Hello All,
Looking for a little help here on properly identifying the polarity of a cap I'm replacing. (The blue one sitting horizontal)
The capacitor in question is C22 near the bottom towards the right.
The schematic shows the polarity different than what I interpret and how its soldered into the circuit.
Maybe I'm reading it wrong?
Thanks!
Looking for a little help here on properly identifying the polarity of a cap I'm replacing. (The blue one sitting horizontal)
The capacitor in question is C22 near the bottom towards the right.
The schematic shows the polarity different than what I interpret and how its soldered into the circuit.
Maybe I'm reading it wrong?
Thanks!
Attachments
Can you post a picture that's pulled back a little more? It's hard to tell what the negative end is soldered to.
It is drawn as a positive earth system which would be consistent with the PNP device used. The polarity is therefore wrong on the diagram. Positive end of the cap should go to ground.
C23 looks wrong as well but C21 is correct.
C23 looks wrong as well but C21 is correct.
I have the negative side removed as I am going to replace it.
After looking at it more, it seems like the diagram is wrong as there are more in the same style of circuit that are backwards and the circuit works.
would it be better to post the entire schematic? Also this is a point to point amp. Its pretty tricky to replace these caps.
All other caps are totally obvious as to their polarity, but these 2 blue ones were causing me to doubt myself.
After looking at it more, it seems like the diagram is wrong as there are more in the same style of circuit that are backwards and the circuit works.
would it be better to post the entire schematic? Also this is a point to point amp. Its pretty tricky to replace these caps.
All other caps are totally obvious as to their polarity, but these 2 blue ones were causing me to doubt myself.
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Definitely replace and reverse C23 also, it has much more reverse voltage on it.
Somebody really messed up when designing this unit.
Somebody really messed up when designing this unit.
So I should just tell the complete story. I thought I could solve my problem with replacing the caps, but obviously I made a mistake somewhere, or maybe a component failed during the process....
I have this old Clairtone T7 amp from a console stereo. Cleaned the pots and switches and ran it a short time before the left channel went out. Cleaned the pots and switches again and it turned out to be dirty contacts on the channel reverse switch. So I was pleased and confident so I went ahead and started to replace the electrolytic caps. Started with the left channel and things were going well, making sure to test the unit after every cap change. Then I got to the 6th one and the right channel went dead.
I've been staring at this schematic for a couple days and really haven't gotten anywhere. I have a cheap o-scope and a function generator (Samsung tablet) and I've been trying to learn how to use an o-scope, and managed to trace the sine wave to the treble and bass pots, but now I'm totally stumped, and embarrassed. I even went against my better judgement and replaced caps in the right channel that seemed like they were in the signal path. Obviously I'm over my head.
Hopefully you guys can point me in the right direction. Let me know what I need to give you. I have the Photofact service manual, as well as one from the manufacturer. You guys are my Obi-Wan at this point!
Thanks for the help so far!
I have this old Clairtone T7 amp from a console stereo. Cleaned the pots and switches and ran it a short time before the left channel went out. Cleaned the pots and switches again and it turned out to be dirty contacts on the channel reverse switch. So I was pleased and confident so I went ahead and started to replace the electrolytic caps. Started with the left channel and things were going well, making sure to test the unit after every cap change. Then I got to the 6th one and the right channel went dead.
I've been staring at this schematic for a couple days and really haven't gotten anywhere. I have a cheap o-scope and a function generator (Samsung tablet) and I've been trying to learn how to use an o-scope, and managed to trace the sine wave to the treble and bass pots, but now I'm totally stumped, and embarrassed. I even went against my better judgement and replaced caps in the right channel that seemed like they were in the signal path. Obviously I'm over my head.
Hopefully you guys can point me in the right direction. Let me know what I need to give you. I have the Photofact service manual, as well as one from the manufacturer. You guys are my Obi-Wan at this point!
Thanks for the help so far!
If in doubt with any of the caps (and for your own experience 🙂) measure the voltage across them and be happy that the measured polarity matches the actual cap fitment irrespective of what the diagram says.
As to your fault the first step is to measure and record the supply voltage (show as -19) and the voltages on the three terminals of the transistor to first of all establish the DC conditions are correct.
As to your fault the first step is to measure and record the supply voltage (show as -19) and the voltages on the three terminals of the transistor to first of all establish the DC conditions are correct.
Yeah I read that somewhere too. I also took photos of each cap to ensure I installed the new ones correctly.
I am confident in the cap replacement so far. I'm just stumped why the right channel went out as I was only replacing caps on the left channel only.
I am confident in the cap replacement so far. I'm just stumped why the right channel went out as I was only replacing caps on the left channel only.
You must begin with the basics which is a check of supply and the DC conditions around the transistor.
Ok, so using a voltmeter, I ground the negative probe to the chassis, and probe the base, emitter, and collector, and report voltages for each?
Do we need the function generator running, and volume at any specific level?
Do we need the function generator running, and volume at any specific level?
