• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Sansui AU-6900 wrong output transistors

Dom123

Dom123

Well-Known Member
Hi everyone,

I bought a non-working AU-6900 off the bay UK. It was advertised as "low output thru speakers and humming". Based on my recent experience with my AU-7900 I figured it shouldn't be too difficult to fix.

I was hoping it would be original inside, but unfortunately all 8 output transistors have been changed to BD317 / BD318 instead of B545A / D188A. The amp has also been recapped (except big filter caps) using mutliple brands, although the majority are Panasonic FC's on the F-2596 & F-2599.
The bipolar caps have been changed to axial bipolar caps, so standing upright, with one wire folded
downwards.

I thought this was a bad sign as I had a hunch that someone had re-capped the amp to fix a problem and didn't manage. However to my amazement I found the 22V/29V output wire connectors on the 2599 board had been mixed up, even though they are labelled orange & brown !

So I fired her up on the DBT, all good then onto full mains. Relay kicks in after a few seconds. The amp works well with no hum or distortion ! One channel is a little louder than the other, but otherwise sound is clean. On good thing is that every single small transistor, diode & resistor are original. I would have thought any mishap that resulted in the outputs to be changed would have cause collateral damage, but apparently not.

The amp measures well too - No issue to dial in 18mV bias, and the DC at the speakers is only 4mV & 6mV respectively.

So my plan is to change out the fuse resistors, VD1212 diodes and change some of the caps on the tone/filter section for better quality ones.
As the DC is very low I think I will leave the 2SA798's alone, and for that matter all small transitors as the amp seems to be hum free.

Can anyone please advise if the BD317/318's are suitable ? They seem to have a lower Ft than the originals. I know the conventional wisdom is to use MJ21193G & MJ21194G, but mouser seem to be out of them, plus I don't want to spend close to 80 Euros on a set. How about MJ15015G/15016G or MJ15003/MJ15004 ?

Also is there any issues in using axial bi polar capacitors in place of the usual radial bi-polar caps ?

Many thanks,
 
Register to hide this ad
Caps? as long as they're low esr and within spec. they're okay.
this is what I have on transistors: httpsaudiokarma.orgforumsindex.phpthreadsonce-more-sansui-transistor-substitutions.583009
 

Attachments

Thanks raycarr, super useful list for the future !

The amp sounds OK with BD317 / BD 318, so I was wondering if they can stay in there or whether I need to change them out.
On my AU-7900 I used MJ15015G/MJ15016G which seem to work perfectly.
 
I hear you on the transistors. I'd wait for someone else to chime in and google the transistor while adding substitute(s).
 
Dom123 said:
Hi everyone,

I bought a non-working AU-6900 off the bay UK. It was advertised as "low output thru speakers and humming". Based on my recent experience with my AU-7900 I figured it shouldn't be too difficult to fix.

I was hoping it would be original inside, but unfortunately all 8 output transistors have been changed to BD317 / BD318 instead of B545A / D188A. The amp has also been recapped (except big filter caps) using mutliple brands, although the majority are Panasonic FC's on the F-2596 & F-2599.
The bipolar caps have been changed to axial bipolar caps, so standing upright, with one wire folded
downwards.

I thought this was a bad sign as I had a hunch that someone had re-capped the amp to fix a problem and didn't manage. However to my amazement I found the 22V/29V output wire connectors on the 2599 board had been mixed up, even though they are labelled orange & brown !

So I fired her up on the DBT, all good then onto full mains. Relay kicks in after a few seconds. The amp works well with no hum or distortion ! One channel is a little louder than the other, but otherwise sound is clean. On good thing is that every single small transistor, diode & resistor are original. I would have thought any mishap that resulted in the outputs to be changed would have cause collateral damage, but apparently not.

The amp measures well too - No issue to dial in 18mV bias, and the DC at the speakers is only 4mV & 6mV respectively.

So my plan is to change out the fuse resistors, VD1212 diodes and change some of the caps on the tone/filter section for better quality ones.
As the DC is very low I think I will leave the 2SA798's alone, and for that matter all small transitors as the amp seems to be hum free.

Can anyone please advise if the BD317/318's are suitable ? They seem to have a lower Ft than the originals. I know the conventional wisdom is to use MJ21193G & MJ21194G, but mouser seem to be out of them, plus I don't want to spend close to 80 Euros on a set. How about MJ15015G/15016G or MJ15003/MJ15004 ?

Also is there any issues in using axial bi polar capacitors in place of the usual radial bi-polar caps ?

