Tone Control Gone Wild...(could use some guidance)

EchoWars · 21st November 2005, 05:56 AM

Got a Pioneer 9500 integrated here that loses a considerable bit of output on the left channel when the tone controls are enabled. I was hoping for the usual dirty controls, but it indeed does seem to be a bit deeper than that... Attached is a scan (big for clarity) that shows what I see...

Fed a 200mV P-P 1KHz signal, I get a 1.2V output on the 'Defeat Out' terminals of both channels, so the FET input and the second transistor seem to be fine. However, the output at the 'Defeat Out' terminals is only about 430mV P-P on the bad channel, wheras it holds steady at 1.2V on the good one.

Both Q5 and Q6 have been changed to new Zetex trannies, the switches cleaned, and the output signal changes very little no matter where the control is set.

The clue, it seems, is that I see a 230mV P-P signal on the base of Q5 on the bad channel, and I almost can't see the signal on the base transistor (Q6) that is working as it should.

What am I missing?

gmphadte · 21st November 2005, 06:35 AM

After confirming the transistor is good, check for bad ceramic capacitors. They can lead to the wrong bias u get.

Gajanan Phadte

EchoWars · 21st November 2005, 07:29 AM

Well, I pulled C31 (the 10pf cap on Q5) and it didn't change anything.

Any ideas on how to go about tracking the problem down?

owen · 21st November 2005, 07:58 AM

Hiya,
after looking at the circuit diagram, you need to check around Q3, not Q5, as the differential in voltages is apparent before Q5... Infact I'd be looking closely at C13 - which judging by the voltage spec is an electrolytic, and almost guaranteed to be defective...

As a general guide - replace all electrolytics, then replace all wax based capacitors and then replace all carbon composition resistors - these are by far the most troublesome componants in any piece of 'older' equipment!

Hope this helps

Owen

EchoWars · 21st November 2005, 08:28 AM

I had checked the voltages on the far side of the cap, but did not show it on the schematic. Here's an update:

The cap appears OK, but as you can see what is on the far side of R23 (L chan) does not match what is on the far side of R24 (R chan). I can barely see the signal at the junction of R24 and R26, but I get 300mV worth of signal on the bad channel at that same place.

I would gladly recap if I thought that was the root problem, but I am doing this for a buddy (who isn't likely to pay well

), and rather than simply shotgun this thing, I'd like to see what is wrong. Plus, if it turns out that there is a switch problem, then the many hours of replacing parts would be a serious waste of time.

There's a bazillion resistors on this board..I couldn't imagine replacing them all.

gmphadte · 21st November 2005, 08:56 AM

Remove/open the base of all the transistors and start comparing respective L and R ch resistor values using a DMM. Just compare...

Gajanan Phadte

gmphadte · 21st November 2005, 09:18 AM

If the problem occurs at all the positions, then ignore those resistors (on the switches.)

janneman · 21st November 2005, 09:24 AM

No offense meant, but I hope we can do something more focused than wholesale blind replacement of parts and hope for a lucky shot...

One thing to clear first is your measurement frequency. What is that? How are the switches set? Try to measure with say 10kHz, that would exercise only the hi controls, see what that gives. If it is OK, then we know it is in the lf tone circuit. Likewise, try measuring with 100Hz to see what the lf controls do.
If you are in defeat out, and all controls are level, is it OK then?

Since the AC at the junction of R21 & R41 is OK, the error appears to be in the chain around S1-2/2F, S2-2/2 and S1-2/2R.
A thing to do is to disconnect the right hand pin of R39 see if that restores the base AC of Q5. Likewise, with the top pin of R35. Do this with sensible test frequencies as suggested above.

Looking forward to your results!

Jan Didden

EchoWars · 21st November 2005, 09:58 AM

Quote:

One thing to clear first is your measurement frequency. What is that?

200mV P-P at 1KHz (first post

)

Quote:

How are the switches set?

I have them set 'flat', but I can't seem to determine just what setting this corresponds to on the schematic. I do know that it doesn't matter where any of the tone switches are set...I can see small amplitude changes as one might expect from a tone control, but no significant change in the output amplitude.

Quote:

Try to measure with say 10kHz, that would exercise only the hi controls, see what that gives. If it is OK, then we know it is in the lf tone circuit. Likewise, try measuring with 100Hz to see what the lf controls do.

I'll see what's up at different frequencies, but I'm betting that it doesn't matter. I'll report back.

Quote:

If you are in defeat out, and all controls are level, is it OK then?

Check the schematic again...it is a little unusual in that a signal is always present at the 'tone in' position of the switch, and at the 'defeat' position. On the right hand side you can see the outputs. The switch is on the 'next' board, and simply chooses which signal path you like...vanilla or chocolate.

I really appreciate your input...I'm losing sleep over this and it's driving me crazy...

