Hey gang,
I've got a power roll-off problem that's making me lean towards a potential bias issue, but I'll give you some background on this poor 858 chassis Washington.
Good:
Output of main regulator set to 13.8v
Recapped with quality electrolytics (this changed nothing in terms of operation/fails)
Mode switch doesn't apply 13.77v to required output section (temporarily bypassed until I can find a replacement mode switch)
Good RX/TX audio on AM
5W out on AM (no roll-off, even after 1 min dead key)
Good SSB RX
BAD:
Stuck at power rolloff on two-tone SSB test, or whistling/testing through mic.
So, the radio will key up and modulate just fine on SSB, but the power will quickly roll off to just a watt. I suspect there's a base bias issue , so I tossed a 180Ω resistor in to drop the 13.77v down, and was able to get the bias fairly close current-wise (within 15% of the updated figures). Still rolls off.
An interesting thing that is occurring is...the output transistors will get warm without any modulation in SSB. Then, if you test audio input, it will have skipped the dramatic roll-off and just put out 1W.
Any thoughts?
Thanks!
-phil
w6sfo
I've got a power roll-off problem that's making me lean towards a potential bias issue, but I'll give you some background on this poor 858 chassis Washington.
Good:
Output of main regulator set to 13.8v
Recapped with quality electrolytics (this changed nothing in terms of operation/fails)
Mode switch doesn't apply 13.77v to required output section (temporarily bypassed until I can find a replacement mode switch)
Good RX/TX audio on AM
5W out on AM (no roll-off, even after 1 min dead key)
Good SSB RX
BAD:
Stuck at power rolloff on two-tone SSB test, or whistling/testing through mic.
So, the radio will key up and modulate just fine on SSB, but the power will quickly roll off to just a watt. I suspect there's a base bias issue , so I tossed a 180Ω resistor in to drop the 13.77v down, and was able to get the bias fairly close current-wise (within 15% of the updated figures). Still rolls off.
An interesting thing that is occurring is...the output transistors will get warm without any modulation in SSB. Then, if you test audio input, it will have skipped the dramatic roll-off and just put out 1W.
Any thoughts?
Thanks!
-phil
w6sfo