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Realistic STA-2100D Outputs and BDT Procedures.

zebulon1

zebulon1

Retired finally and having a blast on the bench
Subscriber
I have a Realistic STA-2100D which had one NPN output, one NPN driver and a 150 ohm resistor blown on the right Amp board assembly. I replaced the toasted pieces with recommendations on this Thread by 69sixpackbee:

http://www.audiokarma.org/forums/showthread.php?t=556573&page=2

When powering up after the repair. The DBT with a 100 watt lamp would not dim. After double checking my work and not finding anything, I decided to put the good (Left board) set of original outputs back in the left amp board assembly (6 way test:Good). The lamp dimmed as hoped.

Do I need to change the lamp to a 75 watt to power this set in safe mode with the new MJ21193G/94G's?

The main fuse is rated as 8 amp's. Lot's of current!

Replaced the outputs: 2SB665/2SD675 with MJ21193G/94G's
Replaced the drivers: 2SD669/2SB649 with KSA1220/KSC2690's
Replaced the VD1221 diodes with a pair of series'd 1N4148 diodes
Thanks for any help, Zeb
 
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What do the power supply voltages look like? If one rail is low, check the filter caps for leakage. Also check for stray solder, bridged traces, broken traces and broken or cold sodler joints. Also check to make sure that any isolation hardware originally used is put back in the original locations (mica isolators, screw insulators, plastic hardware, etc.).
 
I got the left amp board up and running by putting in good MJ21193G/94G's. That appeared to be the problem (self induced as usual). Now the right board is still giving me fits (note: This right board is the one that needed the repair for the blown outputs). I installed good tested outputs, rechecked all resistors and diodes. I checked the trimmers for sweep and inspected the board for any weirdness. Plugged in the amp and the DBT is still bright. I measured the voltage from the right amp boards output and with the 100 watt bulb I am getting +4.89 volts DC. I know that's not good.

I attached one lead from the DMM to the right amp boards output and checked for continuity between the NPN side and PNP side hoping to find a reading that sticks out. Also checked for any grounding to the case from the boards inputs and the transistor outputs, none. I have not found any smoking gun yet. Taking a break and wanted to up date my progress.

Power supply rails (regulated) is spot on at 65.4 volts + and -. New filter caps installed.

Thanks Trio for your help.
 
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Double check the new transistors and make sure they are were they should be. Maybe you got a bad one. Inspect your insulators (no cracks or pieces missing). Inspect all the wires along the board (the wires that connect the board to the outputs). When I replaced my outputs, a few broke clean from the pins/solder points from too much movement/fiddling (wasn't super obvious to find). Triple/quadruple check your work and soldering. Make sure the four plugs on the bottom are oriented correctly during assembly. Check the smaller board transistors out of circuit. Same for the resistors.

Check your power supply board as well. Could be something wrong with the circuit powering your bad channel.

Keep at at it. I bet it'll be something simple. So simple, that you'll overlook it five times before finding it. And keep it on the DBT until cured!

The receiver is worth the extra effort. The designers did a good job, except they goofed the 'loudness' feature. Not the last word in detail or finesse (and I ran/run mine with the weirdest tone control settings), but this thing pounds other receivers into the ground!
 
I installed known good transistors (ones I tested and installed in the good side amp with success). I am really working on this one, and I'm pretty sure that the short is between the regulated rails. I doubled checked everything more than I want to admit. Could a cap short. Pulling the ceramic caps connected to the center output is my next step. All the resistors were lifted and checked, swapped diodes from good board to bad and back along with the transistors I don't have equivalents for. A good lesson is had in this job but I can't help knowing there is an easier way. Measuring the voltage and comparing against the good amp board only makes it more confusing. I am beginning to think all my other repairs were by luck only! Have you been there?

When I check for voltage on the B+ and B- supply pins the voltage is 5 to 6 volts as compared to the good board at 48 volts, DBT connected.
I replaced the VR pots with Bourns units of the same value.

I really like this set as well.

