ft-901dm ham radio repair.

[drforbin]

Hi guys. I don't know if this is the proper place to ask for help related to ham radio repair, but I thought I'd try anyway.

I am currently trying to repair a ft-901dm ham radio.
I'm having problems if transmitter power output.
When turned up on 160, 80,20 I'm getting close to full power on 20m I'm getting only 10 watts.
I tracked it down to ALC circuit (this   circuit is pretty simple. If anyone wants schematics I can provide them) It seems that on some bands, 20m in particular the ALC is preventing the PA from reaching full power. I tested all the caps and traced the circuit, even going as far as changing most of the caps associated with the ALC. NO good. I really cannot figure out whats going on. I really need some help or suggests to put me on a new path.

Ok thank you all for reading.


any suggestions will be welcome.

Drforbin.

PS. I have neutralized the tubes.

[voltz]

Hi,
Been a while since working on one of these but i would download this service manual first and start there (if you dont already have it).

http://www.radiomanual.info/schemi/FT901DM_serv.pdf

Its an excellent manual and covers all sections in detail. And includes a trouble shooting section as well as modifications and updates.

The ALC is not band specific so any faults on it would effect all bands. So i dont think its that. It just reduces TX drive throughout.

Some questions:
Does the Receive side work on all bands? Use the Marker switch and check all bands. Eg (from the manual) 14.2Mhz should read as S9+10 on the meter. Are all the bands able to receive ok? If not, its a clue.
Is it only the 20m band not transmitting well? Do all other bands peak ok up to the correct power? into a 50ohm dummy load of course never ant!!

If the receive side is all ok, and fault is purely 20m TX power low, then i would be looking at the Band Switch wiring to Tank Coil. Check for dry joints. This is also mentioned in the manual.
Other 20m only things to check are the Xtal board (20m xtal and coil can) and VCO unit (coil can). These may be out of adjustment or an actual faulty component. But if the RX side works ok, this is unlikely.

hope this points you in the right direction.

[drforbin]

Hi..I have the service manual. The radio seems to be working on RX on all the other bands.
I'm getting small ALC on 160,80 bands and a great offset on 40m.
What I mean is this. With ALC disconnected I get over 100 watts on dummy load on 40m when connected I get 10 watts.

ALC does not seem to be working at all on 20m at all.
 The ALC seems to be cutting in to early. I checked the entire ALC circuit cannot find anything wrong.

[voltz]

Hi,
And on 20m with ALC disconnected, are you able to get 100watts there too? So is the problem getting worse the higher the band selected? Whats 10m like?

[drforbin]

Yes. I get full power on 20m with alc connected or disconnected.
On 10m same.
On 80m and 160m I can get close to full power.
Measuring ALC line it's producing a small offset on 80m and 160m.
Enough to attenuate power alittle. On 40m it attenuates so much that it cuts power to the 10 watts as stated.

Some others have had same problem...but no one has posted a solution.
please see reference.


https://forums.hamisland.net/showthread.php/17415-Yaesu-FT-902DM-ALC-problem

[voltz]

Ok i see. So the real problem is occurring at 40m. Not 20 as i read it in your first post.
Slight power reduction on 160 and 80m, 40 is very low and all other bands are good. I think thats the summery?

The link you posted suggests the Mode Switch. But how would that effect the ALC / Band selected.

i'll take a look at the diagram. Will get back if i see anything.

[drforbin]

The mode switch is not it. I checked it.
I do get full power on 40m on all modes accept SSB. that is why the poster logically suggested that it was the mode switch. It is not. The reason why the other modes work and SSB does not is because the mode switch grounds ALC for ALL modes except SSB operation.

[voltz]

Well, looking at the Rect A Unit, the ALC is derived from the grid bias (unit pin 5) from the final PA. That signal is then rectified and filtered to produce the ALC line (out on pin 3) which then feeds to the NB/Proc unit to reduce tx signal. And yes it clearly shows the mode switch grounding the ALC on all other modes other than SSB as we mentioned.

That said, its odd that only 40m seems to be having a bad effect on the ALC operation. Maybe some of the ALC rf filtering is not working (looking at that 2M2 resistor, c12 , c11 area).. And its become 'tuned' to 40m. Did you check that area on Rect A? Some small pf caps, they will need testing. Other than that, something else causing the ALC to change badly on 40m, possibly a PA fault / tuning issue. Could even go as far a needing a re-alignment procedure (especially 40).

[drforbin]

Firstly...thank you for helping me.

Ok...I checked and even changed the rec 'a' caps associated with ALC. so we are on the same page.
 I realigned everything including 40m. I also have new Tubes. All this has been done.
You see...it's quite a perplexing problem.

FYI with finals removed ALC signal stops. With Finals in place even with filaments not lit. Problem persists.
I also have neutralized tubes.

[drforbin]

FYI..Alittle bit of theory. I believe the finals are suppose to be CLass B amps.
which means that they should always be negatively biased. The theory of operation best I can tell of the ALC is the peaks which positively bias the grid draw current (of course) where as negative bias (and signal donot). It's  these positive excursions which the alc is looking for thus indicating a over drive condition. The problem is I looked at it on a oscilloscope and even when I attenuate the signal enough for it to be entirely neg biased  I still get alc on 40m and some on the other bands (i.e. 160,80) as before.
It doesn't make any sense. 

