Yaesu FT 901DM RF Power Problem

[Hamsolo]

Hi,
I wonder if anyone out there has a Yaesu FT901 as I aquired this but as I have not owned one before I need to know if it has a problem or not?.
The Transceiver has not been modified or got at in any way in fact it looks original but as some people have told me that it was not meant to produce 100W on all bands ? this seems strange to me but I thought I would ask the question as I have the following RF outputs all taken into a 50 carbon resistive dummy load with AV20 rms reading power meter:
All readings taken by using the "Tune" button and preselect plate/load peaked
160m (1.950) = 100W
80m (3.800) = 100W
40m (7.300) = 60W
20m (14.300) = 80W
15m (21.300) = 80W
10A (28.300) = 50W
10B (28.800) = 60W
10C (29.300) = 50W
10D (29.800) = 40W
If someone who has one of these could say if this is correct or not I would be grateful.
Best Regards to all
Richard

[Cruisin Marine]

Seems about the same as my one, I don't have access to it for some time to confirm though.
They are rock solid rigs- keep it.

[G6Tanuki]

It uses 6146 finals, which if in good condition should do a bit more than 40W on 10M.

[The 6146 is a proper RF valve, rated to work up to around 100MHz - it doesn't suffer the high-anode-capacitance issues that beset transceivers using TV "Sweep Tubes" as the PA, where he high anode capacitance often leads to seriously sub-optimal L@C ratios in the tank circuit on 21/28MHz]

[Hamsolo]

Hi to all,
I have been looking at this for a while now and the output problem does not appear to be a fault with the valves ( I tried 3 brand new sets inc driver all same).
So I decided to go deeper into this and it appears to be low on signal from the VCO ( the J102 rca lead that goes into the rf driver)
I took the following measured with a scope today at the vco lead.

160m 100W (vco output 120mv)
80m 100W ( vco output 120mv)
40m 90W (vco output 100mv)
20m 80W (vco output 70mv)
15m 65W (vco output 60mv)
10A,B,C,D 50W ( vco output 40mv)

As you can see there is a direct relationship between the VCO out and the loss of power but at the moment I am at a loss as to what the fault may be unless this is all correct and just marketing hype.
Today I replaced the VCO board/ RF board/ PLL board/ NB board all had no effect on the dropping driver levels supplied to the driver stage?
This dropping VCO output is all way before PA valve stages so the output stages can be disregarded as they are all off.
This is proving to be a weird fault or im wondering if its how they all are and were just marketing hype yes 100W on some bands but not all?
Hope someone comes along who can help with this.
Thanks in advance to all that reply.
Regards
Richard

[Cruisin Marine]

According to the manual, "it implies" that they should do 100 watts on all bands, mine never did- not that I had access to other valves to try though. I just accepted it as I hardly ever have used the higher bands anyway.
What is the bandwidth of your scope BTW?
https://www.manualslib.com/manual/89...?page=5#manual

[Radio Wrangler]

Does it use the 12BY7A as the driver? Is the one fitted Tosiba branded?

There's a known issue with driver tuning because it's combined with the receiver preselector tuning. There aren't separate trimmers for receive and transmit, so if you don't use the 'right' make of driver valve, the RX preselector peaks at a different knob position to where the transmitter driver stage peaks and it condemns you to re-peaking each time you go between transmit and receive, or vice-versa.

The twin 6146 output stage is known to reduce power on the highr bands, and I think this was why Yaesu did the FT102 with its triple 6146 output. I don't think the double version drops off as far as yours seems to.

How confident are you that your power meter is flat across the bands? There could be a contribution from either it or from whatever you're using as a test load.

David

[Hamsolo]

Hi to all,
To david first yes I did read about preselector peaks at a different knob position to where the transmitter driver stage peaks but this is not the case in this radio as it has a new toshiba 12by7a but even with other new makes its the same and like you say would only affect the peaking by moving the knob and this is all peaked to the max so cant get any more out of it.

The problem is the input to the 12by7a is low on signal from the RF unit(this supplies the Grid) and the RF units input is not being fed with any decent signal:

160M band 170mv going into RF unit
10m band 40mv going into RF unit

This is a huge loss for the RF units transitors to deal with and if you put less into a transistor you get less out and this is what is happening here.
This has been measured using a 100mhz hitachi scope.
The input to the driver valve comes from the RF unit (pb1702) which has been replaced aswell so its really strange.
Hope someone can put a FT901 on 29mhz into a dummy load im sceptical that they can get anywhere near the 100W on those bands.
But as has been mentioned it does seem to suggest it does in the manual?
From Richard.

[Cruisin Marine]

Make sure you do the neutralisation, it tells you how in the manual. If you don't they do weird things especially at higher frequencies.

[Hamsolo]

Hi ,
I did the neutralisation and no effect on this problem as this is a drive issue with hardly any signal coming out of the transistors.

If you take all the valves out and measure what the drive signal is going to the driver valve then its so small on the higher bands that I fail to see how these FT901 or FT902s ever got 100W on 29mhz perhaps its all marketing hype that seems to happen alot and I guess it was the same for these in the 70s when they were made.

[PA3DAF, Erwin]

Richard,

My FT902DM barely makes 70 watts on 10m. I haven't changed the tubes or did any alignment yet. Did you download the "FT901-902 SURVIVAL GUIDE" ? It's full of useful information including lots of modifications by LA8AK.

On this site with all the LA8AK mods you can read that there is a 2K2 10m power reduce resistor that's not on the schematic!

Erwin, PA3DAF

[Hamsolo]

Hi Erwin,
Thanks for your reply and yes I did download the Foxtango info, it was that info that made me want a FT901 in the first place, its a great site full of info.
I have seen the LA8AK (shame he passed away) mods website before but i missed the bit about there being a power reduction resistor on 10m band switch, I think its starting to become more clear that in no way do they produce 100W especially on 10m band.
I went and also found that the Xtal oscillator board that drives everything seems to have all the 10M crystals set at a lower output (mv on scope) from the rest of the crystals so there seems to be something going on at the factory to limit the power in the 10m band.

I dont mind im not after changing it or increasing it if thats how it was intended to be ( I thought it was a fault) but it appears not to be.
I wonder why 10m band was intended to be lower power maybe it was harder and would have been unreliable at that power level on 28mhz maybe?
I thankyou for this info Erwin I seem to be getting 50W max on 10m bands so given that there are tolerances in meter/ dummy load etc then this is probably all fine and working similiar to your FT902.
Best Regards to all.
Richard