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SX980: DBT Fail. What next?

P

palex1080

New Member
I'll start with the question and then fill in some history. Having done some work on an amp and checked it to the best of my ability, I decided to power on with a DBT. Sadly, the bulb glowed constantly (not very dimly). Hopefully, parts were not damaged. No fuses were blown, no magic smoke released. Diagnosis seems to be a short somewhere, but where? My question is: what is the next troubleshooting step, if the DBT fails?

This is an SX980 that was working nicely until one day the relay stopped clicking. Foolishly, I completely disconnected the power supply and amp boards to gain access (do not repeat my mistake -- they are accessible in place). I recapped the boards and reconnected only the power supply board. After working through some problems, with MTF and others, we got the voltages correct. The relay even clicked. Then, I connected the amp boards, along with the many other sundry wires to the power supply. I tried to be careful and double checked each one. But, when I flipped the switch.... light! :sigh:

My current thinking is that either I made a wiring mistake or there was a problem on the amp board that caused the original cascade of ruin and which, not having been fixed, only returns when the beast is brought back to life. I'm pretty confident on the wiring (80%?) so I suspect the latter. All this brings me back to the original question : what to check next?

Many thanks in advance. Without the kindness and generosity of the members here I would be lost.
 
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Wow, how to troubleshoot in a nut shell, not soo easy.
Certainly going at it all the wrong way, blindly changing components without first isolating the cause of the faults. Tough lessons to learn.
These power supplies are unforgiving, as they have no electronic short circuit protection what so ever, other than fuses or transistors/resistors blowing = can be expensive.
Suggest to back track to isolate the power supplies from the loads (un-solder all your wires again). Test the power supplies without loads, then systematically start to connect loads up one at a time, while monotoring the voltages. Leave the power amp sections to the last.
In a perfect world one would test the power supplies independantly with load resistors or electronic loads, totally isolated from the loads in the receivers electronics. You could even power up the receiver circuits with external power supplies that have short circuit protection but that is probably outside of your scope of abilities and equipment, so it remains the hard & difficult way to sort it all out.
I'm pretty confident on the wiring (80%?)
Click to expand...
So that leaves 20% of uncertainty, not a good game plan. You should be 100% confident in your connections, or this is a recipe for disaster. Use your ohmeter to test all the connections first, against the schematics, (sanity check) before applying any power from the line.
Wish I could be more help, but it is difficult to troubleshoot using email.

Rick
 
When the dim bulb glows, you have a tremendous "short".

Now that "short" could be both output transistors turned on hard at the same time due to an amplifier fault, and once their faulty "instructions" are fixed things could be fine.

First thing - WHAT did you do about the thermal bias diodes connected to the amplifier cards, yet mounted to the big heat sink with the output transistors. DID you disconnect their wires instead of un-mounting them from the heat sink? Look for a broken wire.

THEN, what about pins 12 and 22 on the amplifier card? Are there wires connected to them?

DID YOU OPEN UP ANY AMPLIFIER WIRES?

EXACTLY WHICH WIRES DID YOU DISCONNECT? The amp board is connectorized, correct? with the exception of the two wires to the thermal bias diode?

This is self inflicted, and I am not going to be rushing and devoting huge amounts of time in any particular day to this.

Once these questions have been answered, there is a "trick" we'll do to make some measurements on the amp, and yes - the trick will get the bulb to dim.
 
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Thanks for your quick replies.

Mark,
Yes, AMP boards are partially connectorized, but there are also pins 1 - 10 (wire wrap), the STV-4H diodes, pins 22 and 12, and pin 11. I disconnected them all in order to completely remove the amp boards. I'm not sure what you mean by "open up any amplifier wires". If you mean internal connections from one part of the amp board to another, I did not change them.

Yes, I did disconnect the thermal bias diodes instead of unmounting them. The wires are not broken.

Wires 12 and 22 are connected. I used a meter to confirm connections to power supply pins 7 and 8.

Thanks again for your help.

Rick,
I appreciate your guidance on approach. I did check each connection, using the schematic as reference, but my confidence level is only 80% because there's a chance I was not checking them correctly and also there could be connections I didn't know about. Some time passed between disconnecting and reconnecting and many of my markers became detached.
 
