Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Dec Tue 18, 2018 10:58 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 17 posts ] 
Author Message
 Post subject: Transistor Breakdown?
PostPosted: Jun Sun 17, 2018 8:43 pm 
Member
User avatar

Joined: Mar Tue 28, 2017 4:51 pm
Posts: 471
Location: Wayne, NJ
Hi,
when a transistor starts to fail, does it pass more voltage than it's specs?

_________________
Cheers, Steve

Sometimes discussing the wrong path leads you down the right one.....


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Sun 17, 2018 9:50 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1331
Location: Jackson, TN
Hi Steve,

Transistors can fail in several ways, however, I think a common failure would be a short between collector and emitter. This failure would pass positive or negative rail directly to the load. This case would generally blow fuses.

What are you experiencing? A little more information would be help.

Thanks, Tim


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Sun 17, 2018 11:22 pm 
Member
User avatar

Joined: Mar Tue 28, 2017 4:51 pm
Posts: 471
Location: Wayne, NJ
Tkilboy wrote:
What are you experiencing? A little more information would be help.


Hi Tim,
I have this Zenith Troubador record player/radio amp that has a weak left channel which is normally indicative of bad caps.
Gave it a complete re-cap and same symptom. Started to check all transistor voltages and one of the left channel output transistor's emitters is 4 volts over spec and the other ones collector is 4 volts over spec.....everything else checks out.


Attachments:
IMG_3598.jpg
IMG_3598.jpg [ 220.74 KiB | Viewed 1185 times ]

_________________
Cheers, Steve

Sometimes discussing the wrong path leads you down the right one.....
Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Mon 18, 2018 3:18 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1331
Location: Jackson, TN
Hey Steve,

From looking at photo it is looking like a class AB Amp with a positive power supply. The speaker output is through the electrolytic caps which connect to the collectors of the top transistors. If this is an AB Amp then the top transistors are different from bottom, ie NPN VS PNP.

I think you should see roughly 1/2 of the VCC positive voltage at the collectors of the top transistors.

In any case, you should be able to compare right voltages with left and track down the differences. If they don't agree, then the right to left base voltages are probably also disagreeing by a similar amount. This shift in bias is likely being caused by an upstream issue.

Do you have a schematic? I may be complete misreading the photo :shock:

Tim


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Mon 18, 2018 3:26 am 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 24086
Location: Annapolis, MD
addyboy wrote:
[q

Hi Tim,
Started to check all transistor voltages and one of the left channel output transistor's emitters is 4 volts over spec and the other ones collector is 4 volts over spec.....everything else checks out.


You normally can't diagnose with just one voltage reading...you have to measure all the voltages and do the math to see where the current is going. If you are not able to link us to a schematic, then post a snip of the area in question.

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Mon 18, 2018 12:16 pm 
Member
User avatar

Joined: Mar Tue 28, 2017 4:51 pm
Posts: 471
Location: Wayne, NJ
[/quote]You normally can't diagnose with just one voltage reading...you have to measure all the voltages and do the math to see where the current is going. If you are not able to link us to a schematic, then post a snip of the area in question.[/quote]

Here you go......


Attachments:
Troub (1).jpg
Troub (1).jpg [ 57.56 KiB | Viewed 1133 times ]

_________________
Cheers, Steve

Sometimes discussing the wrong path leads you down the right one.....
Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Mon 18, 2018 12:44 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 24086
Location: Annapolis, MD
We need more precision in the base to emitter voltages. If the the drop is really 0, the both transistors are shorted. Measure directly from B to E on a setting that gives at least 2 significant figures.

The. B-E drop is normally 0.2 to 0.3 volts for Ge, and higher for Si Your schematic shows 0.1 which suggests they are operating very close to cutoff.

Also, tell us the transistor type number.

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Mon 18, 2018 1:22 pm 
Member
User avatar

Joined: Mar Tue 28, 2017 4:51 pm
Posts: 471
Location: Wayne, NJ
pixellany wrote:
We need more precision in the base to emitter voltages. If the the drop is really 0, the both transistors are shorted. Measure directly from B to E on a setting that gives at least 2 significant figures.
The. B-E drop is normally 0.2 to 0.3 volts for Ge, and higher for Si Your schematic shows 0.1 which suggests they are operating very close to cutoff.
Also, tell us the transistor type number.


