Now that I got that Avantek YTO to operate properly in the unit, I would like to once again see if I can find an outer case and handle for this thing.
Due to insane international shipping costs I will want it to be in the USA.
Anyone have any leads?
Peter
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On 3/20/2024 11:23 AM, Peter Gottlieb via groups.io wrote:
An update.
The Avantek YTO requires a lot more current to tune to the required frequencies than the HP YTO it replaces.? I was running into trouble because I was actually measuring the 2nd harmonic, which explained why it showed weak on the bench 8566A.? I was seeing minus some odd dBm where I would expect the 1st LO output should be around +10 dBm.? This is why I was having difficulties. I needed to look lower in frequency than I thought for the strongest output.
Tho get the Avantek to run at the needed frequencies required some circuit modifications.
Bottom line, it required adding some extra resistors in parallel to the current sense resistors in the Q301 and Q302 circuits.? In each case, the two parallel 100 ohm resistors had to have added two each precision 76.8 ohm low TC resistors.? This moved the 50 ohm sense resistor to 21.72 ohms.? Once this was done, I was able to get the YTO tuning alignment completed successfully at 3.200 and 6.050 GHz per the manual.
The Avantek main coil is 10 ohms while the HP coil was 39 ohms. To limit dissipation in the drive transistors and sense resistors, I added a 20 ohm 5 watt resistor in series with the main coil, at the YTO.? At the 305 mA required at 6.050 GHz, it dissipates 1.8 watts. The power dissipation in components is calculated to be within limits.
The analyzer errors disappeared and I can work further on it.
I did notice that the drive transistor for the YTF is running VERY hot.? I will need to look into that.
Peter
On 3/18/2024 6:33 PM, Peter Gottlieb via groups.io wrote:
So my adventures in restoring this instrument continue.
The YTO I got from ebay had a definite intermittent short between one side of the FM coil and the -5 volt pin.
I cut it open and this was a manufacturing defect.
The ebay seller sent me another unit, which is fine.? He didn't give me a hard time at all, very honorable.
So I mounted it in the instrument with the adapter plate I had made, hooked up the -5 volt regulator and everything else, and tried the YTO adjustments.
I could set the low to the 3.200 GHz but I couldn't get the high to go to 6.010 GHz.? Not enough range in the adjustment pot.? In the circuit there are R44 and R126, each 100 ohm 0.1% resistors, in parallel, in the current sense part of the current source circuit. I found that an added 1K resistor in parallel would give me enough range so I added a high quality 1% 1K resistor there.
I was then able to go back and forth on the low and high adjustments until they were right.
However, that resistor might not be the right value even though I could get the adjustments to work, because I am getting an error 319, which is a warning that the coarse YTO tune DAC is near its limit.
I will have to look at the DAC output to see if this is for the lower or upper limit and go from there.
This is promising.? It looks like this substitute YTO might just end up working out.
Peter