Many thanks,
Click to expand...
In these lower wattage Sansui’s, new comparable transistors are fine. But understand the Chinese knock offs make it harder to get the DC offset correct. I changed out new transistors in a 9090db for the originals due to voltage requirements. New transistors will burn out sooner because every Sansui I have owned is overpowered and run very hot. This will age cheaper transistors faster. Frankly I had a 9090db in the 80’s and 90’s and the comparable Toshiba transistors were $14.79 USD.The better ones were $26.49 USD. Now transistors are three bucks??? If the transistors you are buying are made in Europe and not Asia you are probably good putting in new ones. If they are too cheap in price, skip it and replace them with originals.
 
raycarr00 said:
Caps? as long as they're low esr and within spec. they're okay.
this is what I have on transistors: httpsaudiokarma.orgforumsindex.phpthreadsonce-more-sansui-transistor-substitutions.583009
Click to expand...
Sorry folks. If the output caps are 20+ years old they can drop out of spec and POOF!! DEAD Sansui. Always replace old caps. I have parted out several Sansui’s that had black crispy boards due to hot resistors and capacitors. And really? You are shaking the room with your Sansui and expecting to depend on 25-45 year old capacitors to pick up the peaks in the music you play. Replace them with the best quality output caps and enjoy for many years to come. Or choose Bzzzttt..POOF!! Dead Sansui.
 
The amp has been recapped entirely by a previous owner, except the 2 big filter caps & 4 caps in the phono equalizer section.
Nearly all caps are Panasonic FC, so I guess the question is how long have FC's been around ? 15 years ? Actually the solder looks fairly new so I guess quite recent, and I must say very tidely done.

For the TO3 output transistors the solder looks very old, so I guess these were changed many years ago. No way of knowing if these are fakes, but they are also identified with 8841 for the BD318 & 8831 for the BD317.

My question is are the BD317's & BD318's suitable ?

20221231_135155_resized.jpg 20221231_135155_resized.jpg
 
So not having received any answers, I decided to run the amp yesterday in my main system at quite loud volumes for a couple of hours.
Happy to announce that it runs well with no distortion or heat generation.

The sound is pretty good with really decent 3D stereo imagining, but just slightly veiled a little on some sources. But the music grabs you which is the main point.
Also the amp is completely quiet, so zero hum or noise.

Still no idea if those BD317/318's should stay in there. Whoever put them in must have done it many years ago. The solder is dark oxidised grey and very neatly done, even looking like the original solder.

Crazy to think this amp was sold as non working just because a couple of wires on the amp board were plugged onto the wrong pins !
 
Good to hear it's playing and sounding wel.
Was difficult to find information about those BD317/318.
Only thing was that they were Motorola originally. And that the fT of 1 MHz was noted as minimum and no curves on the datasheet.
 
Thanks GrisPato, I'll run it for a bit and see how I get on. For the moment it seems good & quieter than my 7900.

I have a new distraction - just bought a 771 in 100% original condition, never been tampered with. Great condition too , no scratches on the outer casing, knobs or front panel. Works well & radio reception is superb. Already have a plan to recap it - it looks really simple and easy to work on, and I love the way the F-1499 board simply lifts off.

I keep accumulating Sansui's (I have 6 now) - time to sell my Marantz 2235B (perfect condition) because the Sansui's simply sound better to me. I can't justify keeping it even though it looks really nice.
 
Been running it again this evening and one channel is definately louder than the other which is dissapointing.
Made a check with my AU-555A pre out/mains in etc and determined that the issue is on the power board of the AU-6900 and not the tone/pre amp.

I still have to change out the fuse resistors & VD1212's, but not sure what could be causing this. Apart from the TO3 output transistors, all other transistors are completely factory original. Any ideas where to check ? Maybe I need to start a systematically measure & compare all resistors & voltages right side & left side
 
Volume definately dropping gradually on the left channel.
Pretty confident this is nothing to do with any switch or potentiometer, but something affecting the gain at some stage on the power amplifier board.

Will start taking measurements. Maybe I need to change out those BD317/318's after all .....
 
Thanks John - to be honest I haven't had a chance to deep dive into it (I got distrated by my "new" 771) but will start tonight.
Will firstly double check again that this is definately related to the power amp board.

I get your point about the output transistors - There is a lingering doubt in my mind that these may be fakes, but I'm not so sure - they have been in there for many years.
Not sure how to go about - of course with one good channel this helps enormously with problem solving. Will check all resistors, diodes and transitors (unpowered)

In your experience, assuming this issue is on the power amp board, which components could cause these symptoms without causing any electrical short or open (no signal) symptoms ?
 
Firstly check the power supply voltages to see if any are out of spec' or collapsing, although I think 'all' on an AU-6900 are shared so this would affect both channels. You might have a reversed electrolytic capacitor (pulling too much current) and overheating a resistor, or a wrongly oriented transistor? Note: there are Fuse Resistors in the power amp of this amplifier - have these been replaced? - even if they test OK, I suggest replacing them.

Which input are you using? - the 'main in' to the power amplifier? - if you are then clearly there is something amiss in the power amp section or its power supply. By not using any of the preamp inputs you can fairly safely eliminate that from your diagnosis, so that has cut the problem in half at a stroke. Does the output return to the right level after the unit has been powered off for a while? - possibly indicating a component heating situation? You are in the fortunate position that you have a working channel, thus the problem has been halved again ;) - use this to compare voltage readings with.

