ft-901dm ham radio repair.

[Mr Simpleton]

Guess we are narrowing in.... I would open up the ALC before the rectifying diode, and look at the RF with a scope.

First try with transmitter on 7 and 14 MHz to see what levels you have, It if is way more RF on 7 MHz, the problem is still there and we may carry on from there.
Do not remember if you still have the high ALC with PA tubes pulled??

[voltz]

The ALC returns to normal once the PA's are pulled.

And a resonance could easily be amplitude related bang on 7Mhz, rather than other RF energy at a different frequency. I agree with Mr Simpleton, that checking the amplitude between bands at the detector would help further narrow in. +1 on that.

[drforbin]

ok guys.

here is that exact same problem.
But no solution was ever posted.

http://www.foxtango.org/phpBB2/viewtopic.php?p=3133

uploading tests now!

[drforbin]

ok...here is latest video enjoy.

ALL comments welcome...thanxs

[voltz]

A few points here:

Was that test showing RF from the main ANT socket (sniffed with your loop)? Because the test i wanted to see was from the RF OUT socket (small phono at the back on the unit). Its much better suited because it connects directly to the ALC grid point in question. Sorry if you already did that.

That second harmonic looks way large from the finals at full power with ALC OFF. But that would be because the PA finals are not being controlled with alc feedback and will be distorting. So thats expected. It was clean enough when you connected the ALC ON.

If i understand what you said later on in the video correctly, you have already tried an SA right at the ALC point in question and you found it clear of all other frequencies. But the 7Mhz rf was very large?.. If that is correct, then this has to be amplitude related and most certainly some kind of resonance on or around 7Mhz.. It explains all the symptoms perfectly, and it also explains when valves are removed the ALC clears because the inter electrode capacitance is being removed and the 'bad' tuned circuit is being either changed or removed entirely.

 

[AF6LJ]

Just because you can explain away the problem in what appears to be a rational manner, doesn't mean you are right.


Good luck everyone, you will need it.

 
 

[drforbin]

That is from the phono jack as you requested.
I used the scope not SA at the alc rectification point. It looked clean.
But the amplitude did peak at when the alc peaked. As I said the freq was the freqs of the primary.

[drforbin]

I will do a spectrum analysis of signal right before rectifier tonight.
You understand I will not be able to sniff it. Generally what U do is first use the scope and turn the signal down enough that it does not harm the SA and then use the SA.
I'll tune everything up. show with finals in place and removed.


Thanxs for all your help guys.

ps...did you read the post from fox tango? Very similar problem.

[voltz]

Oh ok, i saw the sniffer loop and thought that was the pickup point. Sorry for my misunderstanding of the setup. My bad.

So yes, there appears to be no other frequencies involved. Thats clear now.

Mr Simpleton suggested taking a look at the ALC point with rectifier disconnected and comparing the amplitude from 7 to 14 Mhz. I believe we should be seeing a huge difference in the level of RF there. But i don't want to over step someone else's comments here, so please read his post and maybe go that way.

May i just add (sorry this even has to be said); Make no mistake, this will get resolved! Just takes time and patients. And a logical approach. Which is exactly what we have all done so far. Things needed to be explored, ruled out, ruled in. Thats just how it works. Its not an easy problem, we are 'ALL' clueless at this time as to the actual component fault. Luck does not come into it. Its a procedure. And its getting closer all the time. I suppose the real question is how much time are we all prepared to spend on this. Personally, i run my own electronics business so i can find time to log in and read posts on this forum as i operate on my own timetable. So count me in! Right enough said... Back to the problem finding and less of the personal stuff. Which is irrelevant.

Yes, i did read the post on FoxTango. Not convinced about it, he didn't mention anything about it being band specific. It sounded like a general ALC failure. I guess its not the same issue. He was getting just mW rather than your 10 watts too.

[drforbin]

thanxs...Yes..it was not band specific in the case of the post.

thanxs again.

[voltz]

"You understand I will not be able to sniff it. Generally what U do is first use the scope and turn the signal down enough that it does not harm the SA and then use the SA. I'll tune everything up. show with finals in place and removed."

I don't think its necessary to use the SA any more, Your scope is fine for these amplitude tests from now on. Its a clean signal, we are now just interested in level.

[drforbin]

Exciting news guys! Have a look.

[voltz]

Great!. Just a quick response for now, and i will get back later, this was what we predicted.
Having ruled out spurious oscillations occurring (using SA) we were left with a resonance (increased amplitude at 7). Your scope clearly shows that. Notice how at 8Mhz it was actually attenuated? and 9 is almost gone completely..
Two things this could be: after the resonance point its common to see a dip. And there it is. It means its not a clean damped resonator. It has a tail off with varying amplitude. OR this could be a 9Mhz IF trap as its suspicious that the dip seems to be at 9Mhz.

I was going to suggest doing exactly what you did haha, power OFF and inject the stage manually with your wavetek! you beat me to it

Ok, now its all about isolating to the component.. It means soldering iron at the ready. And now there's no power on, its totally safe.

[usual warning notice here, sorry if you already did this] Please physically REMOVE the big mains plug from the back of the unit and take it away from the bench.. Seriously, you know im not kidding

Quick one, try pulling one PA at a time - while the test is running at 7Mhz - live. Pull out, Push in, one side then the other. Are both PAs having the same effect or is this one sided?.. and do the anode caps have any effect?.

