Hojo and the case of the faulty FT-2800M

Hojo and the case of the faulty FT-2800M

Background

A friend, Joe / W2IFB mentioned that he had a Yaesu FT-2800M fail on him.  He had been keyed in a long QSO for more than 10 minutes when the radio failed, catastrophically.  Thinking that he had blown the final, he tried replacing the final transistor.  Unfortunately, it didn't work.  He reported that the radio was basically in a "short" when plugged in.

He gave me the radio and said "It's yours!  If you fix it, maybe I'll buy it back!"  I accepted the challenge.

Initial testing

I took the radio and set the current limiting on my power supply to 300ma, figuring that would be sufficient to demonstrate a problem.  I plugged the radio in, and sure enough, it was drawing current HARD.  Placing my ohm meter across the power leads, it showed 0 ohms.  It's in a dead short.

Investigation

I opened up the radio and snooped around the board a little.  I saw the power transistor that had been replaced, but didn't see any shorts nearby.  It also seemed unlikely that a problem in the final would cause a dead short when the radio was still powered off.   I also observed what appears to be a two-turn inductor coil that may have gotten a bit smushed.  None of this looked like the cause of the short, however.



I pulled out the schematics and took a look at the routing of the power.  I assume the short must be very early in the circuit, since the unit uses a soft power switch, and it hasn't even been powered on. Glancing at the schematic, and board, I saw a few voltage regulators, and a protection diode.

The protection diode was right near the edge of the board, and an obvious first thing to check.  It's a clamping diode, so a surge of some kind may have caused it to fail.

I removed the diode from the board, and sure enough!  It's a dead short.  0 ohms in both directions. Further, I hooked it to the power supply and confirmed that the diode was permitting full voltage rom the supply through it in both directions.  It's blown.

Yaesu is kind enough to list the manufacturers part number in their service manual.  Digi-key has the part for $0.48.  I'll tuck one onto my next order.

Further testing

With the diode removed, the power leads no longer showed a short.  Encouraged, I decided to simply hook power to the board and see if the current draw was reasonable. The head was still detached, so I didn't expect the radio to power on. Sure enough,  the current draw was down in the 10s of milliamps. I think that's reasonable.

Hooking up a more beefy power supply, I was able to run through a full suite of tests, and confirm that it was simply the diode at fault.  The radio was working just fine.

Repair and Alignment

I ordered up the diode and got it in from Digi-Key.  The installation was trivial.

I went ahead and did a few tests prior to aligning the radio.  It was transmitting about -650 hz off frequency at 146.000.  Most other metrics seemed good, though it was only making about 50 watts on high power (should be 65).

The Jig

The perform the alignment, I needed to inject signals into the microphone port.  I decided to make a jig that I could use for this project, and in the future.  

 I built a 6-pin cable to use for the project, and sorted out the pinout.  Yaesu uses a common ground for the Mic and PTT.  The Mic inputs go to the BNC port.  The PTT goes to the toggle switch.

The finished product.  Now I can pump signals right from my service monitor into the mic port on the radio.  The toggle switch makes it easy to engage the PTT without holding a button,

Alignment Details

I followed the alignment procedure from the manual.  Notes here for posterity.

Adjustment	Original Value	Adjusted Value
PLL Reference	-645 hz	-15 hz
TP-TVC	1.547 v	1.502
TP-TVC	0.925 v	Not adjusted.  Broken Trim Pot. Target value was 1.3 v.
TUN 137	1.987 v	2.009 v
TUN 146	2.050 v	2.051 v
TUN 160	2.026 v	2.026 v
TUN 173	2.018 v	2.018 v
TXPW H	50 w	65 w

All other setting values were already optimal when measured, and were left alone.  Current readings on transmit were all well under published maximums.

RCA WV988 VTVM recap and repair

The purchase

I found an RCA WV988 VTVM at a hamfest.  It was labeled "Works - $10".  I figured it would be a fun project to learn a little about recapping and tube based equipment.  It has a minor crack in the face, but I don't really care.  I'm more interested in this VTVM for the "puttering" aspects than for usability.  Still, it'll be nice to have an analog meter on my bench if I get it working well.  It's got a nice big display, and the crack doesn't impede use.

I managed to find a manual for it online.  It's one one electrolytic cap, which I'll replace.  It's also got a selenium rectifier, which can release toxic "magic smoke" at failure.  The interwebz recommend replacing that with a diode and series resistor.

A peek inside

A previous owner had soldered in a D-cell battery to operate the Ohm meter.  I'll pull that out, and put in a battery holder.  Also, clearly in view are the electrolytic capacitor and the selenium rectifier that will need to go.

