Realistic STA-2080 Idle Current Can't Be Set

[Brivan]

Good afternoon,

I've been battling a perplexing issue with the amplifier board for the right channel of a Realistic STA-2080. The entire receiver is great shape, with no heavy wear evident. The amplifier boards are no exception - all components appear as new.

As I usually do with these receivers, I replaced both idle current adjustment pots with Amphenol single-turn 200-ohm pots. I do this prior to power-up, since there are so many of these receivers that have flaky idle current pots, resulting in destroyed components on power-up (if they haven't been damaged already)

The left channel was easily adjustable for 10mV at R509, but the right channel would quickly jump from around 3mV to around 40mV or 50mV as the pot was turned clockwise. I've measured the pot (checked fine) and even replaced it - to no avail. After this, I replaced TR506 & TR507, per my notes. This made no difference. Long story short, I've replaced all the transistors, pulled one leg on each of the resistors and checked their values, temporarily replaced D501, 502 & 503 with 1N4148 diodes (in series, where needed). D506 was replaced with a SB140 Schottky. D504 & 505 were tested okay. All electrolytic capacitors were replaced. Output transistors were tested. Power supply voltages all checked good.

My list of replacement transistors:

TR505 = KSA1381
TR504 & TR506 = KSC3503
TR507 = KSC2073
TR508 = KSA940
TR501, TR502, TR503 & TR511 = KSA992 (TR501 & 502 gain-matched)

I have a spare STA-2080, so I pulled the right-hand amp board from it and tested it in my problem receiver - it worked fine. I installed my problem right-hand amp board in the known good receiver and it exhibited the same jump in voltage measurement across R509 that was witnessed in the receiver it came from. This eliminates anything outside the amp board as the problem, but I'm at a total loss as to what else to check. I've examined traces, solder points under a magnifying glass and they all look fine.

I've read other threads on this subject - some with a definitive solution and others - like mine where every component was seeming replaced / tested - remain unsolved.

Any help I can get on this would be appreciated!

 

[Gnesen]

Good afternoon,

I've been battling a perplexing issue with the amplifier board for the right channel of a Realistic STA-2080. The entire receiver is great shape, with no heavy wear evident. The amplifier boards are no exception - all components appear as new.

As I usually do with these receivers, I replaced both idle current adjustment pots with Amphenol single-turn 200-ohm pots. I do this prior to power-up, since there are so many of these receivers that have flaky idle current pots, resulting in destroyed components on power-up (if they haven't been damaged already)

The left channel was easily adjustable for 10mV at R509, but the right channel would quickly jump from around 3mV to around 40mV or 50mV as the pot was turned clockwise. I've measured the pot (checked fine) and even replaced it - to no avail. After this, I replaced TR506 & TR507, per my notes. This made no difference. Long story short, I've replaced all the transistors, pulled one leg on each of the resistors and checked their values, temporarily replaced D501, 502 & 503 with 1N4148 diodes (in series, where needed). D506 was replaced with a SB140 Schottky. D504 & 505 were tested okay. All electrolytic capacitors were replaced. Output transistors were tested. Power supply voltages all checked good.

My list of replacement transistors:

TR505 = KSA1381
TR504 & TR506 = KSC3503
TR507 = KSC2073
TR508 = KSA940
TR501, TR502, TR503 & TR511 = KSA992 (TR501 & 502 gain-matched)

I have a spare STA-2080, so I pulled the right-hand amp board from it and tested it in my problem receiver - it worked fine. I installed my problem right-hand amp board in the known good receiver and it exhibited the same jump in voltage measurement across R509 that was witnessed in the receiver it came from. This eliminates anything outside the amp board as the problem, but I'm at a total loss as to what else to check. I've examined traces, solder points under a magnifying glass and they all look fine.

I've read other threads on this subject - some with a definitive solution and others - like mine where every component was seeming replaced / tested - remain unsolved.

Any help I can get on this would be appreciated!

For my 2080, it was the temp sensitive double diode (varistor?) that attaches to the top of the heat sink that went bad and prevented bias adjustment. I couldn’t find a direct replacement so I put two diodes in series and encased them in thermal glue that’s heat conductive but electrically insulating. I used the metal tab from the old double diode to attach the replacement to the heat sink. Hope this helps.

 

[Brivan]

For my 2080, it was the temp sensitive double diode (varistor?) that attaches to the top of the heat sink that went bad and prevented bias adjustment. I couldn’t find a direct replacement so I put two diodes in series and encased them in thermal glue that’s heat conductive but electrically insulating. I used the metal tab from the old double diode to attach the replacement to the heat sink. Hope this helps.

