HR500 hardware modifications

[Geert Jan PE1HZG]

While the population of HR500 users will never grow because kits have sold out, perhaps it makes sense to describe some of the hardware changes I made on my HR500. 
Many changes small, but perhaps inspiring to others. So here goes:

1. Finger grill on PSU fan 
   While not fatal, accidentally sticking a finger in the 80cm hole of the running PSU fan does hurt. And our grid voltage is 230V, and that hurts quite a bit if an accidental finger poke touches the input voltages of the power supply.
   There are chrome grills that fit on the outside of the enclosure and prevent a finger getting inside. I found one at a ham fair but "80mm chrome fan grill" will find you one for prices close to a buck - enough said!
   
   
2. Power AC inlet with input net filter
   It is possible to replace the AC inlet with a replacement that includes a net filter. This will prevent PSU noise to get out via the power cable, but perhaps more important, power grid noise from entering the power supply tray of the HR500.
   I found mine at a ham fair (again!) and it wasn't expensive. Note that the HR500 has a substantial startup power peak, you may want to select a filter that is robust enough for higher startup surge currents.
   
   
3. Carry handle
   I got myself a collapsible carry handle and put it to the side of the HR500 for easy transport of the amplifier. One might think that adding a carry handle is difficult because the middle of the side is not a good place to mount, however the RF top half is heavier than the PSU bottom half and experimenting learns that the center of gravity is at the bottom of the top RF half and putting the handle there leaves a unit that is easy to catty because it is balanced. 
   Drilling two holes (be careful not to damage electronics or to get drill dirt in the top RF tray that may short!), two screws, an "equipment carry handle" again from the ham fair and all is well. I would not be without.
   
   
4. Tilt stand
   The ham fair delivered me - in addition to the earlier mentioned fan grill, equipment carry handle and AC input filter, a tilt stand. It allows me to run the HR500 slightly elevated for better display visibility.
   I don't think I would need to explain forum members here how to install an equipment tilt stand I think - just do!
   
   
5. Flatcable connectors with housing
   The HR500 modules interconnecting is via some ribbon cables that connect connectors existing of 2x5 rows of pins. While Jim has been careful that a mis-plug does not cause damage, I have replaced the 2x5 pin headers on the various PCB's with some headers with a case and a polarity lock for ease of mind. This does mean removing the pin headers on the PCB's and hence isn't for everyone, but it is what I did.
   
   Note that while the ribbon cable headers are robust against interchange, the 2-pin power connectors are not. They come with a mechanical lock, but on one of mine, the mechanical lock was broken off. You are well advised to mark the connectors and the cables, and if the lock has been broken off, to replace the 2-pin connector with one with proper lock on it.
   
   And while at it, on my HR500 the coax cables all have colored pieces of schrink tube at the end of the RF jumper cables. I find it easier to see where my internal RF cables are going and for the cost of some color schrink tubing.. just a thought.
   
   
6. Remove of R227 on power PCB for better FT817 support
   R227 on the power board (2K24) may cause wrong band voltages to be sent by a FT817. The pull-up resistor causes too-high BAND voltages.
   Jim told me the resistor is supposed to DNI (Do Not Install) on production boards but the resistor was installed and caused havoc when using HR500 with a FT817. If you open your HR500, check if the resistor is there and if yes, just remove it.
   
   
7. Remove of FB214 on power PCB
   FB214 provides 5V power to pin 1 of the ACC connector.
   However, a possible short of pin 1 with port 6/7 of the same ACC connector (perhaps an accident / mistake connecting a cable) would bring 12V to the 5V circuitry on the power PCB. I didn't want that risk, didn't see the need for 5V (creating 5V from the 12V pin 6/7 seems easy enough with a 78L05 or similar) so I removed FB214. Pin1 of the ACC connector is now N/C which is just as well.
   
   
8. Switch off small fans when PA cold on Rear panel
   U105 (LM 2941), combined with Q101 and Q102 provides a switchable voltage for the small fans that blow the RF module cooling. The circuit provides 4 voltages (6-8-10-max) for 4 different fan speeds controlled by the firmware - if the heat goes up. the fan speed goes up to provide more cooling.
   When the PA is completely idle and has not been used (I listen a lot more than I transmit), the fans run at their lowest speeds but are still audible and the fans do not have to run at all when the PA is not and has not been operating and is stone cold, also reducing dust intake from prolonged unnecessary fan operation.
   Pin 2 of U105 LM2941 is an enable pin. It is currently hard connected to ground, but lifting the pin from ground and bringing it to 5V shuts down U105 switching off the fans completely.
   I have introduced a 74HC132 in SMD (It is what the saturday night junkbox provided) to create a NOR gate that, wen the FAN1 and FAN2 logic signals are both logic 0, the output of the gate will be a logic 1 that drives the U105 enable pin.
   The whole circuit is put on the PCB by connecting pin 14 (VCC) and pin7 (GND) of the gate chip to suitable pads on the PCB, flattening the other pins and using some thin 0.2mm magnet wire for the wiring.
   The net result is that the small fans are completely OFF until the temperature of the RF cooling block raises a bit, at which time the fans start running as usual and speed up when the temp rises further. 
   When the PA has gone back completely idle for a while and the temperature has been low enough that the firmware sets both fan1 and fan2 to a logic-0 level, the fans again switch off.
   So, instead of having 6V-8V-10V-maxV speeds on the fans, I now have off-8V-10V-maxV on the small fans and the difference in fan noise level when the PA is completely idle and cold and the fans not running, is deafening.
   The big 80mm fan still runs but that one is much less audible than the whine of the small 40mm fans controlled by the U105 voltage.
   
   
9. Add rings / check tightening of PA transistor screws
   As others on the list have found too, sometimes temperature cycling of the mounting of the PA transistors can cause the MOSFET mounting screws to loosen up. Once the screws have losened up, heating the MOSFETs may cause them to run without sufficient cooling contact and subsequently fail (expensive!).
   Check postings elsewhere on this forum - I added split lock washers (something I have stolen from how Elecraft does it) and I checked a few times if screws now no longer losen on temperature cycling or over time.

I hope this posting will inspire others to share their hardware improvements. Interested in your mods to make the HR500 even better.

73, Geert Jan PE1HZG