Hello everyone
After a hard battle, I finally came back to life this amp!
The problem was apart from the damaged components, bad welding, wire harnesses with the wire cut (this method of connection I do not like)
Now I have some minor problems, as if I set the speaker selector to "A" .It does not work. Yes the system "B" (I think it is a bad relay output). During this operation, the display will not show that the speaker is working, and when you press the mute button, it lights up on speaker "B" on the screen ...
Sounds like a bad connection (the cables from the front panel straps or the front switch selector in the wrong position?) I will investigate.
When the amp came back to life, I did one test, remove the battery and the amplifier did not stop, as if nothing had happened (a myth?).
Any suggestions would be welcome.
Thank you
Sincerely¨
After a hard battle, I finally came back to life this amp!
The problem was apart from the damaged components, bad welding, wire harnesses with the wire cut (this method of connection I do not like)
Now I have some minor problems, as if I set the speaker selector to "A" .It does not work. Yes the system "B" (I think it is a bad relay output). During this operation, the display will not show that the speaker is working, and when you press the mute button, it lights up on speaker "B" on the screen ...
Sounds like a bad connection (the cables from the front panel straps or the front switch selector in the wrong position?) I will investigate.
When the amp came back to life, I did one test, remove the battery and the amplifier did not stop, as if nothing had happened (a myth?).
Any suggestions would be welcome.
Thank you
Sincerely¨
I guess you had the bad solders in the power amp section, most likely around Q254L too Q257L as these often needs to be reflowed on the TA models (my experience). Right?
As for the parts you replaced, to make those resistors blow someone must have made a damn serious mistake.
I guess you got it in that condition?
As for speaker A not working, it can be a few causes.
1. Speaker selector is badly oxidized in A position.
2. Someone has done a shoddy repair work and cut a cable or two.
3. One or more of the following transistors are bad, Q272, Q274 or Q508. The first two are on the main board, last one on control board.
4. Relay might be broken, RY250, but that doesn't explain why [A] doesn't lit up on the display.
As why only lits up in muted is peculiar, gotta think about that.
As for the parts you replaced, to make those resistors blow someone must have made a damn serious mistake.
I guess you got it in that condition?
As for speaker A not working, it can be a few causes.
1. Speaker selector is badly oxidized in A position.
2. Someone has done a shoddy repair work and cut a cable or two.
3. One or more of the following transistors are bad, Q272, Q274 or Q508. The first two are on the main board, last one on control board.
4. Relay might be broken, RY250, but that doesn't explain why [A] doesn't lit up on the display.
As why only lits up in muted is peculiar, gotta think about that.
Dear Mannegizen.
You're right!!! The connectors were changed. Now the mute control works fine (I can only hear, at a very low volume when pressed)
Now the display is displayed correctly and the "A" output lights up when the switch is on the "A" speaker and you know what? Output "A" now works, when I connect the speakers !!!
That was the problem to solve.
Thank you very much for your contribution.
Best regards from Argentine.
You're right!!! The connectors were changed. Now the mute control works fine (I can only hear, at a very low volume when pressed)
Now the display is displayed correctly and the "A" output lights up when the switch is on the "A" speaker and you know what? Output "A" now works, when I connect the speakers !!!
That was the problem to solve.
Thank you very much for your contribution.
Best regards from Argentine.
Hi Mannegizen
You're right!!! The connectors were changed. Now the mute control works fine (I can only hear, at a very low volume when pressed)
Now the display is displayed correctly and the "A" output lights up when the switch is on the "A" speaker and you know what? Output "A" now works, when I connect the speakers !!!
That was the problem to solve.
Thank you very much for your contribution.
Best regards from Argentine.
You're right!!! The connectors were changed. Now the mute control works fine (I can only hear, at a very low volume when pressed)
Now the display is displayed correctly and the "A" output lights up when the switch is on the "A" speaker and you know what? Output "A" now works, when I connect the speakers !!!
That was the problem to solve.
Thank you very much for your contribution.
Best regards from Argentine.
It had bad welds in the section of the power amplifier and in the control board.
Almost the entire control board had poor welding from the central sector to the part of the input connectors. Probably in the assembly phase of the board, it was not well positioned (horizontal) in the wave welding station.
In the amplifier, poor welding in different areas.
For doubts check and resolded everything.
I agree with you, this amp was with another technician before me.
Provisionally replace 2SK389BL with two BF245 to test. I know it is ********, but here in Argentina it is difficult to get 2SK ...
I bought two originals 2SK389BL and matched on Ebay.
They come in two weeks.
Almost the entire control board had poor welding from the central sector to the part of the input connectors. Probably in the assembly phase of the board, it was not well positioned (horizontal) in the wave welding station.
In the amplifier, poor welding in different areas.
For doubts check and resolded everything.
I agree with you, this amp was with another technician before me.
Provisionally replace 2SK389BL with two BF245 to test. I know it is ********, but here in Argentina it is difficult to get 2SK ...
I bought two originals 2SK389BL and matched on Ebay.
They come in two weeks.
Back to the battery. Purpose of battery is to maintain voltage, thus contents, of SRAM inside microcontroller. Guess only things needing to be saved while ac power disconnected is AM and FM frequency presets. I was speculating that if battery was, liked shorted, at 0V, then SRAM could not function. SRAM is supplied with voltage whenever ac power is on, so battery could be low or even open, and SRAM will still operate okay.
