Capacitor sub?

[Brian G]

So, uh.. Pulling one end of D14 made no difference?

73

So not sure if I answered you. When I lift d14, I get absolutely nothing in the receive in am but s-meter hang about a 5 reading. But again absolutely no sound from speaker on am but still get sound on ssb. Meter also stuck on 5 on ssb too.

 

[nomadradio]

Ahh, that's the exact opposite of what I expected.

Bound to be a good reason. Starting with the simplest, are you sure it's really D14? No, I'm not trying to insult your intelligence, it's where I start when I get a puzzling result.

So, let's assume it really is D14. It's job is to be reverse-biased when no signal is present, or a weak one we don't want the PIN diodes D12 and 13 to attenuate at all. Taking it loose should make no difference in a radio that's receiving properly.

But your's isn't, so the next question is what kind of circuit fault makes it do this?

The zero-signal current through FET2 provides base current to TR10. This causes TR10 to pull collector current from

Whuppsss........ Scratch the dialog above. The diode that feeds AGC current into D12/13 is D68, not D14.

Oops.
Shoulda said to pull one end of D68 to see what changes.
Sorry 'bout that.

Still don't know what made it go nuts with D14 out of the circuit.

If you haven't completely lost patience, put D14 back and try lifting one leg of D68.

If the AGC is being too aggressive, lifting D68 should make the signals stronger.

I have seen an occasional issue with FET2. The 'gain' of a junction FET is typically expressed as the Idss, or the drain current with zero gate bias. The bigger the number, the lower the voltage gain of that part. In this circuit, you want a 2SK19 with the highest Idss, or the lowest voltage gain. That zero-bias current is what holds off the AGC voltage. The stronger the signal, the less current through FET2, and the higher AGC voltage 'turning down' the level of the receiver's input by putting current into D12/13.

Whew!

The "2SK19" part number for FET2 has a suffix letter or letters, corresponding the standard color code. Lowest Idss is "Br" for 'brown', or '1' followed by "R" and "O", "Y" and "Gr". The part specified for this circuit is "Bl" or 'blue' as in '6'. This is the highest Idss version. One with a lower-value suffix letter may or may not function like it should in this circuit.


Anyway, if FET2 has too little drain current when there's no signal it triggers the AGC voltage, same as it does when a signal gets rectified by D8 and D10. The negative DC voltage from them feeds the gate of FET2, reducing its drain current, reducing the base current into TR10, raising the voltage into D12/D13 through D68.

I think.

Anyway, sorry about the bum steer to D14. Still haven't figured out why it should do what it did when unhooked. Might have a look at that part and see that it's a properly-working diode, and hasn't turned into a partial resistor.

73

 

[Brian G]

Ahh, that's the exact opposite of what I expected.

Bound to be a good reason. Starting with the simplest, are you sure it's really D14? No, I'm not trying to insult your intelligence, it's where I start when I get a puzzling result.

So, let's assume it really is D14. It's job is to be reverse-biased when no signal is present, or a weak one we don't want the PIN diodes D12 and 13 to attenuate at all. Taking it loose should make no difference in a radio that's receiving properly.

But your's isn't, so the next question is what kind of circuit fault makes it do this?

The zero-signal current through FET2 provides base current to TR10. This causes TR10 to pull collector current from

Whuppsss........ Scratch the dialog above. The diode that feeds AGC current into D12/13 is D68, not D14.

Oops.
Shoulda said to pull one end of D68 to see what changes.
Sorry 'bout that.

Still don't know what made it go nuts with D14 out of the circuit.

If you haven't completely lost patience, put D14 back and try lifting one leg of D68.

If the AGC is being too aggressive, lifting D68 should make the signals stronger.

I have seen an occasional issue with FET2. The 'gain' of a junction FET is typically expressed as the Idss, or the drain current with zero gate bias. The bigger the number, the lower the voltage gain of that part. In this circuit, you want a 2SK19 with the highest Idss, or the lowest voltage gain. That zero-bias current is what holds off the AGC voltage. The stronger the signal, the less current through FET2, and the higher AGC voltage 'turning down' the level of the receiver's input by putting current into D12/13.

Whew!

The "2SK19" part number for FET2 has a suffix letter or letters, corresponding the standard color code. Lowest Idss is "Br" for 'brown', or '1' followed by "R" and "O", "Y" and "Gr". The part specified for this circuit is "Bl" or 'blue' as in '6'. This is the highest Idss version. One with a lower-value suffix letter may or may not function like it should in this circuit.


Anyway, if FET2 has too little drain current when there's no signal it triggers the AGC voltage, same as it does when a signal gets rectified by D8 and D10. The negative DC voltage from them feeds the gate of FET2, reducing its drain current, reducing the base current into TR10, raising the voltage into D12/D13 through D68.

I think.

Anyway, sorry about the bum steer to D14. Still haven't figured out why it should do what it did when unhooked. Might have a look at that part and see that it's a properly-working diode, and hasn't turned into a partial resistor.

