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 Post subject: [Solved] Help needed with IF stage oscillation/motorboatin
PostPosted: Aug Fri 07, 2020 11:57 pm 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
I've been building a superhet and have completed the audio stage and the IF amplifier. I'm using standard AA5 IF transformers with external 100pF silver mica caps (original caps inside removed due to silver mica disease).
The IF amplifier pentode is a 6Ж1П (6J1P), which is the Russian equivalent of the 6AK5; it :mrgreen: 's sitting inside a shielded socket. The detector is a germanium point contact diode, otherwise everything is pretty standard. The B+ is bypassed to the ground with a 100nF capacitor (not marked on the schematic below).

I haven't built the receiver front-end yet, but I'm using a 100k parallel shunt resistor on the primary of the first IF transformer, to simulate the plate impedance of the converter tube.

I'm getting a really bad motorboating with this IF amplifier and cannot point to any single reason why. Even lowering the screen grid voltage to 30V doesn't help, the stage goes into motorboating once the transformers are tuned up to 455kHz.

Potential factors;

1) The receiver is compact, the transformers are relatively close to each other

2) The external 100pF caps (brown, dipped silver mica) are soldered to the legs of the IF transformers, thus outside of the shielding

3) The 100nF and 68nF bypass capacitors are of foil type, ceramic caps would probably be more ideal

4) Using the (6Ж1П) 6AK5 is not very common as IF amplifier, this tube was designed for WW2 radar use, but its characteristics are very similar to any other pentodes that are commonly used in IF stages. One thing I noticed is its Plate-to-Grid capacitance is in the 0.02pF range, whereas for other, commonly used IF amplifier pentodes the Plate-to-Grid capacitance is an order of magnitude lower, i.e. 0.002pF. Wondering if this higher inter-electrode capacitance is sufficient to completely de-stabilize the stage, and what is the solution to get rid of the self-oscillation.

Would appreciate hints or comments w.r.t. what to look for.

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Last edited by szoftveres on Aug Thu 20, 2020 6:34 am, edited 2 times in total.

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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 1:16 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2395
Location: Saskatoon
Does it still motorboat if you short out the 68n capacitor (effectively removing the AVC)?


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 1:51 am 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
Quote:
Does it still motorboat if you short out the 68n capacitor (effectively removing the AVC)?


It doesn't. It just goes straight into oscillation flat at 455kHz.


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 5:51 am 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
I've got a hunch that those capacitors should be inside the IF cans.

The capacitors with the IF transformer leads form small loops outside the IF transformer (essentially a 3/4 turn air-core inductor), exposing a tiny part of the LC tank. And these tiny "loops" are facing each other, possibly establishing enough inductive coupling between the plate- and the grid tuned circuits. Will experiment tomorrow a bit with moving the caps inside and see what happens..

(I've always thought that the silver mica caps were inside the can just to annoy radio hobbyists with the well known disease - maybe there's another -good- reason for it)


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 6:15 am 
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Joined: Mar Thu 21, 2019 3:53 pm
Posts: 174
Try an EF93 instead. EF 93 is a variable-mu pentode which is suited to be used in stages with AVC


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 6:29 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 1073
Location: Tokyo
I've repaired 455KHz IFTs with external 100pF mica caps, just as you have done, but have had no problems with outright oscillation or motorboating. So, those external caps are not necessarily the cause of your problem.

I'd try decoupling the 6AK5 plate: for example, a 1K resistor and .01 cap inserted between the IFT primary and the B+.

I wonder if you are getting a bit of RF feedback through the AVC line. I'd move that line to the other side of the 47K resistor (that 150pF/47K/220pF pi filter) to get more filtering.

In your wiring photo, I see what seems to be an orange B+ wire coming very close to the 6AK5 plate tube socket lug, then going on to some other place. I'd keep that wire away from that lug.

Btw, I've read that sharp cutoff pentodes like the 6AK5 can mimic remote cutoff characteristics by using a high value screen resistor, say, 47K or so.

Rob


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 6:40 am 
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Location: Tucson, Arizona U.S.A.
The AVC fliter resistor is rather smaller than usual. It is commonly 2.2 or 3.3 megohms. That probably won't solve the immediate problem but may help in the future.

I can't tell from the photo; how are things grounded? Do you have places where ground currents from the output of the stage can flow in input ground circuits?

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Jim Mueller


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 7:16 am 
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Joined: Apr Thu 09, 2020 7:33 am
Posts: 155
You are aware that the green dot on the transformer goes to the grid right? If they are not wired right it will promote oscillation by a positive feedback. I can't remember what terminal the plate goes to but it must go to the correct one and so does the b+ and avc. When you put the transformer back together I'm sure you put the wires back on the right pins?


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 8:10 am 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
Chriscrosley1 wrote:
You are aware that the green dot on the transformer goes to the grid right? If they are not wired right it will promote oscillation by a positive feedback. I can't remember what terminal the plate goes to but it must go to the correct one and so does the b+ and avc. When you put the transformer back together I'm sure you put the wires back on the right pins?

