Luxman L31 intermittent problem

[Depaj]

Hi!

I picked up a beautiful Luxman L31 with only one channel working.
When I opened it up I saw that the two fuses for the postitiv and negative rails of the faulty channel were blown. After replacing those I turned on the amp with a bulb tester and everything seemed ok, compared voltages between the two channels, checked for bias and dc offset and everything was spot on. I hooked up some suicide speakers and it sounded great. The caps had never been replaced so I took care of that. All the electrolytic caps were replaced with nichicon fg and es caps and the filter caps by slightly higher value kg caps.
Once again I checked bias and offset and than tested the amp which sounded much better. After maybe 30 minutes al the sudden there was quite a loud crackling on the channel that had blown its fuses before so I turned of the amp. After a few minutes I turned it on again with bulb tester and suicide speakers and everything was fine again. After a few minutes some crackling appeared for a few seconds and went away again, than slightly louder and a little longer, this went on for a while. Paired with the crackling I measured some AC and some DC on the output sometimes up to 1v.
Touching the volume knob or any other potentiometer doesn't influence the crackling.

Any suggestions on what could be the cause ?

 

[partyface86]

Either a leaky transistor or could be the pots for the bias/offset. Try to spray the pots with deoxit, then move the swipers left and right, if you don`t have the service manual try to remember the position of the bias pot, or measure the bias on the good channel to have a reference when you reset it.

I had the same problem with a Sansui AU-222. If this doesn`t resolve the problem, then one of the transistors is leaky

 

[Depaj]

I might as well change the pots, they are the original ones and might not be very reliable any more.

I would love to keep the original power transistors, any chance those aren't the cause ?

Thanks for your reply

 

[partyface86]

TO-3 transistor usually are very reliable. Could also be the differential input pair.

 

[Depaj]

That's what I was hoping for. I will change the pots first and cross my fingers.

 

[Welcome]

Most of the smaller transistors on old Luxman gear tend to stop working. Definitely the cause here. Replace, and begin with the diff pair as already suggested. KSA992 and KSC1845 will work well in most cases.

 

[partyface86]

Let me know if you have solved the problem.

 

[Depaj]

I won't be able to get the replacement tomorrow but I will post a reply once I changed them.

 

[Depaj]

. KSA992 and KSC1845 will work well in most cases.

How come the original pair is made of two 2SA750 transistors ?

 

[partyface86]

The input differential amplifier are 2 PNP transistors with matched gain, they are not complementary.

 

[Depaj]

that's what I thought but Welcome seems to suggest to put a pair of complementary pnp/npn transistors.

 

[Depaj]

I checked and the actual transistors are 2 2sa763, would BC559 be good replacements ?

 

[partyface86]

They should be OK. BC559 were used a lot in audio amplifiers. Be careful with the pinouts though, as on the BCs are 1.collector 2.base 3.emitter and on the 2SAs are 1.emitter 2.collector 3.base. Also check the Hfe for both, the BCs have 3 types, A/B/C, the 2SAs have 4, 3/4/5/6

 

[Welcome]

that's what I thought but Welcome seems to suggest to put a pair of complementary pnp/npn transistors.

No, I meant KSA992 or KSC1845, depending on whether you need NPN or PNP. You'll need both, because you need to replace all small signal transistors in there. Pre-amp too.

 

[Welcome]

I checked and the actual transistors are 2 2sa763, would BC559 be good replacements ?

Probably not. BC transistors tend to have low gain and high noise, which is not desirable in a a differential pair or other audio applications. It's also heresy to put BC junk in a japanese amplifier.

 

[Depaj]

I recently restored a Sansui au-222 and a lot of people suggested to replace the noisy 2SA871 by BC550, is that also considered heresy ?!!

Okay Welcome, so KSA992 for the diff input pair and you would also replace the 2SA836 by those and 2SC1345 by KSC1845 in the preamp section, is that it ?

What about the 2SC1951 and 2SC945 and the two driver transistors ?

 

[Depaj]

the 2SC945 is the vbe multiplier.

 

[partyface86]

First change those trimpots, if that doesn`t fix the problem only then I would consider changing any transistor

 

[Depaj]

I replaced the trimpots but that didn't change anything so I also changed the diff input transistors and the 2sc1951 by a 2sc2230 which had already been done on the left channel.
The problem is that now I only manage to get 15mA of bias instead of the 45mA I should get. Is there a problem or is it just a matter of slightly adjusting the vbe with those new transistors ?

 

[partyface86]

Did you get the right value trimpot, 330 ohm for the bias and 10 kohm for offset? Try shorting out the input of the main amp, if that stops the crackling, it could be that the tone control preamp transistor is acting up.