Hi petehall347. Good idea. I've already searched pictures -- of course, all the best ones were on this forum -- I found some extra large pictures from JoeDotCom that were even diagrammed. I hadn't thought of posting my own pics for this question, but it might help. Your signature pic "I'm not totally useless. I can be used as a bad example" made me laugh -- it could apply to me in this situation.
 
ok, first it is VERY easy (and forgivable) to put the thermal bias diodes in backwards.

second, pins 1-10 wire wrap is what I was looking for (and what I meant by "open up"). also pins 12 and 22 carry power in to run the driver transistors. BUT the happy part is that ALL the transistor connections are on the connectors, they are NOT energized by power wired in from somewhere else than the sockets - thus TOTAL disconnection of the output transistors is easy.

Remove the transistor connectors, and use an ohmmeter to check for collector to emitter shorts. pins 21 to 19 and pins 14 to 16 in the CONNECTOR side, not the pcb side :D

You are going to need a color list of the wires from another sx-980 to be sure you got them in the right places. While I can identify the sources of each incoming wire, that does not tell me their color.

REDUNDANT documentation, not just markers on each wire, but rather ALSO a paper sketch of what color was where, because as you just found out,markers fall off. And I have, to my sorrow, found out that sometimes a picture leaves doubt between two different shades of color - showing up somewhere in between.

so, double check the thermal bias diode polarities, then after testing the transistors for shorts, LEAVE the transistors UNPLUGGED.
Turn on the unit
measure on both amplifier cards the voltage from pin 15 to ground and pin 20 to ground. You MIGHT hear the protection click on.
post those 4 readings. They SHOULD be 0.6 volts either plus(15) or minus(20). Higher than that is a "runaway", maybe due to a reversed diode. BUT DO NOTHING after taking the readings.

DO NOT leave it turned on for long because a "runaway" is also pulling huge amounts of current through your drivers Q6 & Q7 and the 150 ohm 1/2 watt resistors R15 & R15.
 
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Hi Mark,
Thanks for the help -- your information is excellent as always.

I was very particular about the alignment of the ST-4H diodes since you and EchoWars had helped me test them. Soldering them on was not easy because too many other connections had already been made -- I did it by heating the wire on the "top" side of the card and applying solder to the bottom. I tested the connections between the wire on the top side and other components and they were good, so I think the joints are solid. However, you keep mentioning these diodes, so maybe I should check again...

I tested the lines to the transistors (pins 21/19 orange/blue 14/16 purple/red) for shorts and didn't find any. Thanks for specifying to test the connectors -- in my mind I was picturing the pcb side. I got readings in the 8 to 10 megohm range, and one OL. The OL had me worried, so I got more aggressive with the meter leads. Still OL -- not sure why. Small new problem: the more aggressive action with the meter leads caused some expansion, so when I replaced the connector it wasn't tight. I assume this can be fixed.

I found one of your posts about the importance of documentation after it was too late. I thought -- whoa a guru like MarkTheFixer needs to take notes, and I have none! Never again...

So, I think the diodes are ok, and I'm pretty sure the output transistors aren't shorted, so time to take those voltages.

About the color list -- do you think such a thing exists? I couldn't find it in the documentation and all the photos I have are incomplete. I wouldn't think of asking any AK members to compile one -- I offer to do it myself, once we get this straightened out.

I will test the voltages (quickly) with the transistors unplugged and post again.

Thanks for all your help!

-Alex
 
Ok, I powered on with the transistors disconnected and the relay did click (I'm hoping that is a good sign).

Here are the readings (left and right may not be correct -- they are based on my left and right, facing the front of the receiver):

L pin 15 to Ground: 2.5v
L pin 20 to Ground: 1.3v

R pin 15 to Ground: 2.0v
R pin 20 to Ground: 0.84v

These are very different from expected, but the voltage difference on each side is about 1.2, same as the difference between expected 0.6 and -0.6

Thanks for thinking about it,
-Alex
 
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Forgot to mention: the bulb was glowing during the above test. It was momentarily bright, then less bright, but never completely dim.

The sequence was bright, less bright, relay click...
 
HOW many watts was the bulb? On a receiver of that size 100 watts would be a minimum...

also the idle current pots should have been turned to zero ohms indicated.

Once the threshold of 0.6v is reached, a small increase in voltage yields a dramatic increase in current.

Those two factors could just be your problem, that 1.2v observation is entirely correct.

