Pioneer A-9 Biasing instructions wrong?

[QSilver]

I'm going through the procedures for setting this bias on an A-9 and having some trouble.

Some history: I got it as a non working piece some months back and found it had many poor repairs in the all the regulator boards. I've now been right through the whole amplifier and slowly and surely repaired each fault. The last board to work on was the power amp although I had made a point to replace the trimmers due to the horror stories of these going open once disturbed.

Once I got a good look at the board for recap, I saw that the 2SC pre driver in one channel has been replaced with a TO-92 style transistor and the fusible resistors around it replaced.

I replaced both pre-drivers with 2SC2383 and 2SA1013 and the resistors with fusible resistors of the smae rating and powered it up. Tried setting the first stage of idle current without issue but the undisturbed channel wouldn't even start to get idle current until over half way around the trimmer.

I then replaced its pre-drivers too with the same set. My logic for selecting these transistors is they are used in similar amplifiers with NSA circuits such as the M-90 or A-77X but in these two cases the NSA circuit is within an IC.

Now both channels get idle current at rought the same places on the trimmers but here is my problem.

The service maual calls for 50mV on the first set of trimmers and 75mV for the second set. Every time I bias it for 75mV it drops it back down to 72mV or so within two minutes. I then increase it to 75 again, and it throttles back. If left cold and then powered back up the idle will climb over 100mV untuil the amp warms up.

In the service manual is doesn't mention any pre-warming of the amplifier it just says switch it on and set the idle.

I there a typo? We've had this discussion before about the SX-D5000 whose service manual calls for a whopping 120mV followed by 150mV and this was deemed to be incorrect. Is it another similar case? and would that account for the amp constantly try to push the idle current to 72mV as I keep lifting it to 75mV?

Hope someone can help me figure this out as I'm not sure I should believe the manual anymore!

 

[Tangerine]

Hello,

Was there an answer to your question on this as I have an A-9 that was going into protection now and again, to the point it's full time now "red square" only.
I wanted to check the idle voltages before I did anything else as a pointer towards what the fault may be.

Cheers.

 

[jgmacv]

Wow, what a coincidence, I made the service manual adjustments to my A-9 Tuesday, no issues, all good. Last night I did the same with my SX-D5000, and adjusted at the 120Mv and 150Mv as per the service manual. What are the correct settings (i'm thinking same as the SX-D7000, 56Mv and 70Mv)????? Sorry about going off track from the A-9, I also adjusted per service manual at 50Mv and 75Mv.

 

[QSilver]

Did you wait for the A-9 to heat up before adjusting? Once you had made the adjustments did you watch to see if the idle current changed after you were finished?

 

[jgmacv]

Did you wait for the A-9 to heat up before adjusting? Once you had made the adjustments did you watch to see if the idle current changed after you were finished?

Yes, I let it warm up, then made sure it remained stable before disconnecting DMM. Also, I tried finding a thread about the SX-D5000 adjustments, couldn't find anything. I set mine at the 120Mv and 150Mv. Is this correct?

 

[QSilver]

When I try that, I get it to 75, then it starts cutting it back down again, each time I increase, it cuts back and doesn't warm up either. I'm thinking it might be something to do with pre-driver transistors...

With the SX-D5000, I duscussed this on another thread with rcs16 (I think). He thought that was a typo and that the amp design was very similar to the SA-8800 etc. So, the idle current should be set to 56mV on first adjustment and then 72mV on the second. Quite a difference! I was commenting on how good the SX-D5000 sounded and he suggested it was because the excess idle current was pushing it closer towards class A operation.

I can't remember if I set mine to 150mV or 72mV...

 

[jgmacv]

I've got three of them, the first I adjusted at the 120mV & 150mV. I guess i'll run it at those settings, and see how it does. I won't mess with the others until after that. I'll keep you posted.

 

[Francescone]

QSilver, hai lo stesso problema sia con i toni inseriti che disinseriti? Se il problema si presenta solo con i toni disinseriti, regola il bias con il connettore del segnale staccato in ingresso al modulo pre-driver. QSilver, do you have the same problem with both on and off tones? If the problem occurs only with the tones off, adjust the bias with the unplugged signal connector entering the pre-driver module.

 

[QSilver]

Hi, I'm not sure thats the same amp as the Tone on/off switch here is the line straight switch. I understand your idea with the pre-amp possibly effecting the power amp balance. However, this amplifier uses a DC servo circuit to automatically cancel out DC offset. The pre-amp shouldn't be able to effect the bias at all as its an internal signal that the audio signal effectively floats on.

As a followup to this thread, I did end up replacing the TO-92L case transistors with TO-126 style transistors on the board and re-biased. I was able to get the correct bias in the end, however it took around an hour to get it all to settle down. It seems to be very sensitive to thermal drift.

 

[markthefixer]

Resurrecting this thread.

Your only transistor changes were the "big board" pre-drivers (1.8v base voltage)?
Q29, Q30 2sc1904a TO-126 >>> 2sc2383 to-92L >>> ksa1381 ?
&
Q31, Q32 2sa899a TO-126 >>> 2sa1013 to-92L >>> ksc3503 ?

I am going to come back to this post, editing in transistor and amplifier specifications, so it's all in one spot.

 

[QSilver]

Here's what got changed:

I will use the same transistor to Q number scheme that Pioneer do on their service manuals.

Transistor No. >>> WAS >>> IS
Q9~12, 15, 16 >>>2SC1915>>>KSC2690
Q13, 14 >>>2SA905>>>KSA1220
Q17, 18, 23, 24>>>2SC1400>>>KSC1845
Q19~22>>>2SA750>>>KSA992
Q25, 26>>>2SA872A>>>KSA1381
Q27,28>>>2SC1775A>>>KSC3503

I also renewed all the diodes as far as I can remember. All zeners were definitely renewed.

10E2 diodes were replaced with 1N4004,
1S2471 diodes were replaced with 1N4148
I haven't marked down replacing Q7 and Q8 but if I did, they would have been KSC1845. The rest is unchanged.

All my changes were on the big board, GWH-148. I left the "Drive Ass'y's" as they were with whatever drivers they had in from the factory. I removed the boards to check everything was ok and then put them back in.