Purple MC76 P2P problems - SOLVED !

I am in contact with Sintech and we can't figure out what is wrong with the P2P wiring
in the MC76 unit. Sound is passing through but no compression in both units.

Is someone here ready to have a detailed look into the P2P wiring we have?
I will make the P2P drawings in a few hours and post them here for examination.

BTW, this thread could end up in the 1176 META at the end to offer guidance to all
future Purple MC76 DIYers. :thumb:

Just to add, the point to point is based on an original Urei unit I have here, I'm still missing quite a lot of items from my Purple kit's so i've not been able to realtime troubleshoot this problem. But, I've scoped the original unit through from 'Off switch' to sidechain to get a good idea of what to expect.

I've got to admit.. I'm confused by purple's use of point 22/ 23 and Urei's point 22/23, the Urei schematic I see point 22 as route to C17 and 23 as GND, and on the Purple version point 23 as the route to C17 and 22 as the route from the ratio.. maybe it's just the way this has been drawn out... or maybe me

OK, done. I hope it's not too confusing :?

This diagram works 100% !!!

47K (yellow lead to release) should be 150 Ohm

47K from output should be 56k

OK, corrected. It was 3 o'clock in the morning :green:

the resistor on the attack pot has to be 470R not 470K !!

OK. Will change the resistor and try again although I doubt this is the main reason for not compressing. Could be wrong though.

what potentiometer type do you use for attack?

is the switch really a dpdt swtich?

The switch works fine in the G1176's I have here.

the resistor on the attack pot has to be 470R not 470K !!

Click to expand...

Funny, I actually had 470R soldered there :grin:

Here are some photos of my wiring ...

At first glance, pad 20 wiring looks like it shouldn't go there...

I'm off to look at a schematic.

Wait... It;'s just how it's drawn. -I see what the diagram means now... I didn't understand it at first. I'll look some more.

Keith

unless I read it wrongly, the 10Meg resistor is in entirely the wrong place, (reference to the drawing) surely? -The schematic shows one end of it being PERMANENTLY connected to point 20... not what the layout above shows. So -yes- I see a difference here... the resistor is effectively wired in parallel across the opening contact of the 4:1 switch, and it should be across the opening contact of the 20:1 switch.

HOWEVER... -Not that I can immediately see how that would kill the operation: it appears to be there as an "all-switches open" terminator, and effectively inactive when the unit is in operation.

If you have a 2-channel scope and a PAIR of 10x probes, I'd use a pair of 10X probes (in order to keep the impedances over 10Meg instead of 1meg...) and read the voltages during compression at the 150Ω/470Ω junction, and the other channel at the 20-wire. Are they the same? -You SHOULD read a descending control voltage as you move from 150-470-560-1.5k resistors. Any failure for this voltage to reduce would mean no increase in ratio. On the other poles of the switches, the resistor ladder (56k-68k-56k-56k) just reduces the sidechain signal level by a few dB (about 3,6,12 etc) as the ratio increases, to help reduce the interactive fiddling. You should easily be able to read the audio signal at wire 23 gradually becoming quieter as you switch up through the ratios... this just moves the threshold by a few dB (I estimate about 12dB total between all 4 ratios, without resorting to any serious calculation...) Sounds like this is not your problem, though.

One final thing...

Measure resistance with the power OFF between pad 20 and ground... you should read 150Ω. -Actually do this first. -You should read 150Ω to ground from the blue "20" wire in the diagram above. If not, your control voltage is not being attenuated to change the ratios.

Keith

Thanks Keith for stopping by :thumb:

I read 1K5 between the blue wire on the diagram and gnd. Maybe Sintech can measure his P2P
wiring also. If his wiring gives the same results than we have wired it exactly the same. If not,
I have a mistake somewhere....

I may have also misunderstood the symptom that you're experiencing: I thought you had high compression, unaffected by the ratio control... is that right?

Re-reading, it sounds like you may have no compression at all... or is that wrong? -I think the latter may be your problem, -which is true?

Keith

I have no GR in the sound and on the VU meter as well.

Okay.

Do you have a scope? We need to go hunting for rectification voltages now. -Preferably a x10 probe, since the impedances here are HIGH, and we don't want the scope probe to kill things!

Also, does the FET bias potentiometer have any evvect on the signal level? -It should be able to 'pinch' the signal level down. If it doesn't have any effect throughout the range of it's travel, then that's a clue.

Keith

Sorry, no real scope :?

The only thing I could use is this http://www.recording.org/users/kev/signaltracing.htm

Will that do?

Also, does the FET bias potentiometer have any evvect on the signal level? -It should be able to 'pinch' the signal level down. If it doesn't have any effect throughout the range of it's travel, then that's a clue.

Click to expand...

I twisted mine in both directions. Couldn't notice any signal level change.

There you go then, that sounds like it might be the area where your problem lies...

Look at the bias trim pot area. Do you read DC voltages at each end of the trimmer potentiometer with the wiper in the center and with no signal present? if so what are they? then what voltages do you read at the wiper (wire 18) with the wiper at one end and then the other?

(A real scope is worth it's weight in gold here... make sure you buy one -even an "eBay special"- if you're going to be doing more DIY... best eighty dollars you can spend!)

Keith