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 but 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.

[NQ6U]

Just for clarification, it might be helpful if you tell us how you know that the problem is in the ALC circuit. There are people here who are far better at troubleshooting boat anchors than I am but it seems unusual for an ALC problem to be linked to a particular band.

[N8YX]

Replied to Merlyn in an email but I'll post this to the board at large for reference:

There is a degree of interaction between the trimmer capacitors which peak the various signal stages. The service manual details adjustments - namely, the order in which to do each band segment/stage. Use of a scope with a 30MHz or higher vertical bandwidth is a big help when peaking the pre-driver stages as proper input to the driver stage can be verified as the trimmers are adjusted.

If all of this checks out...and enough drive is present for all bands (see voltage charts in the SM), then have a look at the PA output circuit and check any padding capacitors which are switched in at the 20M spot. They may have failed, and such failure is not an uncommon occurrence with the 90x series.

[drforbin]

Cheers Bernie supporter!

Thumbs up!

Well, when I disable ALC I get full drive. it's not only 40m. It's most pronounce on 40m. For example I cannot get ALC to activate AT ALL on 20m. I have actually found other references to this same failure mode but no seems to ever post a solution. yo give an example. on 40m with ALC disconnected i'm getting well over 100 watts when ALC gets connect that drops to 10 watts. This is on tune up.