There is a thread of mine on audiokarma also. There are, well mostly two tech who are dealing with my issue. An how can I say - It is complicated. As far as I know some thing exclude another and we began from very basics.
Definitely something is wrong with the board because rail voltages and secondary transformer voltage increase to normal if I physical disconnect the board.
DBT of course. Strongest culprit could be rectifier diodes even the dmm shows they are good. I am waiting for the spares to arrive from my supplier and then I go step by step. I examine the board so many times that I could draw the picture of it. Thanks for input, all posts are educational and noticed.
Will keep you informed.
Definitely something is wrong with the board because rail voltages and secondary transformer voltage increase to normal if I physical disconnect the board.
DBT of course. Strongest culprit could be rectifier diodes even the dmm shows they are good. I am waiting for the spares to arrive from my supplier and then I go step by step. I examine the board so many times that I could draw the picture of it. Thanks for input, all posts are educational and noticed.
Will keep you informed.
I’m just going to go ahead and ask the obvious question: You didn’t install any electrolytics backwards, did you?
1N4148 is a very low current signal diode. I think it can only handle 100mA or something like that. Depending on where they’re installed, that may not be enough and they could have failed causing cascading failures.
For your next project, remember the golden rule in electronics repair: Replace as few components as possible (preferably one of that’s possible), test and see if it made a difference. Rinse and repeat. Now you’ve introduced about 100 new potential failure points. If you still have the original diodes, I would give them a quick check on the DMM and reinstall them.
I replace components one by one, I triple check the polarity of caps and once again.
It is none so called sophisticated component installed on that board. Transistors resistors and diodes. Seems to me all hardcore components.
And all of them were checked. Can a DMM do a proper test of all components. A question that remains.
It is none so called sophisticated component installed on that board. Transistors resistors and diodes. Seems to me all hardcore components.
And all of them were checked. Can a DMM do a proper test of all components. A question that remains.
So at which point did it stop working? Which component had you replaced? Reverse that step and re-test!
Don t know anymore. I didn t test play it in between.
We ll see how it goes this week after new diodes arrive.
We ll see how it goes this week after new diodes arrive.
Next time, replace components in small groups and measure after each one. Time consuming, but worth it.
Something doesn't add up here. You say you replaced all transistors. But this is quite an old amp (70's?). Most of the components used here have gone obsolete years ago. Maybe you could find some old stock, but these are often fakes or rejects, especially when found on ebay etc. And no, a simple component tester will not show everything.
So what did you replace all these different transistors with? NOS? Modern equivalents? Which ones?
I repeat myself, but in your place I would start by returning the amp to it's original state. Too many variables introduced now.
So what did you replace all these different transistors with? NOS? Modern equivalents? Which ones?
I repeat myself, but in your place I would start by returning the amp to it's original state. Too many variables introduced now.
Modern equivalents exist. There are several pages even spreadsheets on web for comparison. I did for about 20-25 units till now. Including the three sonys. Two 7055A models. All sucessfull. Folks on audiokarma are very well informed and experienced in vintage pieces. Especially for Marantz there are some best tech in the world. Mouser an Digikey are reputable suppliers.
While others are a go, on this case something went wrong and I must figure it out what was that.
While others are a go, on this case something went wrong and I must figure it out what was that.
There are components that prone to failure. I have several marantz collection including engraved ones. From the beginning I have also exchanged only a few components, and I have to do the repairs from time to time. Then I renew 2285B completely and it is good ever since. Daily driver basically. There are two kinds of tech on AK also. (don t fix it if aint broken - and - get rid of all old components that prone to fail) type. To each his own I say.
Yes, but transistors are simply not among them. A few very rare and well know exceptions aside but I don't see these in this amp. I get it you replace old electrolytics. Maybe some resistors. A relay. Clean the switches and potentiometers. But there is really no reason I can think of why you should replace transistors in a working amp. It's just asking for trouble. As you just found out the hard way.
All that other stuff aside, here are some pointers:
Also, the transistors Q702 and Q703 should be closely matched. Did you match at least their hFE and BE forward voltage drop?
My proposed plan of action is as follows: Verify that you have the specified voltages in the input stage (long-tailed pair). Bring it back to spec, preferably by reinstalling the original components. I can assure you that you did nothing to reduce hum by replacing them.
Once you've fixed the LTP, measure the DC-offset on the output. Measure the current through each output emitter degeneration resistor (using voltage drop and Ohm's law). You'll probably see a nasty surprise there!
Measure all the voltages specified in the schematic in the VBE-multiplier (Q704 and its surroundings). It's CRUCIAL that these voltages are correct, as this is what sets the bias. Again, I urge you to put back the original components, since Q704 and D901 are most likely closely matched at the factory. Also check voltages and currents around the VAS (Q705).
Stick an oscilloscope on the output and make sure it doesn't oscillate. Do that with no load, a resistive dummy load and a speaker. Do the same at idle and with a 1kHz tone. There should be NO high frequency oscillations.
Report your findings back!
