Kenwood KA-7100 needs help
- Thread starter Squidward
- Start date
Squidward said:
No problem... one of my problems is I assume too much and leave
out the details that would make my point much clearer!
I have a working 7100 in my "collection", it's currently my daily driver...
If you need voltage measurements from a working unit... let me know!
Scott
mg196
Johnny Thunders - LAMF
Echowars made my KA-7100 SING!! He can do the same for you: http://audiokarma.org/forums/showthread.php?t=38900
Too funny...
I don't know why, but every single 7100 I've ever worked on needed a new relay driver transistor. Every bloody one...(OK, there might have been one that was OK, but I've worked on a dozen or more of the things and probably 11 of 'em needed a new driver tranny).
The suspect transistor is Qe34, and is at the very front of the horizontally mounted 'motherboard' of the unit, just about dead-center. I believe that the component designations are silkscreened into the board, so you should have no problem locating it. Immediately next to it is a ground terminal with a couple of black wires on it.
The view here is from the conductor (foil) side of the board.
I'm happy to order one and try it out. Forgive me tho, I'm a solid-state idiot, this will literally be the first solid state thing I've tried to fix beyond a good cleaning.
Is there a test to see if the transistor in question is bad? Where do I order another one? Is there a particular one I should look for?
Thank you very much.
Is there a test to see if the transistor in question is bad? Where do I order another one? Is there a particular one I should look for?
Thank you very much.
If there is about .6 ~ .7V on the base of the transistor, and 25 to 30-something volts on the collector, then the transistor is bad.
Even a cheapie RatShack 2N3904 will work fine for a replacement, but the pinout of these RatShack transistors cannot be taken for granted...you must determine for yourself which lead is base, collector, and emitter.
http://www.radioshack.com/product.a...name=CTLG_011_002_017_000&product_id=276-2016
Even a cheapie RatShack 2N3904 will work fine for a replacement, but the pinout of these RatShack transistors cannot be taken for granted...you must determine for yourself which lead is base, collector, and emitter.
http://www.radioshack.com/product.a...name=CTLG_011_002_017_000&product_id=276-2016
Jim H B
Well-Known Member
Squidward, sorry to ask so many questions, but I'd like to learn some of this myself.
How did you make the tests on the transistor? Did you desolder it and remove it from the PCB first?
If you didn't remove it, did you have the power on when testing?
What instrument did you use to test it? If it was a multimeter, where did you put the red and black leads, and what setting did you use?
Use small words, you know how us State fans are...
Jim
How did you make the tests on the transistor? Did you desolder it and remove it from the PCB first?
If you didn't remove it, did you have the power on when testing?
What instrument did you use to test it? If it was a multimeter, where did you put the red and black leads, and what setting did you use?
Use small words, you know how us State fans are...
Jim
Jim,
Here's a post that you might want to take a look at... should explain quite a bit! :thmbsp:
http://audiokarma.org/forums/showthread.php?t=43186
Scott
Here's a post that you might want to take a look at... should explain quite a bit! :thmbsp:
http://audiokarma.org/forums/showthread.php?t=43186
Scott
While we're at it Jim, here's another good one for ya... :thmbsp:
http://audiokarma.org/forums/showthread.php?t=5634
Scott
http://audiokarma.org/forums/showthread.php?t=5634
Scott
Jim H B said:
As long as you're cool with the blind leading the blind, here's my take...
I took off the top and bottom case of the amp. I located the transistor that EW mentioned near the front of the board. I then put the amp on it's side, so that the bottom of the amplifier was facing me and I could see the bottom of the circuit board. I re-located the pins of the transistor. I then put the black lead of my multimeter against the metal chassis of the amplifier, put the red lead in my dominant hand, and put my other hand behind my back. I then turned on the amplifier. With the meter set to take voltage readings, I carefully put the tip of the red proble against each of the three transistor's solder joints on the back of the board, noting the readings. I then turned the amplifier off.
A state fan? Living in Ann Arbor? A stranger in a strange land, indeed... A you fixing a KA-7100? If you need this replacement transistor, let me know, and if I've not ordered mine yet, I'll order you one. A friend was ordering some transistors on-line, and he said he could get me one of these for like $0.40
Jim H B
Well-Known Member
Scott,
Another excellent thread, and actually inspired me to get my multimeter. I've adjusted the offset on a couple of receivers already.
Squidward -
Thanks for the info. I don't have a 7100, but I have a Hitachi HA-5700 with the same problem (no output, I don't know if it's the transistor).
One thing I do wonder about; EW says the transistor can be tested out of the circuit, so I assume that if is being tested in the circuit (ie, still in the receiver), that you wouldn't have to have the power on to test it.
I just get paranoid about doing a test with the power on; I would hate to touch the wrong thing with a probe and fry something.
