Realistic STA-2100D Distorted/No Output

Juanbanzai

Active Member
Hi.
Hoping someone can push me in the right direction for trying to troubleshoot a Realistic STA-2100D that I picked up at Salvation Army. Unfortunately, the service manual doesn’t have a section that says, “No output due to stupidity, try replacing X.”

First, even though this thing has some water/rust damage to the chassis, it WAS working just fine although the left channel was a tad weaker than the right. Since the tone controls and everything were playing fine, I elected to recap the protection board, power supply board, relay board, and the two main power caps. Up until I replaced the power caps, the unit came back up and sounded fantastic. So I decided to finish up by swapping out the power caps, but I made the horrible mistake of putting the first one in very late at night when I couldn’t sleep and wanted to putter around to clear my head. See where this is going?

Long story short, even after checking my work dozens of times and finding nothing wrong, I installed one of the power caps backwards (the one I installed at 2:30 in the morning) and immediately blew the 8-amp fuse on powering the receiver back up. No smoke was let out, and none of the four fuses on the power supply board burned out; just the main fuse. With my head FINALLY clear, I found the mistake, reversed the cap, and powered the unit back up. I was relieved when the relays clicked and the unit came out of protection.

But now when I put a signal into the receiver, I’m getting nothing but an occasional “blurp” out of the left channel, and the right channel is horrifically distorted if I turn the volume up past a whisper. The first thing that’s coming to mind is that I blew the final output transistors on both amplifier boards, but if that’s the case, why would the unit come out of protection? My other thought is that I destroyed one of the new electrolytic filter caps. I’ve honestly never made this kind of stupid mistake before so I have no idea how tough those caps are when they’re installed incorrectly. I’m wondering if I should put the original Elna caps back in to see what happens.

So hopefully, someone can give me a better idea on where to begin finding the problem(s). And the lesson we learned is... having a bad day with lack of sleep do not mix with electronics.

Many thanks.

Aaron
 
Do you have a scope that you can use to make sure you've got clean power coming from the supply?
It's possible you may have damaged the rectifier diodes with the current that would have been pulled due to the backward cap. I'd test the rectifier (or each of the separate diodes depending on what they used) out of circuit to see if they are ok.
 
Hi. I’d like to revisit this thread since I was too busy to work on the STA-2100 for the past two weeks. I THOUGHT I had access to a scope, but it’s older and the trace is extremely blurry. Basically, the scope needs a recap before I could trust it with any accuracy.

I tested the bridge rectifier, and all eight output transistors, the regulators, and the diodes on both amplifier boards out of circuit; each component tested fine. I also performed a basic test on the new replacement filter caps - one of which was installed backwards - by putting a analog voltmeter on them. While I don’t put 100% faith in a test like this, the cap that was installed backwards gave a full-scale reading on the meter, then slowly climbed back down to zero. I did this test on the two original filter caps and got the same result. So would this mean the caps are fine??

I’d like to now perform a voltage test on the unit to see if there’s any damage to the power supply board or the transformer itself. However, after I initially corrected the problem of the filter cap installed backwards, I powered up the unit, it came out of protection, yet the output audio was distorted on the right channel and no output out of the left. I pointed this out in my earlier post. What I forgot to mention was that the heat sink on the left channel output board was VERY hot; the right channel’s heat sink was fine.

If I apply power to the unit to test voltages, should I disconnect the output boards from the unit? I think this is done by simply pulling four wires from each board. I’m also going to see if anything fried on the power supply board. I just hope I didn’t cook the transformer even though the 8-amp main fuse blew.

Thanks again for any replies offering some help.

Aaron
 
Do you still have the old filter caps?
Horse them in with jumpers, bypassing the ones you installed. This time with sleep.
I have a thread on the 2100 in this forum. It has an image of the filter connections in case you need a reference.
The thread has the Fairchild equivalents if needed.
Maybe first:
With the set on the DBT read the voltages. That may lend a clue.
Build a DBT! It would of saved you.
 
Thanks very much for the reply, Zebulon. I do indeed have a DBT, but stupid me thought that simply replacing a bunch of caps... something I've done countless of times without a problem... wouldn't be an issue. Believe me when I say I've got the tester on the 2100 now.

I just finished installing the old caps back into the unit. I also pulled the power supply board and checked the regulator on it. It tested OK. I'm going to power the unit back up (ON THE DBT) and start taking some voltage measurements. Again, I just wish I had found something that was blatantly blown out. Oh well.
 
I just powered up the STA-2100 on my DBT (with a 40 watt bulb in place) and it lit up very bright immediately. I swapped the bulb with a 60 watt and again it lit up right away. Now I now that there's a bit of current when you first power-on a unit you're testing on these things, and I could see that after a second, the bulb dims down a bit but not enough to let the receiver come out of protection mode to operate. I used a DBT on a Marantz 2245 I restored a few months ago so I'm pretty familiar with what's supposed to happen. Since the STA-2100 is such a beast, and probably has quite a bit of current on initial power-up, but do I want to increase the voltage of the bulb I use? Is changing the wattage on the bulb even a worthwhile test?

I should point out that before I started testing the output transistors and other components on the receiver, it DID power up and come out of protection even after I fixed the issue with the backwards filter cap. The original Elna filter caps are back in place and those worked prior to me swapping them out.

Thanks again for the help.
 
what is your line voltage ? if 120v or similar you may need larger bulb . check specs of unit .. and turn vol to min no loads . or inputs .
 
The line voltage in the house has always been a problem even with the antique radios I restore. It sits at 122 vAC. I usually build bucking transformers to power the older gear.

