Friday, April 30, 2021

Repair of a (lightning struck?) Yaesu FT-1000 MP Mark V

The background - and initial assessment:

A month or so ago an acquaintance of mine in the local amateur community asked me to take a look at his radio - a Yaesu FT-1000 MP Mark V.  While I don't routinely repair other people's radios, I decided to make an exception, as I'd known this gentleman for decades.

The complaint was "The receiver is dead" - which is a very broad assessment, but it was an obvious starting point and upon putting this rather large, ponderous piece of equipment on my workbench - which, itself proved to be a challenge - I noted that it took about a 0dBm signal to attain an "S-9" reading:  Yes, the radio was deaf, to the tune of around 70dB!  Knowing that the antenna A/B relay and/or antenna tuner could be a problem with this particular radio, I tried different configurations - even the (differently-routed) rear "RX Antenna" jack - but no difference.

Figure 1:  A blast mark!
Getting it apart:

This radio's technology and construction is what I would refer to as "transitional" -  mostly reminiscent of 80s Japanese radios in that there is a mix of through-hole and surface mount, and the boards are mostly interconnected with a myriad of white wires with unmarked plugs on them that go everywhere in the case - with the occasional gray coaxial cable that use the odd board-mounted plug-in cables.  Fortunately - unlike some of those older radios - the boards are mostly double-sided epoxy rather than single-sided paper phenolic.

Of course, the main receive RF board was buried under the very large heat-sinked power amplifier assembly, so I first did a test on the "sub" receiver board, which was accessible:  That receiver seemed to be reasonably sensitive when I injected a signal directly into it, but there was no sign of RF getting to that receiver via the radio's rear coaxial cable connector.

The next, obvious step was to remove the PA board - which, like many things about working on this radio,  was a real pain:  Only four screws, but two of them were buried under a mass of white wires -  I pulled the amplifier module out from the radio with trepidation, knowing that I'd eventually have to get it back together.

A visual and an olfactory inspection of the receive board - before removing it - was done, and nothing was obviously amiss, so I removed a bunch more screws and cables - marking them as appropriate (you will hate life if you don't mark where they went as you remove them!) I was able to pull the board away and immediately saw the first indications of the problem:  A very obvious black mark on the aluminum chassis under the board and a corresponding area of char on the board itself (see Figure 1, above and Figure 2, below).

What might have happened:

Figure 2:  Carbonized crater in the RF board.
While it is difficult to determine what, exactly, happened to this radio, I suspect that it was likely a nearby lightning strike that did the initial damage, with peripheral damage occurring later.  In inspecting the radio and referring to the service manual, which I found online, I saw that capacitor C1278 on the "RF" unit - and its related circuit board trace, and a portion of the board itself - had been wholly transformed into the black mark on the aluminum chassis.  The fix for this was pretty simple:  Scrape away all of the now-conductive PC board carbonized residue and replace the vaporized surface mount 0.1uF capacitor with an axial-leaded unit to both replace both the component itself and the trace as seen in Figure 3.

Knowing that this was not likely to be the only problem, I half-installed the board again and checked the receiver:  Very slightly less terrible - but still deaf as a post, but with access to the board, I cranked up the signal generator to 0 dBm and started poking around with the oscilloscope to see where RF disappeared.

Figure 3:  New cap, carbon excised!
Fortunately, I was seeing RF at the input to the board - and even though many of the key probing points were on the (inaccessible!) bottom side, I found a few places where I could test on the top and discovered that the RF signal stopped at D1056, a PIN diode found at a low-pass filter stage in the RF path.  Pulling the board again I started probing with the multi-meter and discovered that not only was this PIN diode completely open, but R1157 and L1086 - both part of the DC path of this and (the still-working D1055) were open:  I had a suitable SMD replacement for R1157, but used a small, molded 1mH inductor for L1086 to accommodate the damage due to a blown-off pad and trace that hadn't been immediately obvious and I used a much more rugged PIN diode than the original to replace D1056.  While I was at it I did more probing around and found that R1150 - part of what appears to be a DC drain on the antenna circuit - was also burned open:  Because this resistor can only be "reached" via the 1 mH inductor in series with it, this meant that it was not likely RF that blew it up, but rather a transient with a strong DC component - or at least a low-frequency AC component - maybe lightning?

Initial re-testing:

Putting the board back in, temporarily, I re-checked the receiver sensitivity and found that it was "OK", in that I could touch a screwdriver and put it in the rear-panel RF connector and hear signals, but a quick check with the signal generator indicated that something was still amiss as it seemed to be off by about 15dB based on the specs in the alignment procedure.

At this point I decided to check the transmitter and to my gratification, I was able to get about 90 watts out of it.  My initial satisfaction was short-lived, as I soon realized two three things:

  • I should have been able to get at least 150 watts out of the transmitter.
  • The SWR indicator on the radio was showing a mismatch, with the tuner bypassed, into a known-good load.
  • I smelled epoxy smoke from the "RF Unit".

