FT-101x Neutralizing Cap Mod.

[KA2QFX]

I know the following is likely to invoke a vehement series of flaming replies, but it's been bugging me for quite some time and I think it bears further scrutiny.

Academic replies seeking to correct my assumptions with concurrent or differing data are both welcomed and informative. Flames, to the contrary, will not invoke any reply.
______________________________________________________________________

I have often thought that the practice of changing the fixed neutralizing coupling cap in Yaesu FT-101s was a misguided task. So I've taken some measurements and done some calculations to substantiate my claims.

The primary impetus for this recommended modification is the claim that US tubes have significantly different characteristics from the NEC or Toshiba 6JS6C tubes.  I have collected a number of tubes; Toshiba (green stripe), NEC, RCA GE and Sylvania (ECG) and found them all to have relatively the same Cgp. That is the G1 to Plate capacitance which the neutralization primarily seeks to cancel out.

The available spec sheets for 6JS6C tubes indicates Cgp as 0.7pF. My findings are consistent with this fiqure.
Only the Sylvania tubes exhibit slightly greater Grid to cathode capacitance. I suspect this may be related more to the suppressor structure than anything else but that's difficult to confirm.  So I think it's safe to say that the US tubes do not differ significantly from the Japanese tubes to warrant circuit changes.

Anecdotally, I have likely worked on hundreds of FT-101s and I've never had a problem neutralizing any of them regardless of what tubes I was using.  Similarly, I have read claims that the receiver will experience birdies and the drivers will self-oscillate if these changes are not made. My findings are that the driver will exhibit this behavior (in receive) only when the relay contacts responsible for driver cutoff bias are dirty, or the bias voltage divider is faulty.  In transmit, an unstable driver is usually bad bandswitch wafers, and open bypass cap or a failed trimmer cap in the driver plate stage. Driver coupling cap as well is a known weak point which always deserves a look.

Looking at the mod directly we come up with the following figures; keep in mind that we are only seeking to cancel the effect of 0.7pF of Cgp, which is multiplied to some extent by the Miller Effect due to the gain of the tube. Most, but not all of the US tubes seem to perform with a little less gain than their Japanese counterparts. This difference is generally between -1 and -1.6dB.

The modification seeks to reduce the level of neutralization, inferring that the difference in required neutralization is on the order of -6dB to -0.56dB.  These numbers are determined thusly:
The neutralization cap varies between 10pF and 0.8pF.  In series with 100pF this results in a total Cn of 9.09pF to 0.79pF.
Replacing C27 with 10pF results in Cn varying from 5pf to .74pF.

At it's minimum adjustment the difference in neutralizing voltage is proportional to 20 x log(0.74/0.79), or -0.56dB.  If the intent is to reduce neutralizing levels sufficiently to overcome a significant difference in tube function it would seem this modification fails to provide any substantive difference.

Again, my findings using various tubes do not substantiate the need for this change. Rather, I must conclude there is some other common circuit failure involved which changing this coupling cap ameliorates.  In lieu of any other numerically defining evidence to the contrary I must conclude that this recommended modification is misapplied.

Lastly, I find it similarly notable that the Yaesu service manual, which comprises a body of knowledge from several individuals as well as Yaesu, does not mention any such required change; although several other modifications are clearly described.

Respectfully,
Mark
KA2QFX

[The Slab Bacon]

I do have one that had an interesting "catastrophic failure"

The coupling cap from the plate of the driver to the grids of the finals shorted and started a chain reaction failure taking out all sorts of stuff. After a lot of head scratching and watching the biass filter caps blow to bits twice, I found that the female card edge connector on the main frame had a shorted pair of pins in it. You couldn't see them with your eyes, but after disconnecting everything on that connector, an ohmmeter told the tale.

I think that is why it was sold cheap being in excellent cosmetic shape. I dont think the last guy could find the problem. Now, if I ever get the time, it's off to the AM mods..................

Not exactly the info you're lookin for, but I thought I'd share.

[aa5wg]

Good information!

I have a FT-101FX.  I am the third owner and it sat in its box for most of its life.  It looks brand new.  However, while on the air the transmiter output went dead.  I was on 15 meters.  I tapped the cabinet and the rig came back to life.  This happened about 3 times.  I am guessing, maybe the relay was at fault (not used enough)?  It seems to work just fine now.  

I will copy your findings and place them in the FT-101 file.  

Chuck

[WB6NVH]

There were several notices published by Yaesu about this as service bulletins to dealers.  When I worked in a large ham store in 1981 we had the bulletins and they had also supplied us with a quantity of "Yaesu approved" tubes which were Japanese branded.  I can't remember the make but I think maybe they were National (Matsushita.)  I didn't work in the shop so I have no idea what happened if you switched to American tubes.  The tubes they sent us were supposely matched pairs and were unboxed, they were were in bubble wrap.  I often wondered whether they just wanted to get rid of their tube stock.  The word was that the Japanese manufacturer had ceased production of the tube.  As I recall the bulletins were directed at the FT-101EE, if that means anything.

I rather despised Japanese HF gear at the time and paid no attention.  I can't recall ever selling any of the tubes either, nor of any customers fussing about the issue.

[KA2QFX]

There were several notices published by Yaesu about this as service bulletins to dealers. 

Interesting.  "This" meaning the capacitor modification or using only "approved" tubes?

[WB6NVH]

As I recall, the first bulletin said you had to use only the approved tubes, and here they were.  Later I think they ran out, and then there was advice to change the capacitor.  I should have saved them but hindsight is, well...

Someone must still have copies somewhere...

[KA2QFX]

As I recall, the first bulletin said you had to use only the approved tubes, and here they were.  Later I think they ran out, and then there was advice to change the capacitor.  I should have saved them but hindsight is, well...

Someone must still have copies somewhere...

That's very informative, thank you.  I've contacted Yaesu regarding legacy info for their products but it looks like they've taken the typical route when a company gets bought up; history begins from the time of acquisition.  Sad. 
Although I doubt their service bulletin would provide a thorough explanation of the reasons for the change.

[ke7trp]

The cap change was a Yaesu Musen order. The main reason for the change is when the rig is run on 10/11 meters.  The rig would osclate badly and when it did, it would take out the Fets.  Even with the Cap change, the american tubes do not perform anywhere near the level of the japanese tubes.  Its typical to get about Half output at higher frequencys.

C