FT101ZD MkIII Low output power on high bands
 

Hi all,

Am working on an FT101ZD MkIII at the moment and mostly everything is going fine. Receive side all looks good.

But the power out is low on the higher bands. 80m and 40m are producing close to 100W, 20m is then down at about 70W, but the rest are below 50W.

The current suspect is the RF Board; signals coming in look good, but the output shows the higher band drop-off. The problem I have though is the pc board version in this unit is different from the manuals and schematics. See photo. And so far I have not found a schematic that matches this board.

The manual says to peak tx power by adjusting T104, but this board doesn't have one.

Anybody got schematic for this version?

Thanks,
Ralph

Re: FT101ZD MkIII Low output power on high bands
 

Scratch that- have just found another manual with the right version of RF Board.

Re: FT101ZD MkIII Low output power on high bands
 

I have had valves lose output on higher frequencies when they have got tired.

Re: FT101ZD MkIII Low output power on high bands
 

I found that stated manufacturers output powers are highly speculative and prone to economically biased subjective predictions, based on promiscuous input power errors

Re: FT101ZD MkIII Low output power on high bands
 

Yup, have now concluded that the pa driver side of things is working ok. But I’m just not able to get 12m and 10m to tune up right. I might revisit the neutralisation and see if it is sensitive to that…..

Probably also the 6146’s showing their age.

Re: FT101ZD MkIII Low output power on high bands
 

6146 age.... also are you using a real Toshiba 12BY7A?

David

Re: FT101ZD MkIII Low output power on high bands
 

Hi David,
It’s branded Sylvania.
Is that good or bad?

Ralph

Re: FT101ZD MkIII Low output power on high bands
 

Had another fiddle with this today and am pretty sure everything up to the driver is ok. With the multiplig removed from the back (so the 6146’s are dormant) I can see no big difference when tuning up, say 15m, than i can with 12m.

But with the 6146’s running, there’s very little power coming out on 12m and 10m and it is prone to instability. So something is not right in the PA stage.

Re: FT101ZD MkIII Low output power on high bands
 

Measuring the control grid current on the 6146's will tell you quite a bit, I can't remember if there was a test point or whether you would have to modify to do so.
I have only worked on the FT901 tbh, I hate those Yaesu relays, I changed them for standard type, they were horribly expensive, and maybe impossible to get now?

Re: FT101ZD MkIII Low output power on high bands
 

I have to confess this was me being stupid and forgetting to reset the loading when changing bands :wall:

My excuse is concentrating on not getting another belt from those tuning capacitors.....or age, or something....

Happy to say this unit has tuned up nicely now and outputting a decent amount of power. Happy days.

Thanks to all who took the trouble to respond.

Re: FT101ZD MkIII Low output power on high bands
 

Glad you cracked it, might be worth making some notes on your meaurements to save you time in the future.
That loading can be critical at the higher frequencies too, time for you to grab a:beer:

Keep one hand behind yer back when working on HT/Valved eqpt. and no metal strapped watches, I rememeber those watches were a real b*tch.

Re: FT101ZD MkIII Low output power on high bands
 

Be aware that there's an 'issue' with quite a few Yaesu radios from that era due to failing capacitors between the driver and PA stages; if you see strange fluctuations in the PA anode-current it suggests that you have this problem; and a pre-emptive replacement of the capacitors can avoid a rather-more-expensive replacement of the PA valves!

Harry Leeming G3LLL has written about this in old editions of Practuical Wireless.

Re: FT101ZD MkIII Low output power on high bands
 

Yeah, I’ve heard about the driver capacitor change, but not seen any details.

Are we talking about the dc blocking cap just on the inside of the PA?
1000pF I think. Although I note you say ‘capacitors’, so I guess there’s more than one to change?

Re: FT101ZD MkIII Low output power on high bands
 

The FT101 family have the RF preselector ganged to the driver tuning. They were set at the design stage to track properly with a Toshiba driver valve. Other makes have slightly different stray capacitances and as a result if you peak the preselector on receive, the transmitter winds up a bit off song and it costs you power, worse on the upper bands.

It might be this old issue, but no guarantees.

David

Re: FT101ZD MkIII Low output power on high bands
 

I think the pa grid coupling capacitor failure has also been the cause of the demise of the mains transformer as well on some of these. I have my capacitors to change sometime as well as a general once over as it has not been used on tx maybe 10+ years?
Rob