(Solved) Release pot on G-1176 moves g.r. pointer. Any help?

[gyraf]

In this case, there must be some leakage current out of the gate of the meter FET. Perhaps a faulty or fake jfet?

 

[timpanic]

Thanks for the reply Jacob, my meter can't handle it and I think I'll fit a driver circuit.

I also wanted to ask you for the ATTACK and RELEASE potentiometers if they were logarithmic or linear as in the BOM it is reported linear, while in the logarithmic layout. Thank you in advance.

They should be both linear. I don´t know if there is even a 4m7 LOG available.

 

[timpanic]

In this case, there must be some leakage current out of the gate of the meter FET. Perhaps a faulty or fake jfet?

Hi jakob! Yes I was waiting for a week for new ones. This time I ordered some directly from banzai-music berlin. bf245a and 2n5457. 0,80€. Both stilll show this problem. FIRST, am I right that when I pull out the one before the meter and the problem is gone it MUST be the driver circuit, right?!
I have an old transistor tester here and he shows some leakage in all FETs I have here. When I test the bc107 it shows NONE.
Maybe all FETs available right now doesn´t work with the 1176 and are good enough for stomp boxes?? I am really at the end of my powers now

 

[timpanic]

OKAY WE HAVE A WINNER HERE! After mounths of troubleshooting I soldered one bf245 out of a working mono G1176 and voila the problem is gone!!!!
So where can I get some working FETs right now ?

 

[squib]

Ummm I'm not sure that this has been mentioned... All correctly functioning 1176 will show a small shift in the 0dB point when reading GR and the release pot is rotated from CW to CCW. It's normal

 

[gyraf]

Thanks for reporting back, TimPanic

At this time it seems really tricky to source genuine jfet transistors in TO-92 housing - market is flooded with fakery.

Perhaps try to find a SMD substitution (these seem to be faked much less), and mount by attaching wires?

/Jakob E.

 

[timpanic]

Ummm I'm not sure that this has been mentioned... All correctly functioning 1176 will show a small shift in the 0dB point when reading GR and the release pot is rotated from CW to CCW. It's normal

Small is a little relative. With the old bf245 there is NO movement at all. With the new ones the GR when everything is calibrated correctly ( tracking!), is moving 10db or more so all over the whole VU scale. This can not be right. Also it is showing this when compression is off and then the output is droping when turning the release.

Also the info varies a lot. I was asking mounths ago and people told me here there should be NO movement at.

 

[timpanic]

Thanks for reporting back, TimPanic

At this time it seems really tricky to source genuine jfet transistors in TO-92 housing - market is flooded with fakery.

Perhaps try to find a SMD substitution (these seem to be faked much less), and mount by attaching wires?

/Jakob E.

Thank you Jakob for your support. Yes I saw these SMD ones. At ebay I saw some NOS. But who knows if this is true. But imagine; I have over 20 FETs here right now from different sources and none of them performs like the two old ones I soldered out from a unit which is 8 years old. I didn´t expect that and was swaping Tantals, diodes and caps like crazy.

Can you confirm as mentioned above that every working 1176 has some GR movement? And if YES how much? Thing is, I have the impression that the new 2n5457 which shows this "problem" has a much dirtier sound. The old bf245s without this phenomen sound much cleaner.

 

[gyraf]

I have seen needle movement in some "real" rev#F's - but only just moving, like a needle-width or so..

 

[timpanic]

I have seen needle movement in some "real" rev#F's - but only just moving, like a needle-width or so..

Okay, that´s precise info and exactly the behavior of the old bf245 I have here.
A little blasphemic question: Is there maybe a way to modify the VU Meter Circuit that it works with the newer ones?

 

[gyraf]

referring to schematic: http://gyraf.dk/gy_pd/1176/1176sch.gif

I think it must be drain-to-gate leakage in the "meter" jfet Q10

can you rule out contamination on the PCB in the area?

perhaps the jfet dosen't like the full extent of what is left of the +30/-10V after R49 (or R52). What supply voltage do you have at IC1/Pin7, its positive supply pin? Maybe try making R49 (4K3) larger until you end up having only e.g. +10V here, this would result in lower tension across the jfet

btw, you'll probably preferably want BF245A, not C

 

[timpanic]

referring to schematic: http://gyraf.dk/gy_pd/1176/1176sch.gif

I think it must be drain-to-gate leakage in the "meter" jfet Q10

can you rule out contamination on the PCB in the area?

