400 FM popping

[lethocerus]

Hi folks, just finished overhauling a 400. Everything works great, but there is a steady popping sound whenever the stereo beam is not touching completely. Can anyone advise how to troubleshoot this? Thanks

 

[NIBW]

That's a tricky one. Does it happen in FM mono or all FM? If there is no issue in mono, then it's likely in the MPX circuit.
If all FM selections, try and troubleshoot by:
- Test all tuner tubes
- Confirm tube sockets are clean and pins are tensioned
- Double-check any work done, especially look for any accidentally shorted components, cold solder, touching tube pins, etc.
- Chopstick tuner section
- Look for arcing on tube sockets either by a visible carbon burn or by seeing the arc in a completely dark room.
- Make sure MPX separation pot is clean

 

[dcgillespie]

So it happens even when receiving known strong signal levels? Make sure there are no near by digital devices, which notoriously radiate clicking and static noise into FM receivers.

Dave

 

[lethocerus]

Thanks for the responses.
It goes away with a strong signal (as the stereo beam closes). No digital/phone stuff nearby..
And yes, it occurs both in mono and stereo. I've already cleaned the tube sockets, and also swapped each tube one at a time with no change.
There is no sign of arcing, and I've not been able to reveal any bad joints - everything looks clean and chopsticking doesn't affect anything.

It's a steady beat, slightly faster than the second hand of a clock.
I do have a scope, just unsure where to begin here.

 

[NIBW]

Look into V1 and V2 in the Oscillator unit.

Otherwise, with your scope, you can signal trace from antenna input forward until you see the noise. I’m assuming you don’t have an FM signal generator but the popping should show on the scope.

 

[dcgillespie]

Check also the electrolytic filter caps serving the tuner section — even if they’ve been replaced. I have seen such caps break down and produce a methodical internal partial arcing that could produce noise that rides on the tuner B+ and result the popping sound you’re hearing. With a strong received signal however, the limiter stages would kick in and prevent any amplitude based B+ noise from being produced in the audio output. Just a thought…….

Dave

 

[lethocerus]

Well, I replaced the caps for good measure and the popping persists.
The scope probe was detuning/killing the oscillator signal at V1/V2, so I began working backwards from V6.
The earliest spot I can discern the pop/blip is the plate of V3. I don't see it at the grid or screen, though the trace at pin 3 of Z2 (top of R31) flashes in time with the pop.
Could this be an issue with Z2?

 

[NIBW]

Now you’re getting down to isolating the issue. I would suspect either the 5000pf cap off pin 6 or more likely Z2.
Z2 could be suffering from ‘silver mica disease’ and failing. Those IF transformers can be repaired if need be.

 

[gadget73]

Are you seeing the plate or screen supply bouncing with the pops? Wonder if you have a failing resistor in the IF chain thats arcing across and causing this.

 

[lethocerus]

Now you’re getting down to isolating the issue. I would suspect either the 5000pf cap off pin 6 or more likely Z2.
Z2 could be suffering from ‘silver mica disease’ and failing. Those IF transformers can be repaired if need be.

I was afraid of that.. any idea what the cap values are in this transformer? The schematic shows one on the primary, one on the secondary... but searching by part # ZZ50210-39 suggests a single 24pF

 

[lethocerus]

Are you seeing the plate or screen supply bouncing with the pops? Wonder if you have a failing resistor in the IF chain thats arcing across and causing this.

Sort of, they bounce a bit but not perfectly in time with the popping. I tried a temp gun on all the resistors and nothing is especially hot..

 

[dcgillespie]

Is the noise methodical in nature — that is, very consistent in timing? Transformers with mica disease can produce a popping sound, but invariably it will not be methodical in nature, but quite erratic. The popping will also be accompanied by a loss or improvement in indicated signal strength, as the affected resonant cap in a transformer either connects or disconnects from the circuit. When it does, a pop sound is made, and signal strength changes.

If the sound is methodical or rhythmic in nature, that suggests either oscillation of some kind, or outside influence.

Dave

 

[lethocerus]

Is the noise methodical in nature — that is, very consistent in timing? Transformers with mica disease can produce a popping sound, but invariably it will not be methodical in nature, but quite erratic. The popping will also be accompanied by a loss or improvement in indicated signal strength, as the affected resonant cap in a transformer either connects or disconnects from the circuit. When it does, a pop sound is made, and signal strength changes.

If the sound is methodical or rhythmic in nature, that suggests either oscillation of some kind, or outside influence.

Dave

Yes the popping is consistent. Doesn't sound like motorboating, more like a drum machine. 102bpm.
There is no DC voltage leaking to the Z2 secondary... pulling V1 or V2 stops the pops seen at V3 plate.
I spruced up another 400 recently, and had no issues like this. Confident there is no radiated noise nearby.
So if this points to oscillation, can you suggest a way to divide and conquer?
Thanks!

 

[gadget73]

Since you're seeing it at V3 which is the first IF amp, try pulling V2, the mixer, and see if its still there. If it is, pull V3 and see if it stops.

 

[lethocerus]

Since you're seeing it at V3 which is the first IF amp, try pulling V2, the mixer, and see if its still there. If it is, pull V3 and see if it stops.

The trace at V3 plate flashes slightly in time with the pops. It does the same at V4, and becomes more pronounced at the grid of V5. Pulling any of the tubes, V1-5, stops the flashing (and the signal).

 

[NIBW]

Measure the secondary pins of Z2 and Z1, they should read 0v DC. Any voltage or fluctuations there would indicate a bad IF.
You can replace the silver mica inside the IF with new but you'll likely need a realignment of the FM.

 

[lethocerus]

Measure the secondary pins of Z2 and Z1, they should read 0v DC. Any voltage or fluctuations there would indicate a bad IF.
You can replace the silver mica inside the IF with new but you'll likely need a realignment of the FM.

No DC on either secondary. Thanks though

 

[gadget73]

If its visible at V3, pulling anything downstream would stop it so that makes perfect sense. If pulling any tube shuts it up though that tends to imply the problem is right up at the RF amp or oscillator, upstream of V3 at any rate.

hm, further isolation, does it stop if you ground the grid of V3? I expect it will, just confirming.

 

[lethocerus]

If its visible at V3, pulling anything downstream would stop it so that makes perfect sense. If pulling any tube shuts it up though that tends to imply the problem is right up at the RF amp or oscillator, upstream of V3 at any rate.

hm, further isolation, does it stop if you ground the grid of V3? I expect it will, just confirming.

Grounding V3 grid results in quiet audio, but the pop is still there. Likewise grounding V4 and V5 grids.

 

[gadget73]

so grounding the V3, V4, or V5 grid still has the pop? It should kill any radio signal since there wouldn't be anything coming in to the grid from the front end but not killing the popping is not what I was expecting.

Grid of V4 should just be whats at the plate of V3, so blocking whats coming out of V3 ought to stop it if V3 was the source. Ditto grid of V5, thats just what shows up at the plate of V4. Since it doesn't, that sort of points me somewhere else. Extra interesting that removing any tube stops it, but killing the signal coming in to the grid of any of the tubes does not.

the extreme regularity of it definitely says some sort of oscillation problem like @dcgillespie said. I've had oscillating IF strips before, but grounding any of the IF amp tube grids would kill it.

squarely at the "I don't know" point here since this doesn't give results that seem logical to me.