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 Post subject: Re: Transistor types
PostPosted: Jan Wed 04, 2017 12:59 pm 
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Your are getting lots of help here...

Allow me to comment on some basics:

1. Fix one problem at a time.

2. Don't try to trouble-shoot a circuit without first checking DC voltages, and making corrections required to get them right. This includes resolving potential measurement errors....which is a major theme in this thread.

3. operating equipment with a known problem can lead to further damage. I don't know if this is relevant in your case.


My advice:
In this order
1. Fix the output stage
2. Resolve the measurement issue with the other DC voltages.
3. Make fixes required to get The DC voltages right
4. Then continue trouble-shooting

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 Post subject: Re: Transistor types
PostPosted: Jan Wed 04, 2017 2:44 pm 
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Ok, that's what I will do . I should be getting the transistors sometime this week and I will change the output ones . I ordered all new transistors so if I have another one I will have it on hand. The original symptom was the volume kept getting lower and lower and went out completely. That was why I changed the electrolytics thinking they dried up. I will quadtriple check the electrolytics one more time while I wait for the transistors
Thanks

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 Post subject: Re: Transistor types
PostPosted: Jan Wed 04, 2017 3:30 pm 
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pixellany wrote:
My advice:
In this order
1. Fix the output stage

devildog63 wrote:
Ok, that's what I will do . I should be getting the transistors sometime this week and I will change the output ones .
Just a suggestion. I generally found it best to eliminate the easier possibilities (disconnecting one thing at a time) before making the bigger changes (replacing multiple parts at the same time).

Looking at your schematic it is clear that something is producing a low impedance path to ground at the point where C4, X6 and X7 meet. That's what causes the -.342 V that you have instead of the -4.2 V (circled in green) that you are supposed to have and it causes X6 to get warm.
Attachment:
Output_Stage.jpg
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It's easier to remove C4 than X7 just to make sure there is not something wrong with your replacement capacitor (there is no guarantee that replacements are always good).

Just a thought.

In fact, if you happen to have a 1/8" phone plug that has nothing attached to it, you can plug the unused plug into the earphone jack and this disconnects the other end of C4 from the speaker at the "switching jack" without even unsoldering anything. Then just measure the voltage and see if it pops up to around -4 volts.

If you don't have an unused phone plug, you could even just disconnect one of the speaker wires to accomplish the same thing (easier than unsoldering the C4 capacitor).

If the voltage does not come up with C4 disconnected, then the most likely cause would be a failed X7.

Curtis Eickerman

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 Post subject: Re: Transistor types
PostPosted: Jan Wed 04, 2017 8:15 pm 
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I support Eickerman while those transistors arrive Check that capacitor C4, it could be shorted and could damage your new transistors.


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 Post subject: Re: Transistor types
PostPosted: Jan Wed 04, 2017 8:39 pm 
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Eickerman wrote:
pixellany wrote:
My advice:
In this order
1. Fix the output stage

devildog63 wrote:
Ok, that's what I will do . I should be getting the transistors sometime this week and I will change the output ones .
Just a suggestion. I generally found it best to eliminate the easier possibilities (disconnecting one thing at a time) before making the bigger changes (replacing multiple parts at the same time).

Looking at your schematic it is clear that something is producing a low impedance path to ground at the point where C4, X6 and X7 meet. That's what causes the -.342 V that you have instead of the -4.2 V (circled in green) that you are supposed to have and it causes X6 to get warm.
Attachment:
Output_Stage.jpg
It's easier to remove C4 than X7 just to make sure there is not something wrong with your replacement capacitor (there is no guarantee that replacements are always good).

Just a thought.

In fact, if you happen to have a 1/8" phone plug that has nothing attached to it, you can plug the unused plug into the earphone jack and this disconnects the other end of C4 from the speaker at the "switching jack" without even unsoldering anything. Then just measure the voltage and see if it pops up to around -4 volts.

If you don't have an unused phone plug, you could even just disconnect one of the speaker wires to accomplish the same thing (easier than unsoldering the C4 capacitor).

If the voltage does not come up with C4 disconnected, then the most likely cause would be a failed X7.

Curtis Eickerman


That sounds like good analysis to me. The beauty of the output stage is that it is more-or-less isolated from the rest of the radio and there are only a few parts that could be at fault, rather than anything/everything. With the corrected voltage readings, X7 or C4 (or a short/solder bridge somewhere) is almost certainly the problem.

I also suspect that once the first problem is resolved, there's a good chance all the others will resolved themselves. I really don't believe in multiple independent failures, unless the troubleshooting itself creates them.

Brett


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 Post subject: Re: Transistor types
PostPosted: Jan Wed 04, 2017 9:10 pm 
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Brett_Buck wrote:
I really don't believe in multiple independent failures, unless the troubleshooting itself creates them.
And that NEVER happens, right? :roll:

I was once working on a customer's stereo where one channel was out. While probing the OTHER channel to compare voltages I slipped. The darned amplifier had about 4 "direct connected" transistors in the amplifier chain and I fried ALL of them on the good channel. :cry:

They were not very expensive, but it was really really annoying to have to change 4 transistors that I took out with one slip of a test probe. Not one of my better days.

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 Post subject: Re: Transistor types
PostPosted: Jan Thu 05, 2017 1:17 am 
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How do i check the cap while still in the circuit ?

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 Post subject: Re: Transistor types
PostPosted: Jan Thu 05, 2017 1:21 am 
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Oh by the way the ear phone jack is completely disconnected.

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 Post subject: Re: Transistor types
PostPosted: Jan Thu 05, 2017 1:31 am 
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Can I count on this as far as location of the transistor leads?