Thanks for the reply. I did try installing two 1N4148 diodes in series in place of D503 and didn't have any change in the bias problem. I'm looking for affirmation that 1N4148 was the correct choice for this, so if you recall what diodes you subbed in, it would be helpful!

 

[Racingh11]

Maybe there is something wrong with the current mirror. Check voltage across R509 and R512. They should match. Also compare to the other channel. If they don't match, further testing will need to be done in that area.

 

[Brivan]

Maybe there is something wrong with the current mirror. Check voltage across R509 and R512. They should match. Also compare to the other channel. If they don't match, further testing will need to be done in that area.

Thanks for the info! I'll make a note of it. Strangely enough, I'm able to set the bias correctly this morning. Not sure what I did - I'll give details in the next post.

 

[Brivan]

I posted this same problem on another post, where the OP was dealing with the same problem I was having. I was advised to check voltages between the bases of TR507 and TR508 the the emitters of TR507 and TR508. Here's my follow-up post to that. wish I had more-definitive news to share, but I'm at a loss:

Well, this is just strange. When I initially tried taking readings between TR507 & TR508, I was getting goofy voltages that weren't even close. I starting checking around and found I didn't even have 54v to either transistor. This turned out to be because, I had left the power/audio out/protection connector unplugged, but sitting next to the connector pins on the amp board. Duh. I powered off, waited a bit, then reconnected the amp board.

Here's where it gets strange: I measured 1.3v to 2.3v between the bases TR507 and TR508 and 240mV to 1.2v between the emitters of TR507 & TR508, depending on if VR501 was set fully counter-clockwise, or at about the half-way point (where bias is usually set). These readings seemed pretty normal, so I thought I'd check bias voltage at R509 and, to my amazement, I could now adjust VR501 to the correct 10mV! WTF?

I racked my brain trying to figure what I did different on the board at the end of last week, but I was at a standstill with it. I'm sure of it. So, the only thing I did different this morning was take measurements at TR507 & TR508 and forget to plug in the amp board to the power/audio out/protection connector. My soldering skills are good and I didn't have any loose connections at TR507 or TR508 (or any others) - I wiggled them around and my 10mV at R509 never wavered. I wiggled all kinds of stuff around and the 10mV reading never changed. Wow - baffling! Did powering up the receiver without the right amp board plugged in make a difference? Probably not?

I'm glad it's working, again, but I'm disappointed I didn't find the smoking gun. Sheesh.

 

[Gnesen]

Thanks for the reply. I did try installing two 1N4148 diodes in series in place of D503 and didn't have any change in the bias problem. I'm looking for affirmation that 1N4148 was the correct choice for this, so if you recall what diodes you subbed in, it would be helpful!

Sorry, I don't recall the diodes that I used. I would take a look but they're now encased in thermal glue. Glad to hear you can set the bias now. Hope it stays that way. Did you install new trimmers, too?

 

[Gnesen]

I posted this same problem on another post, where the OP was dealing with the same problem I was having. I was advised to check voltages between the bases of TR507 and TR508 the the emitters of TR507 and TR508. Here's my follow-up post to that. wish I had more-definitive news to share, but I'm at a loss:

Well, this is just strange. When I initially tried taking readings between TR507 & TR508, I was getting goofy voltages that weren't even close. I starting checking around and found I didn't even have 54v to either transistor. This turned out to be because, I had left the power/audio out/protection connector unplugged, but sitting next to the connector pins on the amp board. Duh. I powered off, waited a bit, then reconnected the amp board.

Here's where it gets strange: I measured 1.3v to 2.3v between the bases TR507 and TR508 and 240mV to 1.2v between the emitters of TR507 & TR508, depending on if VR501 was set fully counter-clockwise, or at about the half-way point (where bias is usually set). These readings seemed pretty normal, so I thought I'd check bias voltage at R509 and, to my amazement, I could now adjust VR501 to the correct 10mV! WTF?

I racked my brain trying to figure what I did different on the board at the end of last week, but I was at a standstill with it. I'm sure of it. So, the only thing I did different this morning was take measurements at TR507 & TR508 and forget to plug in the amp board to the power/audio out/protection connector. My soldering skills are good and I didn't have any loose connections at TR507 or TR508 (or any others) - I wiggled them around and my 10mV at R509 never wavered. I wiggled all kinds of stuff around and the 10mV reading never changed. Wow - baffling! Did powering up the receiver without the right amp board plugged in make a difference? Probably not?

