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 Post subject: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Wed 09, 2019 4:27 pm 
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I have a black eye, lost a couple teeth, think I have a cracked rib or two. :(

This set has me whipped badly. It's the one I mentioned in another thread that has background noise at minimum volume. My P-780B with similar circuit had a noisy audio driver, gave same symptom. I've replaced driver, the outputs plus first audio, still has noise. I have not tried the bias regulator(Q9) but fail to see how that could cause problem(read on).

Noise is in 1st audio(all of five parts), if I remove C4 it's gone, remove C3 and it's still there. If I remove Q5, jump B & C with 33mfd NP cap, noise is gone. Of course that kills reserve audio amplification. I've tried three different Ger audio transistors, plus a silicon ECG 159 that gives least amount of noise. Audio is clear with silicon transistor and noise is almost not noticable, I'm about ready to install it and walk away.

Yes it's fully recapped.

Schematic is large enough to be easily readable.

https://i.postimg.cc/m27fFzZP/gep784audiosnip.jpg

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Wed 09, 2019 9:57 pm 
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Well I figured it out. Kinda.

I replaced bias transistor and walah, no more noise. BUT audio now is distorted.. ARRRGHHH!!

I then found distortion was approx same with or without the transistor. Sooo I pull out the trusty resistance sub box and connected it so I could vary bias voltage, dialed it till audio was best. Said to radio, "well I fixed your ass". It replied with same noise as previous, Ha Ha got you, then threw me on the floor. :(

Obviously that replacement transistor doesn't work in bias circuit and even if it did, the noise is still present. Apparently traces back to fact when output transistors are biased harder, they produce noise(I still have no idea why disabling first audio eliminates noise). The replacement SK-3004 outputs are of same design as orig(smaller than one normally sees). Both orig and new replacements have approx 50ua leakage, I dunno if a different type would be better. Problem with that is the heat sinks won't fit any other type.

I'll probably juggle the common emitter resistor for PP outputs to see how much difference that makes. It may wind up with silicon outputs.

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Wed 09, 2019 10:41 pm 
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I replaced the regulator and pass transistors in my old Heathkit bench supply with silicon types, and it works fine. I did have to leave the old germanium current limiter transistor in there, as that circuit won't work properly with a silicon.

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Thu 10, 2019 2:54 am 
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Good luck, Tom---am watching for your report !

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Thu 10, 2019 6:11 pm 
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I'm going to think about the issue a day or two, in other words it's too cold up in bell tower(shop).

Maybe I'll install a 1K bias pot. :wink: The 470 ohm setting was best on sub box and 1K distorted, maybe 600 or 750 ohm etc would be better. Plus I still need to check current draw. Guessing there could be(or at least should be) a "sweet spot" that balances current, noise and distortion.

Swapping to silicon also sort of depends on what I have in stock, fairly sure the majority are NPN types. This radio uses PNP. I could could swap to NPN(probably easier to bias) but that no doubt would require revamping orig bias circuit.

Interesting thing about the original bias transistor is it has almost no gain. It does test OK on B&K 520B.

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Thu 10, 2019 11:19 pm 
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Location: Indianapolis, IN
Tom,

Since that bias transistor is actually being used as a diode, it's possible that it was a "fallout" part that GE made use of which would explain the gain issue.

Even though frustrating, that type of problem intrigues me and hope you find the actual cause before "re-engineering" it...

John


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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Fri 11, 2019 1:28 am 
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I did hit shop long enough to test current. To improve distortion issue, it draws approx 2.5x current over spec. Even with orig bias transistor installed, it draws approx 2x normal current. If I set bias with resistance sub box to listed 14ma spec idle current, it's noticeably distorted.

I did find one of the outputs now installed is drawing less current than other, apparently are mismatched. Going to address that issue before I proceed farther. As a note, current begins to equalize once they are biased at around 25ma(pair). If replacements are mismatched, so were orig.

EDIT

I went through the transistor drawers again and found the unobtainable. Or at least a matched pair of 2N2431(made in Holland no less). Those are repl by SK3004, GE53 & ECG158, that are listed as repl for the orig GE transistors

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Last edited by 35Z5 on Jan Sat 12, 2019 2:51 am, edited 1 time in total.

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Fri 11, 2019 5:35 pm 
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there should be a small value capacitor to ground after the audio detect that grounds any leftover RF. if RF is not grounded there may be some hiss. just a guess


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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Fri 11, 2019 9:32 pm 
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tubeAMP wrote:
there should be a small value capacitor to ground after the audio detect that grounds any leftover RF. if RF is not grounded there may be some hiss. just a guess

There are three bypass capacitors back to ground/B+ on the audio/AGC line, as I've said noise is still there with coupling cap(C3) removed.


