I have an on-going thread about a second Pioneer A-70 amp. I successfully repaired one, this second one is being a little more challenging.
Kinda stuck here, I think the OP-Amp may be shot - perhaps not...
The problem is the "line-straight" mode. If the Amp is turned on with out being in "line-straight" mode, all seems ok and it comes out of protection. If you push the line-straight button, the amp drops into protection and R23 circled in red cooks off produces a nice stream of smoke. It's a 22Ohm 1/2W resistor.
I have checked the switch and it seems to be in correct working order. It's a two-sided 12 pin switch and I get continuity just where I should depending on the position of the push button. Side 1 is in play and is pictured below as S1-2-2. When line-straight is in the off position the equalizer circuit is turned on which allows for the use of the tone control sliders, loudness and sub-sonic functions. The amp seems to operate correctly in this mode - at least it will produce audio to the speakers and the correct voltages are seen on pins 4 and 8 of the op-amp (-15V and 15V) respectively. Other voltages look correct as well
Here is a cut down representative schematic of the surrounding circuitry:
Q63 is the two channel OP Amp (It's a uPC4558C-P)
So, do you think it's the op-amp or is the problem elsewhere - what is causing R23 to smoke?
Thoughts? Thanks for looking.
Kinda stuck here, I think the OP-Amp may be shot - perhaps not...
The problem is the "line-straight" mode. If the Amp is turned on with out being in "line-straight" mode, all seems ok and it comes out of protection. If you push the line-straight button, the amp drops into protection and R23 circled in red cooks off produces a nice stream of smoke. It's a 22Ohm 1/2W resistor.
I have checked the switch and it seems to be in correct working order. It's a two-sided 12 pin switch and I get continuity just where I should depending on the position of the push button. Side 1 is in play and is pictured below as S1-2-2. When line-straight is in the off position the equalizer circuit is turned on which allows for the use of the tone control sliders, loudness and sub-sonic functions. The amp seems to operate correctly in this mode - at least it will produce audio to the speakers and the correct voltages are seen on pins 4 and 8 of the op-amp (-15V and 15V) respectively. Other voltages look correct as well
Here is a cut down representative schematic of the surrounding circuitry:
Q63 is the two channel OP Amp (It's a uPC4558C-P)
So, do you think it's the op-amp or is the problem elsewhere - what is causing R23 to smoke?
Thoughts? Thanks for looking.
Hi Jan,
I had cleaned the switch not long after acquiring the amp. The switch and it's pins are accessible and I have checked continuity with the switch button on and off as the schematic suggests but there is definitely a short somewhere when that button is pushed
I had cleaned the switch not long after acquiring the amp. The switch and it's pins are accessible and I have checked continuity with the switch button on and off as the schematic suggests but there is definitely a short somewhere when that button is pushed
I agree that there's a short somewhere that gets introduced when the switch is in the line-straight position. But how best to find the spurious connection?
I'm gong to ask several questions, but I'll tell you at the outset where I'm headed: I want to explore the semi-radical notion of leaving the switch in the problem state where the resistor smokes, on the assumption it will be easier to locate the defect. But I want to avoid any damage to the amp other than to R23, which you probably intend to replace anyway.
I surmise the amp functions without obvious damage when returned to tone-control mode? I suspect Q63 opamp is ok if the other channel works and if the left channel returns correctly to near 0V when tone control position is selected. Please advise if this not the case.
For perspective, I note that 0.5W dissipation in 220R corresponds to about 10.5VDC applied voltage, and to make obvious R23 smoke will require even more voltage.
Is R23 surviving the smoke producing sessions? Can you make a quick measurement of voltage across R23 while it smokes? Perhaps you could install a temporary higher Wattage while you troubleshoot? But don't let me goad you into a test method that jeopardizes your amp.
If you become comfortable with pursuing this approach, I suggest trying this probing technique: tie one meter probe to the powered end of R23 and poke with the other meter probe until you find a circuit node that yields 0V. That's likely the shorted path.
Good luck!