The right channels B to E voltages are 0.13v


Attachments:
Troub (1).jpg
Troub (1).jpg [ 58.6 KiB | Viewed 1118 times ]
IMG_3600.jpg
IMG_3600.jpg [ 207.47 KiB | Viewed 1118 times ]

_________________
Cheers, Steve

Sometimes discussing the wrong path leads you down the right one.....
Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Mon 18, 2018 3:44 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1331
Location: Jackson, TN
A few observations - suggestions...

The bias is not correct. The bias is set up by the 4 resistors and the two 3 ohm secondary windings shown to the left of the transistors. Want to be sure those six resistance values are all correct. Also verify the two 0.51 ohm emitter resistors. For emitter resistors you should measure resistance and also verify that they each have the same voltage drop when powered up.

Verify that the top transistor case (collector) is properly isolated from the chassis, the bottom one should be electrically connected to chassis. The white grease should be non-conductive thermal grease.

If all the resistances are correct, then the transistors start to look suspicious. They could be swapped out to see what her the problem can be made to move to opposite channel.


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Mon 18, 2018 4:30 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 24086
Location: Annapolis, MD
That circuit is self-balancing, but only if the transistors are reasonably well matched. If the midpoint goes high, it decreases the drive to the upper transistor, and INcreases the drive to the lower. If the the midpoint is low, the opposite happens.
The actual mid-point voltage can be calculated by replacing the transistors and the emitter resistors with resistors whose value is 0.5 ohms times the DC beta of the transistors (90 nominal--from the NTE-121** data sheet) Solving the resulting circuit shows that the mid-point voltage depends on the matching of the transistors. If they are not matched, the circuit still works, but will clip at a lower volume level.

Based on the measured voltages, either one transistor is leaking, or they are significantly mismatched.

I forget if this channel works to some degree, or is totally dead.


**google says this is the replacement for the number in your picture.

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Mon 18, 2018 4:59 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1331
Location: Jackson, TN
Agreed Mark,

The 'higher than balanced' voltage of 25V at the center node should be trying to reduce conduction on the top transistor and increase on the bottom, in effect trying to balance out as you stated. This all assumes that the resistor values are appropriate.

Also just noticed that the feedback loop is DC coupled, so if the node voltage is not at the designed 20V, or so, then the DC offset will also have some biasing effect on the driver Amp. This could be affecting gain and volume. Maybe not significant.

Always fun to go back and look at transistor circuits - been too long :?

Tim


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Mon 18, 2018 8:02 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 447
If you can remove the transistor without risk or major effort, the diode tester on a multimeter is a quick way to check for faults. Just swap probes between base and collector and one direction will show open circuit. Same for base collector.


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Mon 18, 2018 9:04 pm 
Member
User avatar

Joined: Mar Tue 28, 2017 4:51 pm
Posts: 471
Location: Wayne, NJ
Thank guys......will do some more investigating.

_________________
Cheers, Steve

Sometimes discussing the wrong path leads you down the right one.....


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Tue 19, 2018 2:41 am 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 24086
Location: Annapolis, MD
Here is a re-draw of the circuit which may make things easier:
Attachment:
totem_output.jpg
totem_output.jpg [ 24.83 KiB | Viewed 1050 times ]

There are 2 identical circuits stacked together---sometimes called a "totem-pole" configuration. Each half is a current source controlled by the voltage applied to the base of the transistor.

I said something misleading earlier---this circuit is actually relative INsensitive to changes in the Beta of the transistor.
Simplified analysis follows:

First, the output current is controlled by the voltage applied to the base of the transistor. As an approximation, we can assume a very high Beta, and no base-emitter drop, in which case the current is simply the applied voltage divided by 0.51 ohms.

On the base side, we'll start with the divider equivalent circuit, expressed as a negative offset from the +42 (or from +21 when applied to the lower half)
open-circuit voltage: ( 5.6 / 685.6 ) * 21 = 0.17 volt
source resistance: 5.6 || 680, which is ~ 5.6----then add the 3 ohms for a total of 8.6 ohms

To get the base voltage, we next approximate the base load resistance by simply multiplying the emitter resistor by Beta: let's use 100, giving a reflected load of 5.1 ohms.

Finally, find the base voltage = ( 5.1 / 13.7 ) * 0.17 = 0.063. Again assuming no B-E drop, we get the current by dividing this voltage by the emitter resistance:
Ic = 0.063 / 0.51 = 0.124 ====> 124mA (This is the approximate idling current for each 1/2 of the circuit.

To get the AC gain, recognize that the applied voltage is only what is developed in the secondary of the driver transformer. A slightly different analysis is needed to find the resulting collector current.

