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Old 20th Mar 2023, 10:18 pm   #1
Hamsolo
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Default Yaesu FT 901DM SSB RF power control

Hi to all,
I have the user manual for the Yaesu FT901DM but it states that you can adjust the RF output (carrier power to any desired level) on CW/AM/FM/FSK modes ie say 1W to 100W.

My question is that in the SSB modes the RF output power can't be adjusted by the normal carrier control? but is it the RF speech processor that is the RF power control that I should use to reduce power for SSB modes?
Does anyone have any ideas? I can't see anything in the online manual about this?

From Richard
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Old 20th Mar 2023, 11:01 pm   #2
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Default Re: Yaesu FT 901DM SSB RF power control

I have a FT-902DM so very similar and no I can not adjust the RF power on SSB it will be peak power only on SSB. Typically around the 100 Watts peak if Valves and tuning is good.

The speech processor will bring up the average RF power during speech closer to peak that is what it does.

There is a way to fool the set and I did it once but would need to find the circuit again. I did post it one time to the Fox Tango group when the group was on Yahoo (I think) many years ago. What you can not do is change the bias on the finals etc otherwise they will no longer be linear.

Trying to remember I believe I took an output from the output of the driver stage, into a diode probe providing a negative going voltage then via a pot to reduce the voltage feeding this back to the accessory socket ALC line to control the earlier stages. Not sure if there was a better way of doing it.

The set makes it difficult to have low power SSB when needs must for say Top Band or if causing break through to equipment if you wish to run with stealth antennas etc.

Adrian
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Old 21st Mar 2023, 1:22 pm   #3
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Default Re: Yaesu FT 901DM SSB RF power control

Hi Adrian,
Thanks for your reply I had a play around last night with it and it seems that if you set the mic gain to say 12 oclock position I normally have it around that position and then have the RF processor turned on then if you adjust the RF proc control to where the alc just kicks down ie as per the manual then the reading on the Peak reading meter into a dummy load is like you say 100W.
But this is the other interesting thing if you set it up as above mic gain the same but now reduce the RF proc control ie back it off then the peak reading RF output can be adjusted down to 5W peak ssb output I assume this could be how you do it if it were to follow on to a Linear amp etc for example the FL2100Z.
I have not had a chance to listen to the output on a second receiver as yet but if this sounds good it seems a way of backing the power down?.
Also Adrian what power do you get out of yours just out of interest ?
I have all new tubes and peaked and aligned and have the following all into a dummy load and using Tune button only

160m 100W
80m 100W
40m 90W
20m 80W
15m 65W
10A,B,C,D 50W

Thanks for your reply Adrian
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Old 21st Mar 2023, 1:29 pm   #4
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Default Re: Yaesu FT 901DM SSB RF power control

One thing to be careful about is that if you reduce output-power on SSB by reducing the audio-drive, you can compromise the carrier-rejection; this was a bit of a problem in times-past when people wanted a few hundred Milliwatts of 28MHz SSB to drive a transverter for VHF; they'd reduce drive to the transverter by turning down the mic-gain on the main transceiver, which did nothing to similarly reduce the residual carrier.
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Old 21st Mar 2023, 2:03 pm   #5
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Default Re: Yaesu FT 901DM SSB RF power control

This is only as far as I can remember as it has been a while.

If you had transverters etc plugged into the back of the 901/902 and the acc socket, then generally for the transverters the accessory plug removes power to the heaters on the PA valves so they are effectively isolated, RF power for the transverters came out of the driver valve RCA socket. There is a diode detector on the output stages providing a negative voltage that is fed back into the accessory ALC connection.

If you are driving the FL2100Z then the accessory plug has the heater link for the 901/902 PA valves and the RF comes out of them into the FL2100Z. If you look at the output side of the FL2100Z circuit there is a diode detecting the level of output power and feeding this back to the ALC circuit which then can reduce the previous RF stages in the 901/902 to maintain a reasonable power level.

So going off these two scenario's, I basically used a low level diode detector from the driver to feed back to the accessory ALC pin.

This seemed to work well and I believe would not cause the concern that G6Tanuki mentions above as any residual carrier would also be attenuated by the ALC circuit in the 901/902. Doing reduction by reducing the level of audio it would also be possible to cough/sneeze in the microphone and get out a lot more power.

As to power levels I know on 14MHz I can get 100 Watts and I think up at 10m I am getting around 90 Watts, but I have only got a DAIWA CN-101 power/SWR meter so how accurate that is I would not know.

I joined one of the old 901/902 groups again last night and was able to find the very simple circuit I used.

The diode detector was built into an RCA plug the 500 Ohm pot was at the side of the 902 and then the cable fed back to the acc socket.

The CR time constant may want looking at but it is simple to try.

