The AM Forum

I made the mistake of turning on my first home brew receiver to use it as an RF monitor, it used to have a well working IF takeoff for the scope, but I found the receiver all hosed up.
It sounds badly distorted and the scope output looks like the receiver is way overloaded even on weak signals, like something is saturated.
I checked the tubes, replaced some, checked all the resistors, all ok, voltages are normal, AGC works, S meter works, IF cans peak up the signal.
Using the scope, the mixer looks ok, LO is ok since the mixer works and the digital display gets a good reading.
The filter seems fine, the output of the Kiwa filter module looks ok, then it goes into the IF chain and goes to pot.

The IF chain is a rough copy of the Scott SLRM receiver I had when I built the receiver.

So what happened?
I went over all the solder connections, checked for shorted bypass caps, everything seems normal.
The receiver has sat mostly, I might have turned it on 9 months ago.
Very low voltage design, 105 volts B+.
?
This receiver always sounded a bit better then number two but had more drift because of the slug tuned LO coils and octal tubes. Tuned nicer also.

Since I have no schematic -
I would suggest checking for a shorted interstage capacitor near the start of the IF chain - possibly input to first IF grid

should have added
check for an open cathode capacitor

IF transformers couple the IF stages together.
They tune fine.

Next step might be to replace all the bypass caps in the thing.
I checked the grid and cathode (and screen) voltages and they seem normal.

Have you checked the detector??

Fred

I once found the cause of tremendous distortion in a National HRO was caused by an intermittent inter-electrode short between filament and cathode in the detector/first audio tube.  It did not show up on the tube tester, but a substitute tube resolved the problem, and a high resistance ohmmeter confirmed the fault with the tube out of the circuit.

May i suggest using an attenuated signal generator with either 400 Hz or 1 KHz modulation loosely coupled, and look at the signal on the scope throughout the IF path, then at the detector and first audio stages?  Locating the fault this way might narrow the need to change lots of parts to find the problem.  Hope this helps... GL

I did exactly that, put the old RCA signal generator into the RX and it seems ok after the mixer, and after the filter (Kiwa), it hits the 1st IF stage and goes to pot.

I tried to buy one of those Kiwa filters you suggested for my home brew, but they were unavailable.  If I recall correctly, they have a solid state amplifier so they do not incur insertion loss.  Perhaps the problem is the filter?  Maybe try bypassing it and see if all is well with just the tube stages in the IF strip.

I made some progress, I re designd the IF amps and got much more headroom before it clips on peaks.
I need to fine tune things though, and increase the plate voltages to 150 from 105.
I also increased the agc voltage in an attempt to limit the overload on strong signals.
As it is, it looks clean  (and sounds clean) until signals get to be 60 over S9 or something like that.
 Only a few people get that strong here, people I can listen to without any antenna connected.

Maybe I can put a pot in the cathode of the 2nd IF amp and see what that does.
If you add more cathode bias, does that lessen the effect of of the agc voltage?
I will find out.

I know Kiwa was not able to make narrow filters any more but still had wider ones.
GREAT filters, like a good mechanical filter.
Mine are still good, the output looks good (normal) on the scope.

Brett,

Does this describe both your homebrew receivers?

6be6 mixer, 6c4 osc tube, a kiwa filter board, into 2 stages (6ba6) of IF amplification, into a 6bj8 low distortion AM detector with agc.
Also on board is a 12au7 s meter amp, and a 6bh6 455KHz xtal osc with variable output (for zero beating AM signals).

What changes in the dual stage 6BA6 IF did you make?

That is home brew receiver number 2, it still works fine and is the main receiver.
The one I am working on is the first one I built, very experimental, octal tubes.

Some parts of it are based on the scott SLRM which is a very odd duck.

6J5 LO
6SA7 mixer,
6SK7 1st if,
6SK7 2nd if,
6H6 detector,
6SN7 (one part, detector amp,
6SJ7 455 KHz xtal osc (bfo for zero beat and IF alignment aid)
6SN7 S meter amp,
vr105 voltage regulator.

