FT-8900r repair - pre-driver

This particular Yaesu FT-8900r came from a repeat customer of the Cly Institute.  He delivered the bare board for the repair saying to change the final transistor, and even supplied the correct part.  The process was done, but that did not cure the problem, there was still no output on transmit.

With the now fully re-assembled transceiver, the debugging process started by stepping back from final transistor gate to discover where the transmit signal was being lost.  It didn't take long to find the gate of the pre-driver transistor had the transmit signal. This meant that the pre-driver was not functioning.  Surprisingly pressing down on the chip re-established output.  This turned into an easy fix by re-sweating some solder under the chip.  In our experience at the Cly Institute, the RoHS solder they are using today is pretty poor for higher vibration applications.  We only use Lead solder for repairs trusting in its age old reliability in soldering applications.

This would have been the end of the repair story, however with the transmitter set to HIGH output, the final stage began to ring, causing a tone on the received audio, and likely some spurious emissions.  It had to go back to the bench for investigation.  In the process of replacing the final transistor, the chip capacitors had to be removed surrounding it.  This was the likely culprit that one, if not all were overheated and no longer performing their designed task.

The original capacitors were rated 50pf at 500vdc.  They were not ceramic, and were likely heat-damaged during the process of replacing the final transistor.  New ones were only a eBay order away.

The areas circled in red are the ones that were providing trouble.  The solder joints around Q1137 were an issue.  I had to add some serious heat to this transistor to get it re-flowed, but trying to keep the body cool by holding a cold metallic tool on the top of the chip.

Note the new ceramic capacitors around the final transistor Q1134 in the picture above.  These are much higher quality than the original capacitors.  They can take a good bit more heat and should be more stable throughout their lifetime.

The radio was held in transmit attached to a dummy load for an excessive time.  It did not overheat on high, and it did not drop power.  I listened to it through the radio I keep in the shop, and it did not oscillate or produce any spurious tones.  Its time to box it back up and return it to the customer.

Lionel J-38 restoration better than factory

A recent sale of equipment from a local SK presented the opportunity to purchase a Lionel J-38 straight key.  I had been using a JJ-38 Japanese-made key, which was functional, but not nearly as classic as an original.  I snapped up the J-38 despite the tarnish.

The J-38 was designed to be used by the Signal Corps.  This example is missing the bar across the back along with two of the binding posts.  The shorting bar was designed to be closed for listening mode, and open for sending.  For the modern ham, this feature is convenient for tuning up the finals in a tube amplifier by closing the switch and adjusting the tuning and loading controls without having to hold the key down.

The ugly mess you see here is the result of years of laying in a box unused.  Each fastener is brass and can be brought back to a high luster with the right application of elbow grease and Mother's Polish.  The first part is getting the components clean.  For this simple soap and water is enough.

The key is carefully stripped down.  Be very careful not to misplace the insulator parts.  The left side of the key is insulated from the cast base.  There are fiber washers and collars which must go together in the right order.  

The first pass is a scotch-brite pad to remove the top layer of dirt and oxidation.  The second pass is a wet sand with 2000 grit wet sand paper.  This will dull the surface slightly, but the third pass of Mother's Polish brings the brass to a high luster.  The final step is a spray finishing wax like Prestone or Maguires mist.  "The Lionel Corporation" is clearly cast into the frame, along with the stylized letter "L".  This model is particularly nicely styled with the Lionel logo right into the shape of the plate that makes up the bottom contact of the key.  This part was particularly hard to polish evenly.  I just kept at it until I was satisfied by a mirror shine.  Its not always about elbow grease when it comes to Mother's Polish.  Just let the chemicals do their work until the cloth turns black.  Shift to a clean spot in the cloth and wipe away any excess.

The fasteners can be placed in the chuck of a drill, spun at high speed against the polishing cloth to get the best uniform shine with the minimum of effort.  The jam nuts can also be handled the same way by letting the drill chuck grip the shoulder on the inside part of the nut.

The shorting bar was easy to polish after removing a large layer of oxidation.  It has a mirror shine to it.  When re-assembling be sure to put a small drop of oil on the collar of the bolt that retains it.  This insures smooth operation so that the fastener doesn't bind and start to back out of the base.

The finished product is one that any ham can feel proud to display and use on the operating table of the shack.  No J-38 looked this good right out of the factory and into the hands of a Signal Corp Trainee.  This example needs some appropriate cloth-covered wire and a 1/4" plug to fit into the key input of the radio.  I always recommend tinning the wire so that it doesn't fray when inserted into the binding posts.  Also a short piece of shrink tube at the end of the wire keeps the cloth insulation from fraying as well.

The nickel finish on the Japanese key is fine, but the brass is so much nicer.  The knurled nuts are larger on the Lionel and much easier to grasp.  The fulcrum points on the Lionel are actually points.  The ones on the JJ-38 are just squared off and have to be kept very loose in order to keep from binding up.  Here a side by each photo exemplifies the beauty of the original Lionel key vs the Japanese-made key.

73 ES GUD DX de N3FIX dit dit