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 Post subject: Re: Cause of severe ringing in transistor local osc output ?
PostPosted: Mar Sun 15, 2020 12:42 am 
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Joined: Jan Thu 16, 2020 12:29 am
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Usually Lurking wrote:
A couple of things to try would be to add an unbypassed resistor in series with the emitter of the transistor, and to increase the value of R9.

Each measure will decrease the gain.

Hard to say what values might work. It's a chance for cut and try. Might start with around 100 ohms, or 100 ohms extra for R9.

Ted


I agree.


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 Post subject: Re: Cause of severe ringing in transistor local osc output ?
PostPosted: Mar Sun 15, 2020 4:20 pm 
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Thanks much Guys !

Will hopefully give that a try in the next couple days and report back...

John


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 Post subject: Re: Cause of severe ringing in transistor local osc output ?
PostPosted: Mar Sun 15, 2020 6:20 pm 
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After I slept on it I noticed something. Unless that scope trace is a single trace that is stored, the higher frequency is synced to the lower frequency. That would mean the higher frequency is a harmonic of the lower frequency, so it probably isn't simply squegging. However if it was just a harmonic resonance (due to another part of the circuit) it would be more constant. The fix is similar in any case. One thing to look into is the power supply bypassing capacitors, the larger values actually don't work as well at very high frequencies as would be expected by there value. Try adding something like 100 pf ceramics.

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 Post subject: Re: Cause of severe ringing in transistor local osc output ?
PostPosted: Mar Sun 15, 2020 6:40 pm 
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Notimetolooz wrote:
What the scope is showing is not ringing, that usually applies to something like a square wave and is due to the abrupt signal swing.


It's some sort of ringing - you can see it get excited, then start damping out, then get excited again.

Brett

p.s. on further study, it looks to me that you are probably getting some sort of clipping, the discontinuity when it tops out is what is exciting the ringing. I cannot see your scope settings, but the positive swing appears to be flat-topped, presumably when X2 hits the upper rail. I also note that it is asymmetrical, clipping on one side but not the other. I would check the resistors and caps around the oscillator circuit, particularly R10, R12, and C10


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 Post subject: Re: Cause of severe ringing in transistor local osc output ?
PostPosted: Mar Sun 15, 2020 6:55 pm 
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ACORNVALVE wrote:

It is quite surprising that many transistor radios end up injecting less than a perfect looking sine wave into their mixers for the reasons outlined above. The higher frequency harmonics generally don't cause a significant degradation in the performance, but it is really better to have low distortion sine wave (free from significant harmonics) injected into a mixer.


Agreed with that, but given the wide range of BC frequencies, in this case, the "images" are probably the mixed result of the ringing with the carrier, so you can expect that there will be some input from a single carrier at at least two oscillator frequencies. This rarely happens on other bands, like FM, because of the small range of frequencies involved, you can't have a harmonic of the primary oscillator frequency + carrier fall in the range of the IF.

Brett


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 Post subject: Re: Cause of severe ringing in transistor local osc output ?
PostPosted: Mar Mon 16, 2020 11:21 am 
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The CK 760 is a germanium transistor and the mock up is using a silicon
type.

I am wracking my brain remembering stuff about transistor common emitter
amplifiers in terms of how the calcs were done with a damned slide rule.

Your retro project might be to find out the difference between silicon and
germanium , perhaps the rE and re numbers. Evil thoughts about thevenin
way of finding gain..

Given the adjustments using resistors, can you make an oscillator coil with
different ratios, of those two non tuned windings?

Your scope can work with two probes instead of one. In this mode
the ground clips for the probes are not required. Just make them
go away.

The differential mode measures voltages that are not referenced to anything
except themselves. It is just like a voltmeter hanging in space. With the X10
settings, the scope shouldn't disturb anything.

Here is a scan of part the 7th ed. GE transistor manual (1964).
Attachment:
Find an old book.jpg
Find an old book.jpg [ 365.35 KiB | Viewed 814 times ]

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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Mon 16, 2020 2:14 pm 
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Does the scope picture look the same whether or not a station is tuned in?

