Trouble shooting SA-7100 (hum in one channel), difference in voltages at L and R outputs

[Gobstopper]

Hi Folks,
I posted last week asking about filter caps for this unit, and I wanted to follow up with some issues I've discovered. For some background, I swapped out all the electrolytic caps, diodes and transistors on the power supply and electrolytics plus a couple of the transistors on the protection circuit. I swapped out all electrolytics and transistors(other than those mounted to heat sink) on Power Amp and all the electrolytics, tantalums and transistors on Control Amp.Same for Equalizer board. I also tried to deoxit and faderlube the pots and switches. I'm not sure I got any clear into the lever switches though since hard to access and didn't see any obvious openings. Tried rotating plug 180 degrees to no effect All testing done with headphones(various sets that work well fine outside of this amp)

Issue:
Constant steady hum in left channel, does not respond to volume or balance controls.

With pre/main jumpers in: Hum in left, rustling/static in right(not consistent)

Removing pre/main jumpers: Hum in left, quiet in right

While jumpers were removed, I plugged in audio source into Main-In, and was able to hear music in both channels, but hum was quite loud(in left channel) when music stopped.

I took some voltages around the relay(based on wiring diagram from the service manual) and

found the following readings:

Pin DC AC
9 50 mv .007 V
10 136 mv .15 V
This case had rustling/static in right channel

repeated 10 mins later after leaving off for awhile and doing some other things

Pin DC AC
9 22 mv .04V
10 66 mv .007V
This case no rustling/static in right channel.

For the power amp:
Pin DC AC
2(out 1) 69.3 mV .03V
17(out 2) 25.2mv .007
(the rest of the voltages at the pins were within 10% of values listed in SM)


I'm supposing the rustling/static in right channel is due to the control amp or dirty pot/switch although the transistors and caps were changed.

The hum in power amp I'm not entirely sure about. The left channel transistors mounted to heat sink? Bias/offset issue? I haven't tried setting bias and offset since I was a bit intimidated with removing the wire wrap pins from board as required in SM, but if that is recommended as first step I'll do that. Could it be something else entirely?


Any suggestions on what else to check?

Thanks!

 

[Gobstopper]

Thanks inductor. Going to a tech isn't an option right now, so I'll check for bad solder connections, lifted pads, etc. Perhaps use a DIY signal probe to follow the left and right channels on the power amp. From what I've read, given that the hum is only on one side, that would tend to rule out the power supply. Maybe some more concrete steps will come in on the forum. I'm still curious if the higher AC and DC voltages on one channel could be responsible for the hum I'm hearing and if so, how to best find the source of the problem.

 

[Awesomeaudio]

Hi gobstopper,
Was there issues with the amp prior to doing the refurb ?
When I did mine, it would would cycle the protection circuit and being an old amp, it was due for a refurb as it was all original including the known problematic transistors.
I'm not sure if the hum is related to a bias/offset adjustment, but it is very important to do this after having undertaken all that hard work.
I can't remember if I unwound the wires or desoldered the pins. Either way, you should try to set set the bias/offset first.
Don't let it intimidate you. It's not that bad.

 

[Gobstopper]

Thanks Awesomeaudio for the response. I got the unit for free several years ago and at the time it had some odd clicking and hum + static going on. It would come out of protection from what I can remember. I was actually going to PM you to ask how to best remove the pins from the board without removing the wirewraps. I gave it a quick go earlier, but had some access issues when trying to heat from the solder side. I just looked through your original thread and see that you actually didn't specify how you disconnected the pins. Patfont un-wrapped the wire then soldered back on. I'll search around some more to see if I can get some specifics on removing pins. I'll report back when I can get around to trying out the trim pots.

 

[Awesomeaudio]

I'll have to take a look at mine when I get home. At work at the moment.
I may have unwrapped the wires.
Not 100% sure.
If Pat did it that way, then more than likely I would have followed suit.
You can unwrap the wires, then re wrap and solder.
MAKE SURE you insulate the loose wires so they don't inadvertently short to something and then proceed to adjust your bias/offsets.

 

[gslikker]

Generally it is just heat up the pins using the soldering iron and quickly AND gently at the same time, pull them out.
Taking a lot of pictures from different angles helps a lot, when putting them back.

My first restorations I did unwrapping , put together and solder, but it did not look neat. When taking out the pins, the looks stay as it was.