Checking it out

I went through and tried all the basic functions.  They were all pretty good, though it was slightly out of calibration.  I'll go through and attempt a recalibration after repairing it.

Installing a battery holder

Easy enough.  I desoldered the D-cell, and put in a AA battery holder.  I drilled a small hole in the plastic holder, and countersunk it a little. I used the existing screw to install the new holder.  I soldered in the wires and used a bit of heat shrink to insulate.

The Old Battery, Soldered in.

 

Battery removed.  I'll reuse that screw in the center of the old holder.

Hole drilled in AA holder, and countersunk a little, so the screw isn't in the way of the battery.

Installed, with a little shrink-wrap on the wiring.

Now the battery is easily replaced.

Replacing the cap and rectifier

The cap was a 20uF 200v.  That was easily replaced.  The rectifier is rated tat 20mA.  It's easily replaced with a 1N4004 diode and a 2 Watt, 820 ohm series resistor.   I followed the basic recipe in Rich Bonkowski's (W3HWJ) excellent article.  I tried a few different resistors until I got a voltage that matched what I was reading prior to the replacement.

Since I was installing one additional component (the resistor) in series, I drilled two new holes in the board, and ran the diode to one of the new holes, and then connected the resistor from the adjoining hole to the other rectifier pin.  Since there was no trace on the board for that connection, I simply extended the leads to one another, twisted them once, and soldered them together.  The result looked nice and tidy.

The old rectifier and cap.

The PCB from the back side.  The two new holes (bottom box) have leads through them from the diode and resistor.  I simply twisted together and soldered them.  The existing pads from the rectifier and cap are marked.

The final installation of the diode, resistor and new cap.

Testing

Upon testing, it appears that the DC voltage calibration was off, and it is at the limit of the variable resistor for tuning.  It's off by around 1 volt.  More follows as I sort out the issue.

Building and using a Schmitt Trigger Oscillator

Building and using a Schmitt Trigger Oscillator

Inspiration

Alan Wolke / W2AEW has a youtube channel in which he does all manner of informational posts about electronics, RF and Ham Radio.    He did a great video showing how to do TDR measurements by using an oscilloscope and a Schmitt Trigger Oscillator which he built up from some scraps on his bench.  I decided to build one for my own use.

Build and Test

Being anal the way I am, I worked up the schematic and board layout in Diptrace. I decided to use thru-hole parts.  Unfortunately, I'm not a wiz at Diptrace.  It doesn't have great provisions to work in three dimensions.  Some of the traces overlap or take weird routes, where I intended to just route around with some wire jumpers.

I had most everything in my scrap box.  I elected to use a CR-2032 (3V) button battery, seen on the left, since it would keep the whole thing small.  I just had to order some Schmitt Trigger Oscillators from Digi-Key.

Here's the finished product.  There is some kapton tape wrapped around it simply to protect the bare wires soldered on the bottom.  The BNC plug is crazy glued to the perf board.

Running my initial test, I found two issues. First, the clock frequency was quite a bit different from Alan's, though I used the same values for the resistor and cap in the oscillator, I thought!!!!

Upon re-watching his video, he used a 47nF cap. I used a 4.7nF cap.  Oops.  Well, the oscillator frequency came out about 24 khz.  That's still completely serviceable.  I opted not to bother fixing it.

The second issue (which I don't understand) is that the rise time is not as fast as Alan observed.  I'm getting a rise time of about 9 ns.  He was getting about 2ns.  I used the same 72AC14 Schmitt Trigger DIP that he was using.  I'm not sure why it might be slower.  Still, even at 9ns, it's perfectly fine, and still much better than the cheap signal generator on my desk (24ns) which is also much bulkier.

Practical use

As it happened, we needed to know the cable length in our W2CXM shack between a new location and the antennas on the roof.  I used this Schmitt Trigger Oscillator to measure it.

I observed 173ns.  We are using Beldin 9913 coax, which has a velocity factor of 84%.   Using 11.8 inches/second, at 84% that's 9.912 inches/sec.  With 173 nanoseconds that yields 1714.78 inches (142.898 feet) round trip.   That's about 71.5 feet to the end of the cable.

Kevin / WB2EMS used a proper TDR cable measuring device and came up with 72 feet.  So, we're definitely in the right ballpark.

Here's the oscillator hooked up to a small LeCroy 60 mhz oscilloscope up in the penthouse of my building.  That scope is nice and portable, which made it perfect for this task.

Note the boondoggle of adapters one can end up with, if your bag-o-dapters is missing a few things!  I made note of what to pick up at my next hamfest!

All in all, it worked great and served the purpose. It's small and easy to use.  I've popped it in my toolbox with my adapters and other "antenna and coax" tools.   Project successful!