Figure 4:  Original TX isolation relay - welded!
At this point a felt a bit silly:  I should have checked to see that the unit had a functional TX/RX isolation relay - and a quick check with a signal generator and probing with the oscilloscope told me that it did not, the apparent culprit being  RL6414, a reed relay on the "Tuner-Main" unit that was clearly welded - a fact soon verified by multimeter.  The radio's power amplifier had dutifully cut back its power because it was seeing a mismatch caused by the receiver being connected at the same time.

At this point I again removed the RF unit and replaced D1056 (again!) - the apparent source of the smoke that I'd smelled and - without transmitting - restored the operation of the receiver.  In reviewing the service manual and online forums, I discovered that the failure of RL6414 was semi-common, and also that this particular relay - seen in Figure 4 - was difficult to source.  Nevertheless, I sent an email off to Yaesu Parts to find out.

A few days later I'd heard back from Yaesu:  This relay was available - but it would take 6-8 weeks.  In the meantime I'd tried to find an exact replacement elsewhere, but to no avail:  The original relay had a non-standard pin-out and was a brand that was simply not carried by U.S. parts suppliers - and I couldn't be sure if this particularly relay was still made!  In speaking with the owner of the radio I gave him two options:  Order the part and wait 6-8 weeks, or get a more common part and adapt it to fit:  He opted for the latter, so I placed an order with DigiKey, set the radio aside and waited.

It's worth noting that RL6414 is an SPDT relay with the RX signal path connected in the "Normally Open" position with the "Normally Closed" position grounded - that is, the relay must be energized for the receiver to be connected to the antenna.  If the radio is turned off, there is no direct path, so whatever "killed" the receive, must have either happened with the radio turned on, while it was receiving, or it was sufficiently energetic enough to weld the relay and, apparently, blow away the ground "N.C." contact:  Yet more evidence of a "high energy discharge" from lightning.

Resuming work:

Figure 5:  New relay on homebrew carrier board.
About a week later I again had some time to work on the radio and the relay from Digi-Key had arrived.  This relay - a Comus BFH-1C-12C (Digi-Key P/N:  1835-BFH-1C-12C-ND) was an actual RF reed relay with decent voltage and current ratings - but the pin-out was different from the original, requiring me to construct a simple "carrier board" to reroute some of the connections of the relay footprint to match the tuner board:  With plenty of headroom, the extra height of the combination was not an issue.

At this point, I will note again how difficult it is to work on this radio - highlighting, in this case, the antenna tuner.  This module consists of two boards, face-to-face, meaning that no components are accessible unless they are separated.  Unfortunately, there are several wires that appear to have been pulled through from the "lower" board (with the tuning capacitors) and then cut short - and none of the other wires were any longer than they absolutely needed to be.  Disassembly was pretty easy (especially with proper desoldering equipment) but re-soldering the short wires was an exercise of patience and the careful manipulation of small tweezers and screwdrivers to try to align all of the wires simultaneously without causing one or more of them to pop out of place!

Figure 6:  New TX isolation relay - looks right at home!
With the TX isolation relay (RL6414) once again functioning I found that the transmitter's output now exceeded 150 watts with no reflected power on the radio's meter - and no smoke!

Alignment - and more problems:

Not having part of the transmitter signal path in parallel with the receiver input helped the sensitivity a bit, but it still wasn't right so I did a bit more checking with the signal generator and scope, finding no-where that things were obviously amiss on the RF Unit - but I did notice that the secondary receiver was more sensitive than the main:  Since they share the same signal path that more or less ruled out an obvious problem with the RF unit when operated in the normal fashion so I decided to perform a realignment.

To my surprise, the I.F. stages in both receivers were terribly out of alignment - particularly on the main receiver - and after finishing this, the receivers "woke up" and met specifications:  I was surprised to see this on a modern receiver and am at a loss to explain it.  It seems unlikely that it detuned itself with age, so it was either not properly aligned at the factory, or someone else tried to "improve" it.

In performing the alignment steps, I then "discovered" that the "VRF" circuit - an electronically-tuned preselector - was inoperative, about 25 dB of excess attenuation:  This problem was traced back to being caused by RL1005 having welded contacts, not allowing this preselector circuit to be fully switched into the signal path.  This  not unexpected, as this is in the same signal path as the vaporized trace and capacitor.  Fortunately, these exact relays were readily available from Digi-Key as well.

All's well that ends well:

The replacement of RL1005 turned out to be the last problem that was found and the radio easily met its specs when the job was done.  The reassembly of the radio was, as expected a bit of a challenge:  The inboard mounting flange of the PA unit shares the same channel as dozens of thin, white wires and there is practically no visibility as one tries to maneuver the screws into place while trying to avoid them getting knocked off the magnetic driver while, at the same time, trying to make sure that none of the dozens of small, white wires get pinched in the process - all the while trying to align to invisible holes!

In the end, the radio was handed off to the owner, with the recommendation of added lightning protection or, at least, disconnecting things when not in use - good advice for anyone!

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