Well she still has too many volts on the signal path and this goes all the way from the Voltage Amp to the output transistors on the left channel. I leave it on the bench and try a thing or two every day! The layout of the SU-VX920 and SU-VX820 are very close to the SU-VX800, however the VX920 and VX820's use (rare and expensive) MOSFETs in the Class AA section in place of your Q427/Q428/Q429/Q430. I already sourced these once but since then I destroyed 4 output transistors unfortunately, so I hope they are still ok.

Have you also checked the front panel?
The SU-VX800 has a few parts that should have heatsinks but don't. Specifically, there are four 1W resistors on the front panel that run very hot, creating cold solder joints over time and making the PCB traces very fragile.
I also struggle with the PCB quality: I set my iron to 330°C, but if I remove a component just a couple of times, the pads lift right off. This is on top of the traces that were already degraded by the operational heat. As you might have read in my previous posts, some of the issues I encountered were actually due to the board flexing or moving; just a little bit of unintended pressure on a component was enough to break the pad on the copper side.

 

Thanks. I don't remember seeing those when I had the front off (many times!), so will look again. I hadn't thought about the flexing but that seems highly likely too. I've been very careful with desoldering, doing it quickly with flux and copper braid. I also am lucky that the board isn't black and burnt like many of this series. I already fitted extra heatsinks to the Active Servo transistor pair and the Class AA heatsink...but that's for when it works!

 

Thanks. I don't remember seeing those when I had the front off (many times!), so will look again. I hadn't thought about the flexing but that seems highly likely too. I've been very careful with desoldering, doing it quickly with flux and copper braid. I also am lucky that the board isn't black and burnt like many of this series. I already fitted extra heatsinks to the Active Servo transistor pair and the Class AA heatsink...but that's for when it works!

I checked the SM for the VX920: the front panel is actually quite different since it doesn't handle balanced inputs. It also uses the AN7062 IC, which could be responsible for the voltage offset at the output.
In any case, on my unit, the resistor issue I mentioned is located right behind the flat ribbon cable connector.

Also, if you need replacement power transistors, I have some spares available. I bought a large batch from Mouser to be absolutely sure they were genuine and to match their hFE as closely as possible since they are connected in parallel. I was just a few steps away from putting the back cover back on before this happened!

 

Hi all,

I hope you are doing well and that you aren't completely tired of me and my amplifier yet...

Unfortunately, due to a bit of bad handling, I'm pretty sad to report there has been no real progress since my last post—just new issues instead!

Here is the current situation:

• Missing -17.8V Rail: The entire unit is dead now because I've lost the -17.8V rail, so nothing works anymore and it no longer enters "drive" mode. Almost all voltages across the board are completely off.
• Component Checks:
  • Resistors R672, R674, R676, and R680 check out fine.
  • Q652 and Q654 test okay in-circuit.
  • On Q652, I measured -60V on the collector, but only a few millivolts on the Base and Emitter.
  • On Q651/Q653, I have +18V on their bases, but I am missing the +13.8V on pin 8 of IC651 (even though R663 is a 1 kΩ resistor and is properly connected), which seems very weird to me.
• PCB Integrity: I inspected the board and couldn't find any other cracked traces besides the one I already repaired.
• The Worst Part: I even had some smoke coming from the front panel area!

Any ideas on where to start troubleshooting this power supply/regulation collapse safely? Thanks in advance for bearing with me!

 

Oh not smoke as well! This amplifier just keeps on giving! Was the smoke from those large resistors in the front panel you mentioned earlier?

I'm not sure I can add much to help you locate this new problem, I had to take the Technics off the bench for a few days to fix a Yamaha power amp.

Is it somehow possible that the negative supply to this Active Servo has become uncoupled from the chassis ground to give weird voltages? If the components test ok but the numbers don't add up then is the IC651 faulty? I think many other op amp ICs can be stuck in here to test.

I can't quite follow the whole VX800 schematic layout without confusing it with the VX920 but this Active Servo area is a similar hot zone on the VX920 and tends to ruin the circuit boards there. At least the Active Servo area isn't affected by all the signal feedback circuits as I far as I can see.