Also, take a second and contemplate what you just did. A Class AB amplifier is a very delicate piece of gear where every component affects the rest of the chain. Put one part out whack, and rest will be completely out of balance. To make matters worse, they're pretty hard to troubleshoot, since the feedback makes them full of circular dependencies so it's often hard to find the root cause. Since this was a relatively high-end unit back in its days, the fine people at Sony probably spent a fair amount of time closely matching the parameters of each component. You just took all those closely matched components out and replaced them with approximate modern "equivalents". If you ever get it to work, which I doubt, it will never perform like it was supposed to.
Who told you that? It depends what kind of noise. You're mentioning hum. The three main reasons for that are 1) ground issues, 2) poor filter caps and 3) signal wires routed too close and in parallel with AC wires.
You definitely have a short or overcurrent situation somewhere. D701 and R826, along with Q701 form a constant current generator that's driving your long-tailed pair input amplifier. This is the most delicate part of any amplifier and errors here will have big downstream effects. Did you measure the indicated voltages (48V, 49.5V and 50V)? They need to be very close to spec! This is a prime example why you can't just villy-nilly replace components. The original diode was probably carefully selected to provide the correct current through the LTP. And I can assure you that this diode has NOTHING to do with hum. Understand what role a component has in a circuit and THEN think about whether you should replace it. If you have the original diode, I urge you to put it back! Also, put back the original Q701, since it must be matched to D701.
Also, the transistors Q702 and Q703 should be closely matched. Did you match at least their hFE and BE forward voltage drop?
This is most likely a downstream effect of you putting the long-tailed pair out of spec. The LTP is probably unable to correctly use the feedback to stabilize the voltages in the output stage. Have you checked for DC on the outputs? Also, what bias current are you seeing? That's the voltage across the emitter degeneration resistors?
Probably because the output stage is either dumping DC into your speaker or is just grossly over-biased. Are the output transistors and/or degeneration resistors getting hot? One or just both of them?
My proposed plan of action is as follows: Verify that you have the specified voltages in the input stage (long-tailed pair). Bring it back to spec, preferably by reinstalling the original components. I can assure you that you did nothing to reduce hum by replacing them.
Once you've fixed the LTP, measure the DC-offset on the output. Measure the current through each output emitter degeneration resistor (using voltage drop and Ohm's law). You'll probably see a nasty surprise there!
Measure all the voltages specified in the schematic in the VBE-multiplier (Q704 and its surroundings). It's CRUCIAL that these voltages are correct, as this is what sets the bias. Again, I urge you to put back the original components, since Q704 and D901 are most likely closely matched at the factory. Also check voltages and currents around the VAS (Q705).
Stick an oscilloscope on the output and make sure it doesn't oscillate. Do that with no load, a resistive dummy load and a speaker. Do the same at idle and with a 1kHz tone. There should be NO high frequency oscillations.
Report your findings back!
Also, take a second and contemplate what you just did. A Class AB amplifier is a very delicate piece of gear where every component affects the rest of the chain. Put one part out whack, and rest will be completely out of balance. To make matters worse, they're pretty hard to troubleshoot, since the feedback makes them full of circular dependencies so it's often hard to find the root cause. Since this was a relatively high-end unit back in its days, the fine people at Sony probably spent a fair amount of time closely matching the parameters of each component. You just took all those closely matched components out and replaced them with approximate modern "equivalents". If you ever get it to work, which I doubt, it will never perform like it was supposed to.
Also, this indicates that something is SEVERELY wrong, as in a component is oriented the wrong way or something equally catastrophic. Here's a simulation of your original modification with a single 1N4148. I replaced your LTP with a load resistor. It's not going to affect anything, since it's a constant current circuit. The current through an LTP in an audio amplifier is typically of the order of 100-300uA. The current through R826 is that current, plus whatever current flows through the diode. R826 and R704 must drop 100V, minus the diode drop, so let's call it 99.4V. So 99.4/22220=4.47mA. Add the current through the current generator and we land at about 4.5mA, which matches the simulation.
But you said you were able to make the resistor smoke with a single 1N4148. Assuming standard 1/4W resistors, let's calculate how much current that would require: P=I*I*R, So I=sqrt(P/R)=33.7mA. That's 7 times the current you're supposed to have! And I don't think a 1/4W resistor really smokes at 1/4W, so your current is probably 10x too high! And adding another 1N4148 should INCREASE the current, not decrease it. So something is very wrong here. Go through every part of the LTP and its current generator and you might find ONE of the probably many issues you have. Measure the voltage drop across R826. If it's anything other than 4.5e-3*220=1V, you have a big problem in this part. Measured against ground you should see about 49V there.
(BTW, I don't agree with the schematic that you should see 48V at the emitter of Q701. There must be a BE drop there, so you should see 48.6-ish volts)
I used a generic 2N5401 in place of the transistor you used since I didn’t have a SPICE model. It’s shouldn’t make a big enough difference in this particular case.
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With two 1N4148 in series you should have more current. But it’s past my bedtime now, so I’ll calculate it for you tomorrow. 😀
And there’s (at least) one error in my post: LTP tail currents are usually more like 500uA-1mA. So a single 1N4148 might have caused the amplifier to misbehave, but it should NOT have caused R826 to smoke. It would actually cause it to draw a little bit less current.
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Bedtime got delayed. 😀With two 1N4148 you should have about 850uA through the LTP. You would then have about 5.3mA through R826, so it should drop 1.2V.