Pioneer SX-650 - Replacement of Phono Pre-Amp Boards with TA7136AP?

NHDoc

NHDoc

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A friend is having some trouble with intermittent noise in the phono stage of his SX-650 receiver and so I did a little research. It looks like the phono pre-amp came two ways on this model. Each channel has its own phono pre-amp. One version has a Toshiba TA7136P1 (7 Pin IC) and the other has a PC board with discrete components (identified as AWK-065 on the schematic) which are connected to the PC card via ribbon cable into the 7 pin hole that the Toshiba IC fits into. His has the AWK-065 type and I suspect that's where the noise is coming from. There were at least three versions of the IC, the TA7136P, TA7136P1 and TA7136AP. I found that the updated TA7136AP IC is readily available for around $8 each shipped and apparently is an improved version of the P1 (which was an improved version of the P).

I suspect I can but wanted to make sure that; 1) the TA7136AP is a direct substitute for the TA7136P1 and 2) the rest of the circuitry is identical so that no other modifications need to be made to the main PC board and I can just remove the AWK-065 boards and replace the ribbon cable with the IC (plug and play).
 
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Why not just replace the transistors, (which are likely the source of the noise), and recap the circuit?

The 725/726's can be replaced with a KSA992. The 1313 can be replaced with a KSC1815. (A KSC945 or KSC1845 would likely be O.K. as well.)

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The ribbon cables are trashed and so if I have to start rebuilding the boards and replacing the ribbon cables I thought it would just be easier to use the IC instead and it would be done in 5 minutes instead of turning into an afternoon's project.
 
To save anyone else the time and trouble you CANNOT replace those preamp boards with the TA7136AP IC. I tried and got a high frequency noise and no signal. Went back and repaired the old boards and used some small diameter heat shrink tubing over the bare wires from the ribbon cable which had disintegrated so they had insulation. Was worth a try but didn't work out. Turns out the problem seems to have been some cold joints on those old boards.
 
That seems a bit odd, since the SX-750 was produced with both the TA7136P1 linear ICs or the AWK-065 board with discrete components, with no other circuit changes.

Are we certain that the "P" version IC is identical to the "P1" variant?
 
I was actually using the AP version and while I thought it was interchangeable, apparently it is not. I didn’t try the P and the P1 wasn’t available.
 
I was aware that there existed other alternatives but I believe going that route requires modifying the circuit on the main board. Was trying to keep it as simply as possible by just popping a chip in to replace the old board that someone else had already been into. I am guessing the problem was with the choice of the AP variant which has one additional diode shown in its circuitry. It seemed like they went into some high frequency oscillation. Probably had I been able to find a P or P1 chip it would have been fine. As it turned out the old boards were salvageable and are working just fine now. Someone previously went to the trouble of replacing the transistors but didn't replace the 100uf 10V electrolytic caps which I suspect might have been going bad so replaced them, reflowed some bad joints and it's like new again.
 
NHDoc said:
I was aware that there existed other alternatives but I believe going that route requires modifying the circuit on the main board. Was trying to keep it as simply as possible by just popping a chip in to replace the old board that someone else had already been into. I am guessing the problem was with the choice of the AP variant which has one additional diode shown in its circuitry. It seemed like they went into some high frequency oscillation. Probably had I been able to find a P or P1 chip it would have been fine. As it turned out the old boards were salvageable and are working just fine now. Someone previously went to the trouble of replacing the transistors but didn't replace the 100uf 10V electrolytic caps which I suspect might have been going bad so replaced them, reflowed some bad joints and it's like new again.
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I don't believe that any changes are needed with an appropriate op-amp choice. The only thing I can think of that might be needed is one or two small power supply bypass caps right at the solder pads on the EQ board. The OPA604 op-amp will work fine with the voltages on the SX-650 board, it is rated for + & - 24 volts.

I mentioned these adapters as the 7136 is not a great op-amp. And to my ears a modern op-amp with the adapter sounds much better.

BTW the SX-650 has zener voltage regulators on the eq board that could be easily changed to accommodate lower voltage op-amps, perhaps that is the modifications you are thinking of. Not really a modification just a drop in parts change.
 
Thanks. Yes, that was the change I remember had to be made to accommodate the lower voltage of the replacement op-amp. I like to avoid making any changes just because it will make it easier for the "next guy" who is in there to figure out what was done. But it is good to know about the OPA604 as a drop in replacement with the adapter board. If I run into one of these again (or if this one develops another issue) I will give that a shot. Others will probably find this discussion helpful too.
 
 
EvanF said:
7 dollars for one is a scam. Get this and solder in wires. 10 for 1.93 before shipping. Pins included for DIP.


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I do agree they are a bit spendy, but maybe some folks want a plug-n-play solution. There are some board only adapters on ebay but I can't post that link.

There are some other solutions as well if you want to add and route your own wires.
 
mrk229 said:
I do agree they are a bit spendy, but maybe some folks want a plug-n-play solution. There are some board only adapters on ebay but I can't post that link.

There are some other solutions as well if you want to add and route your own wires.
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Soldering in wires is marginal work. Anyone who is cracking into a unit to replace parts should be able to solder wires. 7 dollars for one tiny board before shipping offends me at this point
 
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