Update on my newly acquired Metrix Type 920 HF generator.... making good progress !
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I fixed it ! Yes.... was very difficult but you know how good I am. Was very difficult.... the super low output voltage was solved by.... pressing the probe tip more firmly into the output coax connector... yes, it's all it was ! It's an old style " TV " socket. The inner part is split in two halves. One of the two halves gives zero signal no matter how hard you press on it. The other half, if you press firmly, you get a good and strong signal !
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LF / Modulation signal, no less than 3Vpp !!
So clearly the LF tube is not dead at all !
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And the signal looks like a very nice / clean sine wave ! I thought these old tube generators output super ugly highly distorted signals, somehow... nope, looks like a beautiful sine wave, like I bought one !
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So I need to replace that bastard of a coax connector. No idea where I can get one of these. Panel mount of course, screw type. The body is threaded and there is a big retaining nut at the back to hold it onto the face plate of the instrument. A pig to get to as well : it's inside the attenuator assembly, a big machined metal cylinder / housing as you can see, which is in two parts. First need to desolder a wire, then remove the 4 studs that hold the cylindrical assembly together. This lets you pop the first half of the cylinder housing to the side. This reveals one of the two wafers of the attenuator switch. Pull the second part of the assembly, wafer comes off of its shaft, but don't I panic, it's keyed, phew.
Then you get to see the other wafer of the switch, and the output coax connector.
So since I got a super good strong signal, I proceeded to test the HF output as well. Works very well too ! Again clean nice sine wave. Tested methodically all frequency ranges, sweeping from min to max freq on every band. All bands work just fine, from the 50-150kHz up to the 15-50MHz range !
The dial is also super accurate, on every band, from end to end, top notch !
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It's incredible, I can now generate a nice sine wave at up to 50MHz, using this ancient minimalist tube instrument... when my only sig gen so far, my old '70s Philips can go only up to 1MHz !!! How paradoxical, how funny... and how cool ! I can do 50MHz now ! What a thrill !
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Tried modulated RF as well, works fine
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Then I checked the power supply to see how it was doing, and if the extensive rework it had endured by the previous owner, was working or not.
See schematic below.
So as you can see we have two filter caps, both 8uF, but not in parallel. They are separated by a big dropper resistor.
The cap that got replaced was the first one in line, right at the output of the rectifier tube (which has been bypassed by a solid state diode...)
It was replaced by a 47uF so A LOT bigger than the original 8uF one ! Still, it's not very big.
So DC voltage at this cap is measured at 248V for 240V nominal, good enough. ripple OK-ish at 1,1V (AVG DMM not RMS).
However the second cap I get too high of a voltage, 157 for 135 nominal. Probably because the beefy dropper resistor is supposed to be 10K but got replaced with a 8.2K !!
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Going back to 10K should fix that I think. Ripple on that cap is about 45mV (AVG again, Fluke 11).
As for the LF and RF output, if I wanted to make it perfect, here is what I could turn my attention to :
- LF / modulation signal. Its frequency is a bit off. 360Hz instead of 400. Specs say +/- 5%, so should be at least 380Hz not 360. Have a look at the schematic, what resistor or cap do you think are involved in determining that 400Hz freq ? There are a few caps in there, could be off. Resistors could be off as well... anything could be off !
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- HF level : according to the manual, output level should not drop with frequency. Supposed to be determined solely by the attenuator setting. Of course output is not leveled on such a basic instrument, so you can expect some variations, manual says it can vary up to +/- 30%. Problem is that I observe a lot more than that.. I get almost an order of magnitude difference between lowest and highest ! Highest attenuator setting is 100mV (but I don't know if that's peak, peak-to-peak or RMS. Manual does not say ! ). Anyhow. Strongest signal is at lowest frequencies, I get 250mVpp or so, for 100mV selected.
Then as I go higher and higher in frequency, the amplitude progressively drops more and more. At Fmax, 50MHz, the 250mVpp are loooong gone and I get only 30mVpp !!!
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So I guess the most likely cause is, the HT tube is tired / weak ? It's not like that would be far fetched... the very manual itself tells you that the HF tube WILL weaken and need replacing... so I guess if it was to be expected to replace this tube even back in the day when the instrument was new... not really a surprise to have to replace it 60 years later !
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- Modulated HF : modulation factor seems too high. Supposed to be 30% according to manual, but looks more like 50+ % to me
![Huh? ???](https://www.eevblog.com/forum/Smileys/default/huh.gif)
I guess that's determined by some resistive divider somewhere. Resistors must have drifted and modified the modulation factor. Don't know what resistors are involved though.
So that's it ! Bloody thing does work after all ! That's old French engineering for you !
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Französiche Qualität !
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So I need a replacement tube for the HF oscillator, a 6L6 tube. Then check all resistors hoping to find some that have drifted.
Then need to find a new " TV " style coax socket. Could fit a more modern BNC connector I guess, but I prefer to leave the instrument original and use an adapter instead.
Very happy with this little generator, especially for only 30 Euros !
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