EchoWars · 21st November 2005, 10:14 AM

Well, I disconnected one end of R35, and the output jumped right up to where it is supposed to be.

I meant to mention that the influence of the tone controls on the signal before was pretty minimal (shouldn't be suprised, since it is obviously broken), but after disconnecting R35, the treble works like it should...boosting and cutting the signal in large amounts just like the other channel. 'Course now, there's no bass control...

Any way to nail it down further?

21st November 2005, 05:56 AM	#1
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	Tone Control Gone Wild...(could use some guidance) Got a Pioneer 9500 integrated here that loses a considerable bit of output on the left channel when the tone controls are enabled. I was hoping for the usual dirty controls, but it indeed does seem to be a bit deeper than that... Attached is a scan (big for clarity) that shows what I see... Fed a 200mV P-P 1KHz signal, I get a 1.2V output on the 'Defeat Out' terminals of both channels, so the FET input and the second transistor seem to be fine. However, the output at the 'Defeat Out' terminals is only about 430mV P-P on the bad channel, wheras it holds steady at 1.2V on the good one. Both Q5 and Q6 have been changed to new Zetex trannies, the switches cleaned, and the output signal changes very little no matter where the control is set. The clue, it seems, is that I see a 230mV P-P signal on the base of Q5 on the bad channel, and I almost can't see the signal on the base transistor (Q6) that is working as it should. What am I missing?

21st November 2005, 09:24 AM	#8
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	No offense meant, but I hope we can do something more focused than wholesale blind replacement of parts and hope for a lucky shot... One thing to clear first is your measurement frequency. What is that? How are the switches set? Try to measure with say 10kHz, that would exercise only the hi controls, see what that gives. If it is OK, then we know it is in the lf tone circuit. Likewise, try measuring with 100Hz to see what the lf controls do. If you are in defeat out, and all controls are level, is it OK then? Since the AC at the junction of R21 & R41 is OK, the error appears to be in the chain around S1-2/2F, S2-2/2 and S1-2/2R. A thing to do is to disconnect the right hand pin of R39 see if that restores the base AC of Q5. Likewise, with the top pin of R35. Do this with sensible test frequencies as suggested above. Looking forward to your results! Jan Didden __________________ /Another new issue: Linear Audio Volume 3!

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21st November 2005, 06:35 AM	#2
gmphadte diyAudio Member Join Date: Jul 2003 Location: Dona paula, Goa	After confirming the transistor is good, check for bad ceramic capacitors. They can lead to the wrong bias u get. Gajanan Phadte

21st November 2005, 07:29 AM	#3
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	Well, I pulled C31 (the 10pf cap on Q5) and it didn't change anything. Any ideas on how to go about tracking the problem down?

21st November 2005, 07:58 AM	#4
owen diyAudio Member Join Date: Mar 2004 Location: Shilton	Hiya, after looking at the circuit diagram, you need to check around Q3, *not* Q5, as the differential in voltages is apparent *before* Q5... Infact I'd be looking closely at C13 - which judging by the voltage spec is an electrolytic, and almost guaranteed to be defective... As a general guide - replace all electrolytics, then replace all wax based capacitors and then replace all carbon composition resistors - these are by far the most troublesome componants in any piece of 'older' equipment! Hope this helps Owen

21st November 2005, 08:28 AM	#5
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	I had checked the voltages on the far side of the cap, but did not show it on the schematic. Here's an update: The cap appears OK, but as you can see what is on the far side of R23 (L chan) does not match what is on the far side of R24 (R chan). I can barely see the signal at the junction of R24 and R26, but I get 300mV worth of signal on the bad channel at that same place. I would gladly recap if I thought that was the root problem, but I am doing this for a buddy (who isn't likely to pay well ), and rather than simply shotgun this thing, I'd like to see what is wrong. Plus, if it turns out that there is a switch problem, then the many hours of replacing parts would be a serious waste of time. There's a bazillion resistors on this board..I couldn't imagine replacing them all.

21st November 2005, 08:56 AM	#6
gmphadte diyAudio Member Join Date: Jul 2003 Location: Dona paula, Goa	Remove/open the base of all the transistors and start comparing respective L and R ch resistor values using a DMM. Just compare... Gajanan Phadte

21st November 2005, 09:18 AM	#7
gmphadte diyAudio Member Join Date: Jul 2003 Location: Dona paula, Goa	If the problem occurs at all the positions, then ignore those resistors (on the switches.)

21st November 2005, 10:14 AM	#10
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	Well, I disconnected one end of R35, and the output jumped right up to where it is supposed to be. I meant to mention that the influence of the tone controls on the signal before was pretty minimal (shouldn't be suprised, since it is obviously broken), but after disconnecting R35, the treble works like it should...boosting and cutting the signal in large amounts just like the other channel. 'Course now, there's no bass control... Any way to nail it down further?

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