73

I will check that all out. I will say, I replaced TR10 since I happen to have one. There is MUCH more "noise" now on AM, pretty much the same as SSB however I am still not sure it is actually "receiving" anything and again I could not find any locals on the air to experiment with but again the "loudness" across the speaker is back at least. Not sure what that means though.

BUT NOW I HAVE ANOTHER PROBLEM! So there is a 2 wire bundle (a red and a white) towards the back of the board. I noticed the red wire that solders into the board over near C75 had popped out--no idea why!!. That end seems to go to ground best I can tell by schematic and the other end (white) solders in over next to TR11. I soldered red one back in to the hole next to C75 where it appears to have come out. However, and I don't know if this is related, the radio is now stuck on the same frequency on every channel, AM and SSB and even on the "extras". Showing around 27.7 Mhz on the freq counter. And the radio will not key at all. When you key the "Receive" light goes out, but the "Transmit" light does not come on. Like it is locked for some reason. I am almost out of patience on this thing but I have had it for MANY years and it means alot to me but all these things are driving me crazy! So.....any idea about THIS latest problem and/or on my update above on the receive? THANKS!

 

[loosecannon]

Hi Brian,

I'll try to help out where i can, but Nomad is the authority here.

the two wire bundle issue, if you are talking about a red shielded cable where one wire is white, the other is the shield of the white wire, and the insulation covering both of them is red, then the shield goes to that hole right in front of C75.
the white wire goes to that hole right in front of VR6.
if this is not the wires you are talking about, and there really is a red and a white wire inside a shield, then my guess is that the outer shell would be brown.

if i was right about which wires you meant, then try putting them in the right place without making any other changes, and see if the radio works again.
hopefully no other damage was done.

i know it can get frustrating when chasing down tough repairs, but try to take it slow and not get in a hurry. Remember, the first rule of repair is "do no harm".


now, once that problem is taken care of, we can go back to the perceived lack of receiver sensitivity issue.
C47 is usually a 2pf from the factory, but it can be as high as 5pf when frequency mods are done, as it will broaden the receiver bandwidth somewhat. so 3pf is just fine there.

I am wondering what type of signal generator you have. the cheaper types that don't have a calibrated output will usually not go low enough to do a proper receive alignment.
Is it possible that the first time you put that signal generator into the antenna jack, or worse, into a Test Point in the radio, that it was a super strong signal that pegged the meter and blared out of the speaker? you might have damaged something if that's the case.
If the signal generator does have a calibrated output, how many microvolts, or dbm is the lowest amount the radio will hear? anywhere below .5 microvolts or around -113dbm would be considered good receive sensitivity.

if you get the radio back to where it was before, where the receive seems weak, but all the receive cans seem to show a peak (not flush with the top of the can), then there might be a coil near the PLL that has gotten mistuned, and can cause a "weak receive" symptom.
that is L20 just to the left of the VCO block.
you can tune this coil using receiver noise just like the other cans. just use the receiver hiss and peak that can for maximum sound.
don't mess with this can until you get the radio back to at least where it was before.
LC

 

[Brian G]

Hi Brian,

I'll try to help out where i can, but Nomad is the authority here.

the two wire bundle issue, if you are talking about a red shielded cable where one wire is white, the other is the shield of the white wire, and the insulation covering both of them is red, then the shield goes to that hole right in front of C75.
the white wire goes to that hole right in front of VR6.
if this is not the wires you are talking about, and there really is a red and a white wire inside a shield, then my guess is that the outer shell would be brown.

if i was right about which wires you meant, then try putting them in the right place without making any other changes, and see if the radio works again.
hopefully no other damage was done.

i know it can get frustrating when chasing down tough repairs, but try to take it slow and not get in a hurry. Remember, the first rule of repair is "do no harm".


now, once that problem is taken care of, we can go back to the perceived lack of receiver sensitivity issue.
C47 is usually a 2pf from the factory, but it can be as high as 5pf when frequency mods are done, as it will broaden the receiver bandwidth somewhat. so 3pf is just fine there.

I am wondering what type of signal generator you have. the cheaper types that don't have a calibrated output will usually not go low enough to do a proper receive alignment.
Is it possible that the first time you put that signal generator into the antenna jack, or worse, into a Test Point in the radio, that it was a super strong signal that pegged the meter and blared out of the speaker? you might have damaged something if that's the case.
If the signal generator does have a calibrated output, how many microvolts, or dbm is the lowest amount the radio will hear? anywhere below .5 microvolts or around -113dbm would be considered good receive sensitivity.

if you get the radio back to where it was before, where the receive seems weak, but all the receive cans seem to show a peak (not flush with the top of the can), then there might be a coil near the PLL that has gotten mistuned, and can cause a "weak receive" symptom.
that is L20 just to the left of the VCO block.
you can tune this coil using receiver noise just like the other cans. just use the receiver hiss and peak that can for maximum sound.
don't mess with this can until you get the radio back to at least where it was before.
LC

