Hello,
I am new to this board and would like to ask for some help for bringing my old Cit12 back to life:
One channel is showing all kind of faulty voltages and my plan is to replace all transistors.
After testing all transistors with the "two-diod-method" I thought that I had located the trouble to the pre-driver, but unfortunately not so.
I wonder if someone has a list of replacement transistors that has proved to work in the original design for this unit?
Regards//lasse
Stockholm, Sweden
I am new to this board and would like to ask for some help for bringing my old Cit12 back to life:
One channel is showing all kind of faulty voltages and my plan is to replace all transistors.
After testing all transistors with the "two-diod-method" I thought that I had located the trouble to the pre-driver, but unfortunately not so.
I wonder if someone has a list of replacement transistors that has proved to work in the original design for this unit?
Regards//lasse
Stockholm, Sweden
When using the "two-diode-method" please also check C-E and E-C to check for leakage,
it should be very high resistance, be sure not to touch the base.
Are there fuses in the +/- rails to the amp?
You need to have current limiting, a light bulb current limiter would work well also.
it should be very high resistance, be sure not to touch the base.
Are there fuses in the +/- rails to the amp?
You need to have current limiting, a light bulb current limiter would work well also.
Once we establish how to current limit the amp we can start debugging.
Pull the plug in boards and do the diode test again on the output devices.
Next pull Q704.
Jump C-E on the bias transistor Q708.
Do the following tests with no input and no load on the output.
Measure the voltage at the emitters of the input diff pair and across the 390 ohm in
the collector.
Please tell us the voltages at the rails, the output, and at the pins of the driver transistors.
Pull the plug in boards and do the diode test again on the output devices.
Next pull Q704.
Jump C-E on the bias transistor Q708.
Do the following tests with no input and no load on the output.
Measure the voltage at the emitters of the input diff pair and across the 390 ohm in
the collector.
Please tell us the voltages at the rails, the output, and at the pins of the driver transistors.
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Hi, Pete!
Nice of you to volunteer to help!
I Will be back with the data as soon as I have bought some essential equipment.
Best regards// Lasse
Nice of you to volunteer to help!
I Will be back with the data as soon as I have bought some essential equipment.
Best regards// Lasse
Do you have the service manual? Here it is in .pdf:
http://manuals.harman.com/hk/Service Manual/Citation twelve sm.pdf
I would not buy any transistors until we discuss them.
I do not consider the 2N3055 a good substitute for the 40636 outputs.
http://manuals.harman.com/hk/Service Manual/Citation twelve sm.pdf
I would not buy any transistors until we discuss them.
I do not consider the 2N3055 a good substitute for the 40636 outputs.
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Here is the 1969 Reference design from the RCA transistor manual:
http://www.diyaudio.com/forums/soli...-12-substitute-transistors-3.html#post2355300
http://www.diyaudio.com/forums/soli...-12-substitute-transistors-3.html#post2355300
While the transistors are out check all resistors and capacitors.
If the amp is of any age then the electrolytic's might need changing as they tend to wear out first.
If the amp is of any age then the electrolytic's might need changing as they tend to wear out first.
Hello and thanks for all input.
This nights work is to rig a lightbulb (will 40-60W be ok?) in series with the mains supply to the amp.
Yes, capacitors: That is where all of this started. I replaced all electrolytics and was powering up the amp and was trying to adjust the bias when something happened. Probably, I managed to short something on the board and now, I have rail voltage on the output.
Panasonic FC all over and expensive can-caps in the PS.
Yes, I have the manuals for this amp.
regards//lasse
This nights work is to rig a lightbulb (will 40-60W be ok?) in series with the mains supply to the amp.
Yes, capacitors: That is where all of this started. I replaced all electrolytics and was powering up the amp and was trying to adjust the bias when something happened. Probably, I managed to short something on the board and now, I have rail voltage on the output.
Panasonic FC all over and expensive can-caps in the PS.
Yes, I have the manuals for this amp.
regards//lasse
Hi lasser,
have you investigated how come that connection 23 (and 26) is that much negative ?
The difference from 23 to 26 seems ok, indicating the thermal tracking diode to be likely ok.
You may want to check R720, R718 and C706.
Measure the voltage across these resistors also.
have you investigated how come that connection 23 (and 26) is that much negative ?
The difference from 23 to 26 seems ok, indicating the thermal tracking diode to be likely ok.
You may want to check R720, R718 and C706.
Measure the voltage across these resistors also.
Looks as though the NPN driver has gone AWOL.
With everything unpowered, I'd check its diodes (b-e, b-c) for continuity, or unsolder it and then check.
With everything unpowered, I'd check its diodes (b-e, b-c) for continuity, or unsolder it and then check.
Looks to me like either the VAS and/or the PNP driver have C-E shorts, probably not
both. This is why pulling the VAS will help debug it.
I don't see a problem with the NPN driver.
There is no reason to remove the NPN driver until the VAS device is removed and
voltages taken.
The diff pair look to be blown based on their B-E voltage.
both. This is why pulling the VAS will help debug it.
I don't see a problem with the NPN driver.
There is no reason to remove the NPN driver until the VAS device is removed and
voltages taken.
The diff pair look to be blown based on their B-E voltage.
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Hello guys,
Just a quick word befors I make breakfast (Swedish time...)
I admire you guys who understand what`s goding on in a electronic cirquit like this one.
And I really appreciate the help from you.
I Will be back with results tonight on the feedback basen on post #3 from Pete
Regards// Lasse
Just a quick word befors I make breakfast (Swedish time...)
I admire you guys who understand what`s goding on in a electronic cirquit like this one.
And I really appreciate the help from you.
I Will be back with results tonight on the feedback basen on post #3 from Pete
Regards// Lasse
Those readings are goofy enough to suspect faulty connections from the card to the chassis connectors. I have a Citation 15 tuner with the same connectors and they're sometimes flaky.
G²
Hi, time for some results:
Rail voltage: +/- 35,5V
Output voltage: +33,7V
Emitter voltage on diff pair: +0,9V
Voltage across 390 ohms resistor: 1,9V
Driver Q706: emitter: +33,6V Base: +34,8V Collector: -35,6V
Driver Q710: emitter: +39,0V Base: +34,7V Collector: +35,2V
Best regards//Lasse
Oh, I forgot!
According to the diode-method, the output transistors are healthy. The diode-tester shows abt 0,53V in the right directions and no Reading in the opposite one.
Regards again// Lasse
Stockholm, Sweden
According to the diode-method, the output transistors are healthy. The diode-tester shows abt 0,53V in the right directions and no Reading in the opposite one.
Regards again// Lasse
Stockholm, Sweden
Looking very good so far.
Forgot to ask for the base voltages on the diff pair transistors.
I also want to explain that the original output devices used were what is called single
diffused type of homotaxial type which are not made anymore. Using any newer
faster type will require stability debugging so it is best to just keep the originals if possible.
Forgot to ask for the base voltages on the diff pair transistors.
I also want to explain that the original output devices used were what is called single
diffused type of homotaxial type which are not made anymore. Using any newer
faster type will require stability debugging so it is best to just keep the originals if possible.
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