Hi Barry, Rick and everybody!
I'm happy to report that I finally fixed my problem(s) with my Corsair II.
The problem started as a lack of sensitivity on the higher bands and
then the TX did not switch...
First of all, how to disassemble the LPF-TR board: just partially
remove the back cover (the one with the black final heat exchanger),
remove also the aluminum small rectangular plates on top and bottom.
Now you can access the screws fixing the LPF TR (and its
daughterboard). After unscrewing the 5 screws, unplug the 4 connectors
(66 and the other ones on the daughterboard). Now you should be able
to pull the boards slowly without even removing the band changing
shaft. Of course doing it would make things easier. Be careful to the
short coaxs going to the final amp and to the antenna (disassemble the
SO-239).
Of course, don't rotate the band shaft while the board is outside the radio!
You now have to separate the two boards by unsoldering the
receive/trasmit lines (in my case yellow and blue respectively): these
lines go to ~10V when receiving/trasmitting and the other one goes to
zero.
I checked D1 and D2 (poor man's PIN diodes) and they were fine, then I
verified the 4 rectifying 1N4148, all fine! Let's see the transistors,
Q1 was OK but Q2 was totally burnt inside: all open junctions. Be
careful! Q1 is indicated as a NPN in the TEN TEC schematic (both for
the Corsair I and II).
I decided to use what I had at home: a 2n3906 PNP for Q1 and a 2n3904
for the NPN Q2.
The resistor going to the relay was slightly burnt so I changed it too.
This seems simple but I had to put the board in place a few times and
the Corsair did not TX nor RX at this point!
After one night of work I realized that, since the relay was working,
the only cause for no RX and TX could be a short. It turned out to be
the coax cable from the final amp that fused the insulator due to the
several soldering operations I had to do...
In the meantime I was also able to short the RX line to GND so I had
to replace Q6 and a 10Ohm resistor on the control board... be careful
while measuring voltages with the board outside the radio ;)
I also removed the OPTIONAL HIGH PASS FILTER since I have no high
power MW TXs nearby.
So, now I could do it blindfolded :D
Hope it will be useful for anybody working in the same area of the radio.
Ciao and 73,
marco
IS0KYB
2016-06-08 14:01 GMT+02:00 Barry N1EU <barry.n1eu@gmail.com>:
> Good info Rick. If anybody ever removes the Corsair's LPF-TR board in the
> future, please take plenty of photos to document the process and we can
> post on tentecwiki.org. Actually, this goes for any
> complex dis-assembly for any of the TT radios.
>
> 73, Barry N1EU
>
> On Wed, Jun 8, 2016 at 7:44 AM, Reeve Family <reevefamily@cogeco.ca> wrote:
>
>> Regarding Marco's IS0KYB posting about the Corsair II weak receive
>> condition.I had the exact same issue with my Corsair II that I have owned
>> since new. Like Marco I suspected the problem to be diodes related to T/R
>> switching, however the problem turned out to be in the High Pass Filter
>> circuit on the LPF-TR board (80969). Symptoms were identical to what Marco
>> is indicating, and I found that receive strength was normal if I connected
>> the antenna to the 'RX ANT' connector on the rear panel, which effectively
>> bypasses the SWR and LPF-TR boards (and the High Pass Filter).
>>
>>
>>
>> My original TEN TEC manual has notation showing "OPTIONAL HIGH PASS FILTER
>> NOT INSTALLED" . However once I got the LPF-TR board removed I discovered
>> that the "Optional" High Pass filter was in fact installed contrary to the
>> notation on the schematic. Trouble shooting eventually led me to
>> disconnecting and bypassing the High Pass Filter and receive performance
>> issue was resolved. I did this by opening the circuit at the input to the
>> High Pass Filter and bypassing it with a capacitor between Q2 collector and
>> the RX connector to avoid interfering with operation of Q2. It has worked
>> fine ever since and no noticeable issue without the "Optional" High Pass
>> Filter.
>>
>>
>>
>> The biggest challenge in all of this was getting the LPF-TR board out of
>> the
>> chassis which required careful removal of the Band Switch shaft while
>> ensuring that all of the Band Switch wafer sections were in the correct
>> position when reinserting the switch shaft.
>>
>>
>>
>> Like Marco, I would also like to know of a suitable PIN diode replacement
>> for the TR switching on the board as TEN TEC did not include details of
>> those components on the LPF-TR board schematic. Fortunately mine is still
>> fine after 28 years but I suspect one day I will need to replace those
>> diodes.
>>
>>
>>
>> Hope this helps.
>>
>>
>>
>> 73,
>>
>> Rick VE3ORY
>>
>>
>>
>>
>>
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--
marco cogoni
CRS4
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