UPDATE

I'm happy to report the matched 2N2431 cured the distortion and with original bias transistor reinstalled iidle current is exactly 14.5ma(collector voltages now within .003v). With current reduced, noise is less noticable BUT not completely eliminated. Fortunately there are eyelets in the transistor solder lands, I've now "plugged in" approx 10 different transistors with silicon types producing the least amount of noise. Strange thing is low gain germanium types seem to produce most amount of noise. At this point I may surrender, but I'll wait a few days to see if I come up with a fresh revelation.

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Wed 16, 2019 10:56 pm 
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Well I'm fresh out of revelations.

Apparently the noise is generated within radio but killing all RF/IF function does nothing to change or reduce it. I can hold a small screwdriver near the 1st audio transistor and noise does increase. Adding a ground to case of transistor reduced sensitivity to screwdriver movement, but otherwise made no real improvement.

Installing a Si type for driver reduces noise slightly, requires base bias voltage divider modification. Removing 1st audio and installing a Si darlington for driver produces approx same noise as two transistors installed(and another different base resistor). Problem with darlington is it draws too much current through driver transformer, dragging down the collector voltage. Reducing bias causes distortion. In the end I reinstalled the Ge driver transistor.

Needless to say, I'm stumpt.

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Fri 18, 2019 2:23 am 
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Hey Tom,

Have you tried jumpering C21,23, & 25 with a known good cap just for fun to see what happens ???

John


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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Fri 18, 2019 4:16 am 
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xrhonda91 wrote:
Hey Tom,

Have you tried jumpering C21,23, & 25 with a known good cap just for fun to see what happens ???

John

I tried C21 & C26(lower right, P/S). I don't remember if I tried C23 and/or C24, 99% no to C25.

Thanks John, I'll look at all of these again.

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Fri 18, 2019 8:19 am 
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In my experience, there are some mid century radios that simply refuse to "get with the program". This sound like one of 'em.

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Fri 18, 2019 3:30 pm 
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fifties wrote:
In my experience, there are some mid century radios that simply refuse to "get with the program". This sound like one of 'em.

Unless GE changed something in the design of say the audio driver/output transformer(s), there is no reason set should have noise. The P-783A & 784A are basically variances of the original P-780, AFAIK electrically are 99.9% same as P-780H. I have four 780(no H model), only one of those had any noise at minimum volume and that was leaky driver transistor.

If one believes what's posted on net, it's been declared the 780H is best sounding of all the 780, but I think that's mostly on basis of "has a 9th transistor". As already mentioned, it's functioning as voltage regulator.

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Fri 18, 2019 11:14 pm 
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Hey Tom,

I've got two P-780B's and two "H"'s---the "H" models do have "cleaner" sounding audio which could be due to that bias-regulator circuit having a greater effect on reducing the cross-over distortion...

Am watching for the results of trying those mentioned caps in the driver circuit---sure hope one of 'em is the culprit !

John


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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Sat 19, 2019 4:38 am 
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Hi 35z5

About the noise problem, have you tried to remove C21 ? I see no reason why they put a cap there, it's only killing the sound signal.


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 Post subject: Re: Badly beaten by a GE P-783A -- FIXED!!
PostPosted: Jan Sat 19, 2019 5:31 pm 
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Well I'm embarrassed to say, IT's FIXED! :oops:

I listened to the three P780B & one G, they ALL have a slight bit of this noise(sort of like white noise). It's not really audible at more than a couple ft and after changing outputs in the 783, it's noise level is approx same. Apparently I was just being just too critical. What's really embarrassing is I never really held my ear to the 780s. I've listened several hours to one that has the new SK3010 outputs.

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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Sun 20, 2019 12:19 am 
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Great that it's okay and the "struggle" is over !

Don't feel too bad as we all have done similar things...

John


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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Sat 26, 2019 12:16 am 
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If the bias resistor (R21) is a carbon composition type, you may want to try changing it out. Granted the probability is small, but I have seen carbon composition resistors become noisy with age, usually when they are carrying appreciable current in an emitter circuit, but it may be worth a try here since you have looked at all the "usual suspects". Good luck.

John


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 Post subject: Re: Badly beaten by a GE P-783A(read carefully)
PostPosted: Jan Sat 26, 2019 3:20 am 
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Thanks John, I'll check R21.

It's back together and plays very well, BUT has at least 2X the amount of noise as any of the four P780's I mentioned. After the truce expires, no doubt the battle will resume.

At least replacing the RF amp xistor appears to have fixed the intermittent weak reception issue.

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