I totally agree with you!
Always start with the power supply first - if it is there, what it is like and move on with your research - do not start by replacing components in the amplifier if you are not sure if the supply voltages are good - this is always the beginning!
Always start with the power supply first - if it is there, what it is like and move on with your research - do not start by replacing components in the amplifier if you are not sure if the supply voltages are good - this is always the beginning!
I checked voltages on mainboard b with power to mainboard a disconnected. All the required voltages seem to be present. Ac in for both transformers at 58 volts, left and right channel output to mainboard a colored connector 161 volts DC, power to protection circuit 80 volts DC, speaker outputs both 80 volts DC.
When mainboard a is connected but the channel outputs are not I can see that the left channel bias diode does not light up while the right channel one does since the left channel was the one that died I assume that either the light is dead or, more likely, I did something wrong or missed a defective part.
Currently the assumed working channel is drawing a lot of current. I tried with both channels on mainboard b (so both the right channel with the supposed ok output transistors and the left channel that has the new ones installed.) In both situations there is an excessive current draw which I believe is causing the transformer hum as well. Because of this I would assume that the issue is with mainboard a and not mainboard b but I do not know how I would confirm this.
I did not check if the right channel was ok after the left blew up, I just assumed it was. I did check all of the transistors for shorts and found nothing. I ran the amp without the outputs soldered on for the right channel (with everything except for the power output to MB a and the left channel output to MB a connected. Doing this I got 84B,0C,80E for the A1492 and 84B, 160C, 80E for the C3856 connection points.
When mainboard a is connected but the channel outputs are not I can see that the left channel bias diode does not light up while the right channel one does since the left channel was the one that died I assume that either the light is dead or, more likely, I did something wrong or missed a defective part.
Currently the assumed working channel is drawing a lot of current. I tried with both channels on mainboard b (so both the right channel with the supposed ok output transistors and the left channel that has the new ones installed.) In both situations there is an excessive current draw which I believe is causing the transformer hum as well. Because of this I would assume that the issue is with mainboard a and not mainboard b but I do not know how I would confirm this.
I did not check if the right channel was ok after the left blew up, I just assumed it was. I did check all of the transistors for shorts and found nothing. I ran the amp without the outputs soldered on for the right channel (with everything except for the power output to MB a and the left channel output to MB a connected. Doing this I got 84B,0C,80E for the A1492 and 84B, 160C, 80E for the C3856 connection points.
These voltages do not make sense - are you sure you are measuring them against ground and not against one of the power rails?
Also, please refer to the components using their designations on schematics - transistors are Qxyz etc - will be much easier to understand where exactly you are measuring the voltages.
"84B" means "+84V on base"?
"Mainboard a" means the VAS stage including the bias regulator:
and "mainboard b" means the power stage (power transistors and their drivers:
?
And how are these boards connected - by soldered wires or cables with connectors?
and "mainboard b" means the power stage (power transistors and their drivers:
?
And how are these boards connected - by soldered wires or cables with connectors?
You are correct. I was measuring using one of the power rails, sorry about that. The boards are connected using connectors.
For now I redid the measurements for the output stage with the transistors disconnected.
Q460, Q458, Q456 a1492
Base +6.40 V DC
Collector +5.80 V DC
Emitter +0.424V DC
Q459, Q457, Q455 C3856
Base +6.75 V DC
Collector +81.7V DC
Emitter +0.427 V dC
I will be taking measurements on mainboard A as soon as I have some time.
This is very wrong - these should be negative voltages., especially the collectors which sit directly on a negative power rail and in your case should be around -80 V ...
Edit:
What do you mean by "transistors disconnected" - did you measure the voltage on transistor leads when they were out of circuit?
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Since it draws a lot of current with the transistors connected I disconnected the legs and measured the pads on the board with the channel connected. I do not know why the voltages are positive. It could be some setting on my DMM or it is an actual issues.