No - I don't see any green dot. Where is it? Does this have to do anything with the "outer wiring" of the coil, analog to film capacitor "outer foil" ?

Jim Mueller wrote:
The AVC fliter resistor is rather smaller than usual. It is commonly 2.2 or 3.3 megohms. That probably won't solve the immediate problem but may help in the future.

6AK5 only allows up to 1Mohm between the 1st grid and gnd, but otherwise I agree.

Jim Mueller wrote:
I can't tell from the photo; how are things grounded? Do you have places where ground currents from the output of the stage can flow in input ground circuits?

The chassis is on the ground - as well as the black wires. The white brick shaped capacitor is the AVC 68nF one, its left side is the common point, the bottom of that lug is bolted to the chassis.

shinkuukan wrote:
I wonder if you are getting a bit of RF feedback through the AVC line. I'd move that line to the other side of the 47K resistor (that 150pF/47K/220pF pi filter) to get more filtering.

Will try that (will also contribute to the mimic'ed remote cutoff behaviour to some extent).

shinkuukan wrote:
I'd try decoupling the 6AK5 plate: for example, a 1K resistor and .01 cap inserted between the IFT primary and the B+.

Not sure if that will help as the input of the IF amplifier is literally hanging in the air right now. But will definitely do once the converter is in the circuit.


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 8:36 am 
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Joined: Apr Thu 09, 2020 7:33 am
Posts: 155
Try swapping the two gray wires on the input transformer the input grid and 680k/68n. There is a green mark on the plastic case where the terminal comes out you can barely see it. The other one is a lot more clear and I can see it. It goes to the diode which is correct.


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 9:35 am 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
I tried swapping the transformer leads between the control grid and the AVC, which didn't cause any change in the self-oscillation.

What is the meaning of that green mark? A coil is a very simple component, it has two ends. What does the green mark signify (I'll try to find it, maybe the light)?


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 7:52 pm 
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Joined: Apr Thu 09, 2020 7:33 am
Posts: 155
For maximum rf radiation from primary to secondary the outer part of the winding should go to the tube. I think that's why


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 8:08 pm 
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Joined: Apr Thu 09, 2020 7:33 am
Posts: 155
Temporarily remove the avc wire between the diode and 820k and ground the 820k if it stops oscillating then the avc will have to be re configured. On typical AA5 sets the detector has the anode (in the case of a tube diode plate) is connected on the top of the output IF (green mark) and cathode is ground and bottom of output IF is where the avc and audio input to a 500k audio taper goes and they use a 2.2 Meg from the 500k audio taper input and the other end of the 2.2 Meg to a .05 uf to filter out the audio for the avc. The wiper goes to a .01uf then to the audio amp.


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 8:30 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3914
Location: Lexington, KY USA
I think the AVC has already been checked:

Does it still motorboat if you short out the 68n capacitor (effectively removing the AVC)?

It doesn't. It just goes straight into oscillation flat at 455kHz.


I second the suggestion about grounding. That and lead dress.

For instance, how is the shield in the center of the tube socket grounded? I don' t see any connection right to the chassis. You might try a strap from one socket mounting screw hole across the center and down to the other mounting screw.

It's possible to add some shielding using brass shimstock or raw PCB material. May need some extra holes in the chassis to ground it.

Ted


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 8:48 pm 
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Joined: Apr Thu 09, 2020 7:33 am
Posts: 155
Maybe amplifying too much try lowering screen AND plate voltage. I think those if transformers are impedance matched for AA5 12ba6 or 6ba6


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 9:13 pm 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
Update:

1) I totally grounded the AVC capacitor
2) Also grounded one end of the primary of the 1st IF transformer (it was floating in the air previously)

Oscillation is gone.

I inspected the IF transformers with a bright flashlight and can indeed spot the green marks - will need to swap the wires on the grid circuit.

The tube is still running on 30v screen voltage, will try bringing it back to around 90v.


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Sat 08, 2020 10:13 pm 
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Joined: Apr Thu 09, 2020 7:33 am
Posts: 155
Remove the 100nf on the cathode that just might be all it needs. I built a radio on a cardboard box and cardboard doesn't shield


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Mon 10, 2020 4:13 am 
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Joined: Apr Thu 09, 2020 7:33 am
Posts: 155
How's it going


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Mon 10, 2020 5:24 am 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
Brought the screen voltage back to 90V, learned that the 6AK5 isn't the best choice for tuned IF amplifier (Ratio of Transconductance-to-Cgp is bad, article here: https://www.robkalmeijer.nl/techniek/el ... index.html) so tried with a 12BA6 which I had on hand (providing heater voltage with appropriate wall DC adapter), still oscillating. AVC is shorted to the ground. I have absolutely no idea.


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Mon 10, 2020 8:29 pm 
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Joined: Jul Wed 22, 2009 3:07 pm
Posts: 866
Hi Szoftveres,

Any chance you get feedback between the two IF-transformers, if it comes from the layout?
See if some additional shielding between the two transformers kills the oscillations!
Put a steel plate between them temporarily to test the theory.

Regards, Peter


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