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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:
The distortion is in the right channel only, in both speakers A and B. Tested with two different sets of speakers and have also tried switching the speakers around. Distortion is always on the right channel - ruling out speaker issues. I don't hear any distortion on headphones plugged into the headphones jack on the front of the amp at any volume. There is no distortion at low sound levels. It only kicks in at higher volumes and I was able to confirm this with a scope as well (More details in linked pictures below). There seems to be no distortion in the output if I remove the speakers at any volume levels (confirmed with a scope).
Visual inspection of the circuit board shows no signs of damage, burns, faulty components like bulging capacitors etc. When I first opened the case, I noticed a weird plastic/metallic looking film stuck across a couple of the legs of the IC SVI3205B (which is the power amplifier IC). Removing this made no difference to the issue. Continuity test showed that this material isn't conductive. I'm assuming this was never a problem but thought I'd mentioned this anyways. (I know the film looks little like what might be used in electrolytic capacitors, but I have spent a fair bit of time hunting for any blown caps but I see none.)
Scope output at various frequencies. I used an audio signal generator as source and then plugged my scope in parallel with a speaker to measure amplifier output. Source signal was always a sine wave at various different frequencies. 200Hz Right channel, distorted 200Hz Right channel, clean 800Hz Right channel, distorted 800Hz Right channel, clean 1.6KHz Right channel, distorted 1.6KHz Right channel, clean 3.2KHz Right channel, distorted 3.2KHz Right channel, clean 6KHz Right channel, distorted 6KHz Right channel, clean 12.8KHz Right channel, high volume distorted 12.8KHz Right channel at low volume, clean 12.8KHz Left channel, high volume clean It also seems like the signal starts distorting at lower volumes at lower frequencies and requires a lot more volume before it starts distorting at higher frequencies.
[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.
Circuit top shot (stitched, there is a fair bit of overlap so may look weird)
Circuit top shot
Preamp side of the circuit
Shot of the MOSFET module
Power amp module and the adjacent circuit
Shot of some of the transistors across the L and R channels
The main caps
A shot of the bottom of the circuit board showing the traces (stitched, again there is a fair bit of overlap so may look weird)
Circuit board bottom
Would appreciate any help I can get in hunting down the root cause here. Thank you for your time and help
I'm having trouble with my Rotel slot load CD player (RCD1570)
It's not pulling discs in properly, makes a little bit of noise and jerks/jolts them a bit.
It doesn't happen all the time, but mostly when inserting rather than ejecting a disc.
Item Condition: GOOD (see photos and description) Shipping Options: Pickup available but audition is not available.,Shipping is available at agreed cost. Suburb or Town: ELANORA HEIGHTS State: NSW Payment Method: EFT, Cash on Pickup, or PayPal Reason for selling: Not required Further information:
Serial number (A302)07090 Condition as per photos Videos show playing and fast forward. Rewinds fine too. Counter resets to 000 All lights up as it should Eject button doesn't work (apparently a pretty common issue) Original sales receipt from 22/8/1987 Full service receipt and paperwork from Nakamichi Australia from 18/1/1994 Been in (dry and cool) storage for 15 years. LINK TO GOOGLE DRIVE FOLDER: https://drive.google.com/drive/folders/1OQyx86iqM9qyO0iqAOc9dNVfU22mCOQH?usp=sharing
Plenty of photos in the Google Drive folder. Here's a couple to kick things off!
I just purchased a pair of Acoustic Research AR9LS tower speakers for a change from my JBL 4311Bs as I am predominantly using the latter for mixing at the moment.
I guess I am used to the JBLs and their rather precise monitor sound, but I am a bit unsure as to whether the AR9LS are performing as well as they should be. When listening to more rock oriented music (John McLaughlin's Devotion) I lose quite a bit of cymbals and the brightness of the snare, I don't seem to be getting super big bass/lower-mids on some records either (on the lower-pitched percussion on Ali Farka Toure & Ry Cooder's Talking Timbuktu which hits pretty hard on other setups). But I must say that listening to acoustic guitar instrumentals (Bert Jansch/John Renbourn) there is still a pretty lively, bright sound with a good amount of detail.
I am new to any kind of speaker repair. First thoughts is that maybe these need new caps. The cabinets are pretty worn and someone suggested that these were kept outside on a deck and that someone they know might have looked inside this exact pair and seen some corrosion--but this is pretty foreign to me.
If anyone has any tips on what might be the problem, or what I can do to investigate further that would be much appreciated. Happy to have a go at any repairs necessary!
Please be gentle with me! I'm unsure as to where to post this, as I'm seeking some advice as to the value of some 70's/80's hifi components that my father-in-law has asked me to help him sell.
EDIT - I'VE DONE A DEAL WITH MY FATHER-IN-LAW AND THE GEAR IS NOW OFFICIALLY MINE
I've searched that auction site, Gumtree and a number of forums' sales areas. The values vary a lot, and several of the components I can only find very limited listings for.
I have heaps of photos, videos, etc. I inspected each component yesterday to see how they worked, however I'm no expert.
The items cost quite a lot of money originally, and the deck and tonearm in particular seem to be highly sought-after.
Thankyou in advance!