Jump to content

Repair: Technics SE-A900S | Distortion in right channel at higher volumes only


Recommended Posts

Hey folks,

 

I'm looking for some help with troubleshooting my Technics SE-A900S power amplifier which distorts in the right channel at higher volumes only.

 

Sorry about the long post by the way! I've already spent some time looking for the issue and so have plenty of context, which I'll try and make as succinct as possible.

 

Troubleshooting notes and context:

 

[Circuit diagram of the amp from the service manual (I've tried to trace the Left and Right channel signal path in the circuit diagram using Yellow and Orange colours respectively) as context for the following notes.]

  • I traced the distorted signal through the circuit and can see it all the way up to pin 16 of the preamp IC AN7062N, however, there is no distortion at pin 17. (This is located at the far left of the circuit diagram I linked above, not too for from the signal input posts)

  • I've swapped out the transistors Q402 and Q452 with the equivalent transistors Q401 and Q451 on the left channel respectively and the distortion remains in the right channel.

  • I have also spent a bit of time testing most electrolytic capacitors in-circuit with an ESR meter but don't have much to report here.

  • I've also re soldered some joints along the right channel, but definitely not the entire path... this was random and seemed like a waste of time.

 

While asking for help elsewhere, I took pictures of the actual circuit board, linked below in case they are helpful.

 

Would appreciate any help I can get in hunting down the root cause here. Thank you for your time and help :)

  • Like 1
Link to comment
Share on other sites



I'm currently reflowing most of the components near the power amp circuit, because apparently a dry solder joint is common issue with these and other similar units.

 

I was wondering what components in an amplifier might create distortion at higher power outputs? I just want to try and narrow down and figure out where I should focus.

Link to comment
Share on other sites

Almost looks like phase error during square wave testing. Can you input a 1-2kHz

square wave and note the output.Will take a look at the circuit etc...

Link to comment
Share on other sites

Bet it's the caps. if I were you id be replacing all electrolytic caps as there aging and most likely going resistive now. As you say at low volumes (or headphones) all good but once pushed the juice is running out causing distortion.

  • Like 1
Link to comment
Share on other sites

Did you measure power amp module pin Voltages to GND, 2 sets,with/without distortion. Need to checkout if seperate power supply per channel. Feedback complicates the troubleshooting.

Link to comment
Share on other sites



1 hour ago, mbz said:

Almost looks like phase error during square wave testing. Can you input a 1-2kHz

square wave and note the output.Will take a look at the circuit etc...

 

Appreciate you looking into this for me. I ran a couple of tests with square waves, see results below:

Link to comment
Share on other sites

Yeah, the distorted square waves show missing/bad low frequency response, possibly a coupling cap is bad, need to check schematic for caps in audio path.

  • Like 1
Link to comment
Share on other sites

Ok, I've spent some time learning about coupling capacitors and how they work. I'm going try and mark all coupling caps in the right channel and then use my multimeter to see if they are leaking any DC across. Let me know if there's a better way to do this. Thanks again!

Link to comment
Share on other sites

Won't be coupling caps. If it was then you'd notice a frequency imbalance in the output response. Ive done repairs where one (or both) channels had major bass deficiencies which was fixed by swapping out the coupling caps.

 

Your issue I suspect is due to the main power supply filter caps. As a rule if you don't want to change every single one it's typically the largest ones that tend to fail first.

  • Like 1
Link to comment
Share on other sites

50 minutes ago, Tubularbells said:

Won't be coupling caps. If it was then you'd notice a frequency imbalance in the output response. Ive done repairs where one (or both) channels had major bass deficiencies which was fixed by swapping out the coupling caps.

 

Your issue I suspect is due to the main power supply filter caps. As a rule if you don't want to change every single one it's typically the largest ones that tend to fail first.

 

Very interesting (mostly because I'm learning and so thank you), I used my scope to check for any frequency imbalances. At 30Hz sine wave input, I see an imbalance of about 0.14 Hz, at 60Hz, 200Hz, 2000Hz its out by maybe 0.1Hz here or there at max. I'm assuming these are insignificant?

