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Thread: FT-901 Mods Redux

  1. #1
    Administrator N8YX's Avatar
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    FT-901 Mods Redux

    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.
    Last edited by N8YX; 04-12-2017 at 02:26 PM. Reason: Completed post
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

  2. #2
    Administrator N8YX's Avatar
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    Bumping this one up a bit.

    I managed to get my hands on an FT-902 VCO Assembly, which has 9 discrete bands in a 160-10M sequence (instead of the '901's 160-10/WWV/Aux arrangement with 8 bands). Of course, the Maintenance Manual declares that the 901 and 902 XTAL/VCO Assemblies are NOT interchangeable...

    Wanna bet?

    Subsequent replies in thread will be a detailed explanation of how to get 9 bands (either the FT-902's native complement, the FT-901's original complement plus 30, 12 and its hidden 11M spot - or any variation thereof) with an FT-901. Other than some counter, VCO and XTAL socket rewiring, the only other modification needed is in the form of a latching circuit.

    At least on paper. I'm borrowing from a concept originally published by Jean, ON4AEF - but my mods require no holes to be drilled in the rig. The sticking point is the band switch; it would be so much easier if the '901 used a 12-position part like the '902 did...but with a little electrickery we can work around that.

    One could take the conversion theme even further and swap in a '902-series counter assembly (including red 7-segment readouts) plus the panel-lamp dimmer circuitry and control buy doing that requires a parts-donor '902. I'm not quite ready to tear either of my two basket cases up just yet.

    First steps first. Over the next several weeks/months - time permitting - I'll get the '901 on the bench, rewire the module sockets and counter then test it for correct operation before moving further. Pics and relevant info to follow.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

  3. #3
    'Grumpy old bastid' kb2vxa's Avatar
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    Sorry OM, trying to follow you with my dim memory of what's inside rice burners gave me a headache. My last effort on the bench as "WE Service" (a play on my initials) was making extender boards and untangling the mess CB "power pushers" with golden screwdrivers made inside their Yeasu FT-101 series rigs that still sounded like shit with every knob adjusted for maximum carrier output driving the finals into Class C. This killed the TV sweep tubes in short order that with a 10W plate dissipation rating were NOT intended to glow a bright cherry red. Only one I knew from outside CB circles (a son of the local pharmacist) "Cool Breeze" sounded good even with his FL-1000 amp working because he tuned by the book.

    With that mess off my chest I'll be following your adventures under the hood, I'm confident it will turn out great like the last one.
    "New Jersey, the most American of all states. It has everything from wilderness to the Mafia. All the great things and all the worst, like Route 22."
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    73 de Warren KB2VXA
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  4. #4
    Administrator N8YX's Avatar
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    An FT-901 is a different animal than a -101 in terms of adding extra bands (or enabling transmit on the 11M/Aux spot, which can be used for 12M). To use an FT-901's (hidden) Aux band position you have to populate the XTAL and VCO Boards with parts, modify the Counter Assembly then do some band switch wiring (details of which I captured in the first post of the thread).

    Getting one to operate on 30M requires changing the 15MHz WWV heterodyne crystal with one from a -902; it transceives on 10MHz as delivered from the factory. The Counter Assembly has two WWV readout selectors: One for 5MHz (used in JA-models) and one for 15MHz. Circuitry must be added to the line which selects the 15MHz display in order for it to read 10.xxxMHz properly. You also need to wire the band switch to enable drive and PA tank coil tap switching, as I indicated above.

    Something of merit: I mentioned the 27MHz counter preload line in Note 5 above. EU models apparently came with the band segment/readout preload enabled by default. Bearing this in mind, we have the two WWV preload lines and our (27MHz) Aux Band preload line - for a total of 12 - but only 11 possible bandswitch positions. What I'm scheming will automagically switch a ground between the existing Aux line and the repurposed 5MHz WWV line, allowing for correct readout on all covered band segments.

    Another thing of merit: I mentioned a gray wire from L09 to one of the bandswitch wafers which kills Aux Band drive. Those who wish to use the Aux Band for both CB and 12M but can't keep their fingers off the Heater switch when listening to CB may wish to install a relay which reconnects this wire when the rig is active on 11M. This will be detailed as part of the Latch Assembly construction and implementation.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

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