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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sun 09, 2018 1:18 am 
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Dave Doughty wrote:
35Z5 wrote:
...the 780 has provision for using either NPN or PNP...
Please explain. -Thanks.

Dave

Unfortunately it's not quite as easy Sams would lead one to believe, only requiring jumper swap. The ones I've observed have soldered connections, still swapping ground & B+ connections isn't rocket science. Parts info lists three different alternate output transistors, one being PNP.

BTW now a week later, still very pleased with performance of the SK-3010s.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sun 09, 2018 9:45 am 
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CaveRat, you wrote about putting a resistor in the emitter to prevent excesive currents due to replacing Ge transitors with Si ones...
Quote:
Second it will reduce the overall gain slightly. This prevents self oscillation which is sometimes an issue.


That's what bypass capacitors are for..... To preserve the original gain value of the stage when there is no such resistor at emitter...

The minimun capacitor value should be one whose impedance is a tenth of Re at the minimun desired frecuency.

( Xc<<Re at min desired Freq)

or C=10/(6.28* f *Re) ;where f =freq min desired, Re =Resistor at emitter


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Mon 10, 2018 12:54 pm 
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Good information; I have a P-780 in the restoration pile.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Tue 11, 2018 4:59 am 
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Tim Tress wrote:
Good information; I have a P-780 in the restoration pile.

I've also found two cases of weak reception at lower end of dial, alignment helped neither. Both were outwardly bent blade on tuning cap. On close inspection one was obviously bent(probably from a past service), other was not really noticable. Likely known by most, if adjusting antenna trimmer on low end improves reception, check blades.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Wed 12, 2018 2:08 pm 
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Hi Tom,

Thank you for this post. It gives me some pointers on where to look to chase down the distortion of my GE P-780B radio.

I don't know how you picked the SK3010 transistor. It is listed as a replacement for the original 2N1059 transistor in the Sams Transistor Substitution Handbook. Also listed is the 2N1431 transistor. Unless you already have these old germanium NPN transistors somewhere in your possession, they are very hard and expensive to get.

How about the replacement for the originally listed germanium PNP output transistors? The radio's schematic diagram shows two transistors with GE proprietary part numbers 4JX1E850 and 4JX1E850A. Their specifications could not be found but the 2N1059 basic specifications could be used as a guide for finding substitutes. The 2N1059 has a 180mW max power dissipation, 100mA max collector current and small-signal current gain between 50-100. It's interesting to note that the 2N1059 current gain cutoff frequency is very low at only 10kHz! Since not too many transistors have this low cutoff frequency, this parameter has to be ignored in the search.

One basic requirement for a transistor to be used as a large-signal audio amplifier is that its current gain stays relatively constant over a wide range of collector current. With this in mind, a short list of ge PNP transistors of the same period I could come up with is: 2N321, 2N382, 2N383, 2N526, 2N527, 2N1925 and 2N1926. These transistors have a 225mW max power dissipation, 400mA or 500mA max collector current and a current gain range about the same or better than the 2N1059.

The first two transistors in the list are of particular interest (at least to me) because of their availability and price. The 2N321 was used as output transistors in several GE portable radios in the 700 series, several Silvertone 1959 and 1960 models, and also in several French made radios of the 1960s. This transistor can be had for less than $2 a piece. The 2N382 can be bought at Electronic Surplus for $2 a piece.

35Z5 wrote:
I've also found two cases of weak reception at lower end of dial, alignment helped neither. Both were outwardly bent blade on tuning cap. On close inspection one was obviously bent(probably from a past service), other was not really noticable. Likely known by most, if adjusting antenna trimmer on low end improves reception, check blades.


Bingo! I have a GE P-726B with weak reception at low frequencies. I didn't know what to do to fix it until I saw your note. You gave me an answer without me asking for help!

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Wed 12, 2018 5:18 pm 
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Binh, glad to help. The numbers you listed will be helpful.

I have the transistor stock from a deceased friend's long closed Radio/TV shop, also GE, Syl, & RCA replacement guides from around 1980-82. I took the easy route, looked in guides and made a list of replacements. :mrgreen: Of course other than auto radios, by later '70s most radio service was history.

