Re: [drakelist] 2nd IF Audio Board in TR7

[Garey Barrell]

Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Carel -

First, thank you very much for your informative website.

We seem to have once again run across Drake's penchant for hardware that 
doesn't "quite" match the documentation!  :-)   The only two docs I have 
for the 2nd IF/Audio board are the original(?) in a 1978 version of the 
Service Manual, and another Version 2 Dated October 1980.  The first 
schematic shows CR1101 and the second shows neither the diode nor a 
resistor.  Both schematics show the two diodes in place of R1101, but 
the second one shows the two diodes AND R1101 in series.  Both show the 
resistors R1187 and 1188.  Both show R1112 connected to the +10 VDC 
bus.  Unfortunately the only TR-7 that I currently have access to is a 
9k s/n with the Version 2 board in it.   Now I'll have to find an 
earlier one in one of the storage units!  :-)

Your link below refers me to your Host's "lost" page.  Perhaps it's 
referring to a TMP folder on your machine?  The SpectrumLab plots on 
your website certainly show the effect of that audio filter.

Thanks for replying!

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
<www.k4oah.com>



Carel PA0CMU wrote:
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
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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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