TS-180S - this is getting ridiculous!

I knew it was a mistake putting the lid on properly.

Having restored the functionality of the frequency display I decided to run the radio for a few hours over the weekend to evaluate performance further.   Still no tx, of course or any PLL locking on 28MHz but I knew that already ...

I was listening to a QSO on 7MHz this morning between a very strong Belgian station and a string of UK stations, most of whom I could hear reasonably well when I realised that the frequency seemed to be locked on to 7.150.0MHz.  Tuning either side by a kHz or two made no difference to the display.   I had a funny feeling about this, so I tuned up and down the band to find that lo and behold that the display was only displaying frequency to a 10kHz resolution!

To see whether this was a heat related issue I switched the radio off and let it cool down for half an hour or so, but on re-powering the fault was still present, so I have concluded that I have ANOTHER dead chip!  More than likely a duff 7490 in the counter chain, and that's TTL not CMOS, so another theory goes down the pan.

It is now only sheer stubbornness which is sustaining my interest in this project.  In all the years of building and repairing equipment, I don't think I've ever come across a project like this.   Trying to fix up a radio with numerous faults is one thing (and what I thought I was dealing with here), but fixing one which keeps breaking seemingly randomly whilst under repair is quite something else.   It's a case of either "nolum illigitimus carborundum", or chuck the bloody thing in the (recycling) bin!  I don't give up easily though.  What kind of trauma has this radio suffered in the past?   It's anybody's guess.

TS-180S Repair - Part 4 - That's More Like It!

Great progress to report, and more dead bits!

The frequency counter unit is now working properly.

The two DC-coupled "front end" transistors in the 40MHz amplifier section were suspected as being faulty and were replaced. No exact replacements or direct equivalents were to hand, but looking at the specifications of the original units (2SC460) I figured that more or less any old small signal RF transistor would do, and as I had a bag full of MPSH-10s in my spares box I used those. The pinouts are incompatible and I ended up mounting them under the board.

Anyhow to my great delight the frequency counter then appeared to work, albeit not quite properly. The frequency count was correct at the bottom band edge (e.g. 7.000MHz) but when I tuned a signal at 7.100MHz the counter read 7.060MHz, near as makes no difference. This was consistent across all bands. The counter was proportionally out of kilter receiving at 7.200MHz, the counter reading 7.120MHz

Owing to the frequency scheme in use in this transceiver, and the fact that basically the counter is fed with frequency of 40.0 to 40.5MHz as the VFO is tuned across each band I figured there was a counter timebase problem.   The microprocessor logic in the counter unit adds the base frequency for each band to the difference between the 40.0 to 40.5MHz and 40MHz producing the correct frequency on the display.

Underside view of Counter Unit

Investigation of the divider part of the circuit revealed a problem with Q5, a CMOS BCD counter (4018) which was producing an output of 83Hz instead of the required 50Hz. Ha! That explained everything!  The frequency being too high would mean a shorter gate time for the counter than is required, and hence the frequency count would be too low.  This signal goes through a further division-by-10 process (a 7490 TTL chip) before the 5Hz, or in the case of this faulty radio 8.3Hz gating signal is sent to the counter signal chain.

Oscilloscope display of output from faulty Q5

The repetition rate of the waveform should be 20ms not 12ms!

Below:- the replaced Q5 - the replacement has obviously been in my spares box for quite a while!

 This faulty chip has now been replaced, and the counter is now functioning correctly, and I'm feeling very happy with the way this repair is shaping up.

Here is the old girl on the "operating table" ..

I'm just about ready to see if the set transmits ...

Latest update - no it does't,  so it's on to the next fault (again!) ...