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Old 15th Mar 2017, 3:57 pm   #21
mhennessy
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Default Re: Revox B252 Preamp

Thanks John - good find! The nearest I found from a quick search was MAB8441. 4k is positively palatial for assembly. Room for a couple of Easter eggs
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Old 15th Mar 2017, 11:00 pm   #22
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Default Re: Revox B252 Preamp

I got some time in at it tonight.

There is a small postage-stamp board with a single IC on it and a twisted pair of wires running off down to the lowest level. The edge of this board meets the main CPU board and has three connections to it in the vicinity of pin 12 of the CPU. Two of the connections had broken, and had broken through connections on the cpu board.

Some careful soldering under the microscope restored things.

Fingers crossed, I put the boards back together.

No difference.

I need now to probe the remote cpu board while it's all together and running, but access is impossible. I may need to dismantle, solder on a test wire, reassemble and only then get the scope on the added wire. Could be tedious, but enough for today.

David
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Old 21st Mar 2017, 12:31 am   #23
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Default Re: Revox B252 Preamp

OK, I've traced out the postage stamp.

It contains a quand nand device. It has +5v and ground fro the main CPU board. It has two mystery input wires one coming from the input board, one from the volume control board. The wild bit is the pin on the CPU it drives.... "INT" which seems to be usable as a plain input or for other reasons. In the manual schematic, there is a link in one of two positions, logic high or low to tell the CPU whether its in a B251 integrated amp, or in a B252 preamp. If this is so in this one, at least it's sitting in the B252 selection. The Nand gate circuit has a logical race in it, so it looks like it could make a shortish (4000 CMOS!) pulse on some occurrence between the volume and input boards. I haven't traced what these wires come from. They are hard wired, soldered, not on connectors.

I've probed the remote CPU now (keyboard removed) and its clock is running at about 4MHz.

I need to add some light pull ups to see if the keyboard is being scanned.

The IR preamp is powered from 22v via a dropper from +35v. It doesn't seem to be taking much current so it's getting about 33v.

A few electrolytics have been replaced with Japanese ones on the CPU board (and not on other boards) It seemed to have been built originally with Philips passive components.

Two capacitors, inputs to the metering seem to be loose, but they're not in an area I'd expect to put the CPUs in a sulk.

No real progress, but I'm waiting for a pattern to become clear.

Oh, and there are data and clocks coming out o the ain CPU and via the diverter switch to the display. so that path is working.

David
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Old 2nd Apr 2017, 8:03 pm   #24
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Default Re: Revox B252 Preamp

I've had a break from it doing a few other jobs.

I've dug out a Philips logic analyser PM3580 in the hope that it might have some support for a Philips/Mullard microprocessor, but it doesn't look like it. A one-chip mask programmed job just has I/O pins and nothing as revealing as the data and address busses make it off the die.

The two-wire connection between the processors is interesting. Both machines have their own clock crystals, so it has to be some sort of handshaked asynchronous transfer, though the data rate doesn't need to be much. I've got some time set aside tomorrow evening to have a dig.

First off is to add some light pull-ups so I can see if the switch matrix is being scanned.

Second is to take a look at inter-chip comms. Considering the age and the paucity of memory space, these things must have been written in assembler so the comms could be anything.

PlanB is to just replace all the available silicon on both CPU boards and hope... well it'll be quick and fairly cheap. Which pushes design-my-own-machine into plan C.

David
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Old 3rd Apr 2017, 11:29 am   #25
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Default Re: Revox B252 Preamp

Quote:
Originally Posted by Radio Wrangler View Post
The two-wire connection between the processors is interesting. Both machines have their own clock crystals, so it has to be some sort of handshaked asynchronous transfer, though the data rate doesn't need to be much.
With a Philips processor I would expect this to be I2C - an open collector system with one line for clock and one for data. Your logic analyser can probably decode it.
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Old 30th Apr 2017, 11:15 pm   #26
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Default Re: Revox B252 Preamp

That's what I'm expecting. My logic analyser fairly old (though a Philips product.) I haven't started digging into this properly yet. I had a look to see if there was anything obvious, but it looks like a fairly serious job. I've designed I2C interface logic for FPGAs in the past, doing multidrop connection along long cables or short cables and being agile enough to tell what speed to use. If it is I2C I may be able to determine which processor is waiting for the other. The problem is that the keyboard processor s inaccessible when the unit is plugged together enough to power up.

David
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Old 16th Jul 2017, 10:21 pm   #27
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Default Re: Revox B252 Preamp

I got a non working B251 with a removed MAB8440, I found this web page: http://adamsblog.aperturelabs.com/20...sked-roms.html

Could it be a way to extract the code from the mask?
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Old 17th Jul 2017, 6:25 am   #28
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Default Re: Revox B252 Preamp

Yes, it could. But removing the plastic package is a nasty process and then you need a good camera and microscope.

I've put mine on one side for the summer, I've got some stable roofs to rebuild while the weather is favourable.

Extracting Revox's code would help, but I suspect it would take more effort to extract than the effort involved in coding from scratch.

If I don't find a simple fault when I start seriously digging into mine, then I'm thinking of making a new processor board (or both) around a modern device. If I do this, then at least board designs and code will be available for others.

Thanks,

David
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