I'm glad it's working, again, but I'm disappointed I didn't find the smoking gun. Sheesh.

It sounds like something to do with the plug-in connections and/or a loose wire. I've accidentally broken off a wire on more than one occasion that threw everything off and then had to be tracked down. Anyway, let's hope you're good to go now.

 

[Brivan]

Sorry, I don't recall the diodes that I used. I would take a look but they're now encased in thermal glue. Glad to hear you can set the bias now. Hope it stays that way. Did you install new trimmers, too?

Yes, the trimmers were the first thing I replaced. On these receivers, I don't even power them up without replaced them, first. In a way I wish I would have powered this one up beforehand. Maybe in the future, I'll take resistance readings on each one before I power up, make sure I can set them, then power down and replace them, anyway. This has been a mind-boggling journey!

 

[Brivan]

It sounds like something to do with the plug-in connections and/or a loose wire. I've accidentally broken off a wire on more than one occasion that threw everything off and then had to be tracked down. Anyway, let's hope you're good to go now.

That's what I was hoping for. Damned if I could find it!

 

[Brivan]

I had a sudden realization: I did do something else at the end of last week - I cleaned it. Specifically, I cleaned the pots and switches. This is usually something I save for after repairs and adjustments have been made but, since I was at an impasse, I went ahead with it. Well, I'm pretty sure idle current would be hard to set if there was any kind of signal being fed to the amp board and, with previously uncleaned pots and switches, it would make sense that it could happen. I never connected any speakers to the receiver (for obvious reasons, even though the protection relay was engaging), so I never heard anything. And I don't recall ever looking at the right channel power meter while the idle current was on the high side. I had the volume pot and minimum and the function selector an auxiliary, but that certainly wouldn't eliminate any noise form breaking through. On the other hand, I know I pulled the audio input connector from the board, as well as pulling the pre out / main in jumpers and this made no difference. But I didn't try those things through the whole process. Still no definitive answer, just more conjecture and things to consider if you find yourself up against this same demon.

 

[Matt_ME]

Good afternoon,

I've been battling a perplexing issue with the amplifier board for the right channel of a Realistic STA-2080. The entire receiver is great shape, with no heavy wear evident. The amplifier boards are no exception - all components appear as new.

As I usually do with these receivers, I replaced both idle current adjustment pots with Amphenol single-turn 200-ohm pots. I do this prior to power-up, since there are so many of these receivers that have flaky idle current pots, resulting in destroyed components on power-up (if they haven't been damaged already)

The left channel was easily adjustable for 10mV at R509, but the right channel would quickly jump from around 3mV to around 40mV or 50mV as the pot was turned clockwise. I've measured the pot (checked fine) and even replaced it - to no avail. After this, I replaced TR506 & TR507, per my notes. This made no difference. Long story short, I've replaced all the transistors, pulled one leg on each of the resistors and checked their values, temporarily replaced D501, 502 & 503 with 1N4148 diodes (in series, where needed). D506 was replaced with a SB140 Schottky. D504 & 505 were tested okay. All electrolytic capacitors were replaced. Output transistors were tested. Power supply voltages all checked good.

My list of replacement transistors:

TR505 = KSA1381
TR504 & TR506 = KSC3503
TR507 = KSC2073
TR508 = KSA940
TR501, TR502, TR503 & TR511 = KSA992 (TR501 & 502 gain-matched)

I have a spare STA-2080, so I pulled the right-hand amp board from it and tested it in my problem receiver - it worked fine. I installed my problem right-hand amp board in the known good receiver and it exhibited the same jump in voltage measurement across R509 that was witnessed in the receiver it came from. This eliminates anything outside the amp board as the problem, but I'm at a total loss as to what else to check. I've examined traces, solder points under a magnifying glass and they all look fine.

I've read other threads on this subject - some with a definitive solution and others - like mine where every component was seeming replaced / tested - remain unsolved.

Any help I can get on this would be appreciated!

I posted this same problem on another post, where the OP was dealing with the same problem I was having. I was advised to check voltages between the bases of TR507 and TR508 the the emitters of TR507 and TR508. Here's my follow-up post to that. wish I had more-definitive news to share, but I'm at a loss:

Well, this is just strange. When I initially tried taking readings between TR507 & TR508, I was getting goofy voltages that weren't even close. I starting checking around and found I didn't even have 54v to either transistor. This turned out to be because, I had left the power/audio out/protection connector unplugged, but sitting next to the connector pins on the amp board. Duh. I powered off, waited a bit, then reconnected the amp board.