The strategy is to analyse what the amplifier is doing, use the amplifier controls to see if you can influence the behaviour, reassess, and check your input source is not faulty - i.e. "Challenge Assumptions" & "Divide and Conquer" are the watch words. ;)
 
Last edited:
Dammit, I gave misleading information yesterday !

Just ran a proper trial with my 555A :

AU-6900 pre-amp into AU-555A power amp - Low volume & dull sound on left speaker also very nasty "plop" when switching the 555 on/off
AU-555A pre-amp into AU6900 power amp - Sound is well balanced left speaker - right speaker

All inputs the same : Aux, Tuner & Phono

So the issue is not the power amp, but upstream of the power amp.
The power supply is common for both channels so I don't think there is an issue there

As mentioned this amp has had all electrolytics recapped by a previous owner using a multitude of cap manufacturers + some relatively big axial caps in place of small BP caps with voltage ratings significanty higher than original. All small transistors are original.

My understanding is the equalizer board F2599 apart from containing the PS section & phono equalizer doesn't do much except feed the signal into the volume/Balance boards. Is my reasoning right ?

No switch movement, including tone-in/out, stereo/mono, speaker selection switch etc has any affect on the sound imbalance. In fact all switches & potentiometers work smoothly & silently.

There are a few electrolytics on the tone control board but not many elsewhere - I will start to check for any cap incorrectly installed or wrong value used. Could shotgun the 2ea SA726 & 8ea 2SC1313 with SA992's & SC1845's while i'm in there.

All caps elsewhere tend to be those green film flag type capacitors. Wouldn't know how to detect a bad one of those !

I have ordered some wima caps to replace the 0.47uf & 2.2uf axial bi-polar caps the previous owner installed, which look huge, although that leaves a couple on 10uf axial BP caps. Also have the 220ohm 1/4 W resistors on order to replace the fuse resistors
 
Been struggling with this volume imbalance . All caps have been installed seemingly correctly and can't see anything amiss with anything. all resitors look fine and also transitors.
Thought I'd measure the resistance of the volume potentiometer(s) for L & R channel today, and sure enough there was a large difference in both channels.

Then took a visual look at the wiper positions (from the top they are visible at the uppermost point of both potentiometers, and was surprised to see the wipers were not aligned with each other.
By carefully locking one wiper I could turn the volume knob which would result in one wiper rotating relative to the other. Adjusted both to get the same reisistance at the usual listening position range on the volume knob. Tested with the amp on, and hey presto the volume is equal on the R & L sides !

I was careful, and the volume works smoothly with no scratchiness or apparent damage.
Of course it may be better to take them of the board & try to rebuild them
 
All was well with my AU-6900, then stopped being well.
Turned on the amp on sunday and was nothing but hiss, spitting & popping. After agitating all knobs, the sound gradually came back all clear and worked well.

Then yesterday it was fine for 5 minutes, then suddenly loud spitting & popping like mad on both channels, not affected by volume or any movements of switches or knobs. Music still plays through both speakers, although it is drowned out by the spitting noise. This issue doesn't affect the relay functionning normally.

As a recap, this amp had been recapped by a previous owner but all transistors, resistors & diodes are original except for the 8 output transistors.

Not sure what the exact issue is at this stage, but my plan if to change out the fuse resistors, VD1212 diodes, then swap all 2SA726 & 2SC1313 transistors with KSA992 & KSC1845 and re-flow again all pin connector solders. The tone control board on this amp is only 2SA726 (2ea) & 2SC1313 (8ea). Not sure if it would be necessary to replace the ZD01 & 02 zener diodes (13V 0.5W)

Hope this fixes in the issue. In the meantime listening to my AU-7900, which is....... nice.
 
The AU-7900 IS nice! I’ve owned one since 1977. On your 6900, run your 555 pre out into the 6900 main in again, to assure yourself that the power amp is OK. If so, you can focus on the preamp.
The plan that you detailed, while it is a shotgun approach, is what you’ll end up doing anyway. After troubleshooting and isolating the problem, you’ll still want to replace the VD1212s, the fusible resistors, and the small signal transistors.
 
Reflowed a few solders & the amp started working. I guess there must have been a poor connection somewhere.
Anyway I changed out the 220omn fusible resistors and replaced the VD1212's. Everything working & sounding fine.

Then I sold it to a guy in part exchange for a AU-7900, which eventually turned out to be a total basket case with unspeakeable horrors inside !
He accepted to cancel the sale, and I went to pick up the AU-6900 yesterday after he had it for one week. Normally he never used it. It had just be transported that's all.

But now there is a more fundamental problem. The relay engages quickly, with a horible sound through the speakers, but then disengages after 2 secs, so going into protect mode. The DBT goes almost fully out so no shorts.

Not sure where to start here. Eveything looks OK on the board. The SA798 transistors are original, but the offset was low only one week ago on both channels (<15mV), so I never changed them out. In a state of confusion now - where should I start trying to diagnose the problem ?

As the amp had been transported I guess vibration /shock is a possible cause. Maybe it got bashed around by the guy, but everything including all solders look fine.
 
You must log in or register to reply here.
Back
Top Bottom