Fixed: Mark III RX a little weak

[Sandbagger]

As some of you may know, I picked up a nice MK III from Grumpy a year or so back, and got it pretty much playing again. Transmitter was fine, but the receiver was just a bit weak, only sensitive down to 1 uV, and the S meter readings were way off.

Well after a bunch of voltage checks, tube swaps, resistor tests and what not, and not revealing anything obvious, I finally got a hold of another MK III receiver that worked perfectly, so I did a side-by-side compare of the two to see which stage was weak on my receiver. Turns out the weak stage was the RF front end (not surprising). Swapping nuvistors didn't change anything. After a bunch of part substituting, it turns out that the culprit was choke L2. L2 is in the grid bias circuit that the AGC uses to control the gain of V1, and L2 is used to block RF from feeding back into the AGC line instead of being amplified by the tube. L2 was not shorted or open, but somehow it must have radically changed value, enough that it was shunting some of the RF away from the tube. Changing this part now reveals sensitivity down to .08uV. One other issue was the S meter readings. Normally a meter should read S9 with 50 uV of input signal. The Browning has no S meter range adjustment, only a meter zero. I was getting +25db over S9 readings with 50 uV in (even after receiver "broadbanding"). Well, it seems that V1 is critical in this function. I swapped nuvistors again, and I found one which put the S-meter at a little over S9 at 50 uV, while still giving me the exact same overall sensitivity. So if your S-Meter is reading wildly high, but the receiver is otherwise ok, this is one area to check. This is also why you shouldn't swap 6CW4's for the 6DS4's used in the front end. The DS4 has a more controlled variable gain cutoff, which allows the AGC to work properly.

So now my MK III is now working very well. So I'll probably be running it during the roundup tonight.....


I have a similar "slightly weak" receiver in my Tram Titan IIa. I wonder.......

[2600]

Congratulations! That's a weird one, but not all that rare. L2 almost never looks bad, but when tested may show only a little loss of inductance reading compared to a new, undamaged part.

I tend to blame that one on a past lightning surge that was a "near miss". If your antenna is connected when the neighbor's tree gets hit across the street, you'll still experience a VERY large surge coming down the coax. I have seen it prompt a fat blue arc from center pin to ground on the unplugged PL-259 when a utility pole down the alley was struck. I suppose if a mobile radio with a 250 kW amplifier were to park in your driveway and then just "tap" the mike key you would get a similar effect. Take a guess at the wattage represented by even a small lightning stroke. Think of it as a large, brief pulse of VERY broadband RF. Literally "DC to light" and beyond. A lightning stroke also shines brightly at X-ray wavelengths.

Surge damage to that choke will nearly always be "hidden", and won't make its plastic body look at all damaged.

The one symptom that I associate with this hidden failure is for the front-end stage of the radio to oscillate. Turning up the level of your signal generator should get you a smooth increase in the S-meter reading. If the S-meter reading stays really low below say 400 uVolts, and then jumps up to a 20-over reading at 1000 uVolts, I just replace the choke first and then see if anything else is wrong.

The part we used for years was a JW Miller number that came from Digi-Key. It got discontinued and replaced with a lead-free part. I'll look for the new number. Not all 18 uHenry chokes are created equal. Seems to me we found that the choke's spec for "self-resonant frequency" is important in this circuit. Pretty sure we found that this spec had to be above 27 MHz more or less, to work right.

And if the wire's insulation on L1, the antenna-input coil looks darkened near the middle, this is a clue to one or more previous "near-miss" episodes with lightning, too. And a good reason to just replace L2 and ask questions later.

Oh, and the Titan II uses about the same circuit, but with a single two-section tube. But the equivalent RF choke they use is bigger, and the windings are visible. Don't see it get clobbered often.

73

[mark4]

Had the same problem with a Mark IV.
L102 was not the culprit and neither was L101.
Everything checked out OK but sensitivity is low.
Had me stumped so I changed out the board.
Need to go back to it sometime.
It was from a NOS receiver I picked up a few years back.

[Sandbagger]

Jul 26, 2009 1:21:17 GMT -5 2600 said:

Congratulations! That's a weird one, but not all that rare. L2 almost never looks bad, but when tested may show only a little loss of inductance reading compared to a new, undamaged part.

I tend to blame that one on a past lightning surge that was a "near miss". If your antenna is connected when the neighbor's tree gets hit across the street, you'll still experience a VERY large surge coming down the coax. I have seen it prompt a fat blue arc from center pin to ground on the unplugged PL-259 when a utility pole down the alley was struck. I suppose if a mobile radio with a 250 kW amplifier were to park in your driveway and then just "tap" the mike key you would get a similar effect. Take a guess at the wattage represented by even a small lightning stroke. Think of it as a large, brief pulse of VERY broadband RF. Literally "DC to light" and beyond. A lightning stroke also shines brightly at X-ray wavelengths.

Surge damage to that choke will nearly always be "hidden", and won't make its plastic body look at all damaged.

The one symptom that I associate with this hidden failure is for the front-end stage of the radio to oscillate. Turning up the level of your signal generator should get you a smooth increase in the S-meter reading. If the S-meter reading stays really low below say 400 uVolts, and then jumps up to a 20-over reading at 1000 uVolts, I just replace the choke first and then see if anything else is wrong.

The part we used for years was a JW Miller number that came from Digi-Key. It got discontinued and replaced with a lead-free part. I'll look for the new number. Not all 18 uHenry chokes are created equal. Seems to me we found that the choke's spec for "self-resonant frequency" is important in this circuit. Pretty sure we found that this spec had to be above 27 MHz more or less, to work right.

And if the wire's insulation on L1, the antenna-input coil looks darkened near the middle, this is a clue to one or more previous "near-miss" episodes with lightning, too. And a good reason to just replace L2 and ask questions later.

Oh, and the Titan II uses about the same circuit, but with a single two-section tube. But the equivalent RF choke they use is bigger, and the windings are visible. Don't see it get clobbered often.

73

Yea, I pretty much assumed that this condition was most likely from a surge of some sort. Either a near miss lightning strike, or maybe a high power keyup on an adjacent antenna. I didn't have an exact replacement 18 uH choke so I put 2 10 uH chokes in series, and that did the trick. Or as they say, "close enough for government work". Values from 10 uH to 22 uH are usually effective at blocking 27 Mhz signals. L1 looked fine, with no evidence of excessive current flow.

I haven't looked closely at the Tram Titan IIa yet, but its symptoms are similar. The sensitivity is only good down to about 1 uV, which is not weak enough for the failure to be obvious, but I know the radio is capable of much better. I have looked at the receiver, and it is the same basic cascode arrangement as a Browning. I've already swapped tubes and checked voltages. So it'll be interesting to see what a closer examination of that circuit reveals....

[mark4]

Hey 2600,
Have you seen a problem with Mark IV's loosing sensitivity.
What I usually find is bad tuning by someone.
But that front end board has me stumped.
One thing I noticed is the tuning coils are shorter than any other I've seen. Maybe I should change them out. I am drawing straws at this point.

Good to hear you got that Mark III fixed up Sandbagger. It's a good feeling when you track a problem like that down and solve it.