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 Post subject: GE P-780A with Silicon Enhancement
PostPosted: May Sat 02, 2020 8:39 pm 
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AND..... Drum roll please.... Bias regulator for outputs!!!!

It's a work in progress but I'm not far from a final version. I first set it up with modified bias circuit to support the silicon vs orig germanium transistors.. For that I added a 470 ohm resistor in series with the 110 ohm thermistor, good with that arrangement, but radio still somewhat bassy. Decision was made to settle my question, does the bias regulator actually improve sound? I have bite my tongue and say yes, highs are no doubt more distinct. Speaker in radio is from the late B included in test.

When I started, audio was very distorted with orig Ge xistors(why should I expect anything else?). Yep badly mismatched & one was rather leaky. Since replacement Ge NPN are getting scarce(least round here) I decided now is time to go silicon. With replacement transistors it was still distorted, juggling bias provided only small improvement. Found driver extremely leaky, replaced with the good output. Walah, distortion all but gone and with approx .65v bias on base, audio was clear.

This radio had seen previous romancing, has a newer volume control, speaker felt and part of cone eaten(why speaker from B is installed), broken knobs etc so I don't feel bad about mods. I feel knowledge is worth more than the radio.

BTW these radios may have NPN or PNP output transistors. For NPN the center tap(yellow) of output transformer will be connected to B+, near large filter cap. For PNP, yellow lead connects to ground, near dial lamp socket mounting.


Attachments:
File comment: orig circuit
P780Aampsmip.JPG
P780Aampsmip.JPG [ 388.73 KiB | Viewed 573 times ]
P780siliconmodfinal 001.jpg
P780siliconmodfinal 001.jpg [ 692.68 KiB | Viewed 442 times ]

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Last edited by 35Z5 on May Tue 05, 2020 5:03 pm, edited 1 time in total.
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 Post subject: Re: GE P-780A with Silicon Enhancement
PostPosted: May Tue 05, 2020 3:32 am 
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35Z5 wrote:
It's a work in progress but I'm not far from a final version.

Apparently circuit as drawn is the final version. I was thinking the 15K should be closer to 12K, but all that did was up current from 50ua to 75ua with no difference in performance or regulation, left it as is. The three NPN silicon types I tried as bias regulators did produce slightly different voltages.

Speaking of performance, I've now spent a couple evenings quiet time comparing with P-783A & P-780H(same circuitry). This 780A wins hands down. It's fairly bassy but has real punch at low end(current jumps close to 40ma on strong low notes), plus high freq are much cleaner. Setting tone at roughly 4/5 of max treble is perfect. Now the question is, do the silicon transistors make that much difference? Things altered from orig, bias circuit & 2SC971 transistors plus the audio amp coupling caps, now 4.7mfd vs 8mfd(didn't have 8 on hand). One of orig measured over 32 mfd & other close to 20, neither had more than 10ua leakage @ rated 15v. In operation there is only 4 to 5v across them. Next will toss a recapped B into the comparison, think that one has 10 mfd in amp coupling. GE was somewhat indecisive as to what value to use A, B & H use 8mfd, the others I've seen had 6 mfd. AND that includes the AGC filter as well, apparently GE was making deals.

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Last edited by 35Z5 on May Tue 05, 2020 4:30 pm, edited 1 time in total.

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 Post subject: Re: GE P-780A with Silicon Enhancement
PostPosted: May Tue 05, 2020 5:35 am 
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Location: Indianapolis, IN
Hey Tom,

Glad it's working well ! How much did sound quality differ with the other silicon transistors that you tried ? What about the current requirements ? Why did you choose the ECG 123 as the bias regulator ?

John


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 Post subject: Re: GE P-780A with Silicon Enhancement
PostPosted: May Tue 05, 2020 4:26 pm 
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Hi John. I knew that post was going to be confusing but I was feeling a little rough(something I ate).
I'll edit post and schematic (DONE)....

The 2SC971 is only output I tried, a fairly robust driver/output used in smaller applications. The bias transistor is where I tried three different ones. First was a 2SC971, a 2N5551 & 86-607-2(Sears number & what's installed). There is some confusion in the ECG manual(three 86-607), the last two & 2N5551 all cross to ECG-194, not 123. The 2N5551 pictured was in a sealed 86-607-2 package.

A 2SC971 produced approx .2v over the others in same circuit, pictured is a used example, ones in radio are fresh pair. Also has a nice heat sink, figured I'd have a hard time overheating those. Transistor inside is approx same size as 2N5551. With amount of bass thump(vs thud :wink: ) & and current drawn, output is no doubt greater than orig.


Attachments:
2sc9712n5551.JPG
2sc9712n5551.JPG [ 203.3 KiB | Viewed 444 times ]

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