[voltz]

'With Finals in place even with filaments not lit'

There's only two things i can think of that could cause that. Inter plate capacitance or faulty valves/tubes. uk here so its valves
Either way, its doing bad things to the ALC , as you are painfully aware of.

Yes, the PA stage is biased with a negative grid to keep them in their linear region, turning the valve ON. And the bias - Ic (I current) is set by the user with a preset and panel meter. The ALC looks like its simply detecting anything ac from the bias voltage. I guess that's distortion or clipping of RF at the grid. That looks like its then rectified and produces a negative ALC line (to turn off the dual gate FET at the NP/Proc unit). Now if you have distortion at 40m for some reason, it may show as excess ALC.

[voltz]

Oh, just correct myself there. Rather than detecting distortion, of course this is SSB and the RF rectified would be from a speech envelope of RF. So its detecting the levels of carrier as you speak. Makes more sense.

[drforbin]

yep..that's what I thought "inter plate capacitance" But everything has been neutralized and I even went as far as to change alot of the caps on driver and finals board.
I know i'ts not the correct way to fix something is to throw parts at it but I'm desperate.
Ok..you mentioned distortion. Coming out of the rf unit where the signal and vco are heterodyned  the wave is jumping around alot. It not fully stable. well maybe a problem??
I'll make a video of it and post it to my youtube channel. Would you mind giving me your opinion at that point?

[drforbin]

FYI..As I said I'm at the end of my rope. The ALC circuit is pretty simple and I really think it is function correctly. Also I changed just about everything or tested it on both driver and PA sections. The only thing I have not test is the resistance values of the parasitic chokes off the plate. but I think they are ok. so, again thanks for brain storming with me.
I'm new to ham radio. If you wann have a look at my youtube channel I have fixed spectrum analyzers. tds540 oscilloscopes etc. and this rf stuff has me pulling out my hair.

https://www.youtube.com/user/seaman6661/videos


What I thinking of doing is isolating the driver and PA from the rest of the radio and sending a 7 Mhz 3.5 P-P waveform into the driver and see how the ALC and power output respond. Problem Is I currently donot have a function generator. 



HUmmm what would dave do?

[voltz]

You are certainly welcome to post a video of the signal going in. And if you can, sniff some of the RF out and see if thats a clean sinewave. Hopefully you can see 7Mhz on your scope.
Oh its a tricky one! haha,

[drforbin]

I've looked at it already. The vco is clean and the signal is clean the heterodyned  signal coming from the rf unit is alittle strange. I just didn't think that could be the problem. Your telling me it may be? I'm get the video done by tomorrow for you to look at.


Thank you again.

[voltz]

I am suggesting that taking a look at the RF output (sniffed at dummy load) may give a clue. Just key down for a few seconds for a clean constant carrier. Another thing, if you have access to a spectrum analyser, take a look at the RF out and check for spikes or parasitic oscillations. Again this could be clue and compare 40m with a known good band like 160m.
At this stage, anything that shows up may direct you to finding the fault. Because it sounds like all the likely faults have been dealt with already. Time to start looking for the not-likely ones

oh, i need to ask, sorry, but are the output valves the exact correct type? 6146B's if i remember correct.

[drforbin]

Well. Sniffing at the output of the PA is a problem do to the power. Don't want to blow my test equipment. I will have the video for you tonight sniffing at rf unit output and input.
I have two sets of tubes one set are the B versions and the other the W versions.

Again...thanks for help.


As a said. I did this test already I just will commit it to video.
The output of the rf unit is very jittery. I would like you to have a look at the waves form once I have it on video and see what you think. I didn't think it could matter but who knows.

[AF6LJ]

Don't know what you have for test equipment...
Replacing all those caps may have introduced your current problem.
You need to check what you replaced and make sure it is the proper value. also a way to check if the PA is properly nutralized is to place the transmitter in tune on a band such as 20 or 15 meters tune for max into a dummy load. (let the tubes rest for a minute) then with the radio's meter in the IC position verify that the peak output point agrees with minimum (the dip) in IC reading. If they don't occur at the same point the PA is not properly neutralized. It has been 25 years since I worked on one of those, so I am a little rusty...

[voltz]

'Sniffing at the output of the PA is a problem do to the power. Don't want to blow my test equipment'

Just wrap some wire around the coax to the dummy load, its safe. Not a direct connection!. The term 'sniff' means loosely coupled, a small sample of the larger RF energy. Just a few turns. Not much. Just enough to see the carrier. (assuming your scope can handle 7Mhz.).

[drforbin]

Hi..thanxs for your input.
The radio was doing this before I changed anything.
All caps were changed using values read from old caps and from schematics.
As for neutralizing what I do is run some RF through the PA with the finals filaments not lit.
Using a sensitive DMM I adjust from dummy load for a minima at 29Mhz.

[drforbin]

voltz,  cool! great idea thanks..
I have a 500MHz tds540.


so basically just use inductive coupling right?

[voltz]

Yes. Maybe start very loose, 1 turn, then proceed to increase as needed. Do it 'by feel' .

[drforbin]

wrap it around the coax to dummy load?

[voltz]

somewhere between transmitter and dummy load, its not really important. Around the outer coax plastic, a loose turn.. Increase power slowly, watch the scope, adjust the Y amp.. etc. Until you get a decent trace. The loose turns should be enough to pick up stray RF on the coax. Put your probe across the wire loop. Start small, build it up as needed.