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Author Topic: FT-990 and PIN Diode Mod???  (Read 2099 times)
KB0ETC
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Posts: 248




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« on: October 05, 2001, 09:32:44 PM »

I have a Yaesu FT-990 HF rig.  I have read on several occasions, that some users have/had their rigs modified using PIN Diodes.  

I am finally getting around to upgrading this radio with the optional filters, and TCXO-2 for improvement during contesting.  I am a little confused about the PIN diode mod.

The serial number of my rig is 5J470184.  Did Yaesu do this modification from the factory on later models, and not on the earlier models?

I have the original sales pamphlet from Yaesu Musen and it states: "The FT-990 uses PIN-diode-controlled high LDSS push-pull grounded-gate FET RF amplifier followed by a quad-FET ring mixer, resulting in very wide dynamic range on all frequencies..."

What is the PIN diode mod that everyone is talking about?  Any Information would be helpful.

73's de KB0ETC
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WB9YCJ
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Posts: 5




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« Reply #1 on: October 14, 2001, 04:32:41 PM »

Had a 990 for three years and really liked it EXCEPT for one big thing; Hiss-s-s-s-s-s-s in the receiver I.F./A.F.  And, AF "roll-off" above 2400 Hz to attempt (I believe) to disguise this (sad). AM MW/SWL suffered. This is inherent in the design (QST) and easily noticable, even more so when using headphones. Fatiguing and present in ALL 990's.  Using the scaf and speaker filters was, and is, a poor solution to this design blunder.  Otherwise, its a GREAT radio.  
I pulled my "RF Board" and had Yaesu (Cerritos) install "the" new Pin Diodes which they had in stock. Call them. I would not pay market value for this radio again.
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I7SWX
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Posts: 17


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« Reply #2 on: September 20, 2003, 09:14:21 AM »

Hi, do not modify the FT990, or any other receiver starting from the PIN diodes. You may make it worst as you may have a feeling the mod is OK but then you may discovered you lost 3dB of gain has it happen to a friend of mine on his IC781.
The mod you should look at are around the mixers. I do not have the knoweldge of the FT990 but, normally, for 3 or 4 conversion RXs or RTXs the worst mixers are the second one (DBM) and the 3rd one, simple dual gate mosfet or active mixer (uPC1037).
The best mixer replacement is the H-Mode Mixer in the G3SBI version (3 transformer), see RadCom, Technical Topics July, August and September 1998, or the I7SWX with 2 transformers (See RadCom, Technical Topics April 2003).
After the mixers mod you may discover you have amplifiers and filter and oscillators phase noise giving IMD problems.
If you cannot replace the original DBM with the H-mode mixer and you are satisfied with lower IMD (IP3) performance, you may try the I7SWX switched ring mixer, RadCom, Technical Topics September 2003.
Then, after all these mods and if still not satisfied you may replace the Pin diodes but you will spend money and time.

73

Gian
I7SWX
F5VGU
W1-I7SWX
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