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 Post subject: Chasing Distortion GE P-780B
PostPosted: Dec Sat 01, 2018 6:46 pm 
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Radio had moderate distortion but voltages from 1st IF to audio outputs were spot on. I then fed RF out(detector) to external amp, A-OK. Next coupled AM/FM tuner to radio output, definitely amp issue. I'd been somewhat mystified as weak signal seemed to amplify(pun) the issue. Fired up scope & looked at audio feed from driver transformer to outputs, clearly one had issues. Gain approx 75% of other. Said to self, "driver xfomer isn't likely the problem, pull that output".

Using my handy dandy 1956 GE transistor tester, testing gain here's what I found.
Attachment:
ge780output1.JPG
ge780output1.JPG [ 78.85 KiB | Viewed 449 times ]


Replaced transistor and bingo, distortion gone. Of course I had to test the other output, better but not great gain.
Also worth noting both outputs had only slight leakage.

BTW code dates on both transistors approx 10 months to one year newer than radio, serviced early in it's life.

Image

In case anyone is interested in gain of NOS RCA SK-3010 replacements, not matched pair but both approx same.
Attachment:
ge780outputnew.JPG
ge780outputnew.JPG [ 76.18 KiB | Viewed 449 times ]

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sun 02, 2018 6:12 am 
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Hey Tom,

You only changed one of the matched pair and audio quality good ? Don't think I've ever done that due to the consistent manufacturers warnings on replacement as I like sets to sound as good as possible...

Maybe I worry too much---always replace P-P outputs with good matched pairs from junk radio and small amplifier PCB's...

John


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sun 02, 2018 6:25 am 
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xrhonda91 wrote:
Hey Tom,

You only changed one of the matched pair and audio quality good ? Don't think I've ever done that due to the consistent manufacturers warnings on replacement as I like sets to sound as good as possible...

Maybe I worry too much---always replace P-P outputs with good matched pairs from junk radio and small amplifier PCB's...

John

These are the same manufacturers who recommend using only their brand batteries. It's, "whatever works", lol.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sun 02, 2018 12:57 pm 
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When the transistors in these class B output circuits are replaced, it is desirable to have them closely matched for best results but not mandatory. The important thing is to have them biased at the point where crossover distortion is minimized with the least amount of quiescent (idling) current. Crossover distortion can best be heard at very low volume levels and sounds like a rubbing speaker voice coil.

Dave


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sun 02, 2018 2:53 pm 
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Dave Doughty wrote:
When the transistors in these class B output circuits are replaced, it is desirable to have them closely matched for best results but not mandatory. The important thing is to have them biased at the point where crossover distortion is minimized with the least amount of quiescent (idling) current. Crossover distortion can best be heard at very low volume levels and sounds like a rubbing speaker voice coil.

Dave

That's what had me fooled into thinking it was maybe a IF issue. Didn't seem to be as noticable on locals at low volume as on distant stations.

xrhonda91 wrote:
Hey Tom,

You only changed one of the matched pair and audio quality good ? Don't think I've ever done that due to the consistent manufacturers warnings on replacement as I like sets to sound as good as possible...

Maybe I worry too much---always replace P-P outputs with good matched pairs from junk radio and small amplifier PCB's...

John

I changed both, but honestly it wasn't distorted replacing just the one(or maybe it was so much better, it seemed OK). I could not put radio together without checking second output and once removed, wasn't much chance I was going to reinstall it. I know the pair of SK-3010 now installed are close in gain, can't say how that translates into being a matched pair.

I have another 780 with low level hiss in background at no volume. Since it changes with tone setting, I'm thinking it has a noisy driver transistor. The better transistor may find a home in that one.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sun 02, 2018 4:15 pm 
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Hey Tom---have seen that very problem (low-level hiss) with audio driver transistor in a few radios, easy fix when you have replacement !

John


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sun 02, 2018 7:59 pm 
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A leaky germanium transistor will often cause various degrees of hiss from low-level to objectionable depending on where it is in the circuit.

Dave


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Tue 04, 2018 2:12 am 
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Leaky driver it was, in fact very leaky. Better of two xistors removed from other 780 now installed.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Wed 05, 2018 6:44 am 
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My radio had distortion too and i found a shorted output transistor and as I didn't have Germanium transistors I replaced them with Si transistor like TIP 31 and TIP 32 and put a diode to bias the base of those transistors. Now The sound is great.
While I was there I changed ALL the capacitors of that radio and also changed the second IF transistor with a silicon one, the original had an HFE of 40 and the silicon one had 300 !......this make that radio extremely hot.
I had to change a resistor in the base of that transistor to boost the bias voltaje from 0.2 to 0.7v .
Now My frankestain radio is capable to make really weak stations sound at the same audio level as local ones and all of that without distorting a bit with local and strong stations because that second transistor is not on the AVC (or is it AGC ?) circuit.
If you want details just ask.


Last edited by Huesby on Dec Sat 08, 2018 7:49 am, edited 1 time in total.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Wed 05, 2018 1:24 pm 
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Hello Huesby,

Yes, I think many here would like to see a schematic and details on your audio output mod using silicon transistors ! I've not had any luck with it---seems even the newer radios with silicon transistors in the front end still use Germanium for the push-pull outputs...

John


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Wed 05, 2018 2:26 pm 
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Same here, I'm all for hot rodding about anything.