An interesting thing is that two opposite DC offset voltages can cancel each other out if they are closely matched.

thus -3 volts and +3 volts out wouldn't trigger the protection.

as a side note, even on the dim bulb you CAN dial the dc balance/offset voltage closer to zero, and the adjustment won't drift a LOT during the changeover to direct AC line.
 
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Hi Mark. Great question. I hadn't given too much thought to the bulb. It's 40 watts.

Which are the "idle current" pots? There are two pots on each board. Are you saying that if I use a 100w bulb and set the idle current pot to zero I might get different results?

Thanks,
Alex
 
With 40 watts, absolutely. use 100 watts.

the adjustments and trim pot locations are in the service manual.
page 24 of 62 in the pdf. section 9.4,
the page number 25 is printed on the original manual and in the scanned data.

the dc offset adjust (manual calls it center voltage adjustment) for that channel is the pot "closest" to pin 4.
The outputs do NOT have to be installed to do this.

The idle current adjust for that channel is the pot "closest" to the center of the board.
The output transistors MUST BE INSTALLED to do this.
They ARE the OBJECT of the enterprise.

You MUST insulate the blade of the screwdriver, the chances of it shorting something is almost guaranteed.

BEFORE adjusting them, turn OFF the unit and rotate the pots through their entire range a few times to clean up any "dead spots".
 
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Thanks! The manual was in front of me, but I hadn't thought to check the adjustment section. After studying your post and the manual, the procedure is clear.

Replaced the 40 watt bulb with 100 watt and, transistors still disconnected, powered on the unit. This time the bulb is very dim after startup. Relay still clicks. On both sides, adjusted the center voltage as close to zero as I could using the insulated screwdriver on VR2 -- at one point the meter read 0.00 mv for a second, but then moved to 0.1 , 0.2 etc...

Next I need to fix the expanded crimp connectors. This step is not optional -- the connector assembly pops off the board. Then I will connect the transistors and power up. If the bulb is dim, I will attempt to adjust VR1 to 30ma. I think there's a typo in the manual -- it goes back and forth between mv and ma: "...check if the dc voltmeter reads 30mV. If it doesn't read 30mA, adjust to 30 mV by turning VR1"

I'll post my results here soon.

Thanks,
Alex
 
Yes, the terminology can be confusing, even to the manual writers and translators.

Idle current is in Milliamps, but to measure it without breaking the circuit, it's presence is measured as a voltage across the two 0.5 ohm emitter resistor.

That's one ohm and a direct correlation between amperes and volts and thus milliamperes and millivolts.
 
Good thing you posted when you did -- I was planning to test mA. The one ohm / direct correlation thing is very cool. Is that the basis of the Ohm?

I didn't really find a way of fixing the crimp connector, but I was able to make it stay in place. With the outputs connected the bulb is a bit less dim, but still dim. Idle current set at 30mA.

I repeated the earlier test with these results:

L pin 15 to Ground: 0.721v
L pin 20 to Ground: -0.44v

R pin 15 to Ground: 0.696v
R pin 20 to Ground: -0.476v

I wonder how you thought of that test? How did you know the difference should be 1.2? I'm very interested in the theory.

Next test -- sound. I will hook up some low grade speakers and see what happens. Of course the results will be posted here. I'm optimistic at this point :)

Thanks very much!
 
I'm optimistic too.

A bipolar junction transistor should have 0.6v emitter to base to turn on, it's called biasing and is very basic engineering.

The 0.5 ohm resistors are used primarily for a type of feedback, but they are also useful for measuring the idle current in the output pair(s).
 
UPDATE: Hooked up a speaker and tested each channel separately. Music! At first, I was having trouble with the fm. It sounded like some gremlin was messing with the volume. Finally dialed a reliable Classic station. Rolling Stones and others. I'll know more once I hook up turntable and good speakers, but so far it sounds pretty good. Thanks Mark and the others who helped. Thanks AK!

It seems like the main problem was the wattage of the bulb... Like MTF says, the fools are too ingenious :)
 
Ok, I could ask a hundred follow up questions based on your answer, but instead I'll do a little research on my own. I plan to keep at this.

For this project, instead of quitting now I plan to replace the large capacitors and possibly the capacitors on the Flat and Tone boards. The cover is off, why not go all the way?

Thanks again!!
 
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