23cm pre amplifier


GM6VXB
 

After a few good days on tropo, this morning turned on the 23cm system to find that the pre amplifier had died.

This was a very good VHF Designs low noise high gain pre-amp.

Found that the initial device had gone short. Paperwork says an 'SKY67151', but looking closely it looks like

and is marked as a PGA-103.

Anyway, replaced it with a 103 but the pre amp is still very low (maybe no) gain.


So is there a good source of 'built' pre amplifers as the only ones I can see are the VHF design ones.


Eyesight now not good enough to try to build one of the DDK ones.


Martin, GM6VXB


(QRT 23cm until sorted.)


Robin Szemeti - G1YFG
 

You might want to check the series  L or R that feeds the device, if the device went short it probably blew the series dropper.

If struggling with eyesight, more than happy to repair for you by return of post.


On Tue, 22 Sep 2020 at 13:15, GM6VXB via groups.io <martin.andrew=btinternet.com@groups.io> wrote:

After a few good days on tropo, this morning turned on the 23cm system to find that the pre amplifier had died.

This was a very good VHF Designs low noise high gain pre-amp.

Found that the initial device had gone short. Paperwork says an 'SKY67151', but looking closely it looks like

and is marked as a PGA-103.

Anyway, replaced it with a 103 but the pre amp is still very low (maybe no) gain.


So is there a good source of 'built' pre amplifers as the only ones I can see are the VHF design ones.


Eyesight now not good enough to try to build one of the DDK ones.


Martin, GM6VXB


(QRT 23cm until sorted.)


--
Robin Szemeti - G1YFG


Stephen Tompsett
 

I've used a SBA1250/1450 in my new preamp box:


https://g8fek.com/lna.html


Other possible sources


https://shop.kuhne-electronic.com/kuhne/en/shop/low-noise-amplifiers/




On 22/09/2020 13:15, GM6VXB via groups.io wrote:

After a few good days on tropo, this morning turned on the 23cm system to find that the pre amplifier had died.

This was a very good VHF Designs low noise high gain pre-amp.

Found that the initial device had gone short. Paperwork says an 'SKY67151', but looking closely it looks like

and is marked as a PGA-103.

Anyway, replaced it with a 103 but the pre amp is still very low (maybe no) gain.


So is there a good source of 'built' pre amplifers as the only ones I can see are the VHF design ones.


Eyesight now not good enough to try to build one of the DDK ones.


Martin, GM6VXB


(QRT 23cm until sorted.)

-- 
Stephen Tompsett (G8LYB)
Tel: 01788 578940
Mob: 07956 855816


Robin Szemeti - G1YFG
 

I can't see a VHFDesign 23cm pre with a PGA103  but assuming it is his standard layout ... look for the 12 ohm dropper resistor and an inductor ...  I suspect one or the other is blown open.

image.png


On Tue, 22 Sep 2020 at 14:24, Robin Szemeti - G1YFG via groups.io <robin=redpoint.org.uk@groups.io> wrote:
You might want to check the series  L or R that feeds the device, if the device went short it probably blew the series dropper.

If struggling with eyesight, more than happy to repair for you by return of post.

On Tue, 22 Sep 2020 at 13:15, GM6VXB via groups.io <martin.andrew=btinternet.com@groups.io> wrote:

After a few good days on tropo, this morning turned on the 23cm system to find that the pre amplifier had died.

This was a very good VHF Designs low noise high gain pre-amp.

Found that the initial device had gone short. Paperwork says an 'SKY67151', but looking closely it looks like

and is marked as a PGA-103.

Anyway, replaced it with a 103 but the pre amp is still very low (maybe no) gain.


So is there a good source of 'built' pre amplifers as the only ones I can see are the VHF design ones.


Eyesight now not good enough to try to build one of the DDK ones.


Martin, GM6VXB


(QRT 23cm until sorted.)


--
Robin Szemeti - G1YFG


--
Robin Szemeti - G1YFG


Conrad, PA5Y
 

Hello Martin, the PGA-103 is the 2nd stage of the amplifier. 

Have a look here:


There is also a single stage LNA but that does not have a PGA-103 in it.

Regards

Conrad PA5Y




From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> on behalf of GM6VXB via groups.io <martin.andrew@...>
Sent: 22 September 2020 14:15
To: ukmicrowaves@groups.io <ukmicrowaves@groups.io>
Subject: [UKMicrowaves] 23cm pre amplifier
 

After a few good days on tropo, this morning turned on the 23cm system to find that the pre amplifier had died.

This was a very good VHF Designs low noise high gain pre-amp.

Found that the initial device had gone short. Paperwork says an 'SKY67151', but looking closely it looks like

and is marked as a PGA-103.

Anyway, replaced it with a 103 but the pre amp is still very low (maybe no) gain.