Jim
I'm putting some key words in this post, so I can find this thread in a search:
Testing transistors
How to test transistors
Another excellent thread, and actually inspired me to get my multimeter. I've adjusted the offset on a couple of receivers already.
Squidward -
Thanks for the info. I don't have a 7100, but I have a Hitachi HA-5700 with the same problem (no output, I don't know if it's the transistor).
One thing I do wonder about; EW says the transistor can be tested out of the circuit, so I assume that if is being tested in the circuit (ie, still in the receiver), that you wouldn't have to have the power on to test it.
I just get paranoid about doing a test with the power on; I would hate to touch the wrong thing with a probe and fry something.
Jim
I'm putting some key words in this post, so I can find this thread in a search:
Testing transistors
How to test transistors
I guess that depends on whether you want to look at the voltages on the transistor and determine if its just not doing what its supposed to do even though it has the proper voltages applied, or if you want to remove it to do a basic diode test to see if its blown.
Weak transistors will often pass a basic 'out-of-circuit' diode test, but with power on and proper voltages applied they just won't conduct. Again, this is why people get paid to do this...sometimes is isn't a cut 'n dried operation and without a bit of experiance it might not be apparent what your next move ought to be.
Weak transistors will often pass a basic 'out-of-circuit' diode test, but with power on and proper voltages applied they just won't conduct. Again, this is why people get paid to do this...sometimes is isn't a cut 'n dried operation and without a bit of experiance it might not be apparent what your next move ought to be.
Jim H B
Well-Known Member
Thanks EW, I guess the real goal is to see if it is doing what it is supposed to do when the unit is operating, so I can see where testing it in-circuit makes sense.
And believe me, I have a healthy respect for the years of knowledge that professional techs have, especially you, PunkerX, and the other techs that support AK.
It's just fun to be able to do some of the simple stuff yourself.
Jim
And believe me, I have a healthy respect for the years of knowledge that professional techs have, especially you, PunkerX, and the other techs that support AK.
It's just fun to be able to do some of the simple stuff yourself.
Jim
Well I am a little late returning to the party here, but I put my spare KA-7100's (one working... one not) up on the bench and tried the test...
The working unit (EW... this is not the unit you serviced!) showed these readings...
DC offset about 5mv in both channels and as far as the Q34 transistor is concerned, it shows about .68V on the base and .25V on the collector.. the schematic indicates the collector should be at 1.6V... is this descrepancy a concern?
The non-working unit shows about 45V on the collector, and I can't get any steady reading on the base.... I think it was around .6 or .7V the first time I touched the probes to it... it is ZERO now.
Also, on the non-working unit, the Q29 & Q27 left channel output transistors get very warm.... especially the Q29 it seems.
For the cost of a fresh 2N3904 transistor, I should probably swap in a new one just to see what happens, but I wonder if there is a deeper problem?
The working unit (EW... this is not the unit you serviced!) showed these readings...
DC offset about 5mv in both channels and as far as the Q34 transistor is concerned, it shows about .68V on the base and .25V on the collector.. the schematic indicates the collector should be at 1.6V... is this descrepancy a concern?
The non-working unit shows about 45V on the collector, and I can't get any steady reading on the base.... I think it was around .6 or .7V the first time I touched the probes to it... it is ZERO now.
Also, on the non-working unit, the Q29 & Q27 left channel output transistors get very warm.... especially the Q29 it seems.
For the cost of a fresh 2N3904 transistor, I should probably swap in a new one just to see what happens, but I wonder if there is a deeper problem?
Last edited:
poking a little further...
OK... so I decided to poke a little further with the DMM...., and it looks like the non-working unit has some issues with the left channel.
Voltage readings on the main output transistors are :
Right Channel (Q30 & Q28)
Q30 Q28
E .01V .05V
C -.52V .59V
B -46.7V 47.0V
on the Left Channel (Q27 & Q29)
Q27 Q29
E 34.6V 33.7V
C 34.9V 34.1V
B 43.6V -43.8V
According to the service manual, the specs should be:
E 0V
C .6V
B 47v
It also looks like the R channel is getting the spec'd 47V supply, while the L channnel is only getting about 44V, based on checking voltages at locations C31, C32, C33, C34.
OK... so I decided to poke a little further with the DMM...., and it looks like the non-working unit has some issues with the left channel.
Voltage readings on the main output transistors are :
Right Channel (Q30 & Q28)
Q30 Q28
E .01V .05V
C -.52V .59V
B -46.7V 47.0V
on the Left Channel (Q27 & Q29)
Q27 Q29
E 34.6V 33.7V
C 34.9V 34.1V
B 43.6V -43.8V
According to the service manual, the specs should be:
E 0V
C .6V
B 47v
It also looks like the R channel is getting the spec'd 47V supply, while the L channnel is only getting about 44V, based on checking voltages at locations C31, C32, C33, C34.