Here's where it gets strange: I measured 1.3v to 2.3v between the bases TR507 and TR508 and 240mV to 1.2v between the emitters of TR507 & TR508, depending on if VR501 was set fully counter-clockwise, or at about the half-way point (where bias is usually set). These readings seemed pretty normal, so I thought I'd check bias voltage at R509 and, to my amazement, I could now adjust VR501 to the correct 10mV! WTF?

I racked my brain trying to figure what I did different on the board at the end of last week, but I was at a standstill with it. I'm sure of it. So, the only thing I did different this morning was take measurements at TR507 & TR508 and forget to plug in the amp board to the power/audio out/protection connector. My soldering skills are good and I didn't have any loose connections at TR507 or TR508 (or any others) - I wiggled them around and my 10mV at R509 never wavered. I wiggled all kinds of stuff around and the 10mV reading never changed. Wow - baffling! Did powering up the receiver without the right amp board plugged in make a difference? Probably not?

I'm glad it's working, again, but I'm disappointed I didn't find the smoking gun. Sheesh.

Hi, I'm dealing with amplifier board issues in my 2080 as well. Mine was a barn find from a real barn. It was filthy, and had been out there awhile, I'd say. I have cleaned it up section by section, replaced capacitors, checked resistor values, diodes, transistors, etc. Both finals in the right channel were fried; the left finals tested good, but they were replacements not originals. The tone amp through the pre-out is working fine on both channels. The headphone jack was only working on the left channel. I dug back into right amp board, and rechecked everything (the finals are not installed on the board). Here is what I'm stumped on: without the amp board connected, I'm reading -34mV on the connection for pin 5 (board out to protect) and full negative rail voltage on pin 6 (feed to protect board pin 23). When I connect the board, I now read full rail voltage on both pin 5 and pin 6. Could you verify for me that I am supposed to be seeing negative rail voltage on that grey protect board connection? After reading the manual section on the protect circuitry, I understood that the grey protect wire is supposed to carry overcurrent FROM the amp board to the protect board in the case of a short, and not the other way around. I've tried checking everything I can think of and I'm stumped. I appreciate any help you could offer. Thanks, Matt

 

[Brivan]

Hi, I'm dealing with amplifier board issues in my 2080 as well. Mine was a barn find from a real barn. It was filthy, and had been out there awhile, I'd say. I have cleaned it up section by section, replaced capacitors, checked resistor values, diodes, transistors, etc. Both finals in the right channel were fried; the left finals tested good, but they were replacements not originals. The tone amp through the pre-out is working fine on both channels. The headphone jack was only working on the left channel. I dug back into right amp board, and rechecked everything (the finals are not installed on the board). Here is what I'm stumped on: without the amp board connected, I'm reading -34mV on the connection for pin 5 (board out to protect) and full negative rail voltage on pin 6 (feed to protect board pin 23). When I connect the board, I now read full rail voltage on both pin 5 and pin 6. Could you verify for me that I am supposed to be seeing negative rail voltage on that grey protect board connection? After reading the manual section on the protect circuitry, I understood that the grey protect wire is supposed to carry overcurrent FROM the amp board to the protect board in the case of a short, and not the other way around. I've tried checking everything I can think of and I'm stumped. I appreciate any help you could offer. Thanks, Matt

Hi, Matt

At this moment, I don't have a STA-2080 handy to take any measurements from. I wish I was a little more of an electronic wizard - then I could look at a schematic and hopefully tell you what kind of voltage readings you should see. In this post,
https://audiokarma.org/forums/index.php?threads/collapsing-bias.826818/
I relayed some of the same information as I did above. One of the members I was dealing with was Craig @llwhtt - he may be of some help to you, here. If I'm able to come up with any other info for you, I'll chime in.

 

[Matt_ME]

T

Hi, Matt

At this moment, I don't have a STA-2080 handy to take any measurements from. I wish I was a little more of an electronic wizard - then I could look at a schematic and hopefully tell you what kind of voltage readings you should see. In this post,
https://audiokarma.org/forums/index.php?threads/collapsing-bias.826818/
I relayed some of the same information as I did above. One of the members I was dealing with was Craig @llwhtt - he may be of some help to you, here. If I'm able to come up with any other info for you, I'll chime in.

Thanks very much for your response. I'll check out the link and member Craig.