ICOM IC-910 preamp operation

[VK4UH]

Thanks for reading this

Has anyone any experience with using the ICOM IC910H with mast head preamplifiers?

From my understanding the '910 is supposed to power mast-head pre-amps for 144 and 432 MHz as standard, supplying 12v at 200 mA via the coax to the ICOM AG-25 or AG-35 series of units via a bias-T arrangement.

The manual warns not to place any amplifiers or even SWR bridges in the line to prevent the 12v being accidentally grounded.

The preamp is supposed to be activated by holding down the P.amp/ATT) front panel button. The facility is supposed to bring on the pamp decal on the display This facility has to be activated in the set memory for each band (PrE 144 on or PrE 440 on).

I my case, even though the facility is activated, pressing the preamp button makes the preamp decal
flash on seven times and then go off. This happens on both 144 MHz and 432 MHz. No voltage appears on either antenna socket as it should.

Has anyone else seen this problem? Is there another setting I am missing?

If you are running a 910 I would be interested to know what happens when you hold down the P.amp key on your own set?

Kevin VK4UH Brisbane. vk4uh@wia.org.au

[VK2GOM]

Hi Kevin,

I ran my IC910H for a while with my MiniKits 2m pre-amp on the mast. The IC910H powered it fine when the 2m Preamp menu setting was set to 'On' and holding down the Att/PreAmp button to activate the pre-amp, as you suggest. It sounds like you are doing everything right, so I'm not sure why it's not working?

The only thing I could suggest is a microprocessor reset on the radio.

Any unusual or erratic operation I have had with Icom radios has (to date!) been 'fixed' with a processor reset.

73 - Rob VK2GOM / G0MOH

[VK4UH]

Thanks Rob

A CPU reset was a good idea. I have tried this but to no avail unfortunately.

Interestingly although most of the settings return to factory on reset, and the memories were cleared, the dial frequencies for 144 and 432 were not cleared? What happens with your IC 910 after reset?

I was also wondering if the 910 needs to see a DC load across the coax to hold the preamp facility on? I am still seeing no voltage appearing across either antenna socket.

Kevin VK4UH

[VK2GOM]

Hi Kevin,

I have just tried a few tests.

I don't have a pre-amp on the mast at the moment, so the Pre setting is 'Off'. I unplugged my 2m antenna, then in the menu set it to 'On'. I then put a DMM into the antenna socket, and enabled the preamp. 12.5V appeared at the socket, so it doesn't seem to need a load in order to show Volts.

I am a bit loathe to do any sort of reset for fear of losing memories etc.

It might be time to start looking at the schematic...

But before you do that, have you tried unplugging all antennas, enabling all preamps, and looking for Volts on each socket? Do you have 23cm onboard?

IC701 provides Volts for all the external preamps, and they are switched by Q182 and Q183 for 2m, and Q702 and Q703 for 70cm. 23cm switching is achieved inside the 23cm module.

It is not impossible that IC701 has failed. You really need to open her up and check for Volts on the output of IC701 (a PQ 20VZ51 variable output SMT Voltage reg) which is Pin 3. It's a Sharp Semiconductor device.

Do you have a service manual, schematics and a component overlay for the radio?

73 - Rob VK2GOM / G0MOH

[VK2JDH]

Just clarifing that you have enabled them via Set Mode as detailed on P58 of the manual?
Set mode operation
Push [SET] for 1 sec. to enter the general set mode.
Push [DN Z] or [UP Y] to select the desired item.

Preamplifier controller for 144 MHz
Switch the preamplifier control capability ON and
OFF. When using the optional AG-25, ON must be selected.
Otherwise, the preamplifier is never activated.

[VK2HRX]

....
If you are running a 910 I would be interested to know what happens when you hold down the P.amp key on your own set?
...

I have the Icom 2/70 preamps up the mast for my IC 910H. When I press and hold the ATT/P.AMP button. P.AMP is dislpayed under the MHz reading on the VFO display. Its solid, not flashing. I can't get mine to flash. If I disconnect the antenna it still behaves the same.

I have found Icom to be helpful in a previous support request, so maybe send them a message and ask what a flashing P.AMP symbol means.

Good luck,
Compton
VK2HRX

[VK2GOM]

Kevin, you are not alone:

http://www.amsat.org/amsat/archive/amsa ... 00152.html

And also this page tells of a fault found:

http://www.db0anf.de/app/bbs/messages/show-12907-CX2SA

73 - Rob VK2GOM / G0MOH

[vk1da]

Kevin,

I have a preamp for 432 only and when I press and hold the preamp button, I think I get a beep but anyway the PRE or similar indicator appears on the main display. And the rx noise level increases greatly, I hear distant signals better etc.