 

[Awesomeaudio]

Just had a look at mine. Looks like I unwrapped, re wrapped and soldered.
I think at the time, being my first refurbish I didn't think to unsolder the pins.
I learned about this a bit later.
Mine doesn't look untidy, but I think the pin removal method will keep it original.
It's up the individual I guess and degree of difficulty.

 

[Gobstopper]

Thanks guys. I'll try removing the pins. Gslikker, do you find it best to heat from the solder/pad side?

 

[gslikker]

always heat from the solder side. If not, the pad may not be heated enough or a little bit of solder is hampering the pin to go out.
You might destroy the solder pad, by pulling while the solder not totally melted.
Before doing the removal, add a very little bit of new solder to the pad first, to make sure the solder tip will make good contact when desoldering. Make sure the soldering iron tip is clean.
Of course, if the pin is simply not accessible from the solder side, you have to deviate from the above...

It is always the balancing act between time, temperature and trying get the pin out using almost no force at all, as not only the pin but the solder track itself also loosens from the circuit card, when too much heat is applied for a too long time.

Beware when putting the pins in again. Make sure the solder is removed from the pads and the holes are clean, the pins are free of solder blobs and they will fit in again, BEFORE rewiring the card again. You are better off being prepared than find out one pin is hesitant to get in when done half way.

 

[Gobstopper]

Thanks for the detailed tips gslikker, they were very helpful and I managed to remove the pins. However, I then went to measure the voltages and realized I had nothing to grab on to! I temporarily soldered some pieces of a paper clip into the holes, and was able to set the idle current to obtain the stated 30mv on both channels. The initial voltages were both about 50-60 mv, so no huge discrepancy to explain the hum in one channel. I still have to go back put the original pins and measure the mid-point potential, we'll see if there are any major differences there.

 

[Gobstopper]

Hi Everyone, odd results. I was able to re-insert pins(double checked order against my notes and various photos) and continue with the adjustments. I adjusted the mid-point potential as instructed, and was able to get it down to about 3mv on each channel. I then turned unit off and went to get headphones. Turned back on and plugged in, and still buzzing on left channel(a little bit on right, but mostly left). I checked the voltages at pin 2 and pin 17(both relative to ground) and now Pin 2 was back up to 60 mv. I unplugged headphones and it dropped back to around 20 or 30mv. Re-insert and it goes back to up 60mv. I removed headphones and re-adjusted Vr1 until voltage at pin 2 was about 5mv again. Put headphones back in and shot back up to 60 mv. Pin 17 without headphone was around 12 mv, I lowered it to 5mv(without headphones), with headphones it goes to -5mv. Why is inserting the headphone causing such a change in voltages? Is this expected? If not, could it help indicate where the problem is?

 

[Gobstopper]

It's probably stating the obvious, but the hum only comes on after the relay kicks in. Some more info: I didn't chance the zener diodes on the power amp. Perhaps I'll test those and see if I can find any obvious faults or differences between the two channels. I'm still wondering why the mid-point potential offset changed so much(and in opposite directions) when I plugged the headphones in....

 

[Awesomeaudio]

I am suspecting the hum may be due to the filter caps drying out or something in the power supply. Not sure if you replaced these.
I did read somewhere where someone cured a similar problem by replacing the filter caps.

 

[Gobstopper]

Thanks Awesomeaudio for the response. I already changed the filter caps as well as the caps,transistors and diodes on the power supply and it didn't help. It's also much more prominent on the left channel, which led me to think it wasn't necessarily a power supply issue after all. I'll triple check my voltages though just in case.

 

[Gobstopper]

Finally got a chance to check voltages this evening. I did all the posts on the Power Supply and the Protection circuit(DC and AC), but for brevity will only post info on posts for which the service manual lists values.

Powersupply
Pin Test Service Manual
6 -27.4 -30
7 27.4 30
10 0 E(0)
11 36.1 35
12 30.5 30
13 32.9 34
14 0.1mv E(0)



Protection Circuit
6 -13.1 -13
14 8.5 8
16 35.9 35
17 (AC) 7.9 7.5


I'll try to do the power amp before I head in tonight, and will post those values next.

 

[Gobstopper]

I went ahead and checked the voltages on the power amp. As I mentioned in my post about with setting the mid-point potential, the out (pin 2) for the left channel is higher than it is for the right(~50mv vs 7mv).

