Odd Pioneer preamp IC

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

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 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

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.

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

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

Mark Z.

On Tue, 21 Aug 2007 19:39:23 -0700, "Packrat" put finger to keyboard and composed:

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

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.