Poly 800 mk2 eprom problems

[Circuitbenders]

To be honest i didn't think it would be. 

I think that dump was from a 851005 chip. I do have a 851006 eprom as well somewhere but i can't really see it making much difference.

If theres any readings you need from a working Poly800 mkII i do have one here i could test for you.

[C M O S]

Close inspection of the power supply in the POLY800II i can now see how things function but if anybody knows the circuit in question am i right in saying that the -5v supply line is generated by using a HARTLEY ocillator?? If not is there anyone out there that can explain how this part of the circuit works?? I can see that this must be an ocillator of a sort because there is a stable DC  feeding  into the centre tap. On the seconary side there are four diodes creating a reversed bridge of which the output is fed through an ELEY choke then the final -5. Transistor Q7 clamps the centre tap of the coil being shunted by D7. The value of D7 is not clear in the schematic (68IL ZENER). D794 although using a meter this device checks ok but when mounted in the board the transistor overheats. There does not seem to be any information on the  choke/transformer.

[Gordonjcp]

The wee oscillator thing for the -5 supply seems reasonable.  If the transistor is overheating, it's possible that something is loading down the -5V rail - check you didn't put the zener in "backwards" with the anode to positive ;-)

[C M O S]

Thankyou for your reply gordon.
The ZENER in question has not been removed from the board, i know that sometimes cold checks just don't cut the mustard, got to remove the component and test with suitable dropper on the the bench. Got a nice forward drop on it though so i am assuming that this component is ok. The Drive  transistor responsible for the switching is quite a hefty power type driving the coil under some kind of resonance. What the korg designers have added after the -5v is developed is a small smoothing choke rated at a few uH, with this component taken off the board it allows you to completely isolate the -5 supply line, when i do this the transistor does not overheat meaning there is a load elsewhere. Only problem is the -5v supply line with no load condition is now at -2.86v which is obviously incorrect. The analogue circuits require a 10V P/P swing to work. Close inspection of the schematics show that even some of the MC4051 Multiplexer chips use this supply line also. You mentioned in previous posts that most of the problems you have experianced with the poly800 are with the supply lines, what type of problems have you experianced gordon??? Everything regarding power seems to be ok on the circuitboard apart from this -5 line  and i cannot believe that this line alone will render the poly800 dead. I have voltages just about everywhere else and i would have expected some activity. Do you have any information or spare eley resonance choke as i believe the over heat may have caused a small short in the coil??? I have seen better circuits for the development of a -5v supply line, why korg have done it this way is beyond me.
THANKYOU FOR YOUR HELP

[Gordonjcp]

Normally the 4051s either don't switch at all or get stuck on one output.  I've never heard of them drawing excessive current but I suppose it's not impossible.

[nochtanseenspecht]

i'm afraid i will join this club 

got a broken poly800 mk1 in a trade. the guy said it stopped working after doing the battery mod
(not the one described in this site, but another one) i discovered he forgot to cut a trace, so i did that,
but it didn't help anything. the situation is like this : no display (sometimes strange symbols) at the output
only filtered noise. the moogslayer mod, already there, does function..
i am very much afraid that the firmware has been whiped..

any ideas ?

would it be possible to hack the thing into a filterbank or something, to still use the analogue part of it or something?

cheers nochtanz

[Circuitbenders]

Check for the voltage and the clock signals going into the main synthesis chip. It sounds like you might not be getting proper power to the M5232 to me.

[nochtanseenspecht]

so there's hope still.. thanks! i'll have to wait for aproval of poly800 groups membership, so that i can check pinout of the m5232

[Circuitbenders]

Also make sure the shielding is connected everywhere that it should be. As is says somewhere else in this thread, there is an analogue and a digital ground which should be connected together via the shielding underneath the board. I think i had exactly the display issues you describe while installing some mods on a mkII due the shielding connecting intermittently.

[nochtanseenspecht]

Check for the voltage and the clock signals going into the main synthesis chip. It sounds like you might not be getting proper power to the M5232 to me.

hmm i checked the volages on the M5232, and they're allright 
have to check the shielding a bit better, but so far it seems ok too.. couldn't find a digital ground connection tough