Browning 4A receiver. RF gain drop

[tubefan]

I posted about the transmitter for this radio. Got it from out of state and was supposed to be a working radio bla bla bla. Transmitter ended up needing a mode switch and a bunch of other stuff.

Now on to the receiver. I hooked it up to my antenna and it received. BUT! the signals were off the chart s30 and the RF gain dropped the signal to negative pegged back past the 0 with just a touch. So I pulled the tubes and test them one by one. Half are shorted. The rest weak.

Retube the entire receiver including the 2 nuvista's with all tested brand new NOS high quality tubes. Cleaned all controls with deoxit.

Plug it back in and adjust meter once warmed up. Signals now kind of normal maybe a little weak but no alignment at this point yet.

I touch the RF gain control and with just an 8th of a turn the signal drops to nothing. I figure I have an NOS pot here so why not see if that fixes it. Put the new pot in and same thing. AGC is working. RF gain just drops the signal rapid.

Should I go through alignment and see if that cures this or is there something else I can check? Wanted to have the receiver working pretty decent before I spend the time to align and recap. This is a Nichols built 4A.

Hit me with questions and shoot some tips my way.

Thanks

[tubefan]

Went through alignment. At 50uv I get S8. At 3uv I get s1. At 1uv I can hear 1000hz 30% tone but no signal. At 100uv I am at s9. RF gain still drops rapidly. A couple of things I noticed. If I very slowly turn rf gain down with the s9 signal. At an 8th turn it drops to 0. If I keep going slowly just before half I hear the signal come up a spec then start to drop again. Once past half down I'm negative 0 pegged left. Second thing to note, if I jump out the middle leg on the pot with the end one with the wire on it while receiving a signal I get almost 1s jump up on receive. This is with both pots. New and original. HELP!!

[2600]

Have a look at the DC voltage on the screen-grid pin 5 of each 6BA6 tube. With no signal and the RF Gain full up, should be at least 110 Volts, typically 130. A bad tube can damage the 47k resistor feeding this pin. A close look with a bright light on other resistors in this area might reveal a resistor that looks darkened.

Few things more annoying than a receiver that delivers 95% of the sensitivity it should have. More than one way to lose that last few percent below a couple of microvolts.

73

[tubefan]

Pin 5 of each 6ba6 is 230v dc. Not reading 130v.none of the resistors look burnt or discolored

[tubefan]

Pin 6 off each 6ba6 has 143v dc. Except the middle one has 130v dc. The 47k resistors feeding them are all good

[tubefan]

Referring to schematic that looks close to normal

[2600]

My bad. Pin 6 is the screen grid. Pin 5 is the plate.

Oops.

Well, that's not the cause.

The one other clue to watch for is the behavior of the tuning slugs during alignment. Not all of them exhibit the same sharpness when peaked. The two left-most 455 kHz IF transformers T201 and T202 have a fairly broad peak. T203,204 and 250 are sharper. A slug that won't peak suggests a fault inside that can or in the adjacent circuit.

L101, the antenna-input coil is vulnerable to surge damage if lightning hits a neighbor's tree while the antenna is plugged in. It typically won't show much of a peak at all if it's damaged. A proper peak won't be terribly sharp when L101 is working like it should.

Grasping at straws here. The easy-to-find faults don't appear to be causing the problem.

73

[tubefan]

I noticed l101 peaked but not much at all even with several turns. It did peak just not drastic. I have a junk receiver that has been stripped but that coil is still in there. Maybe tomorrow I will swap it out and see if it helps.

[tubefan]

Ok, changed out the antenna coil and same thing. A friend of mine found a thread on here talking about funky s readings and such on these receivers. It was suggested that the choke L102 can change value.

I figured what do I have to lose? Took it out of my old parts chassis and swapped it out. Put signal generator on it at 50uv and was at 15 over. Hooked antenna up and signals were booming in with DX.

Only locals on were on SSB so I tuned them in and they were pretty much where my other solid state radio was as far a S readings.

Went back to AM and dialed around and some stations were 40 over. So I think we are too high now on S readings.

