Interpret Waveform from Oscilloscope

[Robot17]

Okay. Figured I'd play around with my new (old) test equipment on the cheapest thing I have, a Sansui QS-100 four channel rear amp/quad decoder.

I'm trying to learn how to use the scope and would appreciate feedback from some experienced people. The Scope is a Phillips PM 3244 four trace 50MHZ unit that seems to be working pretty good. I'm feeding a 1khz sine from my computer (my function generator only has one output and I don't have an adapter).

The top waveform is the right channel, bottom is left.

Any input is welcome.

Thanks
Bot

 

[Jon_Logan]

The Left channel looks to have an open device (emitter resistor or transistor) in it's output stage.

Try repeating the experiment with only the scope, but no speakers connected.

 

[pdm4606]

scope

try reversing the channels connected to the scope to check for scope anamolies.

Paul

 

[Robot17]

The Left channel looks to have an open device (emitter resistor or transistor) in it's output stage.

Try repeating the experiment with only the scope, but no speakers connected.

Will do. Have dummys connected now.

try reversing the channels connected to the scope to check for scope anamolies.

Paul

Same-same. Flipped plug. Flipped channels. Flipped probes. Flipped sig. Definite problem with amp.

Thanks to you both. Will report back.

Bot

 

[Robot17]

Here it is without a load.

Bot

 

[Jon_Logan]

Here it is without a load.

Bot

Yeah....

So, there's probably something open in the current stage. That's the driver transistors (and possibly their Base resistors), the output transistors (and possible their Base resistors), and/or the output transistors Emitter resistors.


The other possibility is that a protection circuit is being activated too soon, clamping the Base voltage before it reaches the output transistor.

It's more likely to be an open Output something, though. I'd check all of the Emitter resistors for opens. I'd check to make sure that the output transistors have B+, or a higher positive voltage on their Collectors. Looked for cracked connections about the Output transistors.

It's more unusual for an output transistor to open rather than short, but it does happen.

You have the roadmap (schematic) for this thing? Can you post it? That makes for the quickest and most logical troubleshooting path(s).

It doesn't sound too blown up.

 

[Robot17]

deleted

 

[Robot17]

Here you go. Keep in mind I'm a total noob so you'll have to type v-e-r-y s-l-o-w-l-y.

Thanks
Bot

 

[Jon_Logan]

Thanks for posting the schematic. It helps a lot in this case, as this has a coupling cap connecting the speaker to the amplifier.

The schematic is a bit blurry for me to get the part designations right, but I would check the resistors;
R-833(?) 10 Ohm resistor feeding the Base of the upper output transistor in the Left channel.
R-835(?) 0.5 Ohm Emitter resistor for the same output transistor. Check to make sure these have the right value.

 

[Robot17]

Cool. I'll check them and report back. I'll also try to put up a higher res PDF of the schematic. Thanks again.

Bot

 

[Robot17]

R-833 10 Ohms and R-835 1 Ohm. When I short my DMM probes I get .5 Ohms so I assume that accounts for R-835 being off.

I actually checked all the resistors on that side of the board a couple days ago and they came up okay.

I did test the current adjustments and although it had a hard time settling I was able to adjust the right channel to roughly 15mA but the left channel wouldn't go over 6mA or so. The service manual calls for 15mA both channels.

Time to test the transistors I suppose? I take it I should do that per sticky Echowars has on the site?

Bot

 

[ken kantor]

There is no real protection circuitry, not even output diodes to defend against reactive load spikes. I'd say that there is a 95% chance that this is a classic case of fried output device-itis. The no-load behavior suggests the driver transistors are OK.

Careful with this amp, it's a delicate design. For one thing, it is important not to connect or disconnect speakers with the amp on. Personally, I'd even switch the amp off when changing the A/B speaker selector.

-k

 

[Robot17]

The no-load behavior suggests the driver transistors are OK.

Interesting. Does appear that way now that you mention it. Wasn't certain why Jon asked me to do that. I was still going to check the transistors but I take it your saying there's no need.

Personally, I'd even switch the amp off when changing the A/B speaker selector.

I never switch speakers with any amp on but I'll take this advice. Of course I have no idea what any previous owner has done.

Thanks Ken

 

[Jon_Logan]

Interesting. Does appear that way now that you mention it. Wasn't certain why Jon asked me to do that. I was still going to check the transistors but I take it your saying there's no need.

Just to clarify.

I had you disconnect the load to compare the loaded vs. no load waveforms. I had you do this before it was understood to be an output coupling capacitor design. (These days) Most aren't built like that. This is a troubleshooting step used to determine if there's something open in the current stage of the amp.

Ken is also right (of course) about connecting a speaker while the thing is on. An output cap has enough leakage current to pull up the speaker side of the output capacitor, so that when you connect the speaker, it dumps that cap's charge through the speaker, possibly damaging the speaker.

It does look like this amp does have a bleeder resistor hanging off the output, so maybe the DC doesn't build on this cap?

 

[sregor]

Check the fuses - F002 and F003. They are only going to the output stage, and could cause the problem (there is one for each channel). +1 on the resistors in the output stages. also, since you have a scope, check the signal going into the drivers and output. You should get a maxed out signal before the problem. Also be on the lookout for cold or bad solder connections. Good luck.

 

[Robot17]

Check the fuses - F002 and F003. They are only going to the output stage, and could cause the problem (there is one for each channel). +1 on the resistors in the output stages. also, since you have a scope, check the signal going into the drivers and output. You should get a maxed out signal before the problem. Also be on the lookout for cold or bad solder connections. Good luck.

Thanks Steve. Pretty certain the fuses are fine. Will double check when I get my hands on it again. Not certain what you mean by+1 on the resistors (I'm a total noob).

What points would I measure for drivers and output? Have already checked for cold solders. I try to remember to do the easiest things first.

Thanks again.
Bot

 

[sregor]

+1 just means agree with previous poster about checking the resistors they mentioned.
Check the signals at the bases of the drivers and outputs with load. Also make sure voltage is on the collectors of the outputs. (metal tabs are collectors, as well as the center connector)

 

[Robot17]

Thanks. I'll check and report. Appreciate the assistance.

 

[ken kantor]

Interesting. Does appear that way now that you mention it. Wasn't certain why Jon asked me to do that. I was still going to check the transistors but I take it your saying there's no need.

I never switch speakers with any amp on but I'll take this advice. Of course I have no idea what any previous owner has done.

Thanks Ken

Jon's advice is 100% correct. Even if it appears as if the driver stage is OK, at least do a very quick meter check on the driver devices. There are many possible failure modes, and it is good practice to be sure you don't have a strange one. It will take much less time than replacing the output transistors only to have the new ones pop as soon as you power up!

BTW- Be sure you are measuring the bias current the right way. If you maxed out the left channel control trying to get the current up to 15mA, it is possible that the output stage was accidentally over-biased, and then overheated.

-k

 

[Robot17]

BTW- Be sure you are measuring the bias current the right way. If you maxed out the left channel control trying to get the current up to 15mA, it is possible that the output stage was accidentally over-biased, and then overheated.

-k

I was careful to follow the service manual to the tee. Clipped mini-grabbers with leads to the fuse brackets and the meter never was inside.