Click image for larger version

Name:	FT901ALC.jpg
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ID:	275483

Adrian

p.s. My 902 basically sits here like a large paper weight due to restricted antennas and prying neighbours.
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Old 21st Mar 2023, 3:03 pm   #6
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Default Re: Yaesu FT 901DM SSB RF power control

Hi Guys,
No I dont own the FL2100Z I used that as a wrong example, I meant if I were to get a linear lets say another make Ameritron AL811 then if I were to use the FT901 then it would be supplying full power out into a linear with no way of reducing its RF power seems like they should have put a seperate RF power knob on it as then the Ameritron linear would be at full drive.

If it cant be done by the FT901 front panel then I wont worry about it as im not going to modify it but just wanted to see if I was missing something.
So end of the day it seems that in SSB mode you have full output and thats it.
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Old 22nd Mar 2023, 9:36 pm   #7
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Default Re: Yaesu FT 901DM SSB RF power control

Quote:
Originally Posted by Hamsolo View Post
Hi to all,
I have the user manual for the Yaesu FT901DM but it states that you can adjust the RF output (carrier power to any desired level) on CW/AM/FM/FSK modes ie say 1W to 100W.

My question is that in the SSB modes the RF output power can't be adjusted by the normal carrier control? but is it the RF speech processor that is the RF power control that I should use to reduce power for SSB modes?
Does anyone have any ideas? I can't see anything in the online manual about this?

From Richard
Richard,

you are quite right that adjusting the carrier control will not affect the power on SSB. That's because SSB has no carrier (or very little) to adjust! The carrier control does not affect the power in the sidebands - that is primarily affected by the amount of audio fed to the SSB modulator. Adjusting the transmit audio gain will thus affect the SSB power output up to a point where the system goes into limiting.

As others have pointed out the RF speech processor alters the ratio of the average power to the peak power. Generally the more compression you apply the higher the average power in relation to the peak power of the SSB signal.


Richard
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Old 23rd Mar 2023, 10:40 pm   #8
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Default Re: Yaesu FT 901DM SSB RF power control

Hi Guys,
Thanks to all that replied its a great forum.
Well today I had a chance to have a go at the FT901 as its only recently purchased.

The mic gain has no real control over the output power, I also confirmed from a second receiver the positions that has the best audio on the mic gain and its about mid position thats best audio, the mic gain has little effect on the control of the RF power as it hardly changes much over most of its span so this is best left at mid position for my mic.

So then I moved onto the RF PROC control and when this is at Min i get about 1Watt Peak moving smoothly all the way upto 100Watts so im not sure about any other effects (carrier-rejection) no way of telling that but I did try coughing when it was set at 5Watts and it didnt go above that figure( so far so good) so then I did an on air test, I was able to transmit at 5Watts and upwards and my brother heard this 50 miles away on his receiver and the audio was good no distorsion etc, so at this point im happy that I can now control the SSB RF out power.
But I have to say that you do need a real peak reading RF power meter for ssb as nothing is visible on a normal RF power meter.
Thanks for everyones input its a great Forum.
Ps Adrian love the youtube channel you have, I have been watching that and its very interesting stuff.
Best Regards to all
Richard
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Old 23rd Mar 2023, 10:48 pm   #9
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Default Re: Yaesu FT 901DM SSB RF power control

The 901/902 is a good old fashioned radio, I changed the Main HT caps and a 100pF on mine years ago after reading some horror stores about them. If you can do what you want with the 901 then all is well and good. If I get chance to do the RF probe again I will stick a small video up showing it's working although I think I only used it to just under 20 watts, no mods to the radio just an extra wire stuck in the acc socket.
The youtube stuff is not a serious endeavour, the camera and work area is somewhat restricted, but it can give me the impetus to finish the odd project.
Have fun.

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Old 24th Mar 2023, 1:23 pm   #10
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Default Re: Yaesu FT 901DM SSB RF power control

Quote:
Originally Posted by Hamsolo View Post
Hi Guys,
Thanks to all that replied its a great forum.
Well today I had a chance to have a go at the FT901 as its only recently purchased.

The mic gain has no real control over the output power, I also confirmed from a second receiver the positions that has the best audio on the mic gain and its about mid position thats best audio, the mic gain has little effect on the control of the RF power as it hardly changes much over most of its span so this is best left at mid position for my mic.

So then I moved onto the RF PROC control and when this is at Min i get about 1Watt Peak moving smoothly all the way upto 100Watts.
Best Regards to all
Richard

Richard,

I'm not familiar with the FT-901 but in general the mic amplifier system will limit at some point, so that no greater audio output can be obtained from it. Whether that's because a limiter is built in (I didn't see one on the block diagram) or because the amps run out of headroom it doesn't matter. It will give the effect you are observing of the mic gain not really having much affect on the RF power level at the RF output. The RF processor would indeed affect the power level, which is what you observe.


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Old 24th Mar 2023, 10:01 pm   #11
Hamsolo
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Default Re: Yaesu FT 901DM SSB RF power control

Yeah im happy I can now control the SSB power, everything is obviously working and no faults so thats good.
Many thanks to all that replied
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