So is there a good source of 'built' pre amplifers as the only ones I can see are the VHF design ones.


Eyesight now not good enough to try to build one of the DDK ones.


Martin, GM6VXB


(QRT 23cm until sorted.)


GM6VXB
 

Robin,
The resistor in series from the regular is suprisingly ok, even though it got hot enough to melt the solder
holding it on the board.
Before I re-soldered it a resistance measurement toward the active device showed 'Zero' Ohms which
went to infinity when the device was removed.
Replacing the device with a PGA103 gave me around the correct voltages and current draw.
The second device appears to have feed volts but at this point I was really struggling to focus so gave
up. Put the pre-amp back in line and is still dead i.e. no noticable increase in noise level to the receiver
which normally would be about 2 'S' points when beaming across the village.
Might take you up on repairs but first got a voluntary trip out to the islands (weather permitting).

Martin, GM6VXB


GM6VXB
 

Conrad,

Mistake on my part. Yes, the PGA103 is the second stage. This was definately dead (dead short) .
Will take the pre amp back to the shack later for more checks.
Chances are if the PGA is dead, then the SKY will be as well.
There is no way I would even attempt a repair. Did try with a single stage pre amp but well beyond
my capabilities and eyesight.

Martin, GM6VXB


Robin Szemeti - G1YFG
 

I would be more than happy to assist if you are stuck,  I have a fair amount of SMD experience, and the appropriate equipment, it won't be a problem.

The "trip out to the islands" has me intrigued however, we failed to go sailing this year, and missed our paddling around the coastline. I may bother you offlist for detail ;)


On Tue, 22 Sep 2020 at 16:05, GM6VXB via groups.io <martin.andrew=btinternet.com@groups.io> wrote:
Robin,
The resistor in series from the regular is suprisingly ok, even though it got hot enough to melt the solder
holding it on the board.
Before I re-soldered it a resistance measurement toward the active device showed 'Zero' Ohms which
went to infinity when the device was removed.
Replacing the device with a PGA103 gave me around the correct voltages and current draw.
The second device appears to have feed volts but at this point I was really struggling to focus so gave
up. Put the pre-amp back in line and is still dead i.e. no noticable increase in noise level to the receiver
which normally would be about 2 'S' points when beaming across the village.
Might take you up on repairs but first got a voluntary trip out to the islands (weather permitting).

Martin, GM6VXB


--
Robin Szemeti - G1YFG


Dominique Dehays
 

hi all ,


the PGA103 is prone to go dead when powered under 5v , it is better to use it at 3V

73

Dom

Le 22/09/2020 à 17:27, GM6VXB via groups.io a écrit :
Conrad,

Mistake on my part. Yes, the PGA103 is the second stage. This was definately dead (dead short) .
Will take the pre amp back to the shack later for more checks.
Chances are if the PGA is dead, then the SKY will be as well.
There is no way I would even attempt a repair. Did try with a single stage pre amp but well beyond
my capabilities and eyesight.

Martin, GM6VXB


GM6VXB
 

I think I have it fixed now but too dark and windy to stand on the roof putting the preamp back into the box.
Thanks to Conrad (PA5Y) for pointing out that the PGA103 is actually the second device rather than the first (my brain
not in gear while checking), and also pointing me to a circuit diagram.
So the PGA103 had failed short circuit. From the circuit diagram R3 series supply resistor had become desoldered due to
the PGA getting 'very' hot. Further investigation earlier and I found a small gap in the input side of the PGA where C6
should be. More by luck I found capacitor C6 still inside the box desoldered due to heat. Really difficult to resolder it back with my poor eyesight
even with magnifiers but now when checked I hear noise, which is more than it did before.
No test gear above 1GHz, but listening to a third harmonic at 432MHz  off the Sig Gen it looks very hopeful
So will get it re-fitted tomorrow and hope it is working.

Thanks all for the assistance, at least IO97 will (could/should) be back on air if you have not worked GM4ZUK during
the monthly contest.

Martin, GM6VXB (IO97AQ (as far north on the East coast as you can go))


Martin, GM6VXB


GM6VXB
 

Robin,

Have been out to the islands (in my working life) doing maintenance on lighthouses, so
basically any island with a light and I have been there.
Now though I do voluntary work for RSPB which also involves going out to the islands.
Trip this coming weekend to North Rona (IO79) for last checks before winter makes
these types of trips almost impossible.
Probably not doing any more trips after this one as getting too old (68 now) to stay
on a small island in a cold damp tent and work from first light to dark.
The list of islands activated (when time) are IO99/IP90/IP80 (dead easy),
IO57 (St Kilda), IO69 (Sula Sgeir), IO79 (North Rona) and IO67/IO68 (Hebrides)
and a few others which have not been activiated.

Martin, GM6VXB