Realistic HTX-100 Blinking 28.000 - Fixed!

I picked up a broken Realistic HTX-100 at a hamfest for cheap a while back and wanted to share what I found for the repairs.  The symptom I had was when powering it on, it would just blink 28.000MHz and none of the controls seemed to do anything.  I did some searching around and based on text searches only found old posts saying to send it in somewhere for repairs.  Obviously, these have been long since out of production so that is not a likely repair. My initial guess was a PLL unlock due to bad caps on the PLL board, but that guess was wrong.  I stumbled across a YouTube video about a failed repair of one that had a very similar symptom.  The person that made the video was not able to conclusively determine the fault as the radio started working more or less on its own, but he mentioned finding a metal fragment that fell out of the front switches and noted that if the PTT is held down when the radio is powered on, it will blink the last frequency it is set to and act like it is locked up.

This was as good of a starting point as any for me, so I set out tracing the PTT circuit and found that if there is no 8v going to the front display board it will invoke the PTT line. Long story short, I found a vaporized trace by the 8v regulator and a shorted capacitor (capacitor pictured below).  When the capacitor shorted, the PCB trace burned up like a fuse and took out the 13.8v supply that goes to the 8v regulator. This was capacitor C163 on my rig, but there are others that could short and cause a similar issue as well.

When trying to tack down where the shorted component was, I found it useful to use the PCB jumpers as "divide and conquer" points.  I would pick one, de-solder one side and see which side of it the short was.  Otherwise, you just end up scratching your head wondering which component in that part of the circuit could be shorted.

I'm sorry for not taking detailed pics as I went along, but I hope that someone may be able to find this useful in case they have the same symptom on their HTX-100.

On the Workbench: An Old Dell DImension

A little backstory...

(no, not that kind of backstory!)

This is an old Dell Dimension 3000 PC made around 2005.  I was given it about a year ago, and it appears to have had VERY little use.  It still has the original installation of Windows XP, complete with MS Office 2003.  This was a low end machine when new, and I never cared for these low end Dimensions even when new.  Why am I even bothered to fix this low end, antiquated machine machine?  Simple, this machine will serve perfectly fine running amateur radio logging software for my club (and has already done so in one contest). Also, because I can.

So, what's wrong with it?

When getting this machine prepped in time for the next club contest I discovered a bulging capacitor on the motherboard.  I remember this machine running very sluggish when it was last used (even considering its about 15 years old), and this capacitor failing may explain that.

Is this thing even worth the cost of a new capacitor?

Long answer: It depends
Short answer: No

I am in no way planning to spend $$ on a new name-brand capacitor.  I have a kit of no-name Chinese capacitors and will use one of them.  The original capacitor is a Nichicon 1000μF at 10 volts while my Chineseium replacement is rated 1000μF at 16 volts.

Notes on Testing Capacitors

 

Bad Capacitor

Good Capacitor

I have a fancy component tester that can tell me the Capacitance and ESR of capacitors.  I figured this project was a good excuse to demonstrate it.  ESR means Equivalent Series Resistance and is a good indication of what is a good or bad capacitor as a capacitor can appear perfect physically, and read proper capacitance on a basic LCR meter yet still be bad.  As you can see from the two readings, the bad capacitor in this case is painfully obvious as its capacitance is reading at less than 20% of its rated value!  I believe the original capacitor should be rated at either ±10% or ±20% tolerance.  This thing is reading over 80% off!  In regards to ESR, a typical ESR value chart can be found here.  According to that chart, a typical 10v 1000μF capacitor should have an ESR around 0.12Ω and can be roughly twice as high and still be good.  That wold put our high side tolerance at 0.24Ω.  It reads 6.0Ω.  That's not even in the same country, let alone ballpark.  Definitely bad.

Repair Success!

After soldering in the new capacitor (which was a real pain in the butt for my little 30 watt soldering iron) I reassembled the machine and powered it on.  It booted up much quicker than I remember it taking when I last used this machine.  It was running like brand new.  I suspect this capacitor has something to do with the IDE controller as I originally thought the painfully slow boot times were due to it being a old, slow IDE hard drive.  Of course, I have no idea how long that capacitor will last, but it should last long enough for another free computer that I save from the trash pile to take its place.

Tools and such

Below are some links to tools and supplies that I used in this project.  It is by no means an extensive list, but purchasing using these links will help me out at no cost to you.

Capacitor Kit
PCB Holder 
Component Tester