The AM Forum

I had never tried to operate my old FT-901 on AM until recently, and discovered a 60 hz hum on the output signal. I could see something on my Heathkit TX scope and couldn't tell what the freq was, but one of the guys on 3707 identified it as 60hz with his audio scope.

My first thought was a problem of some sort in the audio chain...ground loop, bad ground on the mic, etc... I listened to the mic audio chain through the monitor function on the rig and it sounded clean as a whistle, but I didn't know where that was sampled. I shorted the mic input and tx'd manually...still had the hum. I poked around in the audio chain with the signal tracer and couldn't find any hum. I finally cut the audio chain completely loose, and still had hum. I could hear it in CW, though very subdued. When in AM, it's wasn't very strong with the carrier cranked up for some reason, but down at the 75ma plate current level I needed to operate at for AM, it seemed to really modulate the tx. 

The driver and finals were soft, so I replaced and neutralized them, just in case it was something in the tubes, but it wasn't.

I looked in the power supply to see what was 1/2 wave rectified (since it was 60hz). I looked in the rect A, B, & C boards and found the -120vdc bias supply had about 30v of 60hz ripple. I recapped the board, no improvement. I experimented a little and found I could connect a 33uf @ 450 vdc cap from pin 6 of rect A bd (-120vdc bias supply) to ground, and would completely clean up.

Here's a couple of pics from my HP scope, of the bias supply. The resolution is set to display one sine at 60hz, at 20vdc per division. The first one shows a ripple of 30 volts! The second pic is after the addition of the 33mF cap to the bias supply.

Dave WB4IUY

THanks for posting this.  Good fix. The only thing that might be helpfull is exactly where the cap goes. I try to take pictures or put pad numbers down when I can.

C

Howdy,

I installed it from pin 6 of rect A bd (-120vdc bias supply) to ground (pin 10). The pins are numbered on the bottom of the PCB sockets, as seen from the bottom of the rig. Here's a pic of how I installed my cap. NOTE...you can't physicaly install it here, if you still have the internal speaked on the bottom cover. I've removed mine (not a very good speaker, on my rig), so there's plenty of space.

Dave WB4IUY

I cant help thinking you overlooked something.

I doubt they built the radio with built in hum.
Adding more capacitance is masking the real problem maybe...

Nothing between the cap and ground?
Does the original cap have a low resistance path to ground?
Are all the diodes good?
Bleeder resistor in spec ?

Just does not seem right...

Brett

Why have 2 threads for the same radio & problem??

Carl

doe syour noise sound like this? I made this recording using my ft-901. It was made by turning the mic gain the whole way up and disconnecting the mic then running the finals at 100 ma.

I often make two posts.  The first post is that I have the problem and it contains info on working through the problem. Then when a solution is found,  A new post like this is made with insturctions, pictures and so forth.  This way, A guy comes along with a problem and finds the answer in a single post.  This entire site is about information sharing.

Thanks for posting that picture of the cap.  901s have had this problem for sometime. Its nice to see a simple fix. I have already passed it along to other 901 owners that are humming along....


C

Hi Brett,

I may have overlooked something, but I can't find it if so. I checked the ground path for the supply, the other surrounding components were good, the diode was good, I replaced the cap, etc. I forgot to mention that I cut the bias supply completely loose from the rest of the radio during the troubleshooting process, and the same ripple seen in the photo was still there. It's a very simple circuit. This fixed my problem, so I'm pretty happy :-)

From the emails from 901 forum, it seems that others have the same problem and decided to live with it. I used the rig, and while it was usable, it was a bit 'hummy'.

Odd.
Without a load it still has hum?
The bleeder resistor is in spec?

Odd that the radio was sold with hum, very odd for the Japanese!
Maybe its got Toyota parts?

Brett

Thats all fine and noble but information dilution by spreading it all over isnt. Especially since it may be just an interim patch of the real problem. And then the next fix starts with a new thread.

Carl