Hi every body
Again with this amp. After a lot, now fail again but in this oportunity, the problem is different.
It work correctly for a some time (30 to 60 minutes) then the sound is off and the green led come to blink.
After awile (power off), 20 to 30 minutes, the amp work fine, and then again the problem begins.
Thanks for any reply.
Best regards.
Again with this amp. After a lot, now fail again but in this oportunity, the problem is different.
It work correctly for a some time (30 to 60 minutes) then the sound is off and the green led come to blink.
After awile (power off), 20 to 30 minutes, the amp work fine, and then again the problem begins.
Thanks for any reply.
Best regards.
I'd suspect the reason for it going into protection is high DC offset, measure it on the speaker relay after it goes into protection.
On the TA-2/3 models there is a 22K Ohm resistor on each channel that goes open with age. On the TA-4 model the corresponding resistor is 47K Ohm.
Worth checking if the value is sane.
If you meassure this resistor in circuit and the reading is less that 47K and falling (less than 5k or so.), swap the probes.
The symptoms you have is consistent with these resistors failing, when they are cold they have a value that permits operating and as they get hotter the resistanse raises and at one point the differential circuit doesn't work any more.
(Tho I've never heard of these failing on TA-4, but you might be the first...)
As you have replaced the dual fet in the differential, this is my secondary suspect.
Maybe you applied to much heat when soldering? (I have no idea on how sensitive these are.)
I have no idea on how to test if this is the case tho.
Third suspect would be bias transistors and/or a heat issue, If the heatspreaders doesnt reach more than 50 degrees celcius it's probably fine in that aspect.
On the TA-2/3 models there is a 22K Ohm resistor on each channel that goes open with age. On the TA-4 model the corresponding resistor is 47K Ohm.
Worth checking if the value is sane.
If you meassure this resistor in circuit and the reading is less that 47K and falling (less than 5k or so.), swap the probes.
The symptoms you have is consistent with these resistors failing, when they are cold they have a value that permits operating and as they get hotter the resistanse raises and at one point the differential circuit doesn't work any more.
(Tho I've never heard of these failing on TA-4, but you might be the first...)
As you have replaced the dual fet in the differential, this is my secondary suspect.
Maybe you applied to much heat when soldering? (I have no idea on how sensitive these are.)
I have no idea on how to test if this is the case tho.
Third suspect would be bias transistors and/or a heat issue, If the heatspreaders doesnt reach more than 50 degrees celcius it's probably fine in that aspect.
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If you change the email that you used to Register for the forum, you need to change it in your private membership information or you will not get notifications or the ability to reset your password. I have done it but I forget. I suggest you post in Forum Problems and a forum expert will help you.
If you want to measure dc-offset after protection is tripped you'll have to do that directly on the speaker relay pins. Between earth and the "middle" pins.
The protection IC uPC1237H can detect 3 types of faults (4 if we count the need for power to the IC).
https://www.promelec.ru/pdf/upc1237ha.pdf
If we can determine what fault it actually detects it will help to find out what the cause is.
First it needs to be powered on. It will typically get 3,3V on pin8, a lack of power to the IC will probably give the same symptom as you have.
It's feed by the positive rail voltage, +59V, tru resistor R295 (18K). A failure here is unlikely tho, but should be checked.
Secondly it needs AC voltage on pin4, a minimum of +0,6V too +0,9V, should be around +2.5V DC (4,4V AC).
This is taken from the rectifier, D901, in the power supply via resistor R902 (27K) and diode D902. There is also a capacitor, C903, that could potentially short this voltage to ground if it's faulty.
Third it detects dc-offset on pin2 from Q266L/R and Q267L/R. According to specification (if I understand it correctly) the maximum voltage here should be between -0,23V too +0,70V. Anything outside these values will trip protection.
Fourth, on pin1 it detects either overheating via the thermal resistor TH250 via Q270 or overload detection via Q268 and Q265L/R. The maximum voltage here (if I understand it correctly) shouldn't exceed somewhere between +0,58V too +0,76V.
Check the values on pins 1, 2, 4 and 8 when it's working and then again when protection has tripped to see what has changed.
The protection IC uPC1237H can detect 3 types of faults (4 if we count the need for power to the IC).
https://www.promelec.ru/pdf/upc1237ha.pdf
If we can determine what fault it actually detects it will help to find out what the cause is.
First it needs to be powered on. It will typically get 3,3V on pin8, a lack of power to the IC will probably give the same symptom as you have.
It's feed by the positive rail voltage, +59V, tru resistor R295 (18K). A failure here is unlikely tho, but should be checked.
Secondly it needs AC voltage on pin4, a minimum of +0,6V too +0,9V, should be around +2.5V DC (4,4V AC).
This is taken from the rectifier, D901, in the power supply via resistor R902 (27K) and diode D902. There is also a capacitor, C903, that could potentially short this voltage to ground if it's faulty.
Third it detects dc-offset on pin2 from Q266L/R and Q267L/R. According to specification (if I understand it correctly) the maximum voltage here should be between -0,23V too +0,70V. Anything outside these values will trip protection.
Fourth, on pin1 it detects either overheating via the thermal resistor TH250 via Q270 or overload detection via Q268 and Q265L/R. The maximum voltage here (if I understand it correctly) shouldn't exceed somewhere between +0,58V too +0,76V.
Check the values on pins 1, 2, 4 and 8 when it's working and then again when protection has tripped to see what has changed.
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