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VX-6000L 37-50 MHz Mobile
To 6 Meters
By John Haserick W1GPO
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Converting the Vertex Standard VX-6000L 37-50 MHz Mobile To 6 Meters By John Haserick W1GPO |
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The VX-6000L mobile is rated at 120W with 22A draw 50% duty cycle, over temperature and SWR protection, so it seemed to be a candidate for repeater service on 6m with the addition of fans, a ventilating top cover, and extra heat sinking. This turned out not to be a good idea, basically because it turned out that the SD1405 output transistors were designed for high frequency (up to around 30 MHz) applications, so while OK for low band, their efficiency drops on 6m. So after about 3 months of repeater usage at 120W output, the PA transistors blew, also taking out the push-pull drivers, and not an easy fix. Brand new SD1405 transistors are almost $70 apiece.
As a follow-up, it turned out the most probable reason for the PA failure in repeater service was the unexpected 6:1 SWR caused by the duplexer being mis-tuned. Apparently the SWR protection in the radio was inadequate. After repairing the PA, resetting the output power to 105W, and correcting the SWR problem, there have been no further problems in over one year of repeater service.
If you can find a VX-6000L at an almost nothing price, they can, at reduced to 100W power, make a reasonably good 6m transceiver.
As with the VX-5500L, the VCO shield needs to be unsoldered, and the interior coil tuning raised in frequency. However, unlike the VX-5500L, there is only one RX side coil, instead of 2 RX coils, so the VX-6000 is probably the same as the VX-4000 VCO. The RX front-end coils also should be tweaked (see the article on the VX-5500L). The photo below shows the entire radio, with the main board to the right (missing its PA transistor) and the PA board on the left. Click on any photo for a larger view.
Warren K2WD wrote a very nice article that describes the VCO tuning procedure. His article can be found on the Yaesu/Vertex/Standard index page.
The good news is that there are only the three harmonic filter coils that need spreading on the TX side. The two larger coils evenly spread to 5/8" end to end, and the smaller coil to 1/2" end to end. The photo below shows mainly the PA circuit board; the harmonic filter coils are at the top of the photo and are hard to see.
The photo below shows the spread coils looking from the rear of the PA circuit board, which is on the bench being diagnosed after failing.
The radio is now good to 54 MHz, and as far as the harmonic filter is concerned, optimally tuned. The spectrum analyzer trace below shows the response.
At 105W output (SVC-49 Software is required to adjust the TX power), the radio should draw 19A or less at 13.8V. See the table below. The radio does just barely exceed its rated specs of 120W out at 22A draw. An external 8 ohm speaker does wonders for improved receive audio. As with all these VX radios, never ground either speaker lead.
| Power | Voltage | Current | Efficiency |
|---|---|---|---|
| 128W | 13.4V | 22A | 43% |
| 105W | 13.5V | 19A | 41% |
| 70W | 13.6V | 16A | 32% |
Update December 2019:
The stock harmonic filter without modifications on the (V9) VX-6000L and its cousin, the Motorola PM-1200 (V2 and V7), is flat without any ripple up to 56 MHz, so it seems no modifications are necessary for 6m operation. However in practice, at least on the VX-6000L, the harmonic filter coils need to be spread a bit to gain transmitter efficiency, probably due to PA output reactance before the filter. The results from adjusting for maximum output and minimum current draw for this VX-6000L V9 are shown in the list below. These coils are along the rear of the PA board. The leftmost coil (closest to the middle of the board) was spread to 14mm, the middle coil was spread to 12mm, and the rightmost coil (closest to the side of the chassis) was left unchanged. By the way, according to the available documentation, the VX-6000L is rated for up to 120 watts but the PM-1200 is only rated for up to 110 watts.
Here's a picture of the VX-6000L PA with the SD1405 transistors.
The PM-1200 uses different output transistors (Toshiba 2SC2879). At first glance, the two blue caps (0.047uF) which are part of the collector to base feedback in series with the big feedback resistors, appear to be new to the PM-1200, but in fact replace smaller surface mount 0.047uF chip caps on the VX-6000, possibly for greater heat dissipation and reliability. Note the RTV on the feedback resistors. In addition, the middle harmonic coil on the PM-1200 is held down with the same nearly clear RTV, and the copper from the grounding strap is of different shape so the copper is now close to this second coil on the PM-1200, possibly raising the frequency. It may not have to be spread for 6m for this reason. Note the possibly stock, leftmost harmonic filter coil on the PM-1200 is already spread to 13mm, so improvement on 6m from further spreading is unknown until we can get the radio to program to 6m.
With 13.65V at the PA terminals, I got the following measurements:
Besides modifying the VCO so the radios will operate above 50 MHz, some sensitivity improvements can be made by modifying the front-end tuning settings with the SVC-49 software, especially to keep the sensitivity the same from at least 46-54 MHz, but if just 6m usage is desired, connect a voltmeter to pin 1 on the rear DB-25 connector, and peak the four front-end coils UPWARD to about what is shown in the photo below, using the RSSI or IF signal strength reading point.
Typical 6m sensitivity for the VX-6000L should be -119 dBM (0.25uV) for 20 DB quieting, -121 dBM (0.2 uV) for 12 DB SINAD with ±7.9 kHZ -6 dB modulation bandwidth, which is wider than the about 7.6 kHz mod bandwidth on the VX-5500, or the 6.5 modulation bandwidth on the GE MASTR II, so because of the bandwidth, the VX-6000L is not as good as the MASTR II on weak signals, but better than the Kenwood TK-6110 or Alinco DT-06 or Yaesu 8900 with the widest bandwidths approaching 9 kHz. On 48 MHz, the modulation bandwidth of the two PM-1200 radios were 8.3 kHz, which puts it just above the VX-6000L, but apparently this can vary with the PM-1200 version, as the latest version PM-1200 and VX-6000L have a front-end change that has less gain, more in line with the lower VX5500L gain ahead of the mixer. This also improved the intermod and adjacent channel specs, the worst being listed for the PM-1200. The VX-5500L, VX-6000L, and PM-1200 all use the identical coils, crystal, ceramic filters, and first mixer, so the difference must be the transistor(s) RF type number and gain between the antenna and mixer.
Apparently the main board indicates the entire radio version. The "01" on the main board indicates Vertex Standard. The only visual difference to the main board for the PM-1200 is that the reverse protection diode near the power wires is larger on the PM-1200, and that board is stamped "02" above the version number. You can see some of these numbers in the photos of the main board above.
Contact Information:
John can be contacted at: jhaserick84 [ at ] comcast [ dot ] net.
Article text and photos by John Haserick W1GPO.
HTML coding and some text by Robert W. Meister WA1MIK.
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This web page created 05-Feb-2019
This web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.