good day my friends i am repairing a ap4040 yorkville amp that keep blowing a zener on both channels i have check every transistor and resistor everything how they are supposed read please help as i can't find cause
Hi
I do not see how the bias would be stable in this circuit since the zener ZD12 is in series with a 470mR-1W resistor that straddles the bases of the output transistors. This voltage could vary considering the bases are driven by the driver stage, in turn driven by the predriver, in turn driven by the input BJTs to the floating amplifer. The bias pot nominally sets the voltage between the input device bases but it does not seem regulated in the way a standard bias circuit does?
The 1W in series with ZD12 would not reach 1W of heat until there is 680mA through it. ZD12 is a 1N750 500mW device which cannot handle that much current (106mA max).
To me it seems like these two parts are totally mismatched. Maybe the 470mR should be a higher value?
I think the only way for this diode to be damaged is if the amp is driven to clipping frequently and/or if D1 or D3 are shorted? Just a guess
Good luck
I do not see how the bias would be stable in this circuit since the zener ZD12 is in series with a 470mR-1W resistor that straddles the bases of the output transistors. This voltage could vary considering the bases are driven by the driver stage, in turn driven by the predriver, in turn driven by the input BJTs to the floating amplifer. The bias pot nominally sets the voltage between the input device bases but it does not seem regulated in the way a standard bias circuit does?
The 1W in series with ZD12 would not reach 1W of heat until there is 680mA through it. ZD12 is a 1N750 500mW device which cannot handle that much current (106mA max).
To me it seems like these two parts are totally mismatched. Maybe the 470mR should be a higher value?
I think the only way for this diode to be damaged is if the amp is driven to clipping frequently and/or if D1 or D3 are shorted? Just a guess
Good luck
Hi again
Actually, the manual states how the bias should work but it still seems a bit less reliable than the usual approach.
There is also a service note related to the bias circuit, where the resistor R10 in series with the bias pot is changed from 4k7 to 3k25. Have you done this?
There could also be an issue with the bias trimpot opening and letting bias go out of control. However, that is more likely to happen just with one channel than with both although everything in the amp is the same age... so who knows?
Peace
Actually, the manual states how the bias should work but it still seems a bit less reliable than the usual approach.
There is also a service note related to the bias circuit, where the resistor R10 in series with the bias pot is changed from 4k7 to 3k25. Have you done this?
There could also be an issue with the bias trimpot opening and letting bias go out of control. However, that is more likely to happen just with one channel than with both although everything in the amp is the same age... so who knows?
Peace
ok nauta going to check the r10 the client told me that is was in use when low voltage came and that was it did a check on the power supply and all check out to be in perfect order thank you for the input
The fault seems shared on both channels. So it is highly likely results from whatever connections are shared.
Brings us back to the power supply assumed shared.
If the zeners dont blow instantly and the amp can be operated. Voltages should be checked under load.
They might read ok at idle or isolated. Under load could sag. And DC offset measured on the outputs.
Being operated at low line voltage is a big hint. There seems to be optical limiters and the amp is protected by a crowbar.
They seem to be operated or referenced with low voltage or 16 volt. Assuming regulated 16 volts. If line voltage was sagging.
And the protection circuits where receiving lower than 16 volts.
Curious if limiting was being false triggered. If the crowbar was false triggered we would expect a lot of damage.
If the voltage current limiters were kicking in. This would introduce a lot of flyback voltages.
Flyback voltages will be quick but often 5x more voltage than the rails. So often we see repaired amps with replaced output devices.
But failed devices in the VI limiters are often missed. So you have a wealth of transistors and diodes in the VI limiters that can be faulty.
And the VI limiter tie in directly to the bias circuit with failing zeners. So DC offset and strange behavior will only be happening when the VI
is being triggered. Often 100 volt diodes like 4148 will or low voltage zeners will seem to test ok but give strange readings.
And is highly likely to be damaged and left in place by any flyback voltages.
Likely or could be likely previous repairs were done. Such as if the crowbar did latch. Output devices would have been changed.
And likely damaged or failed devices in the VI limiters were left in.
Or we have damaged power supply componets and high voltages and low voltages should be tested under load.