Hi- yes, that is the wire bundle I was referring to and I did replace it in front of C75 but it is still showing that bad frequency on all channels and modes. Until I figure that out, I don't want to do anything else with the receive. I used my scope and tuned L20 and 21. I also tried to check L19 with my voltmeter. The specs say to tune it to 2.7 volts. But I cannot get it to go lower than 3.7 or so no matter how I turn it. The frequency changes when I turn it but again its the same everywhere and no TX. I also used freq counter to check L22,23,59. They are all off but turning them does not change them at all. So not sure what's happening. I noticed the frequency was off first then when I went exploring for a reason, I found that red wire popped out. Not sure if that was the cause, but like I said when I put it back, it made no difference. Could I have damaged a component by it coming out in the first place? It appears to be a ground at that point. What is it and the white wire for?
The signal generator is an old tube style Heathkit one. I tune it using my frequency counter before hooking it up. I also have to let it warm up quite awhile first. I think it fairly accurate though the 27 Mhz setting I do notice it drifts off of there a decent amount after awhile but I have used it in the past with no trouble.

 

[Brian G]

Hi Brian,

I'll try to help out where i can, but Nomad is the authority here.

the two wire bundle issue, if you are talking about a red shielded cable where one wire is white, the other is the shield of the white wire, and the insulation covering both of them is red, then the shield goes to that hole right in front of C75.
the white wire goes to that hole right in front of VR6.
if this is not the wires you are talking about, and there really is a red and a white wire inside a shield, then my guess is that the outer shell would be brown.

if i was right about which wires you meant, then try putting them in the right place without making any other changes, and see if the radio works again.
hopefully no other damage was done.

i know it can get frustrating when chasing down tough repairs, but try to take it slow and not get in a hurry. Remember, the first rule of repair is "do no harm".


now, once that problem is taken care of, we can go back to the perceived lack of receiver sensitivity issue.
C47 is usually a 2pf from the factory, but it can be as high as 5pf when frequency mods are done, as it will broaden the receiver bandwidth somewhat. so 3pf is just fine there.

I am wondering what type of signal generator you have. the cheaper types that don't have a calibrated output will usually not go low enough to do a proper receive alignment.
Is it possible that the first time you put that signal generator into the antenna jack, or worse, into a Test Point in the radio, that it was a super strong signal that pegged the meter and blared out of the speaker? you might have damaged something if that's the case.
If the signal generator does have a calibrated output, how many microvolts, or dbm is the lowest amount the radio will hear? anywhere below .5 microvolts or around -113dbm would be considered good receive sensitivity.

if you get the radio back to where it was before, where the receive seems weak, but all the receive cans seem to show a peak (not flush with the top of the can), then there might be a coil near the PLL that has gotten mistuned, and can cause a "weak receive" symptom.
that is L20 just to the left of the VCO block.
you can tune this coil using receiver noise just like the other cans. just use the receiver hiss and peak that can for maximum sound.
don't mess with this can until you get the radio back to at least where it was before.
LC

So I did some checking around the PLL and VCO chips. First, the voltage readings:
VCO; all normal
PLL: Weird!! Following are voltages I got followed compared to the Sam Facts specs. This radio had the MB8719 chip and has channel expansion so I think that changes some of the Sam’s Facts numbers?

Pin 1 3.9/3.4
Pin 2: 4/4.28
Pin 3: 4/5.15
Pin 4: 0/3.44
Pin 5: 0/3.44
Pin 6: 1/8.09
Pin 7: 4.2/4.2
Pin 8: 4.4/4.45
Pin 9: 8.2/8.17
Pin 10: 8/8.05
Pin 11: 8/0.50
Pin 12: 8/0
Pin 13: 8/8.05
Pin 14: 0/8.05
Pin 15: 0/8.05
Pin 16: 8/8.05
Pin 17: 4/3.65
Pin 18: 0/0
At X2 directly and at TP13 (IC1, Pin 8): 25.6xxx Mhz instead of 10.24 (I replaced C85 and 86 just for giggles--no difference)
TR 20,29,30 all ok
Like I said before, L19 cannot be tuned to 2.70 volts--can't get it under 4 volts or so.

that tell you anything? Could PLL or X2 be bad ?

 

[Dmans]

Following are voltages I got followed compared to the Sam Facts specs

Pin 18: 8/0

Pin 18 is the PLL ground. Should be 0.00Vdc. If I am understanding your readings correctly, you are measuring 8.0Vdc here??

I think I would trace that issue down and/or confirm that reading.

73
David

 

[Brian G]

Pin 18 is the PLL ground. Should be 0.00Vdc. If I am understanding your readings correctly, you are measuring 8.0Vdc here??

I think I would trace that issue down and/or confirm that reading.

73
David

You are correct. 18 is zero. I have corrected readings above.

 

[Brian G]

I am going to try and replace the 10.24 mhz crystal. Only because i would think regardless of what else might be going on, the crystal should show the correct frequency as specified if is functioning correctly--and it doesn't whether checking it at IC1 Pin 8 or at the crystal base itself.