to 10 on the drawings and another strip inside the box not numbered. It says to connect via a capacitor, value to be decided/suggested.

But first, find the pin one!
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Old 24th Dec 2019, 10:30 pm   #142
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Default Re: Marconi TF144H/4S

Here's the 2Mhz Xtal waveform.
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Old 24th Dec 2019, 11:07 pm   #143
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Default Re: Marconi TF144H/4S

Thanks. At first glance that looks like the waveform I'd expect to see at the cathode at pin 8. At the cathode I'd expect to see a kind of wonky sine wave but with a flat shape to the negative peaks. This would correspond to the anode current falling to zero for that part of the RF cycle.

At the grid on pin 7 I'd hope to see a fairly good sine wave at several volts rms assuming that the probe doesn't spoil things here. Do you see a sine wave at pin 7?

I see you have a Tek TDS2012/2 scope. I've got one of those here and the counter on it is very good and stable. This is the dedicated frequency counter readout in the lower right corner of the display rather than the menu counter further up on your display. Much better than the vague and jittery software based counter found in many digital scopes!
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Old 24th Dec 2019, 11:40 pm   #144
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Default Re: Marconi TF144H/4S

Quote:
Originally Posted by G0HZU_JMR View Post
Thanks. At first glance that looks like the waveform I'd expect to see at the cathode at pin 8. At the cathode I'd expect to see a kind of wonky sine wave but with a flat shape to the negative peaks. This would correspond to the anode current falling to zero for that part of the RF cycle.

At the grid on pin 7 I'd hope to see a fairly good sine wave at several volts rms assuming that the probe doesn't spoil things here. Do you see a sine wave at pin 7?

I see you have a Tek TDS2012/2 scope. I've got one of those here and the counter on it is very good and stable. This is the dedicated frequency counter readout in the lower right corner of the display rather than the menu counter further up on your display. Much better than the vague and jittery software based counter found in many digital scopes!
Its the TDS2014, the counter on it is very useful, very stable. Very convenient scope and bench space saving. Managed to pick it up for a bargain price when a company I were working for made us all redundant and needed to get the building empty so we all managed to get items at silly prices. I wouldn't have had it otherwise as they aren't cheap, I've got a few cathode ray scopes which although basic, are rather adequate, though I do find the features on this scope very useful indeed, such as measurement, cursor, the trigger menu items and the compactness, portability and space saving is a real bonus.

I've just scoped pin 7 and I do get a nice clean sinewave in the order of 2.15VRMS.
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Old 24th Dec 2019, 11:51 pm   #145
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Default Re: Marconi TF144H/4S

I bought my 2 channel TDS2012 not long after they were first released. This was in the early 2000s and I got it at the (then) bargain price of about 550. These scopes don't get good reviews on Eevblog but I like them a lot for certain tasks. They are small and easy to use and they are silent. If I'm writing any MCU code I always use this scope because I can work in total silence in the room. The only niggle with mine is that it only has 2 channels which often isn't enough for MCU work. the TDS2014 is nicer because it has 4 channels.
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Old 25th Dec 2019, 12:01 am   #146
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Default Re: Marconi TF144H/4S

They do now have 2 channel scopes at reasonable prices. I think these ones were in the order of about 2k a pop at the time our company bought them.

Once I'm happy with this 'pin 1' to connect and audio oscillator up to, it's nearing completion of the checks. I have some other signal generators, rather than audio oscillators, I'm hoping they'll do.
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Old 25th Dec 2019, 12:14 am   #147
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Default Re: Marconi TF144H/4S

They might be referring to the connection '1' in the little square box at the top of the crystal calibrator circuit at FILT 1. This will have a 22k connection (R216) to 250V DC so some care would be needed here along with a high voltage coupling cap. This node in the circuit corresponds to the filtered audio output from the heterodyne mixer at the top of FILT 1. So it would seem logical that this would be the place to inject a test audio signal... but maybe not the safest!

If this is the right connection then I'd have expected a safety warning in the manual because there could be 100-250V DC via R216 here. It might give you a jolt and it might also zap any (modern low voltage) test gear you connect to it even with a DC blocking cap in the way.

They also ask for the range control to be set to a 'between 2 ranges' setting. Will this partial rotation of the range switch put the switch wafer in an open circuit position to isolate the mixer stage and prevent it loading the test point?
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Old 25th Dec 2019, 12:24 am   #148
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Default Re: Marconi TF144H/4S

On page 33, I were thinking it were one of those tag strips, which are inside the RF box.
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Old 25th Dec 2019, 12:35 am   #149
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Default Re: Marconi TF144H/4S

I think it might be the tag strip shown next to figure 4.8 on page 39. This looks to be the same location I described in post #147.

I'm also uncertain what "select CRYSTAL CHECK RANGE Control between two ranges" actually means. Do they mean that the control is deliberately fudged to rest halfway between two range positions?
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Old 25th Dec 2019, 12:46 am   #150
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Default Re: Marconi TF144H/4S

Yes I think so. By the looks of what the test requires, it's to make sure that any 'range' isn't connected, so that any tests injecting voltage is diverted away from other circuits.
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Old 26th Dec 2019, 1:51 pm   #151
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Default Re: Marconi TF144H/4S

Boxing day isn't for cold turkey, it's for fettling and pottering in the 'lab'.

Right, so having located pin 1, it was a tag strip outside of the RF box, just under the HV regulator valves.

So, with Crystal Check selected and Range set to A, I measure at Pin 1, 112V DC. With range between two ranges, 86V DC.

So, that voltage will need to be considered if I'm going to connect a signal generator to it.
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Old 26th Dec 2019, 2:08 pm   #152
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Default Re: Marconi TF144H/4S

A 100V 1nF poly cap will probably be fine for coupling to the circuit, it'll block the DC but allow a small AC signal through.
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Old 26th Dec 2019, 10:00 pm   #153
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Default Re: Marconi TF144H/4S

So, it sort of works. The ALC is certainly doing something, but the levels I can't be sure of at the moment as the presence of any RF may be why Marconi are specifying that this measurement needs to be done with the valve volt meter.

My valve volt meter (Marconi TF1041C) is out of calibration, it does need some work doing to it, but I can roughly gauge that it is giving different results in these areas, but can't be certain of the actual reading, so at some point I will need to sort that out.
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Old 28th Dec 2019, 9:22 pm   #154
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Default Re: Marconi TF144H/4S

So, there's still a few things to do, however the unit is working fine really, as reported earlier, but I still wanted to go through the whole set up.

So regarding voltages, the valve voltmeter is being specified to measure some of these voltages I'm using a modern DVM for, which may be getting upset with present RF.

For the record with my DVM I'm measuring at the phones socket up to 38V DC with my signal generator connected (pin 1 tag strip) at 1khz 20v pk-pk (manual suggests these are DC voltages, however the rectified probe on a valve volt meter is for AC measurements). The max should be 20v and if not then the manual suggests checking V204, MR206 and C208. All appear to be fine. With carrier off this can be 65-70v.

I don't actually have an audio oscillator, just regular generators, but the sinewave appears pretty clean.

So, to try and be sure what I shall do is come back to this with a working valve voltmeter (hopefully) and see if this can be nailed down. Then hopefully I'll be somewhat satisfied with the Crystal Calibrator section of this set up procedure/performance checks.
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