Dave


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 Post subject: Re: Cause of severe ringing in transistor local osc output ?
PostPosted: Mar Mon 16, 2020 4:38 pm 
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radiotechnician wrote:
The CK 760 is a germanium transistor and the mock up is using a silicon
type.



I had forgotten that this was the one with the erzatz transistors! The scope suggests that the bias is not correct -it's not operating in the center of the linear range. That's why it is hitting the rails on one side and not the other. A secondary issue may be that the silicon transistors function better at higher frequencies. They may have been counting on the transistor rolling off to avoid harmonics.

Brett


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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Mon 16, 2020 6:26 pm 
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I second what Brett said. Wrong bias or overdriving the mixer's base could indeed generate harmonic content.

This part of John's thread was originally posted in another forum which focuses mainly on tube equipment.

Dave


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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Mon 16, 2020 7:07 pm 
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Tim , Ted, and ACORNVALVE---Experimented with added series damping resistor and non-bypassed emitter resistance for "degeneration" to lower gain.

Found that increasing R9 to around 2K resulted in a nearly perfect sine-wave across band at "tickler", though the amplitude now decreased at high end. But the reception was worse with more images and oscillations down the line which I didn't have time to investigate further.

Using a small pot, also obtained an un-bypassed resistance which evidently reduced gain and resulted in a lower amplitude, but clean waveform---but oscillation ceased below approx. 900 kHz (on tuning dial)...

Also added a socket on bottom side and experimented with several different old NPN silicon transistors from parts drawer---the results were all over the place, some producing bizarre waveforms I've never seen while "tuning" across dial...

Dave---the waveform does stay similar even when station is tuned in and you can see slight "artifacts" of the "ringing" through the IF chain.

Brett---this is the oscillator circuit, but perhaps there's some kind of feedback from the mixer if transistor not performing properly ? The signal at tank circuit of the oscillator is clean sine-wave of approx. 19 Volts P-P, no clipping there... Agree that the gain and high-freq response of the silicon transistors are problems with this.

Steve---want to use the original Raytheon oscillator coil, so might just go back to germanium transistor if this circuit can't be modified to work...

Thanks Guys !

John


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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Mon 16, 2020 11:52 pm 
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Played with resistors all afternoon and got the oscillator output waveform cleaned up---the hours of experimenting finally paid off !

Have some other business to attend to now, so will post more detail when able...

John


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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Tue 17, 2020 2:00 am 
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Location: Sunnyvale CA
xrhonda91 wrote:


Brett---this is the oscillator circuit, but perhaps there's some kind of feedback from the mixer if transistor not performing properly ? The signal at tank circuit of the oscillator is clean sine-wave of approx. 19 Volts P-P, no clipping there... Agree that the gain and high-freq response of the silicon transistors are problems with this.


The sine wave is driving that transistor. The drive signal is good and the output is bad - which suggest (actually, demands) that the problem is in that amplifier somehow. or in the next one down the line.

19V peak to peak? What is the power supply for this radio?

Brett


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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Tue 17, 2020 5:40 am 
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Dave Doughty wrote:
I second what Brett said. Wrong bias or overdriving the mixer's base could indeed generate harmonic content.

This part of John's thread was originally posted in another forum which focuses mainly on tube equipment.

Dave


To expand a bit, any time a smooth function hits a sharp edge, effectively, that is very similar to a square wave in that it generated wide-band harmonics. Just try to imagine fitting a Fourier series to it, to get the edges sharp, you have to have large components at extremely high frequencies, just like a square wave, just not as much. That's exactly what the scope shows - the top of the sine wave is getting clipped. When it is saturated, there's no high frequency ringing possible. As soon as it comes off the saturation, all that high frequency content rings up some other natural frequency. You can see it then ring down (with, eyeballing it, 5-10% damping) nice and smoothly until it gets all the way to the bottom, and then comes back up, smoothly. Then it hits the limit again, ringing it back up.

I am not sure why or where this is happening, but the fact that the output has an apparent DC/very low frequency offset, and it is clearly clipping on the positive swings but not the negative, suggests that the bias on some transistor is not correct, because the purpose of the bias it to put the transistor in question in the middle of the linear range of operation. Some of them have been changed from germanium to silicon, and the primary difference is that the forward drop went from .3 volts to .7 volts which would certainly necessitate a change in the bias.

