Hello Everyone,
Working on Realistic STA-2080. One side adjusted easily to the correct service manual 10mv. The other side would only be adjusted down to 23 mv. Something unusual about the side that won't adjust is to get to the lowest bias point of 23mv you lower resistance(it's lowered all the way to 2 ohm) wheras the other side that adjusts correct raising resistance lowers bias. The downloaded service manual is missing about half the schematic of amp board so any past experience with this issue would be appreciated.
Working on Realistic STA-2080. One side adjusted easily to the correct service manual 10mv. The other side would only be adjusted down to 23 mv. Something unusual about the side that won't adjust is to get to the lowest bias point of 23mv you lower resistance(it's lowered all the way to 2 ohm) wheras the other side that adjusts correct raising resistance lowers bias. The downloaded service manual is missing about half the schematic of amp board so any past experience with this issue would be appreciated.
It's not easy to troubleshoot something like this.
It might be the MV-11Y bias diode has high Vf or too much current is running through it or it's cold (poor heat transfer to it on heatsink).
I would compare voltage measurements of TR505A, TR506A on the two channels and across the MV-11Y's.
If you can post those numbers might be able to see the problem.
I have the split schematic pages from hifiengine.com, trying to put them together into one sheet. Ran out of time.
It might be the MV-11Y bias diode has high Vf or too much current is running through it or it's cold (poor heat transfer to it on heatsink).
I would compare voltage measurements of TR505A, TR506A on the two channels and across the MV-11Y's.
If you can post those numbers might be able to see the problem.
I have the split schematic pages from hifiengine.com, trying to put them together into one sheet. Ran out of time.
OK I probably have same schematic just thought half was missing.
Here's the voltage measurements:
Good Side: TR506 50.2 1.1 50.8
Bad Side TR506 50.2 .7 50.8
Good Side: TR505 52.4 .68 51.7
Bad Side: TR505 52.3 .03 51.7
Good Side: D503 1.37 across
Bad Side D503 1.4 across
Here's the voltage measurements:
Good Side: TR506 50.2 1.1 50.8
Bad Side TR506 50.2 .7 50.8
Good Side: TR505 52.4 .68 51.7
Bad Side: TR505 52.3 .03 51.7
Good Side: D503 1.37 across
Bad Side D503 1.4 across
DC offset OK? This amp is DC coupled from input to output and the preamp could be causing an offset and screwing with your voltages. If you short the collectors of TR505 and TR506 together the bias/idle current should go straight to zero, if it does you know the driver/output stages are working correctly. The .03 reading on TR505 looks to be the culprit but why?
Craig
Craig
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With bad channel TR505-C low at almost zero V, I expect -1.2VDC offset at the loudspeaker output. Protection relay trips around 3.5V
Need to see if the offset is coming in from the pre-amp or an imbalance in the power amp. Can you measure the input (R501) voltage mV and output V (both sides R506) to GND.
You were not measuring bias current (24mV) with a load connected?
That does not work, as any DC offset, the reading will be both bias+load current.
Need to see if the offset is coming in from the pre-amp or an imbalance in the power amp. Can you measure the input (R501) voltage mV and output V (both sides R506) to GND.
You were not measuring bias current (24mV) with a load connected?
That does not work, as any DC offset, the reading will be both bias+load current.
Attachments
Hello Everyone,
I have not did any new measurements yet-just got messages. I did previously measure dc offset with large 8 ohm resistor according to serv manual. Got 45mv on bad side(just barely under recommended spec of 50mv) and good side is very low around 5mv if I remember correctly. I did use 8 ohm load to do both dc offset and bias adjust. That's whats in manual unless that part is a error. I will check the voltages in a bit.
I have not did any new measurements yet-just got messages. I did previously measure dc offset with large 8 ohm resistor according to serv manual. Got 45mv on bad side(just barely under recommended spec of 50mv) and good side is very low around 5mv if I remember correctly. I did use 8 ohm load to do both dc offset and bias adjust. That's whats in manual unless that part is a error. I will check the voltages in a bit.
Here are measurements:
Good Side R506 .05 V
Bad Side R506 .02 v
Good Side R501 .06 v
Bad Side R501 .05 v
Good side TR504 50.79 9.85 51.04
Bad Side TR504 48.7 10.9 49.4
Note:Just edited voltages as I unknowingly previously had bad side unloaded
Good Side R506 .05 V
Bad Side R506 .02 v
Good Side R501 .06 v
Bad Side R501 .05 v
Good side TR504 50.79 9.85 51.04
Bad Side TR504 48.7 10.9 49.4
Note:Just edited voltages as I unknowingly previously had bad side unloaded
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With TR505-C around zero and loudspeaker output around zero (45mV), that puts the (top NPN) output transistors into cutoff and zero bias current I'd expect. Or TR507 is damaged/leaky. The bad channel should sound distorted.
So I'm confused OP got some bias current readings at all.
So I'm confused OP got some bias current readings at all.
That's why I said to short the collectors of TR505 and TR506 together and see what happens to the idle current, should go to zero.
Craig
Craig
Quote: "So I'm confused OP got some bias current readings at all" I mentioned getting 22ma with pot fully clockwise(opposite of direction that should lower it). Before adjusting when it was at factory position near center I was getting around 200ma bias. I put it back to that center position temporarily and noticed when relay first kicks in bias is fairly normal maybe 12ma then it jumps to high value about a second or two so possibly something making bias go high under a load? All the values taken were at the adjusted 22ma value. Would it be safe to take values again with bias that high?
The service manual has a few mistakes for setting bias current. It should say DISconnect 8R load; it wants 10mV across "emitter resistor R509" (this is incorrect, should be R519 like picture).
Anyhow, 10mV across R519 or R520 (0.5R) is 20mA bias current.
You mention the lowest you can get is 23mV, which is 60mA.
200mA bias current would give 100mV across an emitter resistor and 20W heat. It will warm up but no point to do it until we know what the problem is.
Bias changes with a load connected if there is a DC offset. 50mV offset at 8R load is an imbalance of 6mA not enough to worrry about here.
The voltage across R519 should equal R520 unless we have voodoo occurring.
Anyhow, 10mV across R519 or R520 (0.5R) is 20mA bias current.
You mention the lowest you can get is 23mV, which is 60mA.
200mA bias current would give 100mV across an emitter resistor and 20W heat. It will warm up but no point to do it until we know what the problem is.
Bias changes with a load connected if there is a DC offset. 50mV offset at 8R load is an imbalance of 6mA not enough to worrry about here.
The voltage across R519 should equal R520 unless we have voodoo occurring.
Sorry I meant mV not mA, I did catch SM error about R509 and did do measurements across R519. I was getting 200 mV across R219 before adjustment. I will measure without the load and see how much measurements change.
Unloaded the outputs and good side jumped to 200mv and bad side 260mv when at original factory position. Another thing I noticed is the good side heat sink gets hot fast within 3 minutes and bad side is just barely warm. Not sure of the normal heat characteristics of this amp.
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