FRDX400 S Meter

[Keith]

Just to put it another way. When the set is first switched on the anode of V105 (one side of the meter) is at full HT voltage. The slider of the pot is at a lower voltage (other side of the meter). Current flows from the higher voltage to the lower voltage (and thence to ground through R130).

[milairuk]

Got you Keith - I first disconnected the feed to L115 / V105 anode. The measured voltage at the slider of VR103 is 221v (S Meter -ve). The voltage at the junction of VR102 (preset) and R129 is 233v . Hence a 12volt potential exists allowing current to flow thru VR102/D113 and the S meter.(approx 2.4mA with VR102=5k)
I presume the FSD of the S meter is 1mA, hence the full deflection - ouch !!
Reconnect L115, allow V105 to stabilise and you get 223v at VR103 slider and 215v at VR102/R129. So the voltage difference has "reversed" so to speak and the S meter is at zero. My key error in all this was assuming the S meter +ve went to the slider of VR103, but in fact it is the -ve connection that does.
I cannot see any way round this now except for cutting the HT supply until the valves have warmed up - anyone got any other ideas ?
John

[Keith]

I suppose you could put a 5.6V zener diode between VR102/R129 and VR103 slider. That should limit the current though the meter to just over 1mA. Might upset the linearity at the top end though as you approach the knee of the zener.

[dave walsh]

Cutting off the HT supply used to be done to facilitate a stand by function
John [eg the 358x] just leaving the heaters on. Was it really to protect the S Meter after all? All this extra info is extending the 400 Handbook very usefully. Dave

[MichaelR]

Have you tried powering up with the function switch in the mute postition or stand by

Mike

[milairuk]

Originally Posted by MichaelR

Have you tried powering up with the function switch in the mute postition or stand by

Mike

Mike,
Problem is that the standby position does not cut off HT. It adds a 100k variable pot in series with the RF gain pot, This pot is actually marked "monitor". By heavily biassing back the RF and 1st IF sections you can monitor your transmitted signal without overloading the receiver. In my ham days (>30 years ago) it was quite common, not sure about now though. Only way will be to cut HT to V105 until it warms up I guess.
John

[Station X]

So long as the meter needle shows no sign of bending why worry about it? If the designers thought there was a chance the meter was likely to get damaged they'd have redesigned the circuit rather than risk claims under warranty.