Brett


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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Wed 18, 2020 3:09 am 
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Brett---thanks for the interesting explanation. Could the problem simply be an impedance issue of some sort ? And the B+ is approx. 5.2 Volts at the oscillator... And have built this entire chassis using silicon transistors.

Was able to eliminate the clipping and almost all of the ringing by simply adding damping resistors to oscillator coil. Took a lot of experimenting to get just the right combination so that the oscillator remained stable through entire tuning range. Ended up adding 100K across winding between collector and OSC gang of tuning cap, 22K across winding between OSC gang of tuning cap and 270 Ohm resistor, and replaced R9 with 1.1K. There is still a tiny trace of ringing on waveform which I couldn't eliminate, but the radio seems to perform/sound noticeable better. There are still a couple images on lower end of band, but am going to ignore that for now.

The AGC is now "the problem of the hour"... On the 7RT1 chassis, Raytheon simply reduced gain on the mixer only by lowering bias voltage---and with the low-noise silicon transistors, this works very well when done "manually". Unfortunately, the output of detector diode on very strong signals doesn't go low enough to accomplish this using resistance bridge similar to original with the silicon mixer transistor, so am experimenting trying to come up with a simple AGC amp circuit.

Below is pic of oscillator secondary output after additions of damping resistors...

John


Attachments:
ray osc 2nd out 3-17-20.jpg
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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Wed 18, 2020 3:46 am 
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Could we see the entire original schematic ?

Below is a Sony Amplified AGC technique.
Attachment:
Note amplified AGC via R14.jpg
Note amplified AGC via R14.jpg [ 254.17 KiB | Viewed 710 times ]

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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Wed 18, 2020 4:33 am 
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Hello John,
nice work


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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Wed 18, 2020 12:22 pm 
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Zenith used the amplified AVC technique in some of their sets. Detector AVC is applied to the RF transistor which passes amplified AVC to the mixer and 1st IF transistor in, for example, the Royal 7000 T/O.

Dave


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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Wed 18, 2020 6:46 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
Some oscillator coil damping tricks. Brass cores of various sizes. Try copper rings,
various diameters, widths, on sticks, to slip over coil to see what happens.

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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Wed 18, 2020 10:36 pm 
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Location: Indianapolis, IN
Thanks Guys !

Steve---this radio has been built from "scratch" stage by stage using a home-made plastic chassis with the original Raytheon antenna/tuner/volume control sub-chassis, IF transformers, oscillator coil, and two of the audio transformers along with silicon transistors I had on hand and the necessary resistors/caps for biasing, bypassing, coupling, etc. Did use the original oscillator & mixer circuits, but the others have been modified. Plan to draw a schematic once radio complete and working properly.

Am still playing with AGC design. Decided to add a transistor directly to output of 3rd IFT for "AGC amplifier" and leave the BAT-46 as detector-only. Had read that these diodes perform better with slight bias, so played again with resistances and found that a 220K gave the best improvement in audio quality---and it's very noticeable (may try this on other radios) ! After experimenting with resistor values, the added AGC-amp transistor did make a difference, but still can't get the gain low enough with it to eliminate the strong signal distortion---but a few of my "production" sets also overload on the signal since station is so close, so might just live with it. Tried a couple "overload protection" circuits found in my schematics, but they didn't work well with the silicon IF amps...

On it goes...

John


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 Post subject: Re: Raytheon 8TP-2 "temporary" home-made chassis project...
PostPosted: Mar Sat 21, 2020 1:51 am 
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No luck with a simple AGC amp circuit, so thought I'd try rebuilding the mixer stage with the more common emitter injection of oscillator signal to see how it would perform. The oscillator waveform was now much cleaner and could remove the parallel damping resistors on windings. Mixer output was a bit unstable (oscillations), but would stabilize when scope probe or any other test-lead attached---played with caps to ground and a 12 pF bypass took care of it. Performance similar to other circuit, but just didn't sound as good and lost ability to easily varying mixer gain with base bias as could be done with the original circuit. Am thinking about adding transistor sockets and switching over to PNP germaniums which would solve a lot of problems !

John


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