70's Vintage Midas Console help

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telehumbucker

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I have a 70's Midas Console that is having issues with one of it's modules, hoping maybe someone here is familiar with this one in particular - it's the PR 21A monitor/oscillator module.......I have some schematics for it. I am not a tech but am working with one, though he doesn't have much experience with this type of board/module.

What is going on is, when the module is in the board things go haywire......buss modules that aren't selected are active, if one particular buss (buss 7) is selected the stereo main modules work and meter as normal, if #7 isn't selected than no signal reaches the stereo mains....the oscillator doesn't work properly, giving intermittent or almost arbitrary signals, buss 1, even when not selected is showing a strong signal acting almost microphonic......this happens when a microphone is plugged into a preamp, and most oddly, when a sound is too low for the microphone and preamp to register a signal, it will show up on the #1 buss meter as an extremely strong signal.......i.e., one could be whispering 10' from the SM57 and the signal will show up on buss meter #1 like a screaming guitar amp.

When the PR 21 A module is removed from the board everything returns to normal, all busses function as expected, but the main stereo pair doesn't receive a signal. In the modules documentation it indicates that the stereo main pair receives it's signal from the PR 21 A...so unplugged, no signal.

The tech mentioned that a couple of op-amps appeared to have melted, but he replaced those....Any suggestions, ideas are welcome.
 

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Whoops

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What are the voltage rails?

you should try to measure the voltage rails with and without that module inserted to check if there's a change.

Do you have the schematics for that module?
 

scott2000

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The tech mentioned that a couple of op-amps appeared to have melted, but he replaced those....Any suggestions, ideas are welcome.
The sockets can go bad as well.......

Melted op amps??

What are the voltage rails?

you should try to measure the voltage rails with and without that module inserted to check if there's a change.

Do you have the schematics for that module?
+1
 

telehumbucker

Member
Joined
May 13, 2021
Messages
11
Location
Georgia USA
I have a 70's Midas Console that is having issues with one of it's modules, hoping maybe someone here is familiar with this one in particular - it's the PR 21A monitor/oscillator module.......I have some schematics for it. I am not a tech but am working with one, though he doesn't have much experience with this type of board/module.

What is going on is, when the module is in the board things go haywire......buss modules that aren't selected are active, if one particular buss (buss 7) is selected the stereo main modules work and meter as normal, if #7 isn't selected than no signal reaches the stereo mains....the oscillator doesn't work properly, giving intermittent or almost arbitrary signals, buss 1, even when not selected is showing a strong signal acting almost microphonic......this happens when a microphone is plugged into a preamp, and most oddly, when a sound is too low for the microphone and preamp to register a signal, it will show up on the #1 buss meter as an extremely strong signal.......i.e., one could be whispering 10' from the SM57 and the signal will show up on buss meter #1 like a screaming guitar amp.

When the PR 21 A module is removed from the board everything returns to normal, all busses function as expected, but the main stereo pair doesn't receive a signal. In the modules documentation it indicates that the stereo main pair receives it's signal from the PR 21 A...so unplugged, no signal.

The tech mentioned that a couple of op-amps appeared to have melted, but he replaced those....Any suggestions, ideas are welcome.
Thanks everyone for replying. I thought I was going to be getting e-mail notifications of replies, which I didn't get, hence the late response. It turns out the source of the problems, other than the melted op-amp, was the oscillator channel was put in the wrong slot.....;there are four empty slots on the board that have cards...putting the oscillator in the correct spot fixed the odd behaviors. We replaced the melted op-amp and everything seems to be working o.k...Not sure when or how the op-amp melted, but there was a lot of plugging and unplugging of things, and the power supply fritzed at one point, so maybe that is when it happened...

There is another issue or two though, that needs addressing...the tiny screws that go into the board just above and below the buss meters, that fastens down the enclosures that hold the meters, where can I get replacements? They appear to be an odd size not available in the USA, maybe in Europe? They seem to be either M3.5 with fine pitch threads, or #6 -40 tpi.........with flat phillips heads, about a 1/2" or 10mm long. A lot of the ones in the board have either corroded or simply been lost - anyone who owns an old Midas will know what those screws are, and maybe, hopefully, where to get them...Second issue is all the buss meter
red input lights have all stopped working.....eight of them on this board.
 

musika

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It's possible that they are neither Metric nor Imperial (USA) but BA or British Association (I believe). I am just speculating, but my Allen & Heath MOD 2 UK mixer from ~ 1974 or 1975 has this type of screw (4BA) used to hold down all the modules. There was a period in time where BA was the standard in UK. If that's the case, there are places to buy them online. And some people just replace the strip with a Metric or Imperial thread style since trying to figure out the BA screw can be difficult. There is a table on the British Association screw sizes on wikipedia. I am sure there are gauges just like the ones for Metric too. Just mentioning all this, since my modules are BA from the same era and I needed to find a source for the screws.
 
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diylan

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Nov 30, 2009
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Those old Midas designs are great, cool sound and they didn't skim on component choice. I had an early 80s PR series console that I got a lot of use out of and sold around 5 years ago. I even racked up some spare PR 02A input modules, which I held onto and still use.

I too have an incomplete set of schematics, want to exchange and see if we can fill in the gaps?
 

telehumbucker

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May 13, 2021
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Georgia USA
British Tools & Fasteners

This company in New York state in the US carries the BA screws should you need them.
Thanks. I looked on the linked website, looks like a possible fit. Will call them Monday.
Those old Midas designs are great, cool sound and they didn't skim on component choice. I had an early 80s PR series console that I got a lot of use out of and sold around 5 years ago. I even racked up some spare PR 02A input modules, which I held onto and still use.

I too have an incomplete set of schematics, want to exchange and see if we can fill in the gaps?
Sure, I'm not sure how complete my schematics are, I am not a tech, but let
me know what module schematics you need.
The modules on my board (that I believe I have schematics for) are: Pro5A, Pro3A, Pro25A, PRO12A, PRO 21A.......I need schematics for PRO20A and PRO29.
 

telehumbucker

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Joined
May 13, 2021
Messages
11
Location
Georgia USA
British Tools & Fasteners

This company in New York state in the US carries the BA screws should you need them.
I talked to British Tools And Fasteners today and the screws they have are zinc platted with slotted heads and 1/4" total length measured from the top of the head. The screws in the Midas are 3/8" total length from the top of the head and black....and phillips head......I did find that a M3.5 .6 pitch 10mm screw will screw in 1/4" o.k. but will go no further, due, presumably to slight mismatch in threading, but it will secure the panel, and doesn't appear to be damaging the threads. Would rather use a phillips head rather than slotted screw...did some looking on-line for other British suppliers but haven't found the right size and phillips head 4BA.
 
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