Edit: Could I remove the outputs from this assembly and work the board with out them? I am checking voltages by turning the set on briefly while connected to the DBT. I know the Pioneer SX-1010 has a feedback loop were removing the outputs requires a few jumper resistors installed (Thanks to the member(s) on the Pioneer Forum). I don't know enough about amplifier design to tell if this is possible.
 
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I got voltage every where. 28 and climbing volts instead of .5 volts. I just blip the DBT and check for voltage. I noticed the resistor R224 (150 ohm, 1 watt) is toasting again, so I still have the original problem. I can't figure out where the voltage is coming from.
All the transistors are checking good and the diodes are as well.
 
This is the schematic I am using from HiFi Engine.
All transistors test good.

Replaced the outputs: 2SB665/2SD675 with MJ21193G/94G's
Replaced the drivers: 2SD669/2SB649 with KSA1220/KSC2690's
Replaced the VD1221 diodes with a pair of series'd 1N4148 diodes
Replaced the 2SA872's with KSA992's.
Replaced the 2SA659/2SC1175 with KSA1013/ KSC2383's
Replaced the 13v zener D509A

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Spot on.
The -20 volts is every where on the circuit. I really messed up by not recording my measurements. I was hoping to find the smoking gun. I will take any readings requested. The thing is I have +20 in spots so I have to make sure that I note the polarity.

Thanks for looking and helping!
 
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Are the emitter resistors ok?

It stands to reason that if there's -20v at the emitter of the driver, the outputs should follow suit, which should bring the output (other side of that toasting resistor to a similar voltage) so there should be less than 1volt drop over it. If there's an open between the emitter of the drive transistor to the output line, that resistor is the only thing between the -20V at the emitter and what voltage is present at the output.
 
I lifted the resistors and checked. I used my Simpson to get a good reading and they check good. Now they should be .5 ohms?
Maybe I should check for a good connection at the board. There is a metal tube standoff that the resistor leads slide into and is soldered to the board were there is a flange/rivit arrangement.

I am getting around +6 volts at the output pin 13.

When the set stopped working and went into protection I checked the amp boards and found the high bias on the toasted board. That resistor R524 and both PNP outputs were open along with the PNP driver TR508.
 
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Reading through again we better get some understanding... What voltage is at the output?

What voltages are present at bases of driver transistors? And what voltages at bases of power transistors?
 
Output voltage is +4.5 (Pin 13)
The base and emitter of the driver TR508 is +20.0 volts.
Base voltage at the Outputs (Power transistors): +3.4 volts on both bases.

I read 1.8 volts across the emitters R530 and R532.

Can I test this board without the Outputs?
 
I wanted to double check my measurements:

Whats weird is the DBT is slightly dimmer and the base and emitter of the driver TR508 is now +3.5 volts.
The other readings are the same.
 
R526 and R528 can't be good -20v from driver one side and +3.4v from bases of respective power transistors the other... 23.4v drop over 4.7 ohms would equate to 114.5W power dissipation
 
Check out my correction on the last post.
I have +3.5 now at the base and emitter of TR508.
I noticed at times when I check for voltage at spots on the board, the DBT dims. This last time it dimmed the voltage dropped to the current +3.5v.
I doubled checked R526 and R528 and they are good at 4.7 ohms.
I rechecked R524 as well and it's still good but toasted.
 
SVI2004A, It seemed when I was checking at the base of the outputs is when the DBT dimmed.
 
SVI2004A, I Have a correction: While checking the base voltage at the outputs the DBT dimmed and now I have 3.5 volts at the base and emitter of TR508.

I noticed while checking spots for voltage on the board, the DBT would dim, after lifting the probe the DBT would go bright again. This last time it continued to stay dim.
 
Could be twitchy connection there...

It's a good sign that voltages are coming closer to where they should be
 
I spent four hours yesterday on this amp board and today I had to step away. I realize I am over my head on this. It is something I have not seen. This is why I enjoy this hobby.
 
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