The kiwa filter is on the output of the mixer, feeds to an IF transformer to protect the Kiwa.
The 2nd IF is where things went bad, maybe too much gain, but it used to work very well, and the scope output always looked good, then suddenly is looks like everything is flat topping.
I changed every tube with another one at a time, some three times, all the resistors check out, replaced some of the IF transformers (they plug in on octal sockets).

Bypass caps can be really nasty and cause strange problems.
I did a massive replacement of bypass caps in the I.F. chain of an R390A. And that fixed an AGC problem that I was having. I was having no luck isolating where the problem was.
In fact it was two I.F. modules that had no AGC voltage and I did the bypass cap shotgun effect.
I sent one of those modules to a Rick Mish, Miltronix, and he he supposedly repaired the problem. NOPE! Came back doing the same thing.

I did a lot more work on the receiver and got it working ok, but still not as good as one can work.
It overloads on really strong signals, and seems to have less gain on weak signals then it should.
I replaced some bypass caps but need to do the rest.
I changed the IF gain from changing the voltage to the IF amps to a pot in the mixer cathode, the mixer is the first active stage...

I might leave it as is for a bit and work on things I want to work on!

Your problem might be something with your tube selection for the IFs.  Sharp cutoff vs remote cutoff type tubes.

Fred

I think they are remote.
I tried some other types, the results were worse.

The ARRL handbooks have a section about IF amp tube choices and the remote cutoff tubes are (or were) recomended.
They are what I use in my other receiver and that works well.

I got it working quite well, the book also lists cathode and screen resistor values for each tube type, minor changes from what I had, lower screen resistor mostly.

I thought about redoing the mixer, but that is not where the problem is so left it alone.

The AGC just does not seem quite as effective on this receiver for some reason.
It might be the mixer tube and the grids used.

I looked up both the 6sa7 and the 6be6 mixer (converter) tubes.
Very close to each other, the 6be6 uses a bit less grid 3 voltage, -1.5 compared to the -2 volts on the 6sa7.

Try applying a small amount of AVC to the mixer.

The mixer has full AGC voltage...

How much voltage will the AVC deliver? It should be capable of producing about -20 VDC and that should be plenty for even the strongest signals. If it won't come close to -20 VDC you either have an issue with the AVC or something is dragging it down.

I think its around 6 to 8 volts.
I think that is normal for this design, I THINK the other receiver I built is about the same but will have to measure it.
There is only 150 volts on the plates of the tubes, not the usual 250 volts or more....

Maybe the low plate voltage is the problem.

Fred

The other receiver uses 150 volts and does fine.
This receiver used to use 105 volts and it used to work just as good as the 2nd receiver, it just had more warm up drift.

It now takes a very strong signal to overload the IF, but the 2nd receiver never overloads with any signal from the antenna...

I just wonder what had changed...

Well, this was a lot of fun.
I tried many design changes and experiments, what fun, its a worthless home brew receiver with plenty of space to work on things, so its fun.

After one redesign the sensitivity was way down and I finally found my mistake, I put the B+ for the mixer on the hot end of the IF transformer (should go on the cold end).

No sign of overload or clipping.

For the mixer, IF amps and rf gain control I used the Scott SLRM design with only the addition of the filter and a few minor component value changes.

If you look long enough and far enough you'll find the problem.

B+ to the wrong end of the IF xfmr, novice mistake. We'll let you slide this time, but don't make that mistake again.;D

Anyways, glad you found the problem.

Fred

Its working quite well.
No signal gain is low, or rather the background noise level is very low.

Used it all day today and it is working great.
I bridged all three receivers up using 1 to 2 splitters (.2 to 300 MHz) and feed them all into the Marantz audio amp.
I can switch between them very quickly, home brew 1, home brew 2 and the sdr-iq.
I get scope outputs from both homebrew receivers but number 1 looks a bit funky for some reason, a slight bit more response in the upward direction on the scope then the downward direction...?

They all sound good, #2 sounds a bit better for some reason, the sdr-iq has a lot of background hash on weak signals.

Since I have a fan dipole for 80 and 40 meters, I can put one RX on 80 and one on 40 and just switch the audio to listen to one or the other.
Listened to a nice round table on 3885 in the afternoon while playing with a modulator.
Then worked K9LPA on 40 meters without tuning anything...