Hello guys,
I recently renewed all the electrolytic capacitors on the power amp section of a Ka-7100. Previously working fine, just thought I would recap it.
When I powered it up after recapping, the led light lit up briefly then the protection relay clicked in and remains stuck.
I triple checked all the new caps for polarity and checked all the voltages indicated in the service manual power amp board circuit diagram.
All voltage readings on the entire board are good except for the following:
Qe33 should be 14v, measures 1.9v
Qe34 should be .65v and 1.6v, measures .28v and 47v
RL1 should be 21v, measures 47v.
Re87/88 should read 18v, measures 1.3v
Pin15 should read 9v, measures 0v
There seems to be an error where pins 7 and 8 are reversed on the actual board.
When I installed the new caps, since the board was covered in a combination of grease and dust, I used denatured alcohol and compressed air to try and get the worst off. Maybe I shifted some crud to where it shouldn’t be?
Anyone able to assist in finding the source of the problem, I will be very grateful. I am challenged electrically, but am, or have been, pretty good and building amps and doing the mechanical stuff. This is the first time a project has not cranked up first time.
Cheers
Peter
I recently renewed all the electrolytic capacitors on the power amp section of a Ka-7100. Previously working fine, just thought I would recap it.
When I powered it up after recapping, the led light lit up briefly then the protection relay clicked in and remains stuck.
I triple checked all the new caps for polarity and checked all the voltages indicated in the service manual power amp board circuit diagram.
All voltage readings on the entire board are good except for the following:
Qe33 should be 14v, measures 1.9v
Qe34 should be .65v and 1.6v, measures .28v and 47v
RL1 should be 21v, measures 47v.
Re87/88 should read 18v, measures 1.3v
Pin15 should read 9v, measures 0v
There seems to be an error where pins 7 and 8 are reversed on the actual board.
When I installed the new caps, since the board was covered in a combination of grease and dust, I used denatured alcohol and compressed air to try and get the worst off. Maybe I shifted some crud to where it shouldn’t be?
Anyone able to assist in finding the source of the problem, I will be very grateful. I am challenged electrically, but am, or have been, pretty good and building amps and doing the mechanical stuff. This is the first time a project has not cranked up first time.
Cheers
Peter
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The Q33/34 readings is the protection circuit working. The relay is not stuck.
Something else must have been damaged during the work.
I would not assume that pins 7/8 are labeled wrong, unless you are very certain.
That could lead to reversing the two capacitors that were replaced.
Something else must have been damaged during the work.
I would not assume that pins 7/8 are labeled wrong, unless you are very certain.
That could lead to reversing the two capacitors that were replaced.
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Check the DC offset of both channels. Should be zero volts at the junction of the 0.47 ohm emitter resistors.
I understand it is in protection mode, it’s the “why” I’m trying to figure out.
Here’s a picture of the board showing pins 7 and 8 and their respective transistors. Opposite of the circuit diagram. However I assume that is not a significant problem, just a mistake.
Here’s a picture of the board showing pins 7 and 8 and their respective transistors. Opposite of the circuit diagram. However I assume that is not a significant problem, just a mistake.
Attachments
It can't be related to the problem because you say the amp was previously fine until you recapped it, and after which it was faulty...
Have you measured the offset yet?
Have you measured the offset yet?
I agree. I just mentioned it to acknowledge that the circuit diagram is not infallible.
Center voltage checked at CP points=3mv(as close as I can get to 0v
Bias is 18mv both channels.
This may be a dumb question, but with 47v on both sides of De27, does that indicate a bad diode?
Bear in mind, the relay is “stuck”. There is no click when powering off or on, and of course, no led light. When I first switched the amp on after recapping, the led came on momentarily, the relay clicked and shut down. It’s been like that ever since.
Center voltage checked at CP points=3mv(as close as I can get to 0v
Bias is 18mv both channels.
This may be a dumb question, but with 47v on both sides of De27, does that indicate a bad diode?
Bear in mind, the relay is “stuck”. There is no click when powering off or on, and of course, no led light. When I first switched the amp on after recapping, the led came on momentarily, the relay clicked and shut down. It’s been like that ever since.