I'm gong to ask several questions, but I'll tell you at the outset where I'm headed: I want to explore the semi-radical notion of leaving the switch in the problem state where the resistor smokes, on the assumption it will be easier to locate the defect. But I want to avoid any damage to the amp other than to R23, which you probably intend to replace anyway.
I surmise the amp functions without obvious damage when returned to tone-control mode? I suspect Q63 opamp is ok if the other channel works and if the left channel returns correctly to near 0V when tone control position is selected. Please advise if this not the case.
For perspective, I note that 0.5W dissipation in 220R corresponds to about 10.5VDC applied voltage, and to make obvious R23 smoke will require even more voltage.
Is R23 surviving the smoke producing sessions? Can you make a quick measurement of voltage across R23 while it smokes? Perhaps you could install a temporary higher Wattage while you troubleshoot? But don't let me goad you into a test method that jeopardizes your amp.
If you become comfortable with pursuing this approach, I suggest trying this probing technique: tie one meter probe to the powered end of R23 and poke with the other meter probe until you find a circuit node that yields 0V. That's likely the shorted path.
Good luck!
Most often, people assume, that because they "fixed/cleaned" it, it must be something other.
I would desolder the switch and remeasuere it pin by pin just to make sure. I had a similar fault on a Luxman amp once,
and althoug the Luxman also had a contact in the switch to send dc to a control lamp, it was indded the switch leaking
the dc into the circuit on one channel.
Just saying 😉
So, I have the amp on a dim bulb tester (DBT) with a 100Watt light bulb.
At this point though, R23 has been toasted and is now in an open state. It does not survive puff the magic dragon if the amp is left on.
When I turn it on in NON line straight (signal going through tone control circuitry), the amp the amp settles, the DBT bulb goes dim, and the amp comes out of protection with a nice audible click.
When I turn the amp on in Line Straight mode, the DBT bulb stays bright indicating a short and the amp does not come out of protection.
When in NON line straight mode the op-amp measures the following at the pins:
1 -123mV
2. 1.2mV
3. .2mV
4. -15V
5. 5mV
6. 1.8mV
7. -241mv
8. 15V
More to follow. I'll try to get some voltages in line straight mode.
At this point though, R23 has been toasted and is now in an open state. It does not survive puff the magic dragon if the amp is left on.
When I turn it on in NON line straight (signal going through tone control circuitry), the amp the amp settles, the DBT bulb goes dim, and the amp comes out of protection with a nice audible click.
When I turn the amp on in Line Straight mode, the DBT bulb stays bright indicating a short and the amp does not come out of protection.
When in NON line straight mode the op-amp measures the following at the pins:
1 -123mV
2. 1.2mV
3. .2mV
4. -15V
5. 5mV
6. 1.8mV
7. -241mv
8. 15V
More to follow. I'll try to get some voltages in line straight mode.
It may also be oscillation - do you have any scope to view what is actually going on during the switching phase (or during startup in line straight ON mode) on amplifier output?
Also, have you measured R25 actual value?
I ended up checking all of the resistors around that circuit, and as you might imagine, I had to desolder a led here and there to get correct readings. All resistors read good.
I did another round of deoxit for the switch in question.
Received new 220 Ohm resistors from Mouser and replaced the smoked one.
Left it out of line straight mode, let the amp settle and come out of protection. The DBT was nice and dim.
I checked the voltages on both sides of that resistor and found them to be at or near zero which I believe is correct for no-load and no signal.
Pushed the button to put it into line straight mode and this time there was no smoke and the DBT did not change. Checked the voltages again and they were also on the mark.
At this point, I think the culprit may indeed be the switch.
I did another round of deoxit for the switch in question.
Received new 220 Ohm resistors from Mouser and replaced the smoked one.
Left it out of line straight mode, let the amp settle and come out of protection. The DBT was nice and dim.
I checked the voltages on both sides of that resistor and found them to be at or near zero which I believe is correct for no-load and no signal.
Pushed the button to put it into line straight mode and this time there was no smoke and the DBT did not change. Checked the voltages again and they were also on the mark.
At this point, I think the culprit may indeed be the switch.
