SA 9500 Mk I intermittent crackling on LH channel - advice?

[oz-2]

Hello AK'ers
I have a small problem with my SA 9500 Mk 1

After about 10 - 15 mins of use I get a crackling sound on the LH channel. Observation of the L/S shows that this is accompanied by a large, gradually varying, dc voltage which moves the cone in and out over about 2 seconds. The sound and the movement of the cone then disappear for a while. This is only noticeable in no signal conditions. The problem reoccurs every ten to 15 mins or so but it isn't consistent.

Observations so far:
1. If I separate the control preamp and the power amp - no crackling.
2. The sound appears to be independent of volume control position i.e. it is always about the same loudness
3. It is always in the LH channel. If I turn the switch to Rev, or L etc it still comes out of the LH channel.
4. It appears to happen on any input selection i.e. aux 1 and 2 or tuner etc.
5. Tone controls are switched off.
6. LF and HF cut are Off

As the problem does NOT occur when the pre and per amps are separated I conclude that the per amp section is fine.
As the volume control does not seem to vary the crackling I assume that the problem is after the vol control. My suspicions are on q3 and q5 on the Switch Ct ass AWS-082.

Alternatively I think that q1, q3, q5 on awg-032 could be suspect.

Does anyone else have a constructive opinion that might help please? Is my reasoning ok?

 

[QSilver]

If disconnecting the control amp (preamp) from the power amp stops the crackling, you probably have an issue in the pre amp section after the volume control. If its the left channel only, your probably looking at Q3 and Q5 on the schematic as both of these are known to fail like this.

Q3 is a 2SA725 - replaceable with a KSA992FBU but note that the orientation of the new transistor MAY NOT BE THE SAME as the original.

Q5 is a KSC1312 - replaceable with a KSC1845FTA IIRC. Again, the orientation of the new transistor is likely not the same as the original.

 

[oz-2]

Thank Qsilver. I will get her on the "bench" (dining table!) and perform an extraction and a filling.

 

[oz-2]

Hi All
sorry about the wait on news. I have stripped her down and replaced the transistors Q3, Q4, Q5, Q6. I didn't touch Q1 or Q2.
The fault seems to have gone although there is still a whisper in the lh channel every now and then - but the large deflection of the bass driver has cleared.

But I now have a further question which I would be grateful for a guru to answer if they have time.

It appears hat the lh channel is quieter than the rh channel. To obtain "proper balance" I have to adjust the balance control to about 10 o'clock. I did wonder about setting up channel balance at the time of the little operation but didn't attend to it - so it could mean a further strip down and an adjustment - but to what?

Is there a way (without a 'scope) of injecting a signal and measuring the outputs? Would playing pink noise, or white noise be a suitable comparison reference and if it is out of balance what should I adjust? I did think of testing the transistors I put in for hfe and trying to match them but didn't - perhaps that would have been a good idea.

Any thoughts? Anyone? Thanks for any help received

 

[QSilver]

It sounds like you've fixed the pre-amp but the power amp might still be unwell. To have an imbalance like that means something isn't right. You do no usually have to "balance the channels" ever. Have you measured the DC voltage present at the speaker terminals? I would suggest that changing Q1 through to Q4 with KSA992's might help as these are the same bad transistors you've changed in the pre-amp stage and control the signal being fed into the amplifier.

 

[markthefixer]

Check ALL the switch positions, and exercise them all, then check again. Pay CLOSE attention as to what happens to the "balance" when the MODE switch
is rotated through rev, stereo, L+R(mono), L and R positions. Filters off, TONE off. Don't forget the attenuator switch positions as well.
Problem still there?
Step 1 - thorough deoxiting of every switch (not ALL are on the front panel) and pot, then faderlube at least the pots if not the switches as well.
Step 2 (IF necessary) - check bass vs treble on both channels. See if equal on individual channel. Dying caps pass less bass.


Also there are 2sa726's (if q1, q2, q3, q4 are 2sa763's they are ok) in the power amp as well as POSSIBLE 2sc1451's (q7,q8 in power amp could also be 2sc1628 which are ok) which should be replaced (ticking time bomb) with ksc3503.

 

[oz-2]

thanks all. Will run through those checks Mark asap.
The amp has been recapped c3 years ago right through. I replaced the q1234in the power amp and the 1451's while I was at it last year.

I will get onto the issues asap and let you all know.

Thank you for giving the time and advice. Appreciated.
David

 

[oz-2]

Check ALL the switch positions, and exercise them all, then check again. Pay CLOSE attention as to what happens to the "balance" when the MODE switch
is rotated through rev, stereo, L+R(mono), L and R positions. Filters off, TONE off. Don't forget the attenuator switch positions as well.
Problem still there?
Step 1 - thorough deoxiting of every switch (not ALL are on the front panel) and pot, then faderlube at least the pots if not the switches as well.
Step 2 (IF necessary) - check bass vs treble on both channels. See if equal on individual channel. Dying caps pass less bass.


Also there are 2sa726's (if q1, q2, q3, q4 are 2sa763's they are ok) in the power amp as well as POSSIBLE 2sc1451's (q7,q8 in power amp could also be 2sc1628 which are ok) which should be replaced (ticking time bomb) with ksc3503.

Ok first test using HiFi News Test record pink noise at -20db into L & R channels.
All tone controls OFF
FILTERS OFF
ATTENUATOR @ -15db and then 0dB. Same result.

L spkr quieter than R spkr on STEREO
R spkr quieter than L spkr on REV
Seems equal on L+R
Both spkrs quieter on L
Both spkrs louder on R

After 10mins warm up dc voltages under no signal conditions -9 to -12mV on both spkr terminals l and r with +ve meter on red terminals.

I have rotated all switches. No thumps or scratches. Have rotated balance fully this way and that. No scratches.
Similar effect from tone generator fed through Aux 2.
Any pointers appreciated.
David

 

[oz-2]

Hi Just to give some closure to this.

Following Mark's advice about the stereo/rev/l/r/etc switch and after having exercised it a few times the balance problem seems to have gone. Needless to say I'm pleased and thank you to everyone who helped me try to pin this one down.

David