General Stonewall Jackson half rx

[Harley3315]

Strange issue have a Stonewall here large signals lock it up like it's keyed the needle goes all the way over like its keyed knocks the rx audio out but it isn't actually keyed up. If I cut the rf gain back it does this less but it will also do it with just normal noise or skip running at times unless I cut the rf gain way back. I'm assuming it's an agc issue. Any idea where to start? It's had the schottky dioeds done as well

 

[nomadradio]

It's had the schottky dioeds done as well

If they replaced D15 and D16 with Schottky parts that could cause trouble. Those two are PIN diodes. They act as a voltage-controlled attenuator, cutting down the signal coming in from the antenna in step with the receiver's AGC. I have seen those two replaced with the wrong diode, and the usual result is just overloading and distortion.

Might have a very-close look at the solder side of the board with a good magnifier and a bright light. Changing diodes runs the risk of a solder bridge, a glob that joins two adjacent solder pads that shouldn't.

Just a hunch. Does make me wonder if there is more than one fault at work here.

73

 

[Harley3315]

If they replaced D15 and D16 with Schottky parts that could cause trouble. Those two are PIN diodes. They act as a voltage-controlled attenuator, cutting down the signal coming in from the antenna in step with the receiver's AGC. I have seen those two replaced with the wrong diode, and the usual result is just overloading and distortion.

Might have a very-close look at the solder side of the board with a good magnifier and a bright light. Changing diodes runs the risk of a solder bridge, a glob that joins two adjacent solder pads that shouldn't.

Just a hunch. Does make me wonder if there is more than one fault at work here.

73

Had a look at the solder side of the board no shorts the radio worked fine for a long while after the schottky swap by the way. The pin diodes you mention 15 and 16 weren't ever changed.

 

[nomadradio]

I would have a look at R258. It's a 2.2k resistor, three red bands. It's located about a half inch forward of TP9. If C44 were to fail as a dead short, this voltage won't get past a half Volt with a strong receiver signal.

Here's where a signal generator pays for itself. If you can turn up a steady signal slowly and watch the DC voltage at R258 you should see a steady rise once the signal gets past a few microvolts. It should reach around 2.5 Volts or so and stop rising as rapidly as the signal level increases.

The distortion issues sure sound like AGC trouble. Never learned how to troubleshoot that kind of problem without some source of a steady test signal.

73

 

[Harley3315]

No really distortion issue just the signal causing the radio to read full scale meter and knocks the rx completely out like the radio is thing but it's not

 

[nomadradio]

Wow! Not a common fault. Leads me to wonder if the problem is at the downhill end of the radio, something causing the AGC voltage to jump up?

Or is there a sneaky fault with the transmit/receive switching?

A meter, or better yet a 'scope on the two switched-voltage jumper wires J26 and J27. They're near the front-right corner of the pcb.

J26 has to be less than 0.6 Volts while receiving. If it's higher than that, this causes issues.

J27 should have between 8 and 9 Volts DC while receiving. If it drops off, stuff can happen.

Transmit-side voltage that won't shut down while receiving might be the culprit, but you'll have to catch it in the act.

73

 

[Harley3315]

Wow! Not a common fault. Leads me to wonder if the problem is at the downhill end of the radio, something causing the AGC voltage to jump up?

Or is there a sneaky fault with the transmit/receive switching?

A meter, or better yet a 'scope on the two switched-voltage jumper wires J26 and J27. They're near the front-right corner of the pcb.

J26 has to be less than 0.6 Volts while receiving. If it's higher than that, this causes issues.

J27 should have between 8 and 9 Volts DC while receiving. If it drops off, stuff can happen.

Transmit-side voltage that won't shut down while receiving might be the culprit, but you'll have to catch it in the act.

73

Been a while but jumping back in on this thing here's another random strage thing if you key them mic it unlocks the rx needle goes back down like normal and rx audio is back. The stranger thing I noticed is if the mode switch is moved from am to ssb it also let's it loose switch to ssb then back ro am walla unlocked but switching to fm has no effect. I also had already changed the 2 larger tx/rx switching transistors already and I think a couple dioeds in that area maybe even the other smaller c945 transistor if I remember correctly.

 

[nomadradio]

I should come up with a picture of some sort for this, but for now, find two wire jumpers on the circuit board, to the rear of the meter. J26 and J27 carry the radio's transmit-only and receive-only power feeds to the radio's circuits. They are almost 3/4 inch long, at an angle to the front edge of the pc board. The area around them is mostly open, so they're easy to find.

J26, the rear one is transmit-only voltage. Should be around 8 Volts when transmitting. It's very important to see this voltage drop below a half of a Volt more or less on receive side. If it's too high on receive, this causes mischief.

The front one J27 is receive-only voltage. Should drop below a half Volt while transmitting, and should hold steady around 8 Volts or so while receiving.

If they don't meet those limits, wacky symptoms appear.

Hmm. Wasn't that hard to snatch an image.

 

[Harley3315]

I should come up with a picture of some sort for this, but for now, find two wire jumpers on the circuit board, to the rear of the meter. J26 and J27 carry the radio's transmit-only and receive-only power feeds to the radio's circuits. They are almost 3/4 inch long, at an angle to the front edge of the pc board. The area around them is mostly open, so they're easy to find.

J26, the rear one is transmit-only voltage. Should be around 8 Volts when transmitting. It's very important to see this voltage drop below a half of a Volt more or less on receive side. If it's too high on receive, this causes mischief.

The front one J27 is receive-only voltage. Should drop below a half Volt while transmitting, and should hold steady around 8 Volts or so while receiving.

If they don't meet those limits, wacky symptoms appear.

Hmm. Wasn't that hard to snatch an image.

I'll give that a look see isn't it strange that moving the mode switch unlocks it as well though? Seems to only effect am and fm Id assume and if those voltages are off where do we go from there?