Carel PA0CMU <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Garey,
Not only the demodulator circuit was changed.
1. Drake made some minor changes in the S-meter circuit. R1101 is replaced by
two 1N4148 diodes in series and CR1101 is replaced by a 4700 Ohm resistor.
2. My first generation board was missing the current limiting resistors R1187
and R1188 (feeding the audio preamps Q1111 and Q1113) causing loud clicks in
the speaker when switching to CW and AM mode.
3. R1112 was connected between base and collector of Q1103 in stead of biasing
the base from the +10V-DC (pin11/13) line. The effect was a short pulse to the
RTx relay when switching to CW mode.
All mentioned above is drawn correctly in the first generation schematic Fig.
2-22 2nd IF/Audio Board on page 2-63 of the Service Manual. Unfortunately my
board was not build like that.
I cannot say the TR7 sounds or performs better after the "upgrade" to a second
generation board. I just replaced the demodulator circuit by a MC1496.
Meanwhile I added a switched capacitor audio filter as described on my homepage
and placed it on top of the piggy back mounted PC-board holding the MC1496
circuit.
Here is a link to one of the pictures I still have to add to the "Drake page"
of my website.
http://home.wanadoo.nl/cmulder/tmp/dsc02639a.jpg
The upper board is the audio filter with MAX7400. The lower one is the product
detector. All build very compact to fit in the compartment just behind the fan
connector (changed to a 12V-DC type).
That filter is doing a very great job, clearing the high hiss in my headphones
when listening CW signals.
Carel - PA0CMU
http://home.wanadoo.nl/cmulder
> Date: Thu, 24 Apr 2008 18:58:35 -0400
> From: [EMAIL PROTECTED]
> To: [EMAIL PROTECTED]
> Subject: Re: [drakelist] 2nd IF Audio Board in TR7
>
>
> Garey Barrell made an utterance to the drakelist gang
> ----------------------------------------------------------------------
> David -
>
> I just had this conversation the other day...! :-)
>
> I don't think anyone knows for sure, but I believe the AF board change
> was made around s/n 6000 or so. Early 1980. According to Bill Frost,
> the 1496 was originally designed into the R-7, and was then adopted as a
> cost saving measure in the "second generation" TR-7. The IC replaced a
> hand wound toroid and a couple of alignment problems. It's pretty easy
> to spot in a TR-7, as it's right at the top "right" corner of the 2nd
> IF/AF board at the rear of the card cage. Version 1 has the toroid up
> there and a little U-shaped shield (yet another "costly" item not needed
> with the IC) while the Version 2 board has the IC and a few resistors.
> Personally, I doubt if you would notice the difference by ear. I have a
> couple of each, and can't say that "I" can hear the difference. Yeah,
> the 1496 has a wider dynamic range than the two diode PD, but the AGC
> kinda eliminates the advantage of wide dynamic range! I think it was
> purely cost saving, and a fancier set of specs couldn't hurt! :-) A
> dutch ham did a conversion by removing all the parts associated with the
> Version 1 and putting the 1496 on a small piece of perfboard mounted
> just above the original board.
>
>
>
> I'm sure we'll hear other opinions!! :-)
>
> 73, Garey - K4OAH
> Glen Allen, VA
>
> Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
>
> ----------------------------------------------------------------------
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