Lexicon PCM70 schematics

Hi,

does any good soul have schematics for the thing in a pdf file and would like to share? I've just bought it and I'd like to mod it, remove VCA's, replace comparator in ADC etc.

And the second question: does anyone know anything about Murata low pass filters used in Lexicon and Eventide gear? They are of the same type, but differ in cut-off frequency. What is their order, topology, characteristics?

Thanks in advance.

"Sleeping dog".

If you don't like the sound, sell it and buy something else.

I'd call anyone who fiddles with components in these things a first-grade idiot... what you'll very possibly end up with is a non-functional unit.

Either use it or sell it, but only "modify" it if you want a doorstop that looks like a PCM70.

Seriously.

Presumably you acquired this because you like the sound? then why change it? -Any 'limitations' in the sonic characteristics of this unit will not be altered or improved by a 'comparator-swap'.

My reply may sound a little harsh, but I've seen more than enough of these which can't be repaired because of a lack of available replacement parts, and over 50% were working before someone decided to try and 'improve' them.

Since it's one unit which I have a great affection for, I am appaled that people destroy so many, in the name of 'improvement'.

If you like, I'll save you the trouble and just swap yours with a non-working one...

Keith

Now whats that old saying ?
If it aint BROKE dont FIX IT :thumb:
especially when New :!: :?:

[quote author="SSLtech"]

I'd call anyone who fiddles with components in these things a first-grade idiot... what you'll very possibly end up with is a non-functional unit.

Keith[/quote]

Ouch!

I'd really carefully consider modding it if it was in perfect condition. Unfortunately it is not. The previous owner reported many problems with it (mostly strange noises, but not periodic which would suggest a bad DRAM chip), but I haven't got time to verify them yet.

Keith, I can wholeheartedly assure you that I would be able to modify it and return to previous condition if there is such a need. Really. If not I won't hesitate to change my nick to "First_Grade_Idiot".

So if anyone has the schematics and can send them to me, I would really appreciate it. I'd like to know what's going on inside.

Regards.

Like I said, I KNOW that what I wrote was rather harsh, but I really do want to impress upon you that I have seen a LOT of PCM70s that several good techs have not been able to fix, and usually they have suffered from the tech attention rather than benefitted.

Keith

I would highly recommend following SSLTech's advice and not touching any more in the unit than you absolutely need to. They are sensitive to diddling around. You're probably right, change at least one of the DRAM's out. U30 is the lowest byte, U45 is the highest. You can swap them to find the culprit based on that information.

The chips in there are relatively old so their ESD protection is not very good and they blow up at the slightest provocation. Follow good ESD precautions in there or you'll end up with a dead PCM70. Also, if you accidentally short out the LM311 comparator in the A-D circuit, you'll take out the CMU chip. Remember that the proprietary Lexicon IC's are essentially unavailable so if you blow it up, game over. You need to ensure the grounding method of any possible replacement comparator is exactly compatible with the original one or you'll hit the CMU with a negative voltage and poof.

The Murata filters are around a 8 or 10 order filter, I don't know of the exact topology but the order is really high. I have replaced them with one of the Linear Tech antialiasing filters along with some additional components - a 78L05/79L05 and a 5532 makeup gain amplifier. You need to etch a little board to do that. They have a 0.5 gain, and they are also very sensitive to messing with.

Yea, Keith may have been a bit harsh without any background but his advice on these is sound. I actually just received a couple of DeltaLabs units that were previously 'fixed' by a tech and returned to the customer as unrepairable. I've got one almost working, I know what chip is not working. The tech replaced the output D-A converter circuitry, where the problem was actually in the input A-D circuit.

Have that service manual, if you need it for a repair. Send me an e-mail address. Best way to modify pcm7o is put eprom v 3.o into it.

The Murata filters are around a 8 or 10 order filter, I don't know of the exact topology but the order is really high. I have replaced them with one of the Linear Tech antialiasing filters along with some additional components - a 78L05/79L05 and a 5532 makeup gain amplifier. You need to etch a little board to do that. They have a 0.5 gain, and they are also very sensitive to messing with.

Click to expand...

That's very interesting info dale.
Which Linear Tech filters did you use ?
I was thinking of doing a similar thing to my PCM70 because the Murata filters sound dreadful.
A lot of the programs on the PCM70 are straight out of the 224 and as such I find extremely useful. And I disagree with changing to the version 3.0 ROM (more effects based) I prefer the previous one.

Thanks for advices!

According to Dave Derr, who designed the analog part and converters of Eventide H3000, the Murata filters are of 9th order. Eventide used the same type of filters as Lexicon (AFL89WB), but the cut-off frequency was 20kHz in the H3000 (Lexicon cut-off at 15,250Hz in the PCM70 and 10,500Hz in PCM60). Probably they were used in the 480L as well. I wonder what their topology is. I cannot find any info on them.

Dale, I have to say I really enjoy your posts on Lexicon topology on this and other forums. I've heard that from among reverb algorithms only a "hall" used modulation in the PCM70. Can you verify it?

By the way I have software v3.01, but it seems that v.2 programs (Tiled Room for example) are programmed into user registers. I haven't had the chance to hear them yet though, as I lent my speakers.

LTC1564. It only comes in a SSOP package which needs a PC board done up. But that's not really a problem because then you can make a 'compatible' PC board - one that solders right where the AFL89WB's go.

I do not have PCM70 ROM images, obviously the PCM60 did not use modulation, and the PCM91 ROM is just too big to disassemble and figure out. One way to verify is check if the slave processor is continually updating the control store. Once you get your service manual information, check for a signal called /COPY or something like that.

Thanks Dale. I must check those filters out.
I came across the Murata filters when they were used in the Sony 3324 (although some used Soshin) and always thought that the sound was very edgy. It wasn't until later when Apogee made us aware of the dreadful phase response in these units that I realised why!

Ah. I've just checked the LTC1564 data sheet and it is not a 'linear phase' anti-aliasing filter.
I guess I was misled by the Linear Technology title.
I was working on a design to replace the Murata's but thought that your suggestion might fit the bill instead.
Back to the drawing board.

The linear phase filters they offer don't have nearly the S/N ratio of the one I used, so I didn't use them. I did my board up to fix a Roland SRV-2000 so a different filter didn't bother me much.

Here you go:

http://www.twin-x.com/groupdiy/index.php?cat=10241

My top documentation wants are for:

Euphonix CSll
Synclavier ll
AKG ADR 68K
AES11(MADI) VHDL description file
Publison(any) :wink:

I already got the service info from Bookkeeper, but thanks anyway.

I haven't seen any of the schematics you're looking for. The AKG reverb documentation is on my list as well.
Publison schematics will be particularly difficult to get I think.