Friday, 25 March 2011

The Changing Face of RoPoCo - I'm hopping mad now!

After a long hard week working away from home, I returned this evening to find the latest RadCom magazine waiting.   It is somewhat more pleasant reading than the contents of a brown envelope from HM Revenue and Customs also awaiting me, but upon skimming through the Sport Radio section the word RoPoCo leaped of the page and grabbed my attention.

You may recall from an earlier post that I wasn't too impressed with the change of format of my own favourite contest, I'm even less impressed at a little piece of logic which goes, and I quote : "... participation in RoPoCo1 has generally been in the 50s, and RoPoCo2 in the 40s ...".    They have made the main format change to RoPoCo1!   Go figure ...

Well  "hopping mad" might be just a bit of an overstatement, but I'm certainly puzzled!

Saturday, 19 March 2011

Bringing the TS-180 Back from the Dead - Part 1: The Darndest Fault!

Some little time ago I purchased a "Spares or Repair" Trio-badged TS-180S from an eBay Trader, the price I paid reflected its non-working condition.    I didn't know very much at all about the TS-180S at the time but fancied a challenge.

As I've put my Eden9 70MHz transceiver on the "back burner" for a while and cleared one or two other repair jobs, I finally decided that the time was right to make a start on the '180S.

A preliminary look under the lid on the day the radio arrived revealed that the radio was totally u/s.  The famous Trio/Kenwood "dots" on the display on all bands, and a quick poke around with a DVM revealed that the regulated outputs from the "AVR" board were all over the place.  The whole thing smacked of multiple problems so I was anticipating a bit of a struggle.  The radio may yet prove to be Beyond Economic Repair, only time can tell if that is going to be the case.

Forearmed with this knowledge I decided to have a look at the AVR board for starters.

This consists of three regulator circuits.   One is a DC-DC converter which produces a light duty negative rail.   This appears to be working fine.

The other two parts are shunt regulators (oh dear, I dislike shunt regulators!) producing 8 volts (adjustable)  and 8.1V (non-adjustable) respectively.   These were producing 5 and a bit volts which didn't respond to adjusting and 8.9 volts respectively but are independent of one another save for the fact that the unregulated input rail is common to both.

As it seemed to be the most out of kilter I decided to look at the 8V adjustable supply first.

Having scribbled the circuit out on a pad and made some voltage measurements and jotted them down I homed in on what I felt would be a likely candidate for being faulty.   Shunt regulators are pretty horrible circuits in my view, not so much due to their complexity (they aren't usually all that complex) but due to the way the various stages (voltage reference, error amplifier, regulator driver and regulator) are wrapped up in a tight loop and DC-coupled, fault finding on them can be tricky as a fault in one part of the circuit ripples right through the circuit cutting off or destroying transistors and causing Magic Smoke to be lost from the system.

I didn't appear to have any overstressed components in my circuit, but I certainly appeared to have transistors which were just acting like resistors so I took a punt on the regulator driver being a suspect worthy of investigation.   What I found amazed me.

I removed (or so I thought) some solder from the PCB pads under Q5 the suspect device and tugged on it from the top, and away it came fairly easily.     However I noticed that the transistor legs seemed to be somewhat corroded, surprisingly, and I cleaned them off a bit with my pliers and tested the transistor on my DVM.  It looked absolutely fine.

Well I wasn't 100% convinced that Q5 was the culprit, and with it removed from the circuit, it would make the results of some other resistance tests on the remainder of the circuit easier to interpret, so I wasn't too discouraged.   However, when my attention returned to the board I noticed that I had actually de-soldered the wrong components, probably due to my not having been able to find my near-distance glasses!    The corroded remains of the transistor legs were still firmly attached to the PCB.

I managed to find a similar transistor to the original Japanese one in my junk box and eventually after some mental gymnastics with the different pinout replaced the original unit and re-soldered the remaining components.

The 8V regulator now works and is adjustable, so the original transistor must have had such badly corroded legs that it must have just basically appeared as a resistor network to the circuit.   I've never come across anything quite like that before in my fault-repairing experience.

Even better news, with the 8V supply restored, the radio is now showing some signs of life.   The PLL appears to lock on some bands (10,14 and 21MHz, though not across the whole band.

Nothing is being received even with the PLL locked, but it is a start!

I still have to figure out where the corrosion which ate the legs of the transistor came from - perhaps something liquid was dropped through the grille of the external case (this board is directly under the vent grille as it generates a reasonable amount of heat)?

Sunday, 13 March 2011

Huff and Puff Stabilisers - they really DO work!