Just to be sure:
- on board B (power stage) the transistor legs were lifted? All of them or only the output transistors?
- board B was connected to board A (VAS/bias board) or other boards exactly in what way?
How do you measure the voltages - black (DMM common terminal) wire hooked to amplifier ground and probing around with red wire?
Or somehow else?
I lifted the legs of the right channel 3x2 output transistors.
The boards are connected using CN309 from mainboard B to A, CN 305 and CN 304 for the right channel and the connector on the bottom right of mainboard A that goes to transistor q313. I believe it is CN308 but I cannot read it 100%. I also connected the protection board. Everything else I left disconnected as it is mainly just controls, connections and outputs etc. Yes I ground the negative lead of the dmm to the chasis and test using the positive lead. And as far as I am aware the dmm I am using does display negative voltage.
The green circles are the things I connected, the red square highlights the legs I lifted.
That means that you left the feedback line for the channel hanging in the air.
You should ask @Mooly how to provide the feedback while having outputs disconnected.
And measuring voltage on the solder pads without any current flowing through them does not make much sense.
I haven't followed this one tbh 🙂
Remove (or isolate by lifting one end) all six 0.33 ohm resistors and then connect either end of R402 to the junction of R412 and R415'
If it were me I would rebuild the output stage checking all components for damage such as resistors that have failed open or high and then link (short out) C306 and see if it powers up OK. Use the DBT at all times. It should power up OK provided that cap is shorted and provided you haven't missed any failed parts in the output stage.
For initial testing you need only fit one pair of output transistors and their corresponding 0.33 ohm emitter resistors.
Remove (or isolate by lifting one end) all six 0.33 ohm resistors and then connect either end of R402 to the junction of R412 and R415'
If it were me I would rebuild the output stage checking all components for damage such as resistors that have failed open or high and then link (short out) C306 and see if it powers up OK. Use the DBT at all times. It should power up OK provided that cap is shorted and provided you haven't missed any failed parts in the output stage.
For initial testing you need only fit one pair of output transistors and their corresponding 0.33 ohm emitter resistors.
There is a possibility that the VAS/bias board (board A) is also damaged (one led shines, the other one does not) - so how can the feedback be provided completely without the power stage (board B)?
Prerequisite of the power board connection would be proper voltages on bases of Q401/402.
Well as I said above in post #52 when you tagged me for advice, I haven't looked at this thread before and haven't looked at any of the posts beyond what you asked....
🙂
I assumed the amp was physically complete with everything in place. Doing what I suggested should work in that case.
🙂
I assumed the amp was physically complete with everything in place. Doing what I suggested should work in that case.
that is not a possibility but 100% certain, I already pulled multiple broken transistors from there, two broken zener diodes and two bad resistors). I did replace these with other parts but since it is still not ok I assume I either missed a broken part or mounted something incorrectly. I will be able to look into it more in the weekend.
Thank you for the suggestion. I will take a closer look at the bias board this weekend and go from there but I will keep this in mind. I did try it with just one pair of transistors but it was still pulling a lot of current so I turned it off.
Can the power board (B) be completely disconnected from VAS/bias board (A) so that the A-board still gets powered normally?
If I connect just CN309 board A does get power. I do not know if it acts completely how it would with everything connected but at that point the right led does light up.
If you build up board B and isolate one of the base resistors to the drivers and then link the base of the two drivers you will get a working output stage that can not be biased into conduction (and so fail). Provided all the transistors on board B are OK it should as described. You need only fit one pair of outputs for this to work.
The output of the board would then just follow the driver input but it could never go into conduction from rail to rail. The amp would work and drive a speaker normally like this, just with no bias current (so slightly higher distortion)
You could then hopefully fault find on board A.
The output of the board would then just follow the driver input but it could never go into conduction from rail to rail. The amp would work and drive a speaker normally like this, just with no bias current (so slightly higher distortion)
You could then hopefully fault find on board A.