If I press the preamp button on 144, where I have no preamp, the Preamp indication flashes a couple of times, maybe with some complimentary sound effects/beeps then it goes back off.

This may be due to the 2m antenna having a feed system that is a dc short. I haven't tested to see what happens if I try to turn the preamp on when I don't have any antenna plugged in.

73

[VK4UH]

Thanks all for the suggestions

I have followed your idea Rob and it is clear there is a problem within the '910 itself

I found that the 1296 pre amp control, which I had previously ignored is working normally. The decal stays on and 12v appears on the 23cms socket as it should.

It appears that L-720, which feeds the voltage onto the 144 MHz socket is open circuit. Looks OK visually but is o/c. This needs to be replaced. There is no voltage on the upstream side.

The equivalent inductors on the 432 socket L-182 and L-186 are however OK even though there is no voltage there either.

It appears therefore that the problem is further upstream.

It is not clear to me, from the circuit, if the voltage regulator IC-701 does supply all the external pre-amps or only 144 and 432. If it does supply all three then it must be OK, since 1296 is functioning.

If it only supplies 432 and 144 then this could be the culprit.

I reassembled the rig and have since downloaded the Service manual and board overlays.

Even with the best resolution I can print at the overlays are VERY hard to read in A4 even using magnifying loops. I think I have identified IC 701 midway on the PA board overlay between the two antenna sockets - I think
I can identify Q-702 but not Q-703 on the overlay - 144MHz switching transistors
I cannot find either Q-182 or Q-183 the equivalents on 432

Im not really equipped or experienced in SMD repairs but will explore further.

If IC-701 is OK then there must be at least two (or more faults) to explain the symptoms

Kevin VK4UH

[VK2GOM]

Hi Kevin, I originally looked on the schematic, and of course it only shows 2/70 for the voltage reg. I don't have a schematic for the 23cm module. But since the 1296 module carries it's own antenna socket, I'm just guessing it has its own Voltage reg setup too rather than trying to route the 12V all the way in through the module and to the socket on Rx when you want your preamp on.

I think the links I posted to the forum where someone else had similar problems also pointed to burnt out inductors.

All that can really cause that is having the pre-amp turned on into a short circuit load, although I'm surprised there isn't protection for that sort of thing.

As far as IC701 goes, you really need to locate it on the board and check for Volts output on pin 3 of it. I'm guessing it's dead, and the inductor was taken out at about the same failure time.

Good luck with it, and I hope you get it sorted... if it was busticated when you got it, is there any comeback from the seller?

73 - Rob VK2GOM / G0MOH

[LZ1JH]

Hello,
I have the same problem of 2m.
When I press the button preamp flashes 5 times and stops p.amp. Of 70 cm preamp works.
Does anyone managed to solve this problem.

Thanks,

LZ1JH Rumen

[VK4GHZ]

Hello,
I have the same problem of 2m.
When I press the button preamp flashes 5 times and stops p.amp. Of 70 cm preamp works.
Does anyone managed to solve this problem.

I'm in the process of finishing off a 23cm PA (that allows for an external preamp and separate RX only feed line), and I have just discovered this myself.
And a Google search brings me to this page!

When I enable the preamp in the SET menu "Pre 1200 - ON", and then go to activate the Preamp, "PRE" flashes a number of times, and then the "PRE" disappears altogether.
This is with the feed line connected.

With the feed line disconnected, the "PRE" stays activated, and I measure 12.9V on the 23cm antenna port in the receive state.

I then checked the feedline, and it measures about 0.4 ohms.
The flashing "PRE" warns you that it sees a DC short circuit (or near enough), and shuts down.

The manual is incredibly vague with this, but the (poorly worded) caution is there.

In my application, I will use a Bias-T at the far end to power the MiniKits preamp, so this will inherently eliminate the DC short circuit that the antenna (M2 23CM35) presents.

So if you use a Bias-T, this "problem" will go away, assuming you have not already burnt out the associated 12V feed circuit.

[VK2OMD]

...
The flashing "PRE" warns you that it sees a DC short circuit (or near enough), and shuts down.
...

I just reviewed the circuit to confirm my recollection that the circuit employed on the 1296 output is different to the others.

The 1296 circuit has hints of a current limiter, so it might survive an external s/c.

The others look like there is no current limiter, and that a fault may be interrupted by destroying L194 or L701. These could possibly be replaced by an inductor and polyswitch to make the transceiver more tolerant of accidental button pushes when a DC s/c is applied via the coax socket.

Owen