 

Link to comment
Share on other sites



Suggest use oscilloscope and check for distortion in left channel, maybe present
but not readily audible. A power supply fault typically affects both channels.
A health check of the power supply (when distorting) is not a bad idea.
Include the following dc voltages,
- Q401base
- Q402base

Link to comment
Share on other sites

14 minutes ago, mbz said:

Suggest use oscilloscope and check for distortion in left channel, maybe present
but not readily audible. A power supply fault typically affects both channels.
A health check of the power supply (when distorting) is not a bad idea.
Include the following dc voltages,
- Q401base
- Q402base

 

I haven't seen any distortion in the left channel in my many weeks of testing, however, every time I increase volume past the distortion to try and push left channel and see if there's distortion, the amp goes into protection mode. I THINK, this might be because I have the fan disconnected and not because of a fault in the circuit... and connecting the fan back will mean I have to reassemble the unit as the fan is built into the back plate and so haven't been able to confirm this.

 

DC Voltages at base for Q401 is 43.8v (should be 44.1v according to service manual) and Q402 is 43.7v (should be 44.1v according to service manual). I measured these while the unit was powered on, but no had no input.  I have also tried switching the Q402 with Q401 to check if it was a faulty transistor, but it made no difference to the distortion.

 

If the issue was in the power supply, which caps would I go prodding and checking? In this top shot of the circuit board, you can see there are a bunch of caps in the top right side, near the pre amp circuit. These are powered by a small transformer in the power supply section. And then there's the big transformer, probably 5-6 times the size of the smaller one, which seems to feed the 4 large diodes in the bottom left, next to very large caps. (sorry about the very pointy wavy description).

Link to comment
Share on other sites

Q401,2 voltages look fine. The power supply is mostly common between L&R channels (except Q401,2), so a fault in the power supply would affect both channels. I'd be looking elsewhere.

 

The suggestion was to replace the main filter caps, see below. I would be looking elsewhere. The schematic is a tough read, takes a bit of time...

 

image.png.8da787eca349f585432814d3843c5edc.png

Link to comment
Share on other sites

I think you are going to need a tech with a reasonable oscilloscope. As previously mentioned, the feedback into pin 16 of IC401 makes troubleshooting more difficult. Need a two channel scope that can subtract one channel from the other (basic function) then probe various points in audio path to workout where distortion is introduced.

 

 

  • Like 1
Link to comment
Share on other sites

I would love to hand this amp off to someone with a lot more experience and knowledge to fix, but then I wouldn't learn... and I eventually will end up doing that if I can't figure out the issue. However, I have been looking for an excuse to buy a proper scope and maybe this might be it?!

 

If I were to get a two or maybe even a four-channel scope, after I hook it up to the positive and negative inputs (pin 16 and 17 on the preamp IC) of the differential amplifier, how would I go about probing various points in the audio path? Are there examples that I could check out or is there a name for this troubleshooting method?

 

I think I’m willing to invest more time and money in getting to the bottom of this as long as I continue learning and can get some support from the experienced members here.

Link to comment
Share on other sites



IC401 pin 17 is the input from the RCA's. Pin 16 is from the feedback loop starting between R516 and L504. Select a component then probe input and output,eg, for Q402 probe base and collector on AC. Suggest divide the circuit in roughly half and test there, move forward or back depending on the results. Q454 looks likean ok starting point. probe collector and emitter, keep in mind you are looking for distored shape(sine or square wave) and not focus on size/amplitude differences.

Edited by mbz
format
Link to comment
Share on other sites

  • 4 weeks later...

Okay, I'm back with a proper scope and some measurements! 

 

I spent some time poking around various areas of interest as suggested by Mike. It looks like there are parts of the circuit that have channel imbalance while the amp is not distorting. And the imbalance settles after a certain point in the later stages of the circuit.

I've composed an image with some of the more interesting scope measurements. See image below or check here for high res version.

 

scope-output-small.thumb.png.607a3d72ec290577816ea268deee64f9.png

 

Legend:

  • Yellow signal is Right (faulty) channel scoped at various points on the circuit
  • Pink signal is Left channel scoped at various points on the circuit
  • Green signal is Right (faulty) channel scoped at speaker output
  • Blue signal is Left channel scoped at speaker output
  • Red lines point to the points the scope measurements were taken
  • Top row of scope measurements were taken at a lower volume where there is no distortion
  • Bottom row of scope measurements were taken at a higher volume where the amp distorts in the right channel

 

With the channel imbalance, the signal isn't shifted up (or down), it just has higher amplitude... Any ideas what could be causing this? Any other ideas where I should be looking/testing?

