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 Post subject: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Tue 07, 2017 1:36 am 
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Joined: Jul Sun 12, 2015 4:00 pm
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Location: Monrovia, CA
Hi everyone! I've run into some issues with a 888 involving the audio. It's distorted and I can't seem to figure out why. I've narrowed down the section to the push pull output section using a signal tracer. It seems on the base of the output transistors, the signal is clear but on the collector and emitter (should there be signal here at all?), it's distorted. Anyone have any idea what might be the issue? I've attached a schematic for reference.
Attachment:
schematic (4).pdf [243.54 KiB]
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Attachment:
schematic (5).pdf [120.01 KiB]
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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Tue 07, 2017 3:06 am 
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The bias of the transistors might be off.
That is set by R21 and R20. If one or both have drifted off it would affect the audio.
Also there is a cap, C15, across the output, that could be faulty. That is schematic #1.

In the second schematic, check the voltage at the emitters, should be 5.7 v, check R21 and C12.

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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Tue 07, 2017 4:57 am 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
Thanks for the links to schematics!

How much battery current does the radio draw? Is anything getting hot?

Have you checked the resistor values and the transformer windings?

Subbed-in a known good speaker? This can just be connected at the earphone jack.

The output transistors are PNP, so the emitters and bases will be much closer to B+ than to ground. You might report the voltages seen across R20 and across R21 on the Emerson schematic.

Might the transistors no longer be a matched pair? One failed output transistor would result in distorted audio.

Does the distortion go away at very low volume levels?

Does the service information suggest standard P/N transistor replacements?

It would not be too difficult to change the base bias to make current production silicon transistors work in the output stage. Diodes Inc. / Zetex make some really hot TO-92 transistors that might work well.

Ted


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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Tue 07, 2017 8:42 pm 
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Joined: Jul Sun 12, 2015 4:00 pm
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Location: Monrovia, CA
Here's the specs:
6.18 volts on the emitter of the outputs
The radio draws .15 amps
6.05 volts dropped across R20
0.23ish volts with audio .03 volts without audio dropped across r21

I spot checked all the resistors and only R20 was out of spec so I replaced it. No difference in audio. The distortion doesn't go away at low audio levels.

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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Wed 08, 2017 2:58 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6285
Location: Indianapolis, IN
It's possible that one of the output transistors is degraded enough to cause distortion, but not enough to exhibit a voltage difference---do you have another radio with same type transistors that you could swap the matched outputs into this one to try ?

John


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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Wed 08, 2017 4:44 am 
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Streamline Steamroller wrote:
Here's the specs:
The radio draws .15 amps


Did you mean 15ma? Because if it's truly drawing 150ma, that's way too much.

Also, as westcoastjohn mentioned, there is a paper capacitor in the output stage, I would suggest replacing it.


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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Wed 08, 2017 6:57 pm 
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Location: Monrovia, CA
Woops! Forgot the scale I was on, it is 15mA. My mistake.

In fact I do have another 888 with a leaky oscillator transistor. Will try swapping the outputs.

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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Wed 08, 2017 9:21 pm 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
These measurements do not make sense, referring to the linked schematics.

6.18 volts on the emitter of the outputs This is reasonable, assuming a bit more than 6V from the battery.

The radio draws .15 amps This is not normal, as already pointed out.

6.05 volts dropped across R20 This is not reasonable. At 100 ohms for R20, this gives over 60mA, just for R20. Presumably much more current than this is flowing through the driver transformer secondary and transistor bases. Perhaps the 150mA figure was correct, after all?

0.23ish volts with audio .03 volts without audio dropped across r21 This would be sort of reasonable, but only if the above problems did not exist. But at 10 ohms, R21 will be passing 23mA, which is more than the "corrected" total battery drain figure, but much less than you would see with the nearly 60mA base current.

And I think the original post said that the radio played, though the audio was distorted. I doubt if you would get any sound if the above measurements were correct. You might hear the crackle of smoking hot components.

Ted


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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Wed 08, 2017 9:42 pm 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
I just reviewed my recent post and feel that it seems pretty harsh. I apologize if it comes across that way.

I was trying to show why the measurements don't make sense to me. I'm pretty sure there must have been some confusion about which resistor was being measured. Some double-checking might be in order.

And I failed to suggest turning-down the volume before taking the measurements, since the Class B stage will show different DC voltages depending on the audio level.

Ted


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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Thu 09, 2017 2:33 am 
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Joined: Jul Sun 12, 2015 4:00 pm
Posts: 137
Location: Monrovia, CA
Welp, I'm dumbgoofing left and right, sorry about that. Apparently I was shorting part of the earphone jack to one side of R20 when I made the measurements. With my technique corrected, it drops .13v across R20. I forgot what scale the meter was set to and yet, it only draws 15mA. Sorry about that. So with this in mind, should I swap the drivers?

I tried disconnecting C15 (tone cap) but the distortion didn't go away.

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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Thu 09, 2017 3:21 am 
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Joined: Oct Mon 26, 2009 10:02 pm
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Location: Indianapolis, IN
If the other radio has good output transistors, it would be worthwhile to try them together as a pair and see... Would sure save some time and headaches !

You could also check the resistances of both radio's transformer windings and compare while the transistors are out of circuit just for curiousity...

John


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 Post subject: Re: Emerson 888 Transistor Portable Audio Distortion Problem
PostPosted: Feb Thu 09, 2017 10:35 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3363
Location: Lexington, KY USA
That voltage looks more reasonable. With no transistor base current, the ratio of R19 and R20 would give you about 0.15V across R20. So take away a little for the transistor bias and 0.13V looks about right.

I trust you have checked the audio transformer windings with an ohmmeter. The Sams schematic has ballpark resistance figures.

If you have an oscilloscope, you might use it to see what's going on. It would be easier to understand the waveforms using a sinewave audio test signal. If you have a signal generator...

There is a risk soldering parts in and out, particularly antique transistors. I'm trying to think of something to do without subbing transistors. It would be a shame if your result were two non-working radios.

Lacking test equipment, I'd see if there is any difference between shorting one side and the other of the driver transformer secondary. If one output transistor is dead, shorting across its end of the winding may allow some signal through the other transistor. But if the good transistor's end is shorted, there should be no audio output. This may not work, but just might give you a clue.

If you must try substituting transistors, you might try two of almost any PNP germanium parts for the output transistors. Even something like 2N173s. Obviously they need to be in working order. Results might be better with two similar ones, but matching need not be very good to give you an indication.

Ted


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