Long time - no post !
Well, I've been buzy, changed jobs (twice) and so any time I've had I've concentrated on home and fixing radios.
But now things have settled down a little, I will return to occasionally blogging about fixing kit that I've acquired - in order to help others and record how they were repaired.
TS-950SD
I found this lovely radio on the UK website hamradiodeals.co.uk which was out for a swap and clearly labelled as non working with an interesting fault.
After a little thought I contacted the poster (a lovely guy named Mike) and a deal was done for the transceiver.
I picked it up (never trust couriers even for a faulty radio) as whilst the fault might not get worse the physical condition of the radio may well do so !!
The fault as described was the receiver had a 60db carrier (FSD on the S meter) after warm up and no signals could be received regardless, the TX was perfect - full output on FM and SSB. On the bench sure enough on switch on there was no receive (also no FSD on the S meter), TX was indeed perfect in all modes. So the description was (unusually in many cases in my experience) spot on :)
So left the transceiver on for a few hours and sure enough after a while the s meter went to FSD. During the warm up I checked controls etc - all were fine with the exception of the RF gain control (which apparently didn't function) and a total lack of audio / signal receipt. I also checked the sub receiver - which did have audio and did receiver albeit at a poor (insensitive) level.
So I'd already down loaded the service manual and familiarised myself with it and so I started to look at the receive chain / receiver gain control parts of the transceiver due to the symptoms noted above.
I checked the audio amp by injection a suitable signal into it on the main receiver input to the AF amp and sure enough it worked fine. So onwards.
I doubted that it was a PLL problem as the sub receiver (which shares elements of the PLL) was working fine with low receive, but thought I would check, by carrying out an alignment as per the manual all was fine.
This looked more and more like an AGC type fault (as there was no RF gain control operative) effectively the RX looked liked it was turned hard off when the fault was present. I checked the manual to see where I could check the voltage and noted that there was a couple of nice test points used on the alignment that would give me a good start (see below).
So Fluke to fore, measured on main AGC and to find that it was way off (can't remember the actual voltage and didn't make a note - D'oh!!!) , the sub rx was sligtly off but was adjustable to the required level (2.8V).
Making progress :)
So started to measure voltage in the AGC area and established that they were not correct in the fault state - also they weren't stable (to be expected in an agc circuit - but these varied by volts not mv and were not signal related!). (circuit below)
the voltage (13.5, 14.5 circled in red) were all over the place and are derived from the 15v line (red on the right).
So why is the 15V line variable and where does it come from ?
Answer it comes from a sub board on the control unit called the trx board (circuit below)
And sure enough its a right pain to get to :( - its location is below
So removed the Control unit and decided to remove the trx board from the unit (thank you decent desolder station - makes this a breeze!) to take a look at the board - sure enough R156 and R157 solder flow was poor (in fact when I touched R157 it fell off the board !!
The components check out OK but I decided to replace them both just in case, I also re-flowed the rest if the components on the board as well - just to be sure.
Replaced the trx unit onto the control unit, and ensured that both that unit and the one close to it were bent away from each other to help with any heat issues - although I'm sure sooner or later (an certainly in hot climates) this fault will re-appear, the control unit is located behind the front panel as so there is not much (if any) air movement and so this almost certainly is the root cause. - I considered a small fan but in the decided that this wouldn't really work effectively.
I re-assembled the control unit with all its connectors, rechecked all and then tested again.
First thing I noted was there was noise in the receiver (I forgot to turn down the AF gain!!) and it blasted me :), secondly (when I turned the audio gain down!) - was the the RF gain control now worked correctly - ie as I reduced it the s meter slowly lit up until it was fully lit (ie fully reduced gain).
I checked the test point on the signal unit and it was still wrong (to high at 6.3V) but I could now reset it to the correct level (2.8V).
The receiver was now functioning correctly - I then did a full re-alignment of the transceiver as per the service manual - and it is now totally within spec and working exceptionally well.
Hope this might help others in the future.
Job Done :)
73s
Chris
