Crawls Backward (When Alarmed)

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Hacker Hunter - Recapping the LF Board

Here's the power supply board. This radio was nicely designed for easy service. The plug on the right connects this board to the HT (seems funny calling 18 volts "high tension"), and also carries signal to the volume and tone controls on the top of the radio.

The manual refers to this circuit board as the "LF" (low frequency) board.  This is probably more accurate since the filter and tone capacitors live here.  There is a distinction between this and the "RF" (radio frequency) board for sure.

There are two screws under the cabinet that hold the board in place.  I undid those, and then removed the screw which holds the antenna mount.

The blue and black connectors to the left of the antenna (aerial) are marked "L.S.," for loudspeaker.  Everything is clearly labelled - again, a logical approach to making service easier when it needs to be done.

The LF board just slides off the cabinet like so.

You can see the three screws that hold the cover in place.  Remove these and the board can be accessed.


The LF circuit board removed from the radio.

I'm going to replace all of the blue electrolytic caps.  Some of the values are no longer available, such as the 400 uF cap on the lower left.  I'm just going to substitute 'modern' values, which won't affect performance.  In the case of the 400 uF, I'm putting a 470 in its place.

Note the two black trimmers for adjustment of the power supply voltage.  They are labelled "Piher Spain."  Piher is still in business today - in fact I have some of their trimmers in a parts drawer.

Recapping is just like a tube (valve) radio, only the capacitors are a lot smaller.  I use desoldering braid on the solder side of the board and remove the caps and replace them.

And here's the LF board after the electrolytics have been replaced.  Some of the new ones are quite a bit smaller than the old ones.  I'm saving the originals - I'll keep them in a small bag inside the radio.  Even though they are all bad, they are the originals.  I don't like throwing them away.

One note of interest here:  the green arrows in the image are pointing to the push-pull output transistors.  Just like a tube amp with a p-p output, these give good volume and tone.  The yellow arrow points to the heat sink they're mounted to.  Very tidy.  You just can't service a modern radio like this - radios today have all surface mount devices.  Way too small for humans to remove and replace.

After the recapping, I connected the power and speaker plugs to the board and turned the radio on.  The hum is gone, and it sounds great.  As a bonus, the treble control now actually works!  I figured a bad capacitor was the culprit.

Since I have the radio half apart, I'm going to go ahead and replace the electrolytics on the RF board.  I don't have all of the caps I need on hand, so I have to wait for my order from Mouser to get here.

 
 
 
 

Post a Comment 3 comments:

  • Michael Kim said...
    March 10, 2015 at 10:57 PM
    hi,

    I did recap on this Radio but two registors have been burn out when I turned the switch on.. R15 registor on LF board... if you know the value of this registor, could you let me know?

    R15 registor and the other just above R15...it seems those two registors have same value..Red-Red-Gold is this 22 ohm?

    Thank you in advance.
  • Yr Fthfl Blggr said...
    June 1, 2015 at 4:00 PM
    On the values, red-red-orange would be 22K, and red-red-yellow would be 220K. Gold is only used to indicate tolerance (it would be 5%). Here's an online calculator: http://www.csgnetwork.com/resistcolcalc.html

    When resistors are burned, something else has failed causing too much current to go through the resistor, making it fail. Many times this is because a capacitor in the same part of the circuit has shorted or opened, taking the resistor with it. Replace the caps and then the resistors and see what happens.
  • Yr Fthfl Blggr said...
    June 1, 2015 at 4:04 PM
    Sorry - now I see R16 on the board. I read that as red-black (0)-red, so you are right, it's most likely 22 ohms.

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