(Right-click on the schematic and "save image as" to download gif file to print.)
Credit where it is due Dept.:
The mods are basically stolen from Tim, WA1HLR, with some changes and additions. (See Tim's mods on the AM Window Web Site.) The mods include beefed up modulators, (6550's plug in place of the 1614's), elimination of the crummy audio driver xfmr, improved PTT, solid state power supplies with more C, a regulated and adjustable bias supply for the modulators, and (thanks to Randy-KK7TV), a variable PA screen voltage control to permit adjusting output power.
Mod #1: Remove the tube rectifiers and go solid-state on all supplies.
Mod #2: Replace all HV filter and bypass caps, increasing their values and voltages.
Mod #3: Remove HF rolloff caps C69, C68, C66, C84.
Mod #4: Eliminate R35, HV bleeder, and wire new low voltage 12VDC PTT circuit as shown on schematic.
Mod #5: Replace R3, VFO regulator input, with 18K-10 watt unit, mounted outside of VFO box.
Mod #6: Replace Bias Rectifier circuit with that shown on schematic.
Mod #7: Replace 1614 modulators w/6550's, and audio stages as shown on schematic.
Mod #8: Install PA screen voltage adjust control as shown on schematic.
For the HV rectifiers, I used 1N5408's, rated at 1000V PIV @ 3 amps. I used 2 in series in each leg to replace V12, the 6AX5 LV rectifier tube; and 3 in series in each leg to replace V11, the 5R4 HV rectifier tube. Things sure get a lot cooler with those bottles out of the picture! I replaced HV filter caps C78 with a 470uF @ 450V, and C77 with two 470 uFd @ 450V electrolytics in series, to handle the higher voltages that result from the solid state rectifiers. This also dramatically reduces ripple, voltage sag, power supply "bounce", and carrier shift with modulation.
In mine, I removed the audio preamp V7 since the Turbo-Ranger was optimized for use with an external audio processing chain which operates at a nominal line level of +4 dBu. The input to the unit is now balanced, using a Jensen xfmr, and the input connector on the rear was changed to an XLR. This allows me to use standard broadcast audio processors and limiters for improved audio, and easier experimentation. The xfmr also eliminates ground loops, and the JT-11P-1 model chosen has superb square wave response.
Be alert to the phasing of the feedback around the modulator. If you have it upside down, the modulator will take off and oscillate. If that happens to you, just invert the two leads coming from the feedback winding of the modulation transformer to the 12AU7 voltage amp cathode. (This has caught a few folks, sorry for not calling it to your attention sooner.)
The feedback EQ, (taken from the mod xfmr feedback winding), was flattened out from Tim's circuit to allow the best transient and square wave performance possible. He used additional HF boost in the feedback loop to cheat the high end response up thru the modulator. Although this may help flatten out the high end freq response, it also causes excessive overshoot on transients, or highly clipped audio input, (such as you would likely have at the output of an aggressive audio processor). Coupled with the bad low end response of the mod xfmr, the resulting severe tilt of the waveform causes lower average modulation. This is because you are forced to reduce overall modulation level to keep the overshoot peaks generated by the modulator from overmodulating the xmtr. The real source of the problem is the crummy modulation transformer and its imperfect match to the final. The real solution is to replace the mod xfmr, as Tim details in his mod article. I try to partially make up for the high end rolloff in the xmtr by EQing in the audio chain BEFORE the clipper that feeds the xmtr input. (A band-aid approach, until I can get a decent mod xfmr in there.)
Probably the most useful change made was the addition of a variable PA screen voltage control to allow adjusting the output power of the Turbo-Ranger over a wide range from about 8 watts to a max of about 45 watts. The added control was mounted behind the crystals knob on the front panel, (the xtal socket was removed), allowing easy access without defacing the radio. I needed this because I run the Turbo-Ranger into a modified Heath SB-220 amplifier to get a useful power output level into the antenna. I typically run at around 200-250 watts carrier out of the amp. (The amp could easily do 400 watts, but I'm only running mine on 120 VAC!) This takes around 12-15 watts from the Turbo-Ranger. I tune the Turbo-Ranger normally at full power into a dummy load, and then back down the screen voltage to lower the power to the level I need for the amp. Doing it this way results in much better audio performance. Most folks who have tried de-tuning the stock Ranger to reduce power have found the result less than acceptable. By adjusting the screen, the distortion and linearity are much better. In fact, the positive modulation capability at low power, along with the extra poop the 6550's give, is incredible. This thing will talk clean and LOUD!
When you do all these mods, and solid-state the HV supplies, the voltages will go up substantially. You must make sure your caps are rated sufficiently. On mine, with 120 volt line input, the low B+ goes up to 433V! The hi B+ goes to 670V! In transmit, they drop to 383V and 616V, respectively. You will find that all of the HV caps in the Ranger will have to be replaced to be safe. This includes all the tubular bypasses. In fact, I have taken to running my Turbo-Ranger with a Variac to set the mains voltage at 100-110V, to keep everything happy. A buck-boost xfmr may be in its future...
The end results of all this are much lower distortion, especially on the low end, less tilt through the modulator, greater high freq response, much more modulation power, and greater loudness. Just running barefoot with the power out at full tilt (45 watts) the Turbo-Ranger will easily do >120% positive peaks. At reduced power, (the way I run mine into the SB-220), the positive peaks are only limited by the power reserve in the amp. I disable all positive peak limiting in my outboard audio chain and just let the positive peaks fly. This can make for tremendous loudness without overmodulating the carrier at all. (That is, no negative overmodulation, which is VERY BAD!)
I receive consistently good reports regarding the audio with this setup. The goal was to bring the Ranger as close as possible to broadcast xmtr performance.
Willis, W9FGJ, has also done something neat to his Ranger that I will probably be incorporating into mine eventually. He has added a separate filament xfmr to keep the tubes lit, and switches the primary voltage to the HV xfmr on and off with his PTT, thus keeping all HV completely off until transmitting. Besides being safer and less stressful to the components, it keeps the rig cool except while transmitting. I'd probably prefer to keep HV on the VFO and lower level stages and just switch the PA and modulator, but this is easier. I'd also like to put in a step-start to reduce the stress on the rectifiers. Food for thought...
I'm also about to replace the 0A2 VFO screen regulator with a 150 Volt zener, and maybe regulate the VFO plate as well. The VFO wanders around a little bit on 10M, and stabilizing those voltages should reduce that considerably.
But first and foremost, I'd like to replace that modulation xfmr with a good, larger, multi-tap model to get that hi and low end response and distortion where they need to be. (If anyone out there has one, such as a Stancor A3893 or A3892, or Triad M-15 or M-12A, let's make a deal!) The more the square wave performance of the xmtr can be improved the louder it will play on the air. Louder is mo' betta!
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