I will say I'm pleased with the general replacement SK-3010s, audio very clean. After I finished installing those, I came up with a matched pair of PNP transistors. Since the 780 has provision for using either NPN or PNP, I'll keep those in mind for next one that needs outputs.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Thu 06, 2018 11:42 am 
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Hi there!
The Ge and Si transistor are very like the same but the Ge has less gain (hFE) than the Si transitor, Ge has an hfe value of 20 to 60 vs 100 to 300 on the Si (typical values), besides the voltaje drop between base and emitter (Vbe) is different (0.2 to 0.3 v on the Ge and 0.5 to 0.7 on the Si).

Changing a Ge transitor for a Si one in the output stage involves taking care in adjusting the base voltage to compensate for the bigger drop (0.7 vs 0.2) with a bigger base resistor value, we don't care about the improved gain (hFE) because the behaviour of the output stage BUT things are very different in the IF stages because the improved gain might affect the following stage due to increased signal voltajes.

In the case of this radio I installed Si transistors TIP31 and TIP32 because I already have them but I could have used others like BD137 and BD138 and so on... to compensate the voltaje drop I put a bigger resistor between the Base and earth, in this case lt was from the center of the driver transformer to earth.

I also replaced the second IF Ge transistor X4 and changed the resistor R12 for a bigger one, it's the one which goes from base to earth with to compensate the voltaje drop and I didn't worry about saturating the following stage because it's the detector stage.

The radio works very good and I didn't try to change the other IF Ge transitor because that was gain controlled from voltages coming from the detector stage (AGC network) and that would have meant extra work calculating and experimenting with new values of resistors in the AGC network.

I'd have liked to change the RF transistor too to improve the Q but the radio already have gained an extremely good selectivity due to the changing of the third IF transistor.

if you want (like) more details and the values of the changed resistor please allow me a litte time to open the radio and taking pictures and perhaps I should open a new thread about this subject.


Last edited by Huesby on Dec Fri 07, 2018 10:31 am, edited 1 time in total.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Fri 07, 2018 3:33 am 
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Huesby, we'll see what Tom has to say about including your audio mods in his thread...

I'm mainly interested in using the silicons in the push-pull output stage. You said earlier that you used a diode for bias---would sure like to see schematic with actual part types (especially diode) and values so it could be duplicated ! Think Tom would like to see specifics, also...

Thanks...

John


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Fri 07, 2018 4:53 am 
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Huesby wrote:
I also replaced the third IF Ge transistor and changed the resistor which goes from base to earth with a bigger one to compensate the voltaje drop and didn't worry about over modulating the next stage because it's the diode detector.


Are we talking about the same radio? A GE P-780 does not have a third IF. Has RF, converter, 1st & 2nd IF, audio amp, audio driver and two outputs.

I've seen a couple instances that silicon transistors were used to repl Ge in audio. With rebias usually no issue. I do have a GE P-755A that someone stuck a Si in for the orig Ge in the 2nd IF position, with no other changes that one was distorted. Rather than re engineer the set, I installed correct tr and it's been fine.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Fri 07, 2018 10:08 am 
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Attachment:
p780.PNG
p780.PNG [ 28.85 KiB | Viewed 180 times ]


You are right! P780 only has 2 IF stage ,sorry my bad. :(
I will correct my post inmediatly.

About the resistor I changed, it was R12 from the original value of 6K8 to 10K to boost the bias voltaje of X4 the second IF transistor.

it's easy to change Ge transistor for Si ones, and it's fun too.


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Fri 07, 2018 1:34 pm 
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Huesby---what about the silicons in push-pull audio output stage and the diode you used for bias as well as other component changes in that specific circuit ???

John


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Fri 07, 2018 7:06 pm 
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xrhonda91 wrote:
Huesby---what about the silicons in push-pull audio output stage and the diode you used for bias as well as other component changes in that specific circuit ???

John

As xrhonda91 asks, Help us to understand the reason for the diode, was it added to the base circuit to ground (V-? or B+?) considering that you meant a silicon diode with its .6v Diode drop. Just to clarify please.

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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sat 08, 2018 7:41 am 
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Attachment:
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The voltaje across The diode plus the resistor ALWAYS make more than 0.7V which is enough to prevent the crossover distortion which might happen If there were no such voltaje. Remember Ge transistor only need 0.2v.

I could have the same results with a pair of resistors to get the 0.7v but the diode way is the easy way.

I eliminated the thermistor because I lost it. Its fuction there was to protect the Ge transistos against overcurrent but I dont think the new transistors would need that kind of protection, they are more powerful than necessary.

Hey now, this radio with these mods it's really extra hot I believe it's the most sensitive radio of those that I own and that includes Panasonic Rf2200, Superadios 1,2 and 3, Transocenics, Sony ICF 2010 and so on..


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sat 08, 2018 1:21 pm 
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35Z5 wrote:
...the 780 has provision for using either NPN or PNP...
Please explain. -Thanks.

Dave


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 Post subject: Re: Chasing Distortion GE P-780B
PostPosted: Dec Sat 08, 2018 3:49 pm 
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I have done that mod myself in a few cases where germanium transistors were an issue. One more thing I found though is that at times excessive collector current can occur in the silicon outputs. That I have remedied by breaking the emitter to ground leads and installing a 1 to 4.7 Ohm resistor in each. (Exact value depends on how much reduction is needed). You can determine what value to use by testing, measure the emitter voltage to ground. You want to see no more than about .05 to .1 volts in a battery powered radio. Higher will cause the transistors to run overly warm and shorten battery life, less can cause crossover distortion.

This has the effect of self biasing each transistor. It accomplishes two things; first it makes each somewhat independent so gain matching is not quite as critical. Second it will reduce the overall gain slightly. This prevents self oscillation which is sometimes an issue. The reduced gain is not an issue since the silicon replacements usually are substantially higher gain than the germanium ones they replaced.


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