FRDX400 S Meter

[milairuk]

I am currently restoring a Yaesu FRDX400 to it's former glory, but one thing is troubling me. Upon switching on the Rx, the S Meter bangs hard over at full scale for approx 15 seconds until the receiver starts to warm up.
Looking at the circuit (attached) I cannot see what is passing current in the anode circuit to cause this to happen. Does anyone have a FRDX400 and if so is this normal. Or can anyone explain what is happening ? D113 is not short circuit.

Thanks

John

[dave walsh]

John I got one of these recently. Not much time to look at it but dead at present-could be major power supply problems [lets look on the bright side] so I can't light it up to observe the meters but it doesn't sound right! However, Graham [Station X] very kindly e-mailed me a copy of the manual in response to my general request and he did say that he had worked on one of these for a friend so that seems a good starting point.
I will be interested to see what info comes through. Cheers Dave

[milairuk]

Once you get time Dave, I'll be happy to give you any help with voltage / resistance readings if needed. (Apart from the S meter ) I picked mine up on the usual web auction site, it has all filters fitted, xtal calibrator, FM unit and 2m / 6m converters fitted. After a bit of TLC it seems to be working well HF wise except for the 10m and CB (who cares about that) ranges. May just be the calibrator signal is weaker at 28Megs, I'll have to put a sig gen on it. It was a delight to align apart from the said 10m range.
Have still to work on the 2m / 6m units, lots of noises from the 2m unit, but nothing intelligent.
There appears to have been two versions of the FRDX circuitry wise, mine has a 2 transistor 9v regulator whereas one circuit diagram I looked at only uses a zener.

John

[Keith]

John,

Assuming V105 is an AGC controlled IF stage then an increase in anode voltage (caused by a reduction in anode current due to more negative AGC volts) will produce more meter deflection. The fact that the meter reads full scale implies that there is insufficient anode current to start with and that it takes a long time to get to its correct value. Could be a number of causes - possibly V105 is low emission.

HTH

[dave walsh]

Clearly you know what you are talking about Keith. The Handbook
trouble shooting section refers the reader to the AVC circuit in the event of problems with the meter but you are a lot more detailed. Thanks to John for his offer-I've sent a pm. Dave

[milairuk]

Originally Posted by Keith

Assuming V105 is an AGC controlled IF stage then an increase in anode voltage (caused by a reduction in anode current due to more negative AGC volts) will produce more meter deflection. The fact that the meter reads full scale implies that there is insufficient anode current to start with and that it takes a long time to get to its correct value. Could be a number of causes - possibly V105 is low emission.

Thanks for that Keith - maybe I didn't explain well - the meter "bangs" over full scale on switch on from cold, it goes back to zero as V105 warms up and then works fine. Removing / changing V105 prior to switching makes no difference to this. Apart from C156, C157 and C154 I cannot really see any earth path to cause current to flow. After all the S meter is really just in parallel with R129 to read the voltage change caused as you say by the change in anode current with AGC. The 0.01 caps are all disc ceramics and I have never known these to leak, but when I get a moment I will try disconnecting one by one. It maybe that I am chasing a non-existent problem, but I am not happy about possible damage to the movement.

John

[Keith]

John,

I've just re-read your original post - somehow I'd got 15 minutes into my head rather than 15 seconds! An short initial overshoot is quite normal and, in fact, difficult to avoid in a valve receiver. My HRO does exactly the same, and must have been doing so for 65 years without any damage to the meter movement.

[MichaelR]

From the circuit there is nothing to limit the meter moving hard over while the set stabilises from switch on. slightest voltage over R129 will cause some meter movement , you have an agc line feed on on side which will take time to stabilise. i can think of quite a few military valve comms sets that behave this way.

Mike

[milairuk]

Originally Posted by MichaelR

From the circuit there is nothing to limit the meter moving hard over while the set stabilises from switch on. slightest voltage over R129 will cause some meter movement , you have an agc line feed on on side which will take time to stabilise. i can think of quite a few military valve comms sets that behave this way.
Mike

Thanks everyone for your comments, I think this is maybe flogging a dead horse now, but if you look at the circuit redrawn as attached, how can any voltage appearing on the S meter cause current to flow assuming V105 is not in circuit. Apart from the initial current flowing into the various caps, but are 0.01uf's going to take enough initial current to pin the meter full scale for this time ? Maybe they will given the resistance values in circuit. At the end of the day it may be normal behaviour as many of you say, it's just me being over cautious (or just stupid )

John

[ronbryan]

John

As MichaelR says, the meter will go hard over until the DC conditions are stabilised.