Should I try a realignment since I changed out the coil and choke or what's your thoughts on next step?

[tubefan]

Ok, just went back and re aligned with the new parts. I think it's all set. I will let it run and wait for some known signals and see how it looks. Tons of DX coming in and some are 20 over but they are super bowl power. Keep you posted on how it looks once DX fades and locals near and far pop on.

Thanks for the help. It's a fun hobby if people share.

[Sandbagger]

May 26, 2019 8:49:04 GMT -5 tubefan said:

Ok, changed out the antenna coil and same thing. A friend of mine found a thread on here talking about funky s readings and such on these receivers. It was suggested that the choke L102 can change value.

I figured what do I have to lose? Took it out of my old parts chassis and swapped it out. Put signal generator on it at 50uv and was at 15 over. Hooked antenna up and signals were booming in with DX.

Only locals on were on SSB so I tuned them in and they were pretty much where my other solid state radio was as far a S readings.

Went back to AM and dialed around and some stations were 40 over. So I think we are too high now on S readings.

Should I try a realignment since I changed out the coil and choke or what's your thoughts on next step?

The Browning doesn't really have an S meter level adjustment, only a meter zero. So if your S meter readings are way off (Should read S9 for a 50 uV input signal), then there's something wrong with the gain in one of the stages. When I was redoing my MK III, I found that just swapping out the Nuvistors with different parts made a huge difference in meter readings, even though all of them tested similarly in the tube tester, and the overall sensitivity didn't seem any different. As you found, the choke L102 is a real bugger when your sensitivity is just a bit low. Nearby lightning strikes can fuse this choke into a short, which then allows a portion of your input signal to leak down the AGC line rather than all of it being amplified by the front end tube. That was the final issue in my MK III. So give your radio a good alignment and when you're done, you should be able to hear the signal generator down to less than .2 uV. And if that's the case and your S meter is still "off" at 50 uV, try swapping nuvistors with different parts and see if that fixes it.

[2600]

Rock on!

Almost suggested L102, but the symptom I'm used to is instability, rather than deafness.

Good work!

Just be sure never to substitute the 6CW4 nuvistor for the 6DS4. Tube-substitution books will recommend it. Probably works in some circuits, but not in a Browning receiver.

Good to hear you got it sorted out.

The one odd fault I associate with sensitivity that seems excessive is grid emission from one or more of the 6BA6 IF tubes.

This happens when particles of the white oxide material that coats the cathode come loose and stick to the tube's control-grid wires. This causes a positive DC voltage to be generated by the tube's control grid. That voltage back-feeds into the radio's AGC line and gooses the other tubes' gain and noise output.

Identify three 100k resistors, R201, R205 and R209. With no signal to the receiver, compare the DC-voltage reading at each end of each of these resistors. One end of each is connected to the receiver's AGC voltage line. The other end connects to the 'cold' side of the IF transformer secondary feeding that one of the three 6BA6 tubes' control grids. If the 'tube' side of these resistors shows a higher (more positive) DC voltage than the other end, the one that leads to the AGC, this indicates a tube that has a grid-emission issue. Sometimes referred to as "gas", a small increase of a tenth of a Volt or so is not a big deal. But a half-Volt or more positive-polarity increase indicates a tube with a grid-contamination issue.

Don't see it a lot, but more than one Browning with "super receive" has proved to be caused by this.

73

[tubefan]

All set. I went through alignment one more time to triple check myself. Did all gain stage checks and voltage checks. Everything was right where it should be.

So I took Sandbagers advice and played with several different 6ds4s. Sure enough it solved the problem with a different set.

I then compared this receiver to a known good aligned 4 receiver and then a mint original virgin 4 receiver and all 3 were neck in neck with the signal generator and stations near and far.

Now onto recap and checking and replacing any out of spec carbon comps.

Thank you again for the help.

[2600]

Rock on!

Sounds like you had at least one gassy 6DS4. Sometimes a tube tester will catch this, sometimes not.

Tube testers are not all created equal.

Good work and 73.