That the offset and bias are correct is good news 
So it looks like you have a 'simple' problem with the protection circuit itself.
D27 will have the same voltage on both sides because of the parallel relay coil and the fact the lower end of the coil is not being pulled 'low' to turn on the relay.
So it looks like you have a 'simple' problem with the protection circuit itself.D27 will have the same voltage on both sides because of the parallel relay coil and the fact the lower end of the coil is not being pulled 'low' to turn on the relay.
Check the base voltage on QE33. It should be near zero. If it is 0.6v or thereabouts then that is turning the transistor on and causing your 1.9v (instead of 14v).
The 27k's feeding the base should only be at the DC offset voltage and you have confirmed that is practically zero.
Check the board careful for solder splashes around that transistor. It has to be something super simple, there is nothing else that could turn it on.
The 27k's feeding the base should only be at the DC offset voltage and you have confirmed that is practically zero.
Check the board careful for solder splashes around that transistor. It has to be something super simple, there is nothing else that could turn it on.
Also make sure all the caps in that area around De15 and 16 are correct values and fitted with correct polarity.
Qe33 base 0v. Ce 39, 40 & 41 all correct values and polarity(39 is NP)
Not the best soldering(traces are fragile) but no errant splashes. Examined the area with magnifier and bright light. There seems to be continuity where it is supposed to be. No shorts that I can tell.
Wish I were knowledgeable about how the protection circuit works.
Not the best soldering(traces are fragile) but no errant splashes. Examined the area with magnifier and bright light. There seems to be continuity where it is supposed to be. No shorts that I can tell.
Wish I were knowledgeable about how the protection circuit works.
Qe33 base is correct then.
You need to check the DC voltage across Ce41 (bit blurry) where it says 18 volts. Where it says 9V on De 16 I think will be 9 volts AC and not DC.
If that 18 volts DC is missing then the collector of Qe33 will be pulled to a low voltage all the time.
You need to check the DC voltage across Ce41 (bit blurry) where it says 18 volts. Where it says 9V on De 16 I think will be 9 volts AC and not DC.
If that 18 volts DC is missing then the collector of Qe33 will be pulled to a low voltage all the time.
That sounds to low based on the diagram. That 1.8k resistor is shown as 1 watt and that confirms that there is normally a high-ish voltage there.
You need to to work from that point back to where that 9v goes (or rather comes from).
I suspect that voltage will be missing.
See what voltage is on that diode on the anode (the 9v end). Try measuring both AC and DC voltage to cover all possibilities.
You need to to work from that point back to where that 9v goes (or rather comes from).
I suspect that voltage will be missing.
See what voltage is on that diode on the anode (the 9v end). Try measuring both AC and DC voltage to cover all possibilities.
Has a wire broken or fractured connecting the LED to the other parts of the circuit?
I'll have to leave it shortly but I think your problem here will be a simple one.
I'll have to leave it shortly but I think your problem here will be a simple one.
OK, this has to be something basic...
Look at the circuit and trace it through.
You have 33 volts AC applied to one end of the LED. Check and measure to confirm that is so.
Also if the LED is not lit then there should be zero volts across the LED. If there is lots of voltage across it then it is probably open circuit.
De16 rectifies the AC.
The 680 ohm and 1k8 are in series and go to ground.
So you have an LED across 33 volts AC with a series limiter resistor and diode rectifier. The LED has to light. The low voltage across Ce41 shows that the LED should have its full current available and be really bright.
Something around there is open circuit or missing. Trace it through.
Look at the circuit and trace it through.
You have 33 volts AC applied to one end of the LED. Check and measure to confirm that is so.
Also if the LED is not lit then there should be zero volts across the LED. If there is lots of voltage across it then it is probably open circuit.
De16 rectifies the AC.
The 680 ohm and 1k8 are in series and go to ground.
So you have an LED across 33 volts AC with a series limiter resistor and diode rectifier. The LED has to light. The low voltage across Ce41 shows that the LED should have its full current available and be really bright.
Something around there is open circuit or missing. Trace it through.
