Odd Pioneer preamp IC

[Ken Weitzel]

I found a Pioneer A-X340 amp, it's a rather nice unit someone brought from
Europe and I'm trying to convert it to 110 volts. I was able to find
110-volt replacements for the transformers, but when powered the overload
protector prevents the speakers from energizing. The DC bias voltages are
way out of whack (+ or -40VDC on the bases of all 4 output transistors, for
example), but nothing appears to be shorted. For troubleshooting purposes, I
need to replace or at least substitute two preamp IC's labeled PA0016. I
found a service manual but it does not give a detailed pinout of the IC or
specify what voltages it should have on it. Nor can I can't find any
datasheets online, and I suspect the IC must have a more standard
equivalent.... but what is it?
Any advice would be much appreciated.

Hi Packrat...

Doesn't seem to be any schematics available for free; but if you're
stuck and have to pay, there's a .pdf available here for 15 bucks...


http://www.manualscenter.com/advanc...AX340&osCsid=c4a216560c4a58155504187c10de0f87

Take care.

Ken

 

[Arfa Daily]

I found a Pioneer A-X340 amp, it's a rather nice unit someone brought
from Europe and I'm trying to convert it to 110 volts. I was able to find
110-volt replacements for the transformers, but when powered the overload
protector prevents the speakers from energizing. The DC bias voltages are
way out of whack (+ or -40VDC on the bases of all 4 output transistors,
for example), but nothing appears to be shorted. For troubleshooting
purposes, I need to replace or at least substitute two preamp IC's labeled
PA0016. I found a service manual but it does not give a detailed pinout of
the IC or specify what voltages it should have on it. Nor can I can't find
any datasheets online, and I suspect the IC must have a more standard
equivalent.... but what is it?
Any advice would be much appreciated.

Those voltages are not necessarily wrong, it is the midpoint volts that are
important. This will likely be at the junction point of two very low value
resistors, which are often integrated into a single 3 pin item for each
channel on Pioneers. The midpoint from each channel, will be the actual
speaker output, so will head on up to the protection relay contacts. I would
start by looking at what is on the midpoint of each channel, to see if it is
really a problem in one of the output stages, that is causing the protect
circuit to remain inoperative.

As a slight aside, are the transformers that you've fitted, the genuine 110v
article for that model, or some 'appropriate' substitutes ? The reason I
ask, is that some protection circuits monitor power supply outputs, and
sometimes AC feeds from the power transformers, as well as the output
stages, so if anything was amiss with the replacement transformers, or your
wiring in of them, this might cause you a problem with the protect circuit
that doesn't really exist. Did you try feeding the amp with 240v before you
started on the conversion, to see if it originally worked, or were you
already aware that it didn't ?

Arfa

 

[Packrat]

I found a Pioneer A-X340 amp, it's a rather nice unit someone brought from
Europe and I'm trying to convert it to 110 volts. I was able to find
110-volt replacements for the transformers, but when powered the overload
protector prevents the speakers from energizing. The DC bias voltages are
way out of whack (+ or -40VDC on the bases of all 4 output transistors, for
example), but nothing appears to be shorted. For troubleshooting purposes, I
need to replace or at least substitute two preamp IC's labeled PA0016. I
found a service manual but it does not give a detailed pinout of the IC or
specify what voltages it should have on it. Nor can I can't find any
datasheets online, and I suspect the IC must have a more standard
equivalent.... but what is it?
Any advice would be much appreciated.

 

[Packrat]

Those voltages are not necessarily wrong, it is the midpoint volts that
are important. This will likely be at the junction point of two very low
value resistors, which are often integrated into a single 3 pin item for
each channel on Pioneers. The midpoint from each channel, will be the
actual speaker output, so will head on up to the protection relay
contacts. I would start by looking at what is on the midpoint of each
channel, to see if it is really a problem in one of the output stages,
that is causing the protect circuit to remain inoperative.

Those midpoint voltages you mentioned are way off, about 30VDC to be
exact. I spent quite a bit of time trying to figure out where that 30V was
coming from, and the problem only went away when I removed that preamp
IC..... so I'm wondering if the IC may be faulty. Or perhaps the previous
owner did something really stupid, like trying to apply AC into the speaker
terminals or something?

