I've been working on and off on this M-500T that I just can't seem to fix, primarily because I just don't understand the circuit. Hoping someone can help.
Initially the unit was not coming out of protection. I was able to resolve that by fixing cold solder joints. Ended up re-flowing the entire mainboard and power supply. All output transistors check out, out of circuit on my Peak meter. Once I got relay click, I installed new caps on the mainboard and the smaller caps on the power supply. All voltages from the supply are there at the initial measurement points, but then it starts to change halfway through the schematic, which I also don't really trust because it already has some mismarkings at the power supply, but anyway...
I don't understand the switching power supply and how it works. And i don't know if it's relevant to the issue I'm having, but basically I'm supposed to be seeing a + and - 14v at IC101 at pins 4 and 8, and I'm not. i'm getting around .6v +/- which goes up to maybe .7v if I connect an input signal. I've pulled both zener diodes and checked them and they read fine. I'm presuming that if both voltage rails are missing at that chip, then it's something that would be tied to both and not channel specific. That's what I'm having trouble finding. I'm seeing 74v and 43v +/- at points on transistors where I should be only seeing less than 2v.
This is my last ditch effort to try figuring this thing out. I'm attaching a pic of the area of the schematic where I'm supposed to see the voltages, for what it's worth.
Initially the unit was not coming out of protection. I was able to resolve that by fixing cold solder joints. Ended up re-flowing the entire mainboard and power supply. All output transistors check out, out of circuit on my Peak meter. Once I got relay click, I installed new caps on the mainboard and the smaller caps on the power supply. All voltages from the supply are there at the initial measurement points, but then it starts to change halfway through the schematic, which I also don't really trust because it already has some mismarkings at the power supply, but anyway...
I don't understand the switching power supply and how it works. And i don't know if it's relevant to the issue I'm having, but basically I'm supposed to be seeing a + and - 14v at IC101 at pins 4 and 8, and I'm not. i'm getting around .6v +/- which goes up to maybe .7v if I connect an input signal. I've pulled both zener diodes and checked them and they read fine. I'm presuming that if both voltage rails are missing at that chip, then it's something that would be tied to both and not channel specific. That's what I'm having trouble finding. I'm seeing 74v and 43v +/- at points on transistors where I should be only seeing less than 2v.
This is my last ditch effort to try figuring this thing out. I'm attaching a pic of the area of the schematic where I'm supposed to see the voltages, for what it's worth.
The chip IC101, Zeners ZD101, ZD102, or 6.8K 2Watt Resistors R182, R183 could be toast. Note R119 is 15K, not 15.
NJM072 is a JRC version of TL072.
NJM072 is a JRC version of TL072.
I did pull the zeners and test them and they checked out, but i measured the rest in circuit. I did swap out the TL072 also, originally, but I'm still seeing high voltages at transistors that are marked on the schematic lower, like Q101 for example, where it shows it should be 1.8V, it's 74V.
The collector of Q101 shows 7.4? but it should be about 73 Volts. If the base is also ~73 Volts then Q101 is shorted.
I'll recheck that.... you sound experienced with this schematic; how many known errors are there on this thing?
You must resolve why you are not getting the supply to the opamp first.
R182 and R183 should feel 'hot'. If they are cold (no voltage on them) then check the 22 ohm feeding them (R179 and R181). If those are open then most likely you have a fault around the transistor stages fed by the 22 ohms.
R182 and R183 should feel 'hot'. If they are cold (no voltage on them) then check the 22 ohm feeding them (R179 and R181). If those are open then most likely you have a fault around the transistor stages fed by the 22 ohms.
R182 and R183 both have + and - 74v, and on each end is what I'm seeing at the IC; about .6v +/- on each respective pin. I pulled them both and measured them 6.8k ohms right on the money. I pulled and tested R179 and R181 and same thing; 22 ohms exact. I pulled out Q101 and Q103 because I was seeing that weird voltage and replaced them with KSC1845 / KSA992 and there was no difference.
E= 0v
C= 72-73v (=/- on respective transistor)
B= -0.3v
The base voltage is showing a positive 1.8v and I'm getting a negative voltage. As far as ZD101 and ZD102, any recommendations as to a replacement if I should end up needing them? They are marked XZ147 in the service manual...
E= 0v
C= 72-73v (=/- on respective transistor)
B= -0.3v
The base voltage is showing a positive 1.8v and I'm getting a negative voltage. As far as ZD101 and ZD102, any recommendations as to a replacement if I should end up needing them? They are marked XZ147 in the service manual...
Forget the transistors, concentrate on the opamp supply. The only thing that comes to mind is that either the opamp is faulty, ir it is an incorrect type that has been fitted, or there is a physical short... or the Zeners are in back to front. That would give a supply of -/+ 0.6 volts. Check the Zener orientation.