Attachments:
21dlp36-589x800.jpg
21dlp36-589x800.jpg [ 134.91 KiB | Viewed 799 times ]

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 Post subject: Re: Transistor types
PostPosted: Jan Thu 05, 2017 5:24 am 
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Eickerman wrote:
Brett_Buck wrote:
I really don't believe in multiple independent failures, unless the troubleshooting itself creates them.
And that NEVER happens, right? :roll:

I was once working on a customer's stereo where one channel was out. While probing the OTHER channel to compare voltages I slipped. The darned amplifier had about 4 "direct connected" transistors in the amplifier chain and I fried ALL of them on the good channel. :cry:

They were not very expensive, but it was really really annoying to have to change 4 transistors that I took out with one slip of a test probe. Not one of my better days.

Curtis Eickerman


No, I have never done anything like that. Well, except for that time I did something almost exactly like that - checking voltages, probe slipped, blew a whole row of transistor like poof. At least this was on my *own* amplifier.


Brett


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 Post subject: Re: Transistor types
PostPosted: Jan Thu 05, 2017 1:41 pm 
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devildog63 wrote:
Oh by the way the ear phone jack is completely disconnected.
Since the speaker is connected to the radio by way of the earphone jack switching connection and since that is "completely disconnected" it might be good if you describe where you connected the speaker to the radio.

If you connected it to the otherwise unconnected side of C4 that is fine (the side that went to the earphone jack that isn't there). If you connected it to the side of C4 that attaches to X6 and X7 that would not be fine.

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 Post subject: Re: Transistor types
PostPosted: Jan Thu 05, 2017 2:42 pm 
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What I did is connect the speaker to the yellow lead that was attached to the earphone jack and the other jumper to the black wire that was attached to the speaker , so it should be fine. As far as that goes. I took it completely out of the case to work on the thing.
Thanks

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 Post subject: Re: Transistor types
PostPosted: Jan Thu 05, 2017 2:55 pm 
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devildog63 wrote:
What I did is connect the speaker to the yellow lead that was attached to the earphone jack
I believe that will be fine.

So if you want to determine if C4 is OK simply disconnect that yellow wire from the speaker and see if the voltage at the junction of X6, X7 and C4 (the place that you had as -.342 V that should be -4.2 V - circled in green above on this page) changes to being something closer to -4 volts.

If having the speaker disconnected lets that rise to -4.2 V (or thereabouts), then C4 is shorted (internally or externally) or is installed backwards. If the voltage stays down around -.342 V then it is likely that X7 has failed (Collector-to-Emitter short).

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 Post subject: Re: Transistor types
PostPosted: Jan Fri 06, 2017 1:12 am 
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I checked the electrolytic capacitor. I had it installed backwards. I replaced and installed correctly. But the voltages are still out of whack. I wonder if it burnt up X6 answer X7 ?

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 Post subject: Re: Transistor types
PostPosted: Jan Fri 06, 2017 1:18 am 
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Assume the worst, and replace them, unless you have a way of testing them.

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 Post subject: Re: Transistor types
PostPosted: Jan Fri 06, 2017 2:38 am 
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devildog63 wrote:
I checked the electrolytic capacitor. I had it installed backwards. I replaced and installed correctly. But the voltages are still out of whack. I wonder if it burnt up X6 answer X7 ?
You didn't say what the voltage were.

Curtis Eickerman

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 Post subject: Re: Transistor types
PostPosted: Jan Fri 06, 2017 2:52 pm 
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Sorry Curtis, but voltages remain the same. I should get all the transistors today, so which one should I change? X7 or X6 . I don't want to change anymore than nessary, it's my mothers radio so i just want it working .
Thanks

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 Post subject: Re: Transistor types
PostPosted: Jan Fri 06, 2017 3:11 pm 
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devildog63 wrote:
which one should I change? X7 or X6 . I don't want to change anymore than nessary
The way I see it, X6 was getting warm so it was "conducting" (i.e., turned ON). The junction of X6, X7 and C4 was close to ground potential which would mean that something was pulling that voltage to ground. X6 won't do that, and X6 appeared to be ON. Since C4 does not appear to be a factor (you have replaced it and installed the new capacitor with the correct orientation), that kind of leaves X7 as the potential culprit.

Before replacing X7 you should do a resistance test with the radio turned OFF. Check the resistance between ground and the junction of X7, X6 and C4. The reading should be something over 2000 Ohms. Note: The resistance reading will start out at a low value and then keep going up until it is over 2000 Ohms. This is because the Ohm meter is reading C4 being charged up by the meter. If the meter readings don't seem to do this, try swaping the meter leads and check again.
Attachment:
Output_Stage.jpg
Output_Stage.jpg [ 66.29 KiB | Viewed 743 times ]
If or when it comes time to change X7, first remove X7 and recheck the above reading to make sure you are getting the resistance reading of greater than 2000 Ohms before installing the new device. If you do not get a resistance reading of over 2000 Ohms something else might be causing a problem (a short circuit across R21 for example).

Curtis Eickerman

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 Post subject: Re: Transistor types
PostPosted: Jan Sat 07, 2017 6:08 pm 
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What does the red mean on the side of the transistor? I looked and believe it's the collector?

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 Post subject: Re: Transistor types
PostPosted: Jan Sat 07, 2017 6:30 pm 
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devildog63 wrote:
What does the red mean on the side of the transistor? I looked and believe it's the collector?
I have no idea without seeing the transistor in question. Are you talking about the old one or the new one? Also the new one should have something with it to show which is which. If it does not, again a picture would be helpful along with the specific part number.

I am not aware of any transistors that used a red mark to indicate the collector, but I suppose it is possible.

The bottom line is that you have to install the new transistor in accordance with the layout diagram that shows where the Emitter, Base and Collector must be attached to the circuit board.
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X7.jpg
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Curtis Eickerman

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