Improving the Drake TR-7 by Scott D. Prather, N7NB (ex-KB9Y)
Reversible Modifications Enhance Performance
Please Note: The mailing address included on the first page of this article is no longer current. Please address all correspondence to Scott Prather at: sdprat@worldnet.att.net.
Abstract.
The Drake TR-7 is an outstanding all-band, solid-state transceiver that was a trendsetter
when it was unveiled to the amateur radio market in 1978. However, the TR-7 was not
without its problems, some of which included:
 | Poor receive performance below 1.5 MHz |
| No full break-in (QSK) CW operation |
| Compromised signal/noise ratio when using narrow CW filters |
| High cooling fan noise, and the stock fan operates only when the radio is powered from an AC source |
My article, which originally appeared in the Summer, 1992, issue of Communications Quarterly, addresses these four problems with modifications that require no drilled holes, making them 100% reversible.
| Notes
Improving the TR-7 is reprinted here in its entirety, and it appears on the Web with the permission of Communications Quarterly. I scanned the original 19-page paper and converted it into an Adobe ® PDF file. The size of this file is approximately 1.9 MB, so it will take a while to download if you are using dial-up Internet access. For example, at 28.8 kbps it took me about 14 minutes to download the file. |
| Component Availability
At the time that this article was written, all of the components required for these modifications were readily available. However, there are a few components that are currently difficult to obtain. The most difficult component to locate is D2005 (MA4P4002D), a stud-mount, high-power PIN diode manufactured by M/A COM. It's my understanding that this diode is no longer available from the manufacturer, and I've been unable to locate a diode that I'm convinced will function as a substitute. The MA4P4002D has the following specifications at 25 degrees Celsius: |
Series Resistance @ 100 mA forward current: 0.5 ohms
Carrier Lifetime: 6 microseconds @ 100 mA forward current
Voltage Rating: 200 Volts
Power Dissipation: 15 watts
Initially, my decision to use M/A COM's stud-mounted PIN diode was driven by the requirement to support 100% duty cycle operation, such as RTTY or SSTV. However, for low duty-cycle modes such as SSB and CW, I have successfully used the MA4P4002B axial-lead PIN diode with a small heat sink on the diode body. Unfortunately, this diode is not readily available either. Generally, any axial-lead PIN diode capable of dissipating about 5 watts and meeting the series resistance, voltage rating, and carrier lifetime specifications listed above should function in a low duty cycle application at the 150W power level of the TR-7. I will continue my search for a substitute, and I'll post my findings here. If anyone has a suggestion for a substitute, please send the information to me at the e-mail address listed above.
The second component that's difficult to obtain is reed-relay K2001, manufactured by Magnecraft. I've been unable to locate a source for this part, however, it appears that an EAC P/N 20-1121-30 reed relay should work in its place. The primary difference between the original Magnecraft part and the EAC substitute is the lead pinout. The EAC relay has short in-line pins, while the Magnecraft part had longer leads which would permit a "triangular" PC pad configuration at each end. Also, the NC contact on the EAC part is on the opposite side of the relay from the Magnecraft part. However, minor redrilling of the PC board along with a jumper to the NC contact will solve these minor compatibility problems.
| Miscellaneous: A minor change in operation is required if a TR-7 is modified with my QSK card and subsequently used to transceive with a Drake R-7 receiver using the Drake 1548 cable. Because the QSK card RX Mute line only functions when the TR-7 is turned on, the R-7 will be muted if the "Mute" button is depressed and the TR-7 is turned off. To use the R-7 when the TR-7 is off, just release the R-7 "Mute" button. |
| Errata: Although I carefully reviewed this article prior to its publication, in a project of this scope some errors are bound to occur. The known errors are: |
Page 22, Figure 3: Two minor labeling errors exist in this schematic. There are two capacitors labeled C2002, one connected to point "C" and the other to the drain of Q2002. The capacitor connected to point C should be C2006. Also, there are two capacitors labeled C2005, one connected to point "I" and one from L2002 to ground. The capacitor from L2002 to ground should be C2007.
Page 24, Figure 6: The RX coax ground connection point (K) is shown incorrectly. This ground should be tied to the RF ground point that comes up to the NO contact of relay K2001. This point is just to the left and below the ground point (K) shown in Figure 6. Photo E on page 29 details the correct location for (K).
Page 32, top left-hand column: The wires connected to points F and G on the QSK card were reversed. The list of wire colors should read as follows:
4) Connect the tan wire to point D
5) Connect the pink wire to point E
6) Connect the red wire to point F
7) Connect the blue wire to point G
Page 37, Fan Control Parts List: Although it's not specifically stated, temperature-sensitive zener diode D4001 is in a type TO-92 case. Also, C4001 should be 1.0 m F, 35 volts, not the 25 volt capacitor listed.
If anyone has questions concerning the modifications in this article or availability of components, please contact me via e-mail at sdprat@worldnet.att.net.
73
Scott Prather
N7NB
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