IC-705 module and ALC

[Randy Diddel]

Hi,

I just switched to a 705. I have the Bluetooth control module. Works well with WFVIEW app on the Mac.

Does the IC 705 interface provide any ALC functionality? I am finding that I have to continually adjust output power out of the 705. For instance, I only need to have 3.2W drive to get ~500 out on 75/80M and need about 6W to get about ~400. Drive power varies from band to band. 

I asked the IC-705 group if there was a way to set output per band but there is not. One OM suggested that the solution to my problem was ALC. So, does it and how do I enable it?  I am new to high power PA's and I don't want to hurt those MOSFETs if I forget to set drive on a band and TX.

Thanks and 73

K5RHD

/randy

[jcve...@gmail.com]

There are provisions on the LPF board in the HR500 to create a negative ALC voltage. You'll need to provide a way to get it back to the IC705 and there is a pot for adjusting the level that may not be populated. You can also make it a positive voltage by reversing the ALC diode.

I have not looked at the specs on the IC-705's ALC input so be careful to make sure everything is compatible.

The negative voltage ALC dates back to vacuum tube exciters that needed a negative grid voltage to reduce the output.

73,

Jim WA2EUJ

[Randy Diddel]

Thanks, Jim,

I did solder in a pot and an SMA connector when I did assembly but I was just wondering if there was any sort of pass through the serial harness and TRS connector into the 705. I guess I will wait until I feel motivated enough to open the 500 back up and tinker. Enjoying the amp.

As always, thanks for your support and knowledge.

73

/randy

[jcve...@gmail.com]

There aren't any unused pins on the DB9 connector so you'll need to add a RCA jack to the rear panel, probably in the lower section. My bet is that you also want a pot mounted on the back to adjust the ALC level without opening the case and removing the ATU. 

There are so few QRP transceivers with ALC that it didn't seem important.

73,

Jim WA2EUJ

[Geert Jan PE1HZG]

In addition to "no connector available", I notice that the HR500 also doesn't have the circuitry to generate the ALC signal.

The "no connector available" you can solve by modifying the power board - I would consider removing +5V to pin 1 of the DE9 connector, which I don't use, however I do wonder about the use case of said ALC signal.

The problem is going to be to add circuitry to provide the ALC signal. There's also no space on the internal cabling.

ALC is used to tune-back a transmitter that is generating too much drive. When ALC operates, it creates distortion so your better bet, IMHO, would be to avoid the need for the signal at all.

I can think of two reasons to do ALC - if you're accidentally drive up the drive power too much on your QRP radio, like a 705,

There is also a Very Wrong Reason, to drive back a 100W-capable radio like a 7300 to make it compatible with the HR500. For the latter, my advice is simply "don't" - it take RF power to trigger the ALC circuit, that RF power (of a 100W radio) is already damaging your FETs before the ALC circuit is able to kick in and reduce the drive. If you're lucky, you create horrible distortion and splatter, if you're unlucky, you need new FETs. There are rumors that hobbypcb is adding a feature for this, but frankly it would defeat the point of the HR500 to me - just to bring an RF signal from 100W to 500W, I don't see the point for a HR500.

With a QRP radio, like a 705, 817, K3, K2, the max output power is much less and (hopefully) doesn't cause damage if too much drive is given before the protection in the HR500 kicks in. I have modified the HR500 firmware, and in my case, when a protection event happens (excess input drive, excess output, poor SWR, excess current) the KR500 disables, latches the event and requires the operator to manually acknowledge the mistake he made. I accidentally triggered this a few times and can tell that is quite effective.

So, in-brief, IMHO I agree with Jim that an ALC is not required, not for the 705. You may want to watch when you go to a new band (which you need to do anyway, since the gain of the HR500 varies per band). And perhaps you may want to ask ICOM to add an output-level-per-band feature - afaik firmware development of the 705 is still active.

73, Geert Jan PE1HZG

[Randy Diddel]

Hello Geert,

I have been out of town and just got back. Just getting around to answering emails.

I am not a programmer and do not really know how to begin to re-code the firmware as you have. Would you be interested in sending me a copy of the version that "have modified the HR500 firmware, and in my case, when a protection event happens (excess input drive, excess output, poor SWR, excess current) the KR500 disables, latches the event and requires the operator to manually acknowledge the mistake he made?" This seems like a sensible workaround. I would rather manually reset the amp than replace those expensive MOSFETS again.

I did put the ALC provisions in the HR500 when I built it-the pot and the SMA connector but if there is no provision on the 705, I will not persue it.

73,

K5RHD

/randy