Ideally (and this is confusing if you are not experienced) but you have to think in terms of a 'positive' earth... so that actually means in this case the meters positive lead goes to chassis and you probe with the negative lead. All your readings will be negative but that is actually the correct result and method for this unit.
For the DC checks we have NO signal applied. We are looking at just the static voltages.
For the DC checks we have NO signal applied. We are looking at just the static voltages.
And just to finish off for tonight...
If the DC voltages look reasonable and particularly the all important -9 volts or on the collector of the transistor then you progress to using the generator and scope.
Connect the scope (and here you do have to connect the ground clip to chassis) and set the generator to give any reasonable value of signal at 1kHz. Say 1 volt peak to peak. Generator ground lead to chassis as well now.
Now check you see that signal voltage on the top left of the diagram going into R98 and R4.
Check to see if anything is on the collector of the transistor.
Use AC coupling on the scope so that the trace is not altered by the DC conditions in the circuit.
If nothing on the collector then look at both sides of C21 for signal.
Good luck for now 🙂
If the DC voltages look reasonable and particularly the all important -9 volts or on the collector of the transistor then you progress to using the generator and scope.
Connect the scope (and here you do have to connect the ground clip to chassis) and set the generator to give any reasonable value of signal at 1kHz. Say 1 volt peak to peak. Generator ground lead to chassis as well now.
Now check you see that signal voltage on the top left of the diagram going into R98 and R4.
Check to see if anything is on the collector of the transistor.
Use AC coupling on the scope so that the trace is not altered by the DC conditions in the circuit.
If nothing on the collector then look at both sides of C21 for signal.
Good luck for now 🙂
Ok!
Had the Negative probe of the meter on the chassis, but the meter reads as follows:
Right channel X20 XB3
Base -2.03
Emitter -2.01
Collector -2.15
Left channel X12 XB3
Base -1.16
Emitter -1.07
Collector -8.88
Obviously we have a problem... Will go now and finish the rest of the checks.
Had the Negative probe of the meter on the chassis, but the meter reads as follows:
Right channel X20 XB3
Base -2.03
Emitter -2.01
Collector -2.15
Left channel X12 XB3
Base -1.16
Emitter -1.07
Collector -8.88
Obviously we have a problem... Will go now and finish the rest of the checks.
C22 AND C23 are wrong, C21 is fine.
And in genral, measure actual voltage across capacitor legs or ends, and orient it accordingly.
And in genral, measure actual voltage across capacitor legs or ends, and orient it accordingly.
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Yup, schematic is wrong. Double checked with meter, and checked left channel, and they are in the correct polarity on the amp itself.
I see a proper sine wave at the junction of R98 and R4.
Set scope to A/C and probed the collector of X20 and found no sine wave. (right channel)
Probed the collector of X12 and confirmed proper sine wave. (left channel)
Probed C21 on both sides and found no sine wave. (Right channel)
Probed C10 on both sides and found no sine wave. (Left channel)
I was expecting a sine wave at C10. - Reminder - Left channel works properly.
Yes, a problem around the transistor voltage readings. The collector volts is way out for some reason.
As the unit worked before recapping the obvious thing to do is to go back over the work done. In the first instance try disconnecting (just one end) of C21, C22 and C23.
(Your measured base and emitter voltage reading is very similar. Make sure you haven't blobbed some solder between those points. Easily done 🙂 With it OFF do a resistance check between B and E and make sure it is not showing a near short)
(I wonder if it is a germanium transistor, the good voltage readings suggest it is... although it doesn't matter at this point though)
With the caps isolated the DC voltages around the transistor should now be correct. If they are then you have a problem with one of the replacement caps (polarity or voltage rating incorrect).
Lets see where that gets you 🙂
(what I said earlier about the positive earth and positive meter lead to chassis was wrong in that the readings done that way would all have been positive numbers, not negative I said. Positive earth means all the voltages in the circuit are negative with respect to ground)
Ok!
The resistance between base and emitter of X20 XB3 is 20.5 ohms. Visually inspected along all 3 electrodes and they look clean.
Disconnected one leg of each C21, C22, and C23. Resulted in no change in voltage:
Base= -2.02
Emitter= -2.01
Collector= -2.14
triple checked for any solder mistakes. None found.
I did check the voltage at resistor R105. One side goes to the collector, and the other to -18v, and it was -17v.
suspect faulty resistor?
.PS. I always make sure to clamp on a clip to sink the heat from the transistors during soldering.
The resistance between base and emitter of X20 XB3 is 20.5 ohms. Visually inspected along all 3 electrodes and they look clean.
Disconnected one leg of each C21, C22, and C23. Resulted in no change in voltage:
Base= -2.02
Emitter= -2.01
Collector= -2.14
triple checked for any solder mistakes. None found.
I did check the voltage at resistor R105. One side goes to the collector, and the other to -18v, and it was -17v.
suspect faulty resistor?
.PS. I always make sure to clamp on a clip to sink the heat from the transistors during soldering.
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