Purple MC76 P2P problems - SOLVED !

[Purusha]

I have 0V on both ends of the trimmer and the same with pad 18.

One theoretical question. Where should I surly see some V on the PCB?
I used a ultra fast diodes instead of fast ones in the PSU section. Could this be a problem?
It's strange, I don't see any voltage coming out on the +V for the meter light. But on the other
hand the meter is working OK and so is the output volume pot. I guess some V is coming
across the PCB otherwise this would not work, correct?

 

[SSLtech]

If you're getting audio through, and the output volume control etc. works, then you're okay.

Look at the Gyraf version of the 1176 schematic, it's easier to read in this area: http://www.gyraf.dk/gy_pd/1176/1176sch.gif

R82 (R35 in the Purple numbering scheme) has -10Volts at one end, the other end goes through a 2kΩ pot in parallel with 3.9kΩ to 0Volts.... that means I'd expect to read about 1.5Volts across the pot. The bias trim wiper should vary between 0V at one end, and one-point-something at the other.

You DO measure -10Volts at the negative end of CR9 (ten volt zener diode...)... -right?

Keith

 

[Purusha]

OK, I took another Ammeter and I see now the -10.9V on one side of the R35 resistor.
Than one side of the trimmer has -3.8V. I couldn't get to the other pin but I see -4.1V on the 18 PAD now.

 

[Purusha]

When I go fuly CCW with wiper I see - 5.2V on one side and 0V on the other.
When fully CW I see 0V on both sides of the trimmer.

 

[Purusha]

You DO measure -10Volts at the negative end of CR9 (ten volt zener diode...)... -right?

Sorry, couldn't find any CR9 diode on my PCB.

 

[Purusha]

Let me see if I can DIY this probe and run it with some software scope for now.
I promise I'll buy a scope ASAP :wink:

 

[Purusha]

OK Keith, I have a free software scope working like a charm :thumb:

Waiting for your command. What do I have to check?

 

[sintech]

The 10V zener must be the stud diode on the purple??

 

[sintech]

Regarding the resistance test, I also get in the region of 1k5 from pad 20 to ground

 

[enthalpystudios]

yup cr9 is the stud diode

 

[sintech]

cool, CR28 on the Purple :thumb:

 

[Purusha]

The top of the stud diode measures 30VDC like described on the PCB.

 

[Purusha]

This is where I am at right now:

I sent a sine wave through the input, scoped point 23. and saw the signal when the attack switch is on and not when it's off. The sine was at it's largest at 20:1 and at it's smallest at 4:1 so I gues the P2P wiring seems to be OK.

Any idea what to check next?

BTW, is anyone selling a scope? My software version will expire in 6 days :green:

 

[Purusha]

I don't know if this will help but here is the detailed photo of my comp amp section

 

[Infernal_Death]

Probably doesn't help but the right side of R37 and left side of R41 doesn't look like it's soldered. Maybe you want to resolder it. Doesn't hurt i guess.

Flo

 

[Purusha]

I have soldered some of the stuff underneath so it's fine. Is it possible that I left out an important element :?

 

[SSLtech]

Wait... you guys have it wrong... CR9 should be the -10V Zener. The stud Zener is the +30V Zener.

I'm looking at the MC76 schematic for my component references... with occasional references to Jakob's site schematic, which is the same in this area, and different in the input, output and VU meter buffer areas.

So now you say you read a few volts of difference at the wiper of theb ias pot... but a solid tone doesn't change in level when you set it to read 0VU at the output meter and then change the bias trimpot from end to end???

By the way a software scope is basically no use for this sort of stuff... it uses your soundcard input, which is useless for DC or slow-change circuits... Also for high-impedance circuits like the attack/release area of the 1176, a soundcard will be WAY too low impedance... you need 10-Megohms.

Keith

 

[Purusha]

So now you say you read a few volts of difference at the wiper of the bias pot... but a solid tone doesn't change in level when you set it to read 0VU at the output meter and then change the bias trimpot from end to end???

I hear nor see any volume change at the output of the unit by changing the bias pot.

By the way a software scope is basically no use for this sort of stuff... it uses your soundcard input, which is useless for DC or slow-change circuits... Also for high-impedance circuits like the attack/release area of the 1176, a soundcard will be WAY too low impedance... you need 10-Megohms.

I am already checking the e-bay... :wink:

 

[sintech]

Ok, the Point to point is good, after a few unrelated issues the first unit here has correct operation.

 

[Purusha]

Oh, great work Sintech :thumb:

I am almost there :?