Using a Heathkit RF-1U signal generator for MW radio alignment

[Jolly 7]

I have a working Panasonic DR28 radio but MW stations are appearing on wrong parts of the dial and some are missing. I have the radio alignment instructions and also a signal generator. I am going to make my first attempt at aligning both the IF and RF stages of the radio using a signal generator, but was wondering if an oscilloscope is absolutely essential for this. Is it possible just to tweak the IFTs etc. in the correct sequence, while listening for peak audio signals from the radio speaker during adjustments ?

[G4_Pete]

Hi Oscilloscopes have their place but to be honest I only use mine as a front end for my frequency counter for these type of jobs. Often the probe can be very minimally coupled or laid across the circuit with no contact to get the counter to clock thereby not loading the circuit. Loading and changing what you are measuring always has to be borne in mind with oscilloscopes. Also clipping on an earth probe is problematic from introducing stray signals to closing an earth loop.
The human ear is a remarkable set up instrument with log type of response and selective filtering!!
Pete

[Skywave]

For performing the re-alignment of a radio designed for A.M. reception, an oscilloscope is not necessary. In fact, it is likely to be counter-productive for that task on account of its input capacitance which will appear across various parts of the circuitry where that 'scope (and its probe) will be connected. All the 'good books' on re-alignment refer to using an output meter in addition to - or as a substitute for - the loudspeaker. Generally, one's own hearing is quite adequate.

Although the 'standard' technique for re-alignment is IF first, then oscillator and R.F. amp. (if fitted), in your case I would suggest you check the oscillator section first, since you have reported "MW stations are appearing on wrong parts of the dial and some are missing". The one really important caution you will have to take is that you do not accidently set the oscillator freq. to the 'image' freq.

Al.

[Station X]

Not forgetting to make sure the dial pointer is in the correct place relative to the tuning capacitor vanes.

[ms660]

Also a 'scope or a meter is better than your ears when peaking up.

Lawrence.

[Skywave]

Yes, to some degree, that is correct. But in practice with 'ordinary' A.M. radios, I have found the difference to be of no real consequence.

Al.

[G4_Pete]

Hi in the original post I am not sure if you have an oscilloscope? But another well publicised option is to use an oscilloscope or a volt meter , analogue preferred, to monitor the AGC line. You then have an "S" meter for set up.

Pete

[Jolly 7]

Hi. I don't have a usable oscilloscope at present. I bought an oscilloscope kit recently but haven't found the time to build it yet. I am planning to make a loop to radiate the signals from the generator to the radio as described in the link below. I should be able to place it loosely over the MW antenna coil inside the radio.

http://radio-quaderno.blogspot.com/2...storation.html

[G4_Pete]

Hi,
That is some fancy injection coil, You can experiment with just a turn .The key thing is whatever you use make sure it is mechanically rigid as if it moves about during setup the injected signal level will change.

Ref Grahams post above
1. Check all the mechanical parts , is the dial indicator tracking the capacitor
the right way round?, Check drive wheels or whatever you have are not slipping on the shafts Lowest frequency with the capacitor fully meshed. Cords can be wound the wrong way , that would certainly give stations in the wrong place.

Before attempting any alignment you need to do a basic function check against your signal generator then work problems from there.

Whilst it is a good exercise to go through the alignment I personally would not touch any IFT /oscillator cores if it is not necessary. Do you have a set of nylon alignment tools to match the cores ?

[ms660]

And careful how that coil is coupled otherwise what you might think is aligned won't be aligned when you remove it.

Lawrence.

[Sideband]

Also of course you should check that there are no other faults (other than alignment) causing the incorrect dial readings. However the DR28 is a synthesised 'world' receiver is it not? In which case it doesn't have a 'dial' but a digital readout? Or is there another DR28 that uses analogue tuning? Or are there two types of tuning...one analogue and one digital?

[Jolly 7]

The DR28 has both an analogue and digital frequency readout with MW, four shortwave bands covering 1.6 to 30 MHz and FM. The situation with my radio is a bit complicated because the main board currently in place belonged to a DR29, which a seller on a well-known auction site sold to me as a DR28 board. The DR29 board had LW and only 3 SW bands on it. The reason why I replaced the original DR28 board is because I accidentally damaged it in the past and became unrepairable. However, with quite a lot of work involving adding/ removing certain components and careful comparison of both boards, the 'transplantation' was successful. It is only the MW band that is problematic.
My plan is to use just the analog dial for calibration with the signal generator and then reconnect the frequency counter, which should do the job automatically I hope.

[Jolly 7]

Originally Posted by G4_Pete

Hi,
That is some fancy injection coil, You can experiment with just a turn .The key thing is whatever you use make sure it is mechanically rigid as if it moves about during setup the injected signal level will change.

Ref Grahams post above
1. Check all the mechanical parts , is the dial indicator tracking the capacitor
the right way round?, Check drive wheels or whatever you have are not slipping on the shafts Lowest frequency with the capacitor fully meshed. Cords can be wound the wrong way , that would certainly give stations in the wrong place.

Before attempting any alignment you need to do a basic function check against your signal generator then work problems from there.

Whilst it is a good exercise to go through the alignment I personally would not touch any IFT /oscillator cores if it is not necessary. Do you have a set of nylon alignment tools to match the cores ?

Thankfully the radio has no dial cord and judging by the FM section, it is rotating in the right direction. I have a full set of nylon alignment tools including one with a metal driver to allow sufficient torque.

[G4_Pete]

I just looked it up (perhaps should have done earlier!). I bit above the average for a 1978 portable!

Obviously a lot of history here with this one so cant add much more.

Pete

[Skywave]

Regretfully, the revelation in post #12 has the potential to make many of the above posts not applicable. So for myself, there is nothing to add.

Al.

[Sideband]

Originally Posted by Jolly 7

The DR28 has both an analogue and digital frequency readout with MW, four shortwave bands covering 1.6 to 30 MHz and FM. The situation with my radio is a bit complicated because the main board currently in place belonged to a DR29, which a seller on a well-known auction site sold to me as a DR28 board. The DR29 board had LW and only 3 SW bands on it. The reason why I replaced the original DR28 board is because I accidentally damaged it in the past and became unrepairable. However, with quite a lot of work involving adding/ removing certain components and careful comparison of both boards, the 'transplantation' was successful. It is only the MW band that is problematic.
My plan is to use just the analog dial for calibration with the signal generator and then reconnect the frequency counter, which should do the job automatically I hope.

I suspect there is more to it than just adding/removing components. Basically this is an analogue tuning set with a frequency counter for the digital display.