Under no circumstances whatsoever do not take the switch apart and clean it properly 😉
Attachments
Not sure these are at all serviceable like the ones above. All three would have to come out for a total of 30 pins to unsolder. That metal bar is clamped onto all three. After a couple more dosed of DeOxit and working that switch it seems to be behaving now,
You are correct, I have tried to work on one of this type of tact switch but failed to reassemble it - components are too small inside and I do not have a microscope.
Unsoldering 30 pins is "trivial" - once you get the rhythm and hand movements correct - input selector switch sets have usually more 😉
Attachments
Last edited:
Yes, and after three bouts of cleaning, it's still not quite right. With no input, volume all the way down, in Line Straight mode, I can hear some static noise in the left channel. It's not present out of line straight mode.
I think it's going to be difficult to source one of those old switches.
I think it's going to be difficult to source one of those old switches.
Have you measured the contact resistance wit a DMM?
I would replace the signal path lytics as first step.
Do you have any kind of oscilloscope? If yes then you could get lucky and spot the stage where noise starts.
These types of switches I know at least in two dimensions (from ALPS) - the older ones I could not find, newer (and smaller) ones or similar copies can be found e.g. on eBay:
https://www.ebay.de/itm/195955424750
https://www.ebay.de/itm/256276569466
Perhaps the remaining problem is now noisy bass or treble pots. Do they "feel" noisy or erratic? Is noise absent with pot wipers at extreme ends? Maybe try cleaning them.
Good luck!
Good luck!
No not yet.
I just finished up replacing all of the 'lytics yesterday (except for the 4 big ones in the power supply) prior to powering on. Sadly, I have no O'Scope.
My guess at this time is it's the switch or perhaps one of those small caps (C27, C9, C801)?
This switch is two sided - left and right channel - 6 pins per side for a total of 12 pins. It's just like the one pictured in post #11.Only the left side is noisy and it's the same side that was smoking that resistor. I may have found a source here in the states. If the place has them, they are $25 per
Unfortunately you may have reached the situation where you need one 😉
Without a scope it will be quite difficult to find out what is going on at various points on the signal path when you press the switch .
I have used a Picoscope for many years in similar situations:
but you can also use soundcard as a scope provided that you place an attenuator in its input signal path (if you feed it with more than 2V then you may fry it).
Used analogue scopes (when you need to go down to 2mV per division level when looking for noise sources) are also not that costly on eBay.
C27 could be worth replacing to check if it has any effect.
Have you checked (for breakage) and resoldered the ribbon cable ends?
There is also quite a lot of reading in these forums about the paired transistors getting noisy over time - I wonder if shorting D27 (or 28) would eliminate possible noise from the input stage - without causing any damage to the first stage transistors...
Well, I was looking around for something to use as a probe to trace signal with. I don't have an oscilloscope (and it's been way too many years since I actually used one) all I have is a multi-meter. But, I do have a pair of old powered computer speakers. So, ground to chassis and then to ground on rca jack and one end of a clip to a channel on the rca jack and the other as a probe. With the Amp turned off and a portable CD player attached to the Aux input, I can trace the signal through all of the switches. Did not have a lot of time to really get into it tonight, but it seems to be doing the trick. I'll let y'all know how it goes.
I must say, that worked really well! With the amp turned off, I could trace the left and right channel signals from input all they up to the first transistor it goes to. With this amp, it's a long trip to those transistors. The signal run from the input jacks, through the equalizer (for phono), through the function switch, through the tape1 and tape2 switches, through the sub-sonic switch, through the muting switch, through the balance, and then through the volume, and finally into the components of the DC Servo. It gets a little confusing for this novice but it basically goes from there to the line-straight switch for optional tone-control.
So, the issue ended up being the Tape 1 switch. Looks like I need to go another round of cleaning and exercising the switches.
Just had the amp up and running with both channels clear and strong.
Thanks for the help guys!
So, the issue ended up being the Tape 1 switch. Looks like I need to go another round of cleaning and exercising the switches.
Just had the amp up and running with both channels clear and strong.
Thanks for the help guys!