PL-518 mod. Step 1, tap into the 18V supply on PWR-035

[John Little]

Hi everyone!

I have a PL-518, which I recently acquired and need to service. Any advise will be welcome on this, so many things to do:

• The tonearm isn't returning very smoothly compared to some of the youtube videos I saw. What do you need to oil or grease?
  
• The plastic cover is all scratched and yellow. How do you guys deal with that?
• The feet are missing. I haven't read all the threads yet about suitable replacements.

In any case, I purchased a AT95E cartridge and the TT sounds fantastic.

I also got my mittens on a pristine copy of HSF75, which I'm sure will be really fun to run and check the output on a half descent soundcard or an oscilloscope. For example, the effect of antiskatting on L/R balance, or the TT / preamp frequency response.

Anyway, onto my first mod. For now, I have a PP-400 preamp which required its own 12V PSU. What a waste!

Prodding around service manuals, I looked for a voltage (any voltage) to tap into. Good news is, there is 18V available on the PWR-035 board, between E (earth?) and +B on the PWR-035 board, presumably to power the motor when the tonearm is moved from its resting position.

A trip to ebay located some "Mini-360 DC-DC Buck Converter Step Down Module" which fit the bill nicely and cost next to nothing. I really like those.

• 96% efficient
• Input voltage up to 23V
• 1.8A load continuous

So I wired a module in, between E and +B for the input (I used dupont connector pins), and on the output, to a bit of wire and barrel connector suitable for the PP-400 (thick centre pin), which I managed to feed through a slot next to the coax output. I then tweaked the module pot to have exactly 12V on the barrel connector.

Is it working? Yes! No noticeable noise and the preamp now only comes on when it's needed. And it frees a wall socket, which is what I wanted all along.

I put some additional shrinkwrap on, but that's about it for this fully reversible mod:


Would I advise doing this? mmh... there is one issue with the setup. When the tonearm returns and cuts the supply to the preamp, C3 and C5 are still charged for a little while and the preamp outputs a low frequency square signal (I think) for up to 5 seconds.

I'm thinking of wiring a resistor across E and +B but haven't given much thought as to what value it should be. Enough to discharge the capacitors, but would that be too much load for the transistors to handle? I'll think about it.

So, work in progress I would say, and by all means if you have any comments or advise, I'd be more than happy to listen

Cheers,
John

 

[John Little]

I tried a couple of approaches to reduce my farting noises problem, thought I would share.

• 10K in parallel with the input did sweet F all.
• 4.7K increased the period between farts but at what cost? Stealing an additional 68mW from a circuit that may not have been designed for it.
• ... time to try something else?

Introducing the SEPIC buck-boost converter. The one in this video to be precise (warning at 2:55 about output voltage, careful with these things!), and available for a quid each from this seller on ebay. It's bigger than the buck converter I was using before, but can be mounted on short plastic poles, and secured in place with a screw through one of the holes on the bottom plate.



Then engage the tonearm and tweak the trimpot to reach whichever voltage you require (12V in my case). Again, mind your fingers!

So what happens when the motor switches off and the caps are still charged? This is where the buck boost converter switches from being a buck converter (18V -> 12V) to a boost converter (remaining -> 12V). This drains the caps pretty quickly and eliminates the farting / machine gun noises! Unfortunately, there's still a tiny little "wow" sound at the very end of the discharge (maybe after a second of silence).

Am I pleased with the result? You bet! I tried to listen to any other noises with headphones plugged into the preamp, but couldn't hear any (even compared with the original 12V power supply).

If I get a chance, I'll upload some samples of the "wow" sound and of the "motor on" and "motor off" noises.