As a slight aside, are the transformers that you've fitted, the genuine
110v article for that model, or some 'appropriate' substitutes ? The
reason I ask, is that some protection circuits monitor power supply
outputs, and sometimes AC feeds from the power transformers, as well as
the output stages, so if anything was amiss with the replacement
transformers, or your wiring in of them, this might cause you a problem
with the protect circuit that doesn't really exist. Did you try feeding
the amp with 240v before you started on the conversion, to see if it
originally worked, or were you already aware that it didn't ?

Arfa

Unfortunately I didn't power it with 240 before converting, but I'm very
certain my conversion is correct because all the power supply voltages match
the schematic within 1 - 2%.

 

[Packrat]

Already bought it from www.user-manuals.com .... like I said it has
schematics and parts list but is not very detailed on voltages.

 

[Arfa Daily]

Those midpoint voltages you mentioned are way off, about 30VDC to be
exact. I spent quite a bit of time trying to figure out where that 30V was
coming from, and the problem only went away when I removed that preamp
IC..... so I'm wondering if the IC may be faulty. Or perhaps the previous
owner did something really stupid, like trying to apply AC into the
speaker terminals or something?

Unfortunately I didn't power it with 240 before converting, but I'm very
certain my conversion is correct because all the power supply voltages
match the schematic within 1 - 2%.

It's quite possible then that the preamp 'IC' is faulty. Is it one of those
hybrid voltage amplifier modules that look a bit like a small STK device ?
If so, then I have had them cause problems like this. The midpoint volts
should be zero, or close enough that it doesn't matter. A very large
midpoint offset such as you appear to have, is usually as a result of a
missing rail ( I'm assuming that you have both the HV+ and HV- rails making
it to the output transistors ), a short circuit output transistor with an
open circuit emitter resistor, or sometimes incorrect drive as you think
that you have. If the midpoint volts drop to zero when you remove the IC,
that would tend to suggest that you are right with your diagnosis. Good luck
with it.

Arfa

 

[Mark D. Zacharias]

It's quite possible then that the preamp 'IC' is faulty. Is it one of
those hybrid voltage amplifier modules that look a bit like a small STK
device ? If so, then I have had them cause problems like this. The
midpoint volts should be zero, or close enough that it doesn't matter. A
very large midpoint offset such as you appear to have, is usually as a
result of a missing rail ( I'm assuming that you have both the HV+ and HV-
rails making it to the output transistors ), a short circuit output
transistor with an open circuit emitter resistor, or sometimes incorrect
drive as you think that you have. If the midpoint volts drop to zero when
you remove the IC, that would tend to suggest that you are right with your
diagnosis. Good luck with it.

Arfa

The PA0016 is a 16 pin (IIRC) DIP IC. They do go bad, but usually only if
there has been an associated amplifier failure.

I'd still be looking for power supply issues, especially a floating ground.
For example a broken eyelet at a power transformer, where they solder to the
pins of the transformer.

Mark Z.

 

[Packrat]

As per my original post.... is there an equivalent IC I can replace this
with?

 

[Mark D. Zacharias]

Not that I'm aware of. I just get them from Pioneer.

Mark Z.

 

[Franc Zabkar]

I found a Pioneer A-X340 amp, it's a rather nice unit someone brought from
Europe and I'm trying to convert it to 110 volts. I was able to find
110-volt replacements for the transformers, but when powered the overload
protector prevents the speakers from energizing. The DC bias voltages are
way out of whack (+ or -40VDC on the bases of all 4 output transistors, for
example), but nothing appears to be shorted. For troubleshooting purposes, I
need to replace or at least substitute two preamp IC's labeled PA0016. I
found a service manual but it does not give a detailed pinout of the IC or
specify what voltages it should have on it. Nor can I can't find any
datasheets online, and I suspect the IC must have a more standard
equivalent.... but what is it?
Any advice would be much appreciated.

Is the manual available online? If not, can you scan the relevant
section of the circuit?

- Franc Zabkar