Edit... the Zeners if fitted back to front would give the voltages you are seeing on Q101 and 103. They can be replaced with any 0.5 or 1.3 watt 15v Zener. In use they dissipate less than 0.1 watt.
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I hear you. I've checked and double-checked the Zener orientation and even did a continuity test from each one to it's connection at IC101 (pins 4/8) just to confirm and that is correct. I also replaced the Zeners (temporarily) with some 16v TZX16C diodes I had on hand, which made no difference. Then I swapped the TL072 out for a new one.
What doesn't make sense to me is how R182 and R183 check out fine out of circuit, but I can't help but think they're breaking down under load. I'm getting +73v on the way in to R182, and -0.37v on the way out, which translates to what I ultimately see at pin 8. R183 gives me -74v on the way in, and -0.33v on the way out which also matches what I'm seeing at the op amp, pin 4. I would just swap those out if I had them on hand but I have to order them; I just want to make sure I'm understanding this correctly because I can't see how the voltage can drop that much into the negative.
What doesn't make sense to me is how R182 and R183 check out fine out of circuit, but I can't help but think they're breaking down under load. I'm getting +73v on the way in to R182, and -0.37v on the way out, which translates to what I ultimately see at pin 8. R183 gives me -74v on the way in, and -0.33v on the way out which also matches what I'm seeing at the op amp, pin 4. I would just swap those out if I had them on hand but I have to order them; I just want to make sure I'm understanding this correctly because I can't see how the voltage can drop that much into the negative.
Very odd.
Just to be sure the end of the Zener that goes to pin 8 (+ opamp supply) should be the end with a stripe and the Zener that goes to pin should have the non stripe end connected to pin 4.
R182/3 If they are hot they are basically OK, They are dropping 74 volts by your measurements and so they dissipate 0.8 watt. That will heat them quickly. If they are cold they are faulty.
If you have doubts just tag a 10k across them, even a half watt or quarter watt will survive long enough to prove the point.
There is nothing else 🙂
Just to be sure the end of the Zener that goes to pin 8 (+ opamp supply) should be the end with a stripe and the Zener that goes to pin should have the non stripe end connected to pin 4.
R182/3 If they are hot they are basically OK, They are dropping 74 volts by your measurements and so they dissipate 0.8 watt. That will heat them quickly. If they are cold they are faulty.
If you have doubts just tag a 10k across them, even a half watt or quarter watt will survive long enough to prove the point.
There is nothing else 🙂
Yes, the Zeners are oriented correctly. Stripe to 8, non stripe to 4.
Thanks for the assistance also; I’ve been tinkering on and off with this thing since February!
Thanks for the assistance also; I’ve been tinkering on and off with this thing since February!
One thing you could try is to remove the opamp and see if the rails come up. They have to because there is nothing else low impedance connected to them, just the opamp and the 15k's.
This gets more interesting then...
The circuit shows there is nothing else connected to those rails.
You have to be logical. I would look at the print from the resistor to the Zener and make sure nothing else (that might not be on the circuit) connects to that print. Follow the print to the opamp and again make sure there is nothing else. I know there are the two 15k's but they can't load anything. There are the two caps across the Zeners but both faulty... nah but you have to remove them and test again. It will work fine without those caps as a test.
The circuit shows there is nothing else connected to those rails.
You have to be logical. I would look at the print from the resistor to the Zener and make sure nothing else (that might not be on the circuit) connects to that print. Follow the print to the opamp and again make sure there is nothing else. I know there are the two 15k's but they can't load anything. There are the two caps across the Zeners but both faulty... nah but you have to remove them and test again. It will work fine without those caps as a test.
The feed to the left goes to another 15k (R119 has a typo, 15 rather than 15k).
Just saying this without thinking it through... is the opamp in the right way around ? Could the Zeners be marked the wrong way around on the board?
There is nothing here that can pull the rails down. The -/+ 0.6 volts is suspicious as it is exactly what you would get with forward biased junction.
I'll look in later 🙂
Just saying this without thinking it through... is the opamp in the right way around ? Could the Zeners be marked the wrong way around on the board?
There is nothing here that can pull the rails down. The -/+ 0.6 volts is suspicious as it is exactly what you would get with forward biased junction.
I'll look in later 🙂
I dunno man, the IC is oriented the right way and the zeners test proper continuity. It is weird, right? I mean part of my misunderstanding with this circuit is how it's supposedly a 'switching' amp, so I didn't know if that somehow complicated things. I've done a bunch of shotgun repair on it already, including resoldering every single solder joint on the main board which actually resolved the relay issue it originally had. I've also done a blanket re-cap on the main board (not the supply, other than the small electrolytics). I figured I may as well anyway. But this wasn't working from the start, so there hasn't been any extra harm done...
