Hard to believe almost 8 years has passed since I posted the last entry in this thread:
https://forums.hamisland.net/showthr...l=1#post191175
All of the FT-901s and 902s referenced in said thread are gone - have been for a while - and I find myself in possession of another '901/'902 pair, both of which need attention from the test bench to realize their fullest potential.
Alas, my stash of '90x VCO and XTAL-Board components was depleted to the point where I needed to find some alternatives when it came to some things. The '901's XTAL Board uses the same tunable inductor in every oscillator circuit from 160 through 10M, and (for the most part) the same collection of discretes so that assembly could easily be fitted with components for its Aux Band option.
The VCO Board is a little more tricky. There are different coils for each tuning range, and depending on what you're going to add in the way of an extra band you need one which will cover the appropriate tuning ranges. Ditto when it comes to the particular capacitors (NP0 and N750 types) used in the circuit.
I bought a spare, complete VCO Board from a UK vendor. Growing impatient for its arrival I decided to sacrifice my two remaining surplus OEM VCO coils to the reverse engineering process and attempt to figure out winding details. The coils in question are Yaesu P/N L002094, and 095/096/097 are also used in the VCO Board Assembly. 097 is the one which is spec'd for 10M operation.
Refer to the following links for details on the VCO coils:
http://www.lodestonepacific.com/coilforms.php
http://g4hup.com/DFS/Rewinding%20Tok...es%20Coils.pdf
The Yaesu coils are Toko 10mm types. Using the VCO circuit 'C' values and VCO operational ranges from the FT-901DM Service Manual then calculating required inductances, Lodestone's and 'HUP's winding data can be used to get you pretty damn close in the inductance ballpark but what it DOESN'T tell you is the tap point on each type of coil. Bear in mind that total turns is only one part of the equation; if the tap point isn't the correct impedance then the circuit won't work owing to over or under-coupling.
So...what I found out about an L002094 is that it has 24t total with a tap point 4t above ground. Pin 3 on the coil form is "hot", Pin 2 is "tap" and Pin 1 is "ground" as used in the circuit. Playing around with winding these things, a good sub for an -096 was built with 10t total; tap 4t above ground - though I'd try this with 3t too.
-097 was proving to be a bastard. About 11PM one evening I had a flash of inspiration and went back to the lab to wind yet another test coil. What I ended up with was 7t total, tap 2t above ground. When assembled into the VCO unit and placed into operation, The Majick Frequency Range (actually, anything from 24-30MHz) became available. Measured VCO drift was on the order of 100Hz total overnight, after which it completely stabilized - even before the components were potted. This is better than Yaesu's spec for the assembly. They call out a 12pF NP0 and a 15pF N750 (15UJ) part. I used what I had in the junk box; who knows what the temp coefficients are. Overall, the rig is more stable than any other '901 I've had through here.
Some corrections to my earlier "Hacking" thread which will help others who want to modify their 901s for WARC:
1) Switch wafers E, G, K, I and O need attention. There's a grey wire connection to the "Aux" band spot on one of those wafers which - by way of L09 - will kill drive on that band. ('No CB transmit for YOUUU!'). We need to remove this for use in transmit mode.
2) If you modify the rig for 30M TRX it's normal for the Preselector indicator to fall between 80 and 40M when peaked. An FT-902 (which is labeled with 30M) peaks exactly in this spot and the RX/TX-driver stages are identical between the two.
3) The PA Tank Coil benefits from an optimized LC ratio if one doesn't use the 20 or 40M tap points for the 30M addition, as is directed by a number of previous modification articles. Scrape away insulation on a winding turn between the 40 and 20M tap points then run your 30M shunt connection from S1O to it.
4) 24-30MHz PA operation can use the 10M tap position if jumpered appropriately.
5) Yaesu included a "hidden" 27MHz counter preload connection in their design but didn't wire it to the "Aux Band" connection. One can perform the widely published Counter Assembly mod and get that spot to read correctly on 12 or 17M, and the WWV spot to read 10.xxxMHz. I'm eventually going to set mine up with Aux-band jumpers which will allow me to select the band display at will, depending on what I've got in the rig with regards to VCO and XTAL Board additions.
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