Basically boils down to there are few Ge types(mostly SK-300x) that possibly could be used in these early radios. I felt lucky to dredge up the two 3010s I used. I found a third(actually first) that had been installed, gave considerably less gain than the two still sealed in package. It likely would have performed OK with the better of two I removed, but I needed a 2N1059 driver for second 780B.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Mon 17, 2018 4:02 am 
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bb.odin wrote:
How about the replacement for the originally listed germanium PNP output transistors? The radio's schematic diagram shows two transistors with GE proprietary part numbers 4JX1E850 and 4JX1E850A. Their specifications could not be found but the 2N1059 basic specifications could be used as a guide for finding substitutes.


Using GE's replacement guide, 4JX1E850(no listing for A version) crosses to a GE-8. Apparently used in P-780G, probably others. The '82 RCA SK manual calls for SK3862/103, with 103 being the ECG replacement. I see no reason the more robust SK-3004 would not be fine. Of course doesn't hurt that I found a couple of those in my stock.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Jan Sun 13, 2019 10:47 pm 
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Back to "hijacking" Tom's thread...

Thanks to Huesby's info, finally got my Arvin 9562 which uses difficult to find NPN audio outputs back into service, please see old thread for details: viewtopic.php?f=4&t=297341

Hope the circuit details will be of help to others...

John


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: May Tue 24, 2022 3:36 pm 
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I have a bad NPN output transistor in my P-780A, so I am going to try the silicon modification. I have quite a few NPN transistors on hand, with a little heat sink attached; will start with a 200 ohm resistor in series with a diode, for bias.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: May Tue 24, 2022 11:13 pm 
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A series resistor of 240 ohms seems to be the best compromise between current draw and distortion. The output signal is very clean, until I start to drive it below about 50 Hz, probably due to transformer losses. Resting current is about 40 ma; I kept the original 4.7 ohm emitter resistor. The next thing will be finding replacements for the dial bulbs.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: May Wed 25, 2022 4:22 am 
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I suspect you've seen this, copied from a P-780H, what's in my 780A. H uses PNP xistors, I retained the NPN. Beginning with late B, I'm 99.9% all versions used PNP.

I'm extremely pleased. Best I remember, idle current less than 20ma. My notes are up in bell tower & I'm still sore from two hernia operations. I'll recheck first chance I get.

EDIT
Rather than go ladder climbing, I pulled out a Simpson 461 meter I have here in house. With fresh battery, operating at 9.3v, idle current is 12ma. Normal volume is approx 20 ma and loud, can easily bump 50 ma.


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: May Wed 25, 2022 6:18 pm 
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With the transistors I used, if the current is reduced, I started to get crossover distortion on low frequencies at high volume. It idles at 40 ma, and goes much higher at high volume. For outputs, I used small silicon power transistors, with a beta of 216. I kept the original bias resistors on the output transistors.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: May Wed 25, 2022 9:49 pm 
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A listening test on my Part 15 station shows good fidelity at normal volume, but distortion on bass peaks with the volume turned up. I might try different output transistors.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: May Thu 26, 2022 1:22 am 
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Success! Tom, I took your lead and replaced the outputs with a pair of SK-3010 transistors that I found in my stash, converting the bias circuit back to original. The resting current came back down to about 18 ma, bouncing up to about 55 on the peaks at high volume. Evidently, the silicon subs that I used were too "hot".

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: May Thu 26, 2022 2:59 am 
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Glad it worked, one I have with SK3010 has been fine.

In addition to gain, I suspect there are other matching considerations. The 2SC971 I installed are matched pairs, no hint of distortion. No idea of gain as at time I didn't have a tester that I trusted. Pinned scale on the germanium testers I have.

Check this out, I suspect crossover voltage is as, if not more important than matching gain.

Mid page discusses crossover distortion, biasing for high idle current pushes transistors into class A operation.

https://www.vintage-radio.com/repair-re ... ages1.html

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: May Thu 26, 2022 3:23 am 
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From the way that the output stage behaved with bias adjustments, I had a feeling that it was turning into class A operation; it kept demanding more current as I tried to reduce the distortion. I was up to 500 ohms on the series resistor, and it still wasn't enough; by that time, the resting current was up to about 80 ma. At that point, I realized things were going in the wrong direction.

It sounds MUCH better with the SK-3010s. I did try to match the gain closely with the silicon subs, so that wasn't the problem.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: May Thu 26, 2022 3:03 pm 
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I wonder if separating emitters and using a 10 ohm resistor on one with a 15/20 ohm pot on other could compensate for mismatch?

Yeah, easier said than done on a 780.

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