In an earlier post I alluded to the fact that I was building an X-lock by Cumbria Designs with a view to incorporating it into my 30-year-old Yaesu FT-107M as that particular radio has (or had) a bad case of "driftitis".

X-Lock is a software based derivation of the "Huff and Puff" oscillator stabilisation technique whereby the frequency of an oscillator is sampled, compared with a crystal or other high stability reference, sampled again and frequency nudged in the opposite direction to any detected drift.

A couple of sessions with the soldering iron saw the kit completed without any issues, and then followed quite a bit of "circuit-bashing" (study of schematics!) plus reading up of other implementations of X-lock into old equipment.   After all this I had on the back of an envelope a plan for interfacing the two pieces of equipment.

Last weekend I lashed up a test which looked promising, and finally, this weekend the project was completed and I now have an "X-locked FT-107M".

The X-lock fits very snugly into the radio, and all of my interconnections are such that no mechanical or electrical changes have been made, and it could all be removed without a trace, if required.

This will all be written up properly in due course and posted on my web site - here and now isn't the place for detailed explanations, but the following two "Argo" screen grabs illustrate just how effective the unit is on received signals.

They both show reception of the 14MHz beacon IW3ICH and were grabbed withing minutes of each other, one with X-lock controlling the frequency, and the other with just the bog-standard built-in circuitry in control.

The above trace is with X-lock disconnected, in other words how the "vanilla" FT-107M performs even after the radio has been powered for some considerable time.
Now look at the same signal with X-lock in charge :-
I would say that's quite an astonishing result and the screen grabs speak for themselves!

One of my long-term objectives has been to improve the frequency stabilityh performance, if possible of my old radio gear to the point where they could be used without any embarrassment on modern modes such as WSPR, and I would say that the X-locked FT-107M is now "good to go" on WSPR.  I just might try it today ...

Thursday, 3 March 2011

Thank Goodness for Electrolytic Capacitors (they keep lots of folk in work!)

I'm on a bit of a roll as far as fixing stuff goes.

The latest bit of gear to succumb to my endeavours is an old D-Link DE-812TP Ethernet hub rescued from the scap pile at work and which had failed in industrial use. It was being discarded as being "beyond economic repair".  Had the unit been somewhat more youthful it may well have gone back to the company's repair shop but as it dated from 1997 it was regarded as "consumable" and should have been destined for recycling.  In a previous life (or so it seems) it could well have been down to me to repair the hub myself!

Seeing an opportunity to acquire a potentially useful piece of equipment at no cost, and, frankly, fancying the challange,  I opted to have a quick look at it, as having seen the fault symptoms I thought there was a reasonable chance of making a repair.

The fault symptoms were rapidly flashing LED indicators (ALL of them were flashing) which strongly suggested a power supply fault.   There appeared to be no blown fuses either, a good sign.

Opening up the unit I discovered a small switched-mode power supply on a separate sub-board bearing a strong resemblance to a standard computer type supply, albeit a miniature version, without cooling fans and the like.   The general arrangement looked very familiar so confidence was high.

The first step was to prove that the main motherboard was OK, so I disconnected the built-in supply and provided a temporary 5V feed to the motherboard which immediately sprang into life with "normal" indications on the LEDs.   I was now more confident than ever that the PSU was the culprit so out it came for a closer examination.

Not having the circuit I had to make rough guesses about where and what the various circuit sections were, but I quickly identified what was probably the main capacitor filter section for the +5V supply and decided to test the reservoir capacitors with my Peak Electronics ESR60 Capacitor Analyser.   Years of experience of repairing electronic circuits teaches one to suspect electrolytic capacitors before anything else!   Strange readings were obtained with the capacitors in circuit (this is often due to other components connected to and across the capacitors and doesn't necessarily mean that there is a problem), so I unsoldered and removed them from the PCB in order to check them out of circuit.

See the photo below.

Once capacitor came out minus one of its legs (not a good sign!) and the other measured "Low Capacitance" on the meter.  They were both branded as 1000uF 16V units.    There was possibly some evidence on the PCB in the form of a small light brown stain resembling a drop of dried out spilt coffee that there may have been some electrolyte leakage, but it was clear that both of these capacitors were u/s.   As you can see from the photo they were also branded as 105 deg C capacitors so it would have been reasonable to expect a long life from them.    Still the hub had been in service 24/7 for around 12 years which I suppose is reasonable.  Around 100,000 hours of  continuous operation before failing.   As the two capacitors are connected in parallel in this circuit one can ponder that perhaps they had been dying gracefully for some considerable time, but one never knows.

Happily, the capacitors were replaced with new units purchased cheaply on EBay, and the PSU and its parent hub are now back up and running, seemingly as good as new.