I checked voltages at the various transistors for both channels, and for all other than those attached to the heat sink, they are more or less the same for both channels. The interesting bit is for the heat-sink mounted transistors. Reminder, the left channel has most of the the loud hum/buzz going on and after setting the mid-point offset, it wouldn't stay where set after turning power off and on, and the voltages at pins 2 and 17(the outs) would change dramatically if I plugged in headphones.

For the 2SD371's

Right Channel:
Pin 21: Collector 28 V
Pin 22: Base .61V
Pin 20: Emitter 10mv-8mv

Left Channel
Pin 31: Collector 28 V
Pin 32: Base .56V
Pin 30: Emitter -36mv

For the 2SB531s

Right Channel
Pin 24: Collector -28V
Pin 25: Base -.63V
Pin 23: Emitter -25.3mv

Left Channel
Pin 28: Collector -28V
Pin 29: Base -.68V
Pin 27: Emitter -71mv

So, it seems that the voltages at the bases and collectors are similar with respect to each other on the right and left channels, but that the emitters are very different, with those on the left channel having a larger voltage(magnitude) at the emitters in both cases.

SO, the million dollar question: given that the voltages leading up to these heat-sink mounted transistors look relatively similar for both channels, is this most likely a sign of bad transistors on the heat sink or did I miss checking something?

Thanks in advance!

 

[w_barton]

recheck your solder connections on all the coupling caps you replaced, check for cold solder joints or missing solder. If you don't have scope, use your AC function of your meter to find that hum.You have a ac signal floating on your dc that is why you are seeing that fluctuation. Measure your bias( ?) test point with your meter set at AC volts. There should be no AC there and I think you will see your hum there. Check any wire you disconnected, especially ground wire.

 

[Gobstopper]

Hi W, thanks for the response! I took AC readings earlier of the power amp, at the mid-point potential measuring points(same as the outs(pins 2 and 17) and the AC was 0.03V for both channels. The AC readings were also similar at the heat-sink mounted transistors for both channels. All the collectors (+/- 28 V DC,) had an oscillating AC reading. The right channels measured 0.021-0.024V AC at the bases and emitters, while the Left Channel bases and emitters measured 0.03V AC. I checked the solder connections and didn't find anything obvious, but will go back and repeat. Also, I didn't disconnect any wires, and I thought I checked all the grounds, but another go can't hurt. Given that the sounds is mostly in the left channel and occurs even if I remove the pre-main jumpers, would that suggest that if it were a bad connection, it would be in the portion of the power amp dedicated to the left channel?

 

[w_barton]

okay, that sounds right on the AC. so lets start isolating. without no speakers hooked up, (with the pre and power jumper installed) measure the DC and the AC across the + and - speaker termination in the right channel. Repeat on the left channel. You should see a difference between the left and right channel measurements. If there is a difference, we now need to find out what portion(preamp-poweramp) of the the suspected channel is causing it. if you have a rca to rca connector, then using the rca cable connect the right preamp to the left amp and right repeat the left preamp going to the right preamp. If the measurement reading swapped then the issue is in the preamp. If the measurement did not change, then the power amp is the culprit. I think that the left channel is the problem. You can also check the Signal path of the preamp and power amp to find the hum. Check C3,7,47,and C35. Hope this helps

 

[Gobstopper]

Hi Barton, I checked the voltage at the speaker outputs.

With the speaker selector switch set to A,

Jumpers In:
Right channel -1 mV DC and .054V AC
Left Channel -49.2 mV DC and .055V AC

(However, when I switch the speaker selector to B (and still measuring on A speakers)
Right Channel .4mv DC 2.05V AC
Left Channel .7mv DC 2.15V AC)

Speaker Selector Back at A:

No Jumpers:
Right Channel -13.1mv DC .018V AC
Left Channel -30mv DC .12V

Switched Jumpers
Right Channel -18.8mv DC .13V AC
Left Channel -43.3mV .081V AC

So it seems that the left channel has consistently more DC voltage than the right, but the AC moves a bit depending on jumper configuration
My RCA cables (for switching) were a bit hard to get on, so I don't know if that was affecting things.

In my limited understanding, it would seem that something is definitely up with the left channel on the power amp, but that there might be something less obvious going on at the pre-amp as well.

Are the capacitors your suggested checking on the pre or power amp?

Would faulty transistors give this kind of noise, or would one expect more "popping"?