Once I get the Yamaha buttoned up I'll get the Technics apart again to see what mood it is in tomorrow! They sure are a mystery and a nightmare.

 

Oh not smoke as well! This amplifier just keeps on giving! Was the smoke from those large resistors in the front panel you mentioned earlier?

I'm not sure I can add much to help you locate this new problem, I had to take the Technics off the bench for a few days to fix a Yamaha power amp.

Is it somehow possible that the negative supply to this Active Servo has become uncoupled from the chassis ground to give weird voltages? If the components test ok but the numbers don't add up then is the IC651 faulty? I think many other op amp ICs can be stuck in here to test.

I can't quite follow the whole VX800 schematic layout without confusing it with the VX920 but this Active Servo area is a similar hot zone on the VX920 and tends to ruin the circuit boards there. At least the Active Servo area isn't affected by all the signal feedback circuits as I far as I can see.

Once I get the Yamaha buttoned up I'll get the Technics apart again to see what mood it is in tomorrow! They sure are a mystery and a nightmare.

If you need to work on the Technics VX920 (or any amplifier), we have to create a new thread for that otherwise it will get confusing.

 

If you need to work on the Technics VX920 (or any amplifier), we have to create a new thread for that otherwise it will get confusing.

Definitely was going to create a separate thread…..these amps are confusing enough without me making things worse! Would love to see this VX800 get going again.

 

Thanks @pumptronics for your help and wishes.

Smoke from the front panel could be not a real problem as it already happened on the same components, I interrupted power and put the finger to see if it was the same place and it was.

I'm worried about the missing 17.8 as I'm not understanding how it is possible.

@MrChips
I see you did no longer reply to my posts
Please don't feel under any pressure to reply—I know you've already spent a lot of time helping me (which I'm already incredibly grateful for!), and I don't want to be a nuisance. If you have the time and will to take a look with no hurry, your insight would be highly appreciated, but if you need to step away from this project, I completely understand.

Thanks again for everything!

Alberto

 

Thanks @pumptronics for your help and wishes.

Smoke from the front panel could be not a real problem as it already happened on the same components, I interrupted power and put the finger to see if it was the same place and it was.

I'm worried about the missing 17.8 as I'm not understanding how it is possible.

@MrChips
I see you did no longer reply to my posts
Please don't feel under any pressure to reply—I know you've already spent a lot of time helping me (which I'm already incredibly grateful for!), and I don't want to be a nuisance. If you have the time and will to take a look with no hurry, your insight would be highly appreciated, but if you need to step away from this project, I completely understand.

Thanks again for everything!

Alberto

No. I have not abandoned this repair project. Just know that there are two things at play here.

(1) Sometimes, life gets in the way and I don't have the spare time to look at this right now.

(2) When the situation gets complex, one needs to spend the time to analyze the problem carefully and plan a trouble shooting strategy.

In brief, when the magic smoke is released, it indicates that something is drawing too much current (the obvious thing).
If the supply voltage is low, it means that either something is loading down the power supply or the power supply is at fault (another obvious thing).

The trouble shooting strategy is to disconnect the load from the power supply in order to isolate the fault.
(I have yet to study the latest situation.)

 

No. I have not abandoned this repair project. Just know that there are two things at play here.

(1) Sometimes, life gets in the way and I don't have the spare time to look at this right now.

(2) When the situation gets complex, one needs to spend the time to analyze the problem carefully and plan a trouble shooting strategy.

In brief, when the magic smoke is released, it indicates that something is drawing too much current (the obvious thing).
If the supply voltage is low, it means that either something is loading down the power supply or the power supply is at fault (another obvious thing).

The trouble shooting strategy is to disconnect the load from the power supply in order to isolate the fault.
(I have yet to study the latest situation.)

Thank you so much, MrChips! Take all the time you need, I really appreciate your reassurance.
To help you analyze the puzzle when you have a moment, i'll post the exact chronological order of events and what led to the current state: