Philharmonic -- AC uA Meter

[Jay Forbes]

Where can I purchase an AC uA meter? 

Need one to perform an alignment on a Philharmonic.

I cannot find one online.  I do see plenty of DC uA meters...

[Norman S Braithwaite]

A DC microamp meter is identified in Scott literature for monitoring AGC voltage.  At the time the service data was prepared, a microamp meter was more accessible to service shops than a vacuum tube volt meter.  A vacuum tube volt meter will work very well for alignment of the Philharmonic chassis.  You may find the clarified Philharmonic alignment instructions at the following link to be helpful.

https://ehscott.ning.com/photo/albums/am-philharmonic-alignment

Norman

[Jay Forbes]

This is what I was looking at for the alignment. 

I used a DC mA current meter and meter did not deflect at all. 

Are you SURE that a DC uA meter will work?  Or at that low current, it will also measure AC current?

Norman S Braithwaite said:

A DC microamp meter is identified in Scott literature for monitoring AGC voltage.  At the time the service data was prepared, a microamp meter was more accessible to service shops than a vacuum tube volt meter.  A vacuum tube volt meter will work very well for alignment of the Philharmonic chassis.  You may find the clarified Philharmonic alignment instructions at the following link to be helpful.

https://ehscott.ning.com/photo/albums/am-philharmonic-alignment

Norman

[mike hadley]

I have tried both alignment with a wobbulator and the method posted by Norman both work.

The meter needs to be dc as you are looking at the signal after the detectors and the signal here is dc,

over here in the UK the nearest I could get to a 30ua meter was an Avo8 meter on the 50ua range

that's near enough,

make sure you have the correct cap and resistor on the output of your signal generator as you dont need to inject much signal.

When performong the alignment the top cap to the 6L7 is removed and the capacitor and resistor are fitted, the resistor provides a method of terminating the grid to chassis and the cap provides dc isolation from the signal generator,

follow Normans instructions and pay attention to where you set the IF bandwidth control and you should be good to go.

[Norman S Braithwaite]

Yes, a DC meter is required.  Make sure the meter is working (connect to 1.5-volt battery through a 100,000-ohm resistor for 15uA), make sure the meter is connected with positive to chassis, and make sure the negative side of the meter is connected in front of the capacitor (circuit wise) if you have a later version Philharmonic.  Also as Mike mentioned, start with the selectivity control at or near full CCW (sharp tuning).

Another potential cause of no AGC is loose variable selectivity rotors.  Remove the four square shields and check to make sure the rotor plates are tightly fastened to the shaft.  If a little loose but correctly phased, loosen and retighten the set screws for a good electrical connection.  I have encountered a couple Philharmonic sets where the rotors were so loose that their position was dictated by gravity rather than the control shaft.  If you find them to be so loose, they will need to be properly phased (report back if this is the case).

Norman

Jay Forbes said:

This is what I was looking at for the alignment. 

I used a DC mA current meter and meter did not deflect at all. 

Are you SURE that a DC uA meter will work?  Or at that low current, it will also measure AC current?

Norman S Braithwaite said:

A DC microamp meter is identified in Scott literature for monitoring AGC voltage.  At the time the service data was prepared, a microamp meter was more accessible to service shops than a vacuum tube volt meter.  A vacuum tube volt meter will work very well for alignment of the Philharmonic chassis.  You may find the clarified Philharmonic alignment instructions at the following link to be helpful.

https://ehscott.ning.com/photo/albums/am-philharmonic-alignment

Norman

[Jay Forbes]

Mike, Norm,

Thanks for all the input.  I have a DC 0-50 microamp meter on order.  

All IF tuner plates are secure.

Due to previous repair, I still am not sure if position of the IF tuners (open/closed) is correct as I have nothing to base it on.

The issue I have been having is that after alignment in Sharp position, then radio tuned into a station, the station de-tunes when HiFi is turned to Broad.  Then in maximum Broad, and station tuned in, station comes in whisper quiet even at higher volumes.

Have either of you run into this issue?

[Norman S Braithwaite]

I haven't.  There is a bias potentiometer to compensate for change in gain when adjusting selectivity although it normally reduces gain when bandwidth is increased.  Check it and make sure it is wired correctly.  After tuning in a station in the narrow bandwidth position, do you have to retune after increasing bandwidth?

Norman 

[Jay Forbes]

Yes, I have to retune just a bit from broad to sharp, and vise versa.  That gain adjustment was set to max gain.

[Norman S Braithwaite]

Retuning from broad to sharp is expected.  Vise-versa is not normal.  It is an indication that the aprons of the IF bandpass characteristic are not balanced.  However, minor retuning is not an indication of the problem you are experiencing with gain.  I would look carefully at the gain compensating potentiometer attached to the selectivity shaft (not the sensitivity control on the front panel).

Norman

[Jay Forbes]

Norm, I had been doing the alignment with the control in CW position instead of CCW position....  I had my customer send this pic to verify the Selective position.

I realigned in the CCW position and now the control works as it should, except it is still detuning when going from Sharp to Broad. 

I am waiting for the DC uA ammeter to perform a proper alignment.

Norman S Braithwaite said:

Retuning from broad to sharp is expected.  Vise-versa is not normal.  It is an indication that the aprons of the IF bandpass characteristic are not balanced.  However, minor retuning is not an indication of the problem you are experiencing with gain.  I would look carefully at the gain compensating potentiometer attached to the selectivity shaft (not the sensitivity control on the front panel).

Norman

[Norman S Braithwaite]

Use of a meter for aligning the IF amplifier is good for peak alignment at the narrow bandwidth position. Balancing the bandpass aprons to prevent the need to retune can be accomplished with a meter but will be difficult and time consuming. Sweep alignment with the bandpass characteristic shown on an oscilloscope is best for balancing bandpass aprons.

Norman