Mark,
On Sun, 11 Feb 2024 11:02:15 -0800
"atlasstuff" <g4fph@...> wrote:
@Mike: Thanks for you post. I hope you are in good health. I will send you an e-mail regarding the power supply board. TI and others have some amazing switching regulator IC's available these days. If I had time for a design challenge, it would be to have a go at replacing all of the linear regulators with them. The +24 and -5 Volt rails must run hardly any current. Has anyone measured the current draw on the +10 and +5 Volt rails. In any case, must be less than 5-Amp and 1-Amp respectively.
The +24 supplies a boost voltage to the PLL for faster lock. The -5 back-biases the PIN diodes on the IF filter board and typically measures arounf -3 to -4. Not very critical it seems. I wouldn't go nuts about replacing the regulators. The original design is very good. There was a blurb in QST years ago about adding some bypass caps to the regulators to reduce noise. It may be in the group file section but if not I can probably dig it up.
@Group: I now have the TR-7 transmitter working OK. The Drake 7077 mic that came with the radio sounded somewhat average, so I plugged in the Shure 444 that I use on my other TR-7. It didn't work - what!? The reason was that this low s/n TR-7 did not have the 470k resistor installed between pins 1 & 4 on the rear of the microphone socket. It now has, and the 444 sounds good.
I was going to mention that eventually. I have a 7077. Not impressed with it. Most any other mic sounds better in my experience.
The transmitter produces 100 W RF out up to and including 20 metres. On 15 metres, it's down to around 85 W. On 10 metres, it's down to about 25 W, so some work to do. Despite having a low s/n of just over 1000, this TR-7 has the MkII pre-driver board in the PA, whic I guess is a good thing.
The ALC threshold is lowered by a pair of resistors on the band switch for 15 and 10 meters due to lower gain of the transistors at higher frequencies. A good PA deck should easily do 140+ on 20m and very close to 100 0n 15/10 when adjusted to service manual spec. The version 2 predriver was a standard upgrade by Drake service. Apparently they had a lot of trouble with the original.
I have some 'talkback' / low-level undemodulated SSB getting back into the speaker when I transmit. Think I read somewhere that the early models were prone to this and that separating out the Hi- and Lo- Amps lines in the DC power lead (like Atlas did!) helps. I will try this. Any other tips for getting rid?
That somewhere is probably here:
Also check that you have a 470uF capacitor on the bottom side from pin 10 of the Power Supply board to chassis.
I did have a nice find when I went digging to do the transmit carrier balance - an AUX 7 card installed, with the three Drake diode modules fitted for full 0 - 1.5 MHz receive coverage, plus a previous owner had made some, Christmas-tree style, modules for 30, 17 and 12 metres. So, I did have a bit of luck after all the pain!
Lucky score!
--
73
-Jim
NU0C
Jim Shorney