For example if the HT comes up first, before V105 is conducting, the full HT will be applied to one side of the meter via two series resistors and the diode and be compared with a potted down version of the HT on the other side of the meter. This will cause a deflection dependent on the current flowing.

When the dc conditions do stabilise, the potential difference either side of the meter will be very small and the meter deflection will be correspondingly small, proportional to the pot settings.

When radio signals are received, they will be rectified by the diode and smoothed by the filter capacitor and this dc voltage will cause a deflection proportional to signal strength.

Are you able to try delaying the application of the HT until the valve heaters have warmed up? This might help the DC conditions to stabilise more quickly and put less strain on the meter movement.

Ron

[Keith]

John,

As Ron explains, you have to consider the circuit as a bridge. The potential difference is between what will be almost full HT (before the valve has started to conduct) and the lower voltage on the adjusment pot slider. It may be exacerbated by the use of solid state rectifiers in the power supply - an indirectly heated valve rectifier should give less of an problem due to the delay in HT. I think the diode is there just to protect against reverse deflection when the set is switched off - the meter is simply a way of indicating anode current variation in the IF amplifier due to AGC action.

[milairuk]

Thanks Ron & Keith - you do have differing views on the function of the diode though. I like Ron's suggestion to delay the HT - there is a standby position on the main function switch which is linked to the monitor pot for monitoring your Tx signal - basically biases back the grids heavily, maybe I'll change that to feed a relay to open the HT feed as I don't have a Tx at present. (Don't want to add a new switch as this will spoil the original look)
Point taken about a bridge - but surely a bridge has a +ve and -ve connection ? This is my logic to the problem - where is the -ve connection here if you remove V105 ?
John

[ronbryan]

John

Keith is quite correct about the function of the diode. I overlooked the decoupling capacitor at the top of the tuned circuit, which will remove most of the RF.

The -ve connection to the bridge is the potential divider formed by VR103 and R130.

Ron

[Station X]

I've just had an email from a mate who has an FRDX400. He confirms that the S Meter bangs the end stop on switch on then drops back.

So either this is normal operation or both sets have the same fault.

[MichaelR]

Originally Posted by milairuk

Point taken about a bridge - but surely a bridge has a +ve and -ve connection ? This is my logic to the problem - where is the -ve connection here if you remove V105 ?
John

A bridge measures a differnece in potential.

if you look at the circuit carefully you will see there is voltage feed ( irrespective of the valve) through the coil and as I said AGC from the other side.

I owned one of these sets 12 years ago

[milairuk]

Originally Posted by Station X

I've just had an email from a mate who has an FRDX400. He confirms that the S Meter bangs the end stop on switch on then drops back.
So either this is normal operation or both sets have the same fault.

Thanks Graham, I'll go back to sleep now

[Keith]

John,

Just to clarify I've attached a simplified circuit showing the bridge arrangement.

HTH

[milairuk]

Originally Posted by Keith

John,
Just to clarify I've attached a simplified circuit showing the bridge arrangement.
HTH

Thanks Keith, but my understanding has always been that current flows from HT to Ground via whatever circuitry is connected between ?? If you physically remove the valve from your circuit where is the current path to ground for the S meter ?? As far as I can work out there isn't one ? When replaced V105 warms up and draws anode current, thus making the "ground" connection.
This is really the point I am trying to get over - or maybe I am just very dumb.
John

[Station X]

The resistor and pot connected between HT+ and ground have current flowing through them as a result of the HT voltage being across them. Whenever current flows through a resistor a voltage appears across it. The voltage is given by current times resistance. (IR).

This voltage is also across the S meter and the components in series with it. So the meter deflects. You might be able to understand more easily if you mentally remove all the components except the resistor, pot and meter.

[MichaelR]

Originally Posted by milairuk

This is really the point I am trying to get over - or maybe I am just very dumb.
John

John you are not dumb just try and reason an understanding to this , once it "clicks" it will help you tremendously in developing your understanding electrical theory.

Station X is right in what he says , the bridge sees the volt drop (potential difference) across the resistor.

mike