Getting back to the issue with the circuit:
From the above we can see that the quiescent output voltage will be relatively insensitive to Beta, unless of course one of the transistors fails to ZERO beta. I think the 25 volt reading at the midpoint points to the upper transistor being leaky.

None of this changes the suggested diagnosis of simply removing the transistors and testing them. (Checking resistors first would be a normal 1st step, but they probably cannot be correctly tested in-circuit. This may favor testing transistors first)

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Tue 19, 2018 4:22 pm 
Member
User avatar

Joined: Mar Tue 28, 2017 4:51 pm
Posts: 471
Location: Wayne, NJ
pixellany wrote:
Here is a re-draw of the circuit which may make things easier:
Attachment:
totem_output.jpg

There are 2 identical circuits stacked together---sometimes called a "totem-pole" configuration. Each half is a current source controlled by the voltage applied to the base of the transistor.

I said something misleading earlier---this circuit is actually relative INsensitive to changes in the Beta of the transistor.
Simplified analysis follows:

First, the output current is controlled by the voltage applied to the base of the transistor. As an approximation, we can assume a very high Beta, and no base-emitter drop, in which case the current is simply the applied voltage divided by 0.51 ohms.

On the base side, we'll start with the divider equivalent circuit, expressed as a negative offset from the +42 (or from +21 when applied to the lower half)
open-circuit voltage: ( 5.6 / 685.6 ) * 21 = 0.17 volt
source resistance: 5.6 || 680, which is ~ 5.6----then add the 3 ohms for a total of 8.6 ohms

To get the base voltage, we next approximate the base load resistance by simply multiplying the emitter resistor by Beta: let's use 100, giving a reflected load of 5.1 ohms.

Finally, find the base voltage = ( 5.1 / 13.7 ) * 0.17 = 0.063. Again assuming no B-E drop, we get the current by dividing this voltage by the emitter resistance:
Ic = 0.063 / 0.51 = 0.124 ====> 124mA (This is the approximate idling current for each 1/2 of the circuit.

To get the AC gain, recognize that the applied voltage is only what is developed in the secondary of the driver transformer. A slightly different analysis is needed to find the resulting collector current.

Getting back to the issue with the circuit:
From the above we can see that the quiescent output voltage will be relatively insensitive to Beta, unless of course one of the transistors fails to ZERO beta. I think the 25 volt reading at the midpoint points to the upper transistor being leaky.

None of this changes the suggested diagnosis of simply removing the transistors and testing them. (Checking resistors first would be a normal 1st step, but they probably cannot be correctly tested in-circuit. This may favor testing transistors first)


Wow, thank you for that!

_________________
Cheers, Steve

Sometimes discussing the wrong path leads you down the right one.....


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Tue 19, 2018 7:10 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 447
Possibly leaking capacitors could alter transistor voltage. DC electrons are free to flow through the dielectric. If a radio is really getting on in years it may be an idea to change the capacitors regardless. In fact, the audio improves a heck of a lot. And when you charge the old caps and discharge across a speaker, the pop is hardly audible. If after all that the transistors seem off spec, be gentle on removal. I tend to wiggle out a leg at a time with quick heat applied. Too much heat and tracks may peel. I removed before AF117s and found leaks to screen lead and case.


Top
 Profile  
 
 Post subject: Re: Transistor Breakdown?
PostPosted: Jun Fri 29, 2018 5:08 pm 
Member

Joined: Sep Thu 23, 2010 6:37 am
Posts: 10473
Location: Powell River BC Canada
C106 and R114 are feedback net parts.

Your issue is weak left channel.

If feedback is removed, channel gets louder. If feedback is the
problem, then the weak channel may be the good channel.

When it is decided which channel is bad, remove output transistors
and check with transistor tester, and/or multimeter.
,
If it is determined that output transistors , both R & L channels, test OK,
then on the bad channel , with output transistors out, connect a test speaker
across each 3 ohm secondary of driver transformer, and make sure
the quieter signal you hear, is the same for both windings.

Right channel / left channel A B this test to determine if the driver
transistor stage is root of problem.


Transistors in an amplifier like this that test OK for shorts,
and beta, compensate themselves because of how the circuit
is designed.

A cranky boss might have growled at a technician who decided
to go all-academic on a record player. Just fix the damn thing !

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


Top
 Profile  
 
Post New Topic Post Reply  [ 17 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 3 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB