QRX-5500 Protection issues

moodydan

moodydan

I love the punk rock.
I'm working on a QRX-5500. Initially it would start up, sound was good, but after about 15 minutes of playing it would kick into protection mode and not come out. So, I replaced the caps on the protection board (except for the bipolar caps, because I didn't have any on hand; also, the 220uf/75v and 100uf/75v I replaced with 160v capacitors because that's what I had on hand - the uf values did match, though).

Now, the protection relay clicks upon being powered on, but immediately trips. I removed the output board, and the protection relay clicks and is fine - at least, for the 20 minutes or so that I had it turned on, it did not click again.

I tried setting the DC offset and bias, but it didn't get me anywhere. Where bias is supposed to be 25+1mv, I'm getting about 33V, and the variable resistors don't change anything for me at all. Bad trim pots, maybe? Suggestions are appreciated!
 
assuming you are measuring bias across emitter resistors ........bias that high suggests emitter resistor/s open .
if checking across a removed fuse your meter is incorrectly set on volts instead of amps ..
i just checked the manual and if you are using those instructions as i guessed your meter is on wrong setting .. it says 25ma not 25mv ..
 
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Got that sorted; all values were off by a little (most were at around 17ma). However, still doing the same thing. Clicks and holds without any issues when the output board is pulled, clicks and pops right back into protection when board is installed. Sometimes won't click at all. I'll check and make sure I didn't burn any wires while I was soldering, there may be something crossing over, but other than that I'm pretty stumped!
 
It wasn't high before, I was testing on the wrong setting, as you pointed out. It was low, but adjusted now. I tested and set with no load.
 
I'll be receiving my "new" 5500 on Monday. It has the EXACT same issue as yours Dan. I plan on taking mine in to the local repair person and if I should learn what the problem is before you do I will pass the info along to you.

I would, of course, appreciate it if you would do the same for me.
 
Of course! I'll be working on it some this coming week; if I can figure it out, I will be certain to update the thread and let you know. I've had entirely too much help here to not try to pass that on to someone else it may benefit.

tygr said:
I'll be receiving my "new" 5500 on Monday. It has the EXACT same issue as yours Dan. I plan on taking mine in to the local repair person and if I should learn what the problem is before you do I will pass the info along to you.

I would, of course, appreciate it if you would do the same for me.
Click to expand...
 
moodydan said:
Pete,
Those pins are for the QRX, correct? I could not find them...
Click to expand...

just been looking again and cant see what i was looking at ..bit frustrated with this badly scanned document ..
i have another plan which is a bit easier ..
first check every fuse in unit ...
if found to be all ok pull fuses out of all driver boards and power up and see if relay clicks in ..
if so replace the pair of fuses for a driver board .then power up to see if relay clicks ..if so replace another pair of fuses ...etc etc until you find which stops it clicking in ..
if it doesn't click in with all driver board fuses removed there is a protection circuit problem ,,
if any fuses are blown it needs investigating before replacing them .
 
That will be where I start on Monday, Pete. I accidentally started down this track earlier, but didn't follow through with all sets. None of my fuses are blown, so I'll take that as an initial good sign!
 
Installing all fuses by sets resulted in the protection relay clicking and holding every time; however, there is only sound from the left channel, and it's very distorted. There's no sound at all from the right channel. I guess my next steps are I'll pull the outputs and test them, then test the driver transistors, then the emitter resistors.
 
The one I got today is also out of protection and staying that way. It also only has left channel sound, not distorted though. I have rotated the front channel to the rear (1/2 turn) and I have sound from both speakers though not in stereo.

Some weird issues with this unit.
 
I'll have to go through the rotations again and see if that changes anything, I didn't think of that today when I was messing with it. I ran through that before, but it'll be good just to check again, before I get into all the testing!

tygr said:
The one I got today is also out of protection and staying that way. It also only has left channel sound, not distorted though. I have rotated the front channel to the rear (1/2 turn) and I have sound from both speakers though not in stereo.

Some weird issues with this unit.
Click to expand...
 
I pulled all the output transistors and emitter resistors today, they all tested fine. Tomorrow I'll pull the driver transistors and report back; if nothing else, at least I'm finding what the problem isn't!
 
I didn't get a chance to work on the Sansui today, I was running around all day. I'm on it in the morning though, hopefully with some progress to report!
 
I have yet to pull the driver transistors, haven't had the time, but I did put everything back together and test just to remind myself of what the symptoms are between working sessions. I have sound out of the right channel, but it's acting as if it's the left.

When I turn the balance all the way to the left, I get strong sound from the right speaker. When I turn balance all the way to the right, there's nothing, no sound. The left speaker does not produce sound.

I'm checking the driver transistors tomorrow or Monday, but I really have no clue what would cause the output to cross over like this...
 
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