DX-7 Mk 1 trimpot adjustment Reference voltages

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Jamie77
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Joined: Thu Jan 10, 2019 1:01 pm

DX-7 Mk 1 trimpot adjustment Reference voltages

Unread post by Jamie77 » Fri Jan 11, 2019 3:01 am

Hi all,
New to group and recently bought a DX-7 Mk1. A bit of TLC required but all in all in good shape.

Whilst inspecting the internals I found a couple of the trim pots hard over and well, this just does not feel right.
I was wondering if anyone has some documentation detailing the proper adjustment procedure for the various trim pots?

Noting some are for voltage regulator circuits which is easy enough to set. One in particular I am interested in is VR9 which is the 5AV regulator (assuming 5A constant current, attached image of circuit). The output of this then feeds the +2.5V regulator circuit, so need to set the 5AV reg first (Vref set by VR9).

Any assistance would be greatly appreciated.

Kind regards,
Jamie
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shadowmask
sector101.co.uk
sector101.co.uk
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Re: DX-7 Mk 1 trimpot adjustment Reference voltages

Unread post by shadowmask » Fri Jan 11, 2019 8:18 am

Hi,

I would take a guess that the +5AV is actually an analog +5V supply rather than a 5 Amp supply.

Before adjusting VR9 check that the +15 rail is within spec. IC57 and TR18 are buffering the divider voltage at the VR9 wiper so probe the cathode of D1 for +5V during adjustment. After that do the VR8 adjustment.

Brian

EDIT: Typos are always visible AFTER posting ;)
User avatar Australia
Jamie77
Posts: 2
Joined: Thu Jan 10, 2019 1:01 pm

Re: DX-7 Mk 1 trimpot adjustment Reference voltages

Unread post by Jamie77 » Sat Jan 12, 2019 1:36 am

Thanks Brian, will look at what you suggested.

The annotation of the circuits are a little confusing. Regulator name is described as +5AV but the output is +5A (may just be a means to distinguish between the +5V supply on the DC PCB (7805 fixed voltage regulator output).
I don't like adjusting things without understanding what the change will do first.
Looking at the 5AV and 2.5 regulator circuits, it makes sense that the 5AV output is 5V as the 5AV is the input to the 2.5V reg and the voltage divider is halving the input to generate the 2.5v reference voltage output of the non-inverting buffer amp (adjusted by VR8).

I totally understand what you mean about typos AFTER the fact :-)

Kind regards,
Jamie
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