As luck would have it, I put in a 150 watt bulb and the unit powered right up. I tested the voltages coming from the D1401 (S15VB) bridge rectifier and I'm getting 91.3vDC. According to the schematic, I should be getting 63.1vDC with no input and 53.9vDC with a full 120 watts out on the receiver. The AC going to the rectifier should be 74 vAC and it's measuring 76vAC on my unit. I'm concerned about the high DC voltage so I'm wondering if the rectifier should be replaced since it should have bore the brunt of having that cap installed backwards. I honestly thought the rectifier would be toast, but it tested fine out of circuit.
 
If the positive and negative voltages are the same on the filters then something is messing with the voltages/current in the amps.
 
I just checked. The voltages are indeed the same on both filter caps... but I don't understand how they COULDN'T be because of the positive and negative leads coming from the bridge rectifier to the caps. The "plus" side is going to the positive of the first cap, and the "negative" side of the rectifier is going to the negative of the second cap. If I'm testing the voltages at the caps, I'm just checking from a jumper point. Am I missing something?
Thanks again for the continued assistance.
 
Meter on DC. Black probe to chassis ground. Red to the voltage terminals on the filters.
Tell me what your reading.
Not sure what you are wondering about.
 
Ugh. I'm treating this like I've never done this before! Of course you put the negative from the meter on chassis ground. I was going straight from the regulator which is just plain wrong. I'm sorry, man. I'm getting 56.4vDC coming from the regulator to the caps with both power amps disconnected. 45.9vDC with the boards reconnected.
 
I decided to recap the amps even though I wasn't planning on doing so until well after everything else in the power and protection circuits were finished. Destroying that filter cap wasn't part of the plan so the amp recap was initially put aside. I started taking a hard look at all the voltages going into and out of the amps and nothing was making sense. I checked all the transistors and resistors (out of circuit where necessary) on both amp boards and everything checked fine. I actually replaced a few resistors that were slightly out of tolerance. That's when I checked the voltages coming from the transformer itself. Nothing was right. At the outlets on the back of the receiver, I was only getting 99vAC. Even though I was still testing on the DBT, I didn't think I'd get a voltage drop that severe. Once I plugged the receiver directly into my mains, everything started checking out. Now I'm getting 60.2vDC at the filter caps... which is pretty close to spot-on if all things being equal.

I did see on another thread concerning the STA-2100D that the transistors and diodes on the amp boards were usually replaced. Is that a matter of course for a rebuild like this?
 
I would say most of the transistors used in the amplifier circuits are fine and left in with the exception of the 2sd400 mounted on the heatsink. I would replace that and insure it is installed as the factory did it.
The reason some will replace silicon is due to blown components, making the amps equal in operation.
I had rebuilt a 2100D for a shorted outputs and blamed the 2SD400 for the destruction.
You can concentrate on the components that reside in the side of the amp the backwards cap was on.
Were you able to set the amp DC Balance and idles after the cap issue?
 
I haven't gotten that far. I was just amazed that the thing fired up after thinking I had destroyed everything by the backwards cap. Was hoping to finish up the recap of the amp boards this afternoon, but I ran out of 100uf/80VDC caps so it's another order to Mouser. I'm glad you responded in time, I was just about to start looking through Mouser's website for the replacement transistors... but now I'll just see about the 2SD400s... which I just found out has been discontinued. What is the recommended replacement?

I've had the receiver on at low volume for about two hours now while I was working on other things and it's holding its own. I do have to spray the volume control again since it's still cutting out on the left channel every once in a while. As soon as I move the control, the volume comes back. I'll try the voltage adjustments later today and report back.
Thanks again for standing by with this one.
 
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Hi Juanbanzai,

I have had an experience with a receiver similar to yours and I would recommend the following:

1. If you are not completely sure that the voltages coming from the main power source are the correct ones, disconnect the four wires that go to each amplification PCB. You must be sure that the voltages that reach the amplification PCB are adequate (60 ~ 64VDC) and are symmetrical with respect to ground. (+ 60VDC 0 -60VDC) Otherwise, the DC Offset and Bias will probably be incorrect and the 4 power transistors on each PCB amplification will be damaged.

2. Perform the tests with the original ELNA capacitors, if you still do not obtain the correct voltages you should check the status of the rectifier diodes and the voltages that feeds the amplifier PCB of the transformer that should be 45 0 45 VAC (Previous voltage to rectification).

3. Before you decide to change the pieces of the amplification PCB, check each piece out of the plate and evaluate if it really needs to be changed. The compensating transistors 2SD400P-1 are prone to fail when there is a problem with DC offset or bias, however if you want to replace it look for the exact part (2SD400 is not the same as 2SD400P-1, the difference is the maximum power of collector dissipation 900mW for the first and 1W for the second). Zebulon1 recommended me the site BDent.com

4. Put special attention to the trimmers of the amplification PCB: VR501 & VR502, it is the weakest point of the amplification stage of this receiver. If you decide to change the configuration of these trimmers, do so with the power transistors out of place and check the voltages you should obtain according to the service manual before replacing the power transistors in their place. (500 mV for each base with reference to ground with the equipment turned on and the volume to the minimum) If you notice that the heatsinks get hot immediately after turning on the equipment, something is wrong and it could get even worse. Adjust the bias and voltage compensation.

Let me know if I can help you in something else, I spent several weeks to restore a receiver of this type and after wasting several pieces and asking several questions to people in this forum, I solved my problem. Regards!
 
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