More later.
 

[voltz]

More questions / tests,
did the Band Switch have any effect? You tested the Preselect, how about the band selector rotary.

[drforbin]

I didn't try the band selector, I will, but it should not make any difference all the band selector stuff for the finals is to the right in the circuit (not connected to this segment). I agree with with your suggestion. The only problem is I have changed all the caps in that area. I already checked the two parasitic chocks (resistors and inductors).
That's why this problem has been so difficult, the first thing I thought of was resonance and checked all that stuff.

But it's clearly resonance of some kind and the voltage is really to low to be moving electrons around in the tube themselves, so this potion of the circuit should be isolated.

more to come.
will do your suggestions.

[voltz]

One thing further. Was the driver wire disconnected at the PA finals (feeding C01, 100pf) when you did your tests? If not, i would try de-soldering it and check again. This is because the resonance may be at the driver side (PB1092 trimmer board for example). It really is about disconnecting stages and testing to get closer to the problem.  This may take a few iterations.

Just to recap:
PA finals, unplug one at a time test - any difference between them or both the same?
Anodes off one at a time - any effect?
bandswitch - any real effect?
Disconnect drive wire to C01. retest for resonance. Is it still there with driver stage disconnected?

That should help isolate the problem.

thanks.

[drforbin]

ok guys...I need your opinion. Here are results and surprise!

[AF6LJ]

That coil is part of a 8.9MHZ trap, The trap keeps IF from feeding through when you are on 7.0MHZ. Remember that dirty looking signal you saw on the scope???
I told you about that also.
I told you there was one, someone misstuned it to 7..MHZ (more or less).


I just wanted to say I told you so, but y'all were not going to listen.

[drforbin]

I am listening. I'm sorry you think I am not.
From what I can tell it has nothing to do with tuning. it seems dead.
Am I wrong?

[drforbin]

FYi I like the picture of your cat.
I'm a cat lover too.

[voltz]

Ok, so that red wire disconnection takes us left, back to the driver direction.

You remember the scope did actually DIP at 9Mhz (not 7)? It was a clear dip with fairly hi Q.
Well, as said, if it was either a resonance at 7 with an overshoot tail off OR a 9Mhz Trap at work, then that scope test the other day would suggest its working at the desired 9Mhz frequency and not 7.
So i'm afraid from me, i'm still not convinced [that its a faulty coil].

Some things here:
Testing an RF choke may not show correctly if you test it with a general tester at audio frequencies. If its a dust iron core (and it probably is) then you may get the wrong reactance showing. To test the coil, put it in a tuned circuit with a capacitor in parallel and work out a where it should resonate. [fo = 1/ (2pi root LC) ]Then check that with your signal generator and scope for a dip across the coil. The calculated resonance point and measured point should be the same, if not, the coil is faulty.

It should be easy to rule this choke in or out, simply short it out in the circuit. If this is a misaligned trap, the resonance will stop.

[voltz]

Oh guys, 4 cats here!! We are all cat people lol.

AF6LJ - we are listening my friend, please dont think we are not. Its an open forum, anyone can comment.. Please do. But it would be really helpful to focus on what the actual fault could be. Did you get a good look at the diagram? Is this the coil you believe to be the IF trap and you believe to be faulty? That would be interesting. Its too broad a statement to say 'its an IF trap'.. where?

[AF6LJ]

I am listening. I'm sorry you think I am not.
From what I can tell it has nothing to do with tuning. it seems dead.
Am I wrong?

If it is mistuned you will have low output on forty.

[AF6LJ]

FYi I like the picture of your cat.
I'm a cat lover too.

She was my best friend for over thirteen years.

[AF6LJ]

In these designs that use this type of conversion scheme, IF feedthough is a problem on the 7.0 MHZ band, for one thing without a properly tuned IF trap somewhere after the mixer but before the PA input it becomes possible for an operator to tune up the transmitter to operate on the IF frequency, that would get said operator in trouble.

The ALC circuit is seeing more signal than it should, and doing what it does best, provide a signal to lower the gain of a stage or stages in the transmitter chain.

Before I had a spectrum analyzer and all the pads and directional couplers, sniffing loops and DC blocks you need to take meaningful measurements I would use a sniffing loop with an adjustable attenuator connected to a receiver tuned to the transmitter IF frequency and use the receiver as a selective volt meter.
Works great for tuning those traps and general troubleshooting.
Which is why I know what that coil is used for. (notice the manual doesn't provide alignment instructions for that coil Yaesu manuals Suck deeply.)
 
And i will mention this again. None of those circuits internal to that radio are designed to work into a fifty ohm load such as your spectrum analyzer, which is why I suggested using a sniffing loop. Nobody back when that radio was being built was designing their subsystems to work into fifty ohms, nobody making amateur radio gear that is. So when you plug one of those cables into your SA you alter how that circuit works.

At this point you should be able to put that coil back in, place the radio in transmit on  40 meters and tune it through its range and observe the change it has on the transmitter's output. That coil does have to be tuned to 8.9MHZ to notch out the IF signal, you might find things working a whole lot better.

Good luck.