  • Like 1
Link to comment
Share on other sites

2 hours ago, bob_m_54 said:

Either power supply to IC401, feedback components to IC401, or IC401 itself. Check the components to the left of Q401, and a likely candidate is the electrolytic cap C405 47uF 25V.

 

Thanks Bob. Q401 and C504 are both on the left channel. The equivalent on the right channel are Q402 and C406, both of which I've already swapped around with the same components on the left channel to see if these were causing a problem and it made no difference. I've also tested all the resistors to the left of Q402.

 

If there was an issue with the supply to IC401, wouldn't that affect both channels?

 

Is there a relatively easy way to test if IC401 is faulty?

 

P.S. I've been to Port Stephens and its beautiful out that way! My mates parents live up in North Arm Cove and I've been up there a few times sailing across the bay. I miss those day, good times :)

Link to comment
Share on other sites

Yes sorry, got the L and R chans ass about. and yes the P/S itself would affect both chans, but check the voltages right at the pins on the IC, to see if they are what they are supposed to be, at the chip.

 

As for testing the IC, no easy way that I can think of off the top of my head.

 

A scope trace of the inputs to both channels would have been good, just to be sure that the same signal was going to both channels.

Link to comment
Share on other sites



Spent more time poking around with the scope. See image below or here for full res version.

 

Scope output images 1 and 2 are at the input of the differential amplifier (pin 16 and 17) on the right (faulty) channel. The white trace is subtracting the two channels in both screenshots 1 and 2. Scope output 2 is at a higher volume when the amp is distorting. Equivalent Left channel pins have clean/equal sine waves.

 

P.S. Maroon scope channel represents maroon line on the diagram and yellow = orange lines. 

 

I also noticed a small oddity around C414 which is a ceramic cap. There is a small dip in the signal, right in sync with the input 30 Hz sine wave. See scope output '4. Right channel, yellow signal' vs '3. Left channel, yellow signal' which is flat. Looking around I noticed the exact same signal with a dip at all the points marked with orange X. I further zoomed in to get a better look at the dip, this can be seen in scope output 5. Scope output 6 is the same as 5 but at a higher volume at which the amplifier starts distorting.

 

2scope.thumb.png.c411a798c2f1eed3344ef0de1587111a.png

 

This difference in the signal between L and R channels when the amp is not distorting feels like the strongest clue yet. However, I'm still not sure what component might be responsible for this.

Edited by bulletim3
Updated image
  • Like 1
Link to comment
Share on other sites

First up, excellent work in putting together the oscilloscope traces

 

Yeah, I'd also be leaning towards IC401. Odd that the faulty channel has larger amplitude, IC probably going non-linear. The feedback signal is more or less ok then the output of IC401 is badly distorted. Minor point would be to test at Q402base to rule out Q402 as source though I recall that you switched Q401,2. Suggest contact @SVI2004A at AUDIOKARMA, something of a Technics specialists

 

I'd stay away from ebay replacements, likely to be fakes.

 

You could try a squirt of freeze spray onto the IC while it's distorting and note any affects.

  • Like 1
Link to comment
Share on other sites

On 23/08/2021 at 7:04 PM, mbz said:

Minor point would be to test at Q402base to rule out Q402 as source though I recall that you switched Q401,2.

 

Hey, yeah, I did swap Q401 and Q402 around some time ago.

 

I just also realised I never looked at the signal on pin 11 up close (output of the differential amp) of IC401... and I see the same signal as pin 14/spots marked X in my previous post. This could explain the higher signal amplitude seen past Q402.

 

Considering IC401 (AN7062N) is very hard to find, I need to be sure if it is actually faulty. And I think the only way to do this would be to de-solder IC401 and flip it to see if distortion moves to the left channel.

 

I've also reached out to SVI2004A. Will post an update if/once I have one. Cheers

 

  • Like 1
Link to comment
Share on other sites



  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...
To Top