Brings us back to the power supply assumed shared.
If the zeners dont blow instantly and the amp can be operated. Voltages should be checked under load.
They might read ok at idle or isolated. Under load could sag. And DC offset measured on the outputs.
Being operated at low line voltage is a big hint. There seems to be optical limiters and the amp is protected by a crowbar.
They seem to be operated or referenced with low voltage or 16 volt. Assuming regulated 16 volts. If line voltage was sagging.
And the protection circuits where receiving lower than 16 volts.
Curious if limiting was being false triggered. If the crowbar was false triggered we would expect a lot of damage.
If the voltage current limiters were kicking in. This would introduce a lot of flyback voltages.
Flyback voltages will be quick but often 5x more voltage than the rails. So often we see repaired amps with replaced output devices.
But failed devices in the VI limiters are often missed. So you have a wealth of transistors and diodes in the VI limiters that can be faulty.
And the VI limiter tie in directly to the bias circuit with failing zeners. So DC offset and strange behavior will only be happening when the VI
is being triggered. Often 100 volt diodes like 4148 will or low voltage zeners will seem to test ok but give strange readings.
And is highly likely to be damaged and left in place by any flyback voltages.
Likely or could be likely previous repairs were done. Such as if the crowbar did latch. Output devices would have been changed.
And likely damaged or failed devices in the VI limiters were left in.
Or we have damaged power supply componets and high voltages and low voltages should be tested under load.
whitedragon i think is a power supply problem i'll b going over the powersupply and check the components and the voltage
I'm still studying the bias spreader, but can offer some opinion/speculation.
I believe the ZD12 zener is intended as a protection device that isn't active in normal operation, and that R85 is probably intended as a fusible resistor when things go terribly wrong. Which seems to be the case here.
A there's a couple of circuit details that support this theory. Aside from several small value resistors, ZD12 is in shunt between the positive and negative bases of output transistor array, i.e. voltage generated by the bias spreader. At idle, the voltage between between the bases will be about 1.2V, way less tan the 4.7V zener. Further, the bias spread is derived from the voltage difference between the bases of Q1 and Q2. This is about 2 Vbe, as the drops across R3 and R4 are minimal.
The destruction of ZD12 was probably sourced by current flow through the driver transistors. The spreader is essentially a shunt regulator, with the reference being the 2 Vbe spread between Q1 and Q2. There could be an open somewhere in this control loop. If this is the failure cause, you have to find and correct it to prevent repeat failures. I'm not rejecting the notion that the problem is provoked by a power supply defect, since you see the problem in both channels. Just saying the issue may lie within the spreader.
I saw a tip from another member advising how to debug a spreader using a 9V battery, with the amp otherwise unpowered. The idea may have come from @JonSnell Electronic
I believe the ZD12 zener is intended as a protection device that isn't active in normal operation, and that R85 is probably intended as a fusible resistor when things go terribly wrong. Which seems to be the case here.
A there's a couple of circuit details that support this theory. Aside from several small value resistors, ZD12 is in shunt between the positive and negative bases of output transistor array, i.e. voltage generated by the bias spreader. At idle, the voltage between between the bases will be about 1.2V, way less tan the 4.7V zener. Further, the bias spread is derived from the voltage difference between the bases of Q1 and Q2. This is about 2 Vbe, as the drops across R3 and R4 are minimal.
The destruction of ZD12 was probably sourced by current flow through the driver transistors. The spreader is essentially a shunt regulator, with the reference being the 2 Vbe spread between Q1 and Q2. There could be an open somewhere in this control loop. If this is the failure cause, you have to find and correct it to prevent repeat failures. I'm not rejecting the notion that the problem is provoked by a power supply defect, since you see the problem in both channels. Just saying the issue may lie within the spreader.
I saw a tip from another member advising how to debug a spreader using a 9V battery, with the amp otherwise unpowered. The idea may have come from @JonSnell Electronic
hello guys over the weekend went through the power supply and a transistor bad mpsa13 and checking back zd12 found that in the board it read short but i can't see how that happens well when apply power to the amp (which i ran through a bulb as a protection ) the bulb keep puslsating when the 2 channels are connected no relay clicks and the power led and protection led lights up