perhaps the jfet dosen't like the full extent of what is left of the +30/-10V after R49 (or R52). What supply voltage do you have at IC1/Pin7, its positive supply pin? Maybe try making R49 (4K3) larger until you end up having only e.g. +10V here, this would result in lower tension across the jfet

btw, you'll probably preferably want BF245A, not C

I check this out. Damn, I just ordered some Bs and Cs cause I had no luck with the As.
Also I saw that some reads BF245 and some HF245 which have a different pin out according to my transistor tester.
What do you think about the 2n5457 which should be used for the MS76? For the bfs I had to put them in reverse on these boards.
The pcb contamination can not be the problem. I can demonstrate the same behavior with two MS76 pcbs on all channels and when I put in the 2n5457s I have here into the old mono g1176 the needle also drops so it must be the Fets. It also varies a little by changing them but only the old bf245a are showing no movement at all.
I also orderd some smd ones bf545.
I will report if changing r49 or r52 is doing something for us.

 

[gyraf]

for meter circuit, it's probably not important what sort of fet you use - meter precision is a concern very low on the list

 

[timpanic]

for meter circuit, it's probably not important what sort of fet you use - meter precision is a concern very low on the list

Totally get you but it´s just that it´s also droping the output level which is a little annoying.

 

[gyraf]

yeah, we don't want audio affected - also because this reduction of gain comes with distortion cost

Try lowering the meter-fet voltage as suggested above?

 

[timpanic]

yeah, we don't want audio affected - also because this reduction of gain comes with distortion cost

Try lowering the meter-fet voltage as suggested above?

Jakob, you are the man. Amazing! This really made it. I kneel. I measured around 17 volt at pin 7. I soldered out the 4k3 resistor and put a 10k pot there to bring the voltage down to 10V at Pin 7 and guess what the needle stays at zero like a brick no matter the release knob is turned. I measure 6k4 at the pot now.
I really wonder why this "issue" and this fix doesn´t appear here before but I am convinced this will help a lot of people.
I can only repeat myself: Thank you very very much and props to your knowledge, help and tone !!!

 

[mata_haze]

I am going to revive this thread as I am a bit stuck working on a Purple Audio MC76 (stock so non DIY).
when I got it the unit was stuck in max attenuation so I could only get max 6dB out of it.
massive battery leakage around Q1. cleaned all of that and (momentarely) disconnected the active circuitry.
stripped the PCB on the preamp-gain reduction circuit, scrubbed and cleaned. recapped almost every where reinstated the FETs and its' working. I can now line it up and so on, however....
I am having the same problem as everybody else here, but here is what I have found.
-I have disconnected the sidechain Diodes (CR2+CR3) as well as C19 and C20
-disconnected C 27 ( on the attack pot) AND disconnected the attack pot from the release pot.
-removed C18 and C814
-removed C21 and C813
-removed C17
and still if I measure at the release pot where it connects onto the PCB basically at R7/C22 I get a DC voltage swing between having the release pot CW or CCW.
(caps replaced so far, C1, C28+C817,C2,C5,C600+C816, C22, C3 and C6)
when Release is full CW I get 0.222 VDC at the gate of Q1
when Release is fully CCW I get 0.1543 VDC at the gate of Q1.
why is this DC getting through C3 and C6 first of and secondly where is it coming from? I have replaced C3 and C6 with ceramics (that is all I had) but even if I remove them it's still happening. if I remove C22 I get pretty much no DC swing.
so it looks like is coming from the Release pot attack pot combo?

any ideas/pointers/previous experience?

many thanks

 

[jonathanmorbin]

I am going to revive this thread as I am a bit stuck working on a Purple Audio MC76 (stock so non DIY).
when I got it the unit was stuck in max attenuation so I could only get max 6dB out of it.
massive battery leakage around Q1. cleaned all of that and (momentarely) disconnected the active circuitry.
stripped the PCB on the preamp-gain reduction circuit, scrubbed and cleaned. recapped almost every where reinstated the FETs and its' working. I can now line it up and so on, however....
I am having the same problem as everybody else here, but here is what I have found.
-I have disconnected the sidechain Diodes (CR2+CR3) as well as C19 and C20
-disconnected C 27 ( on the attack pot) AND disconnected the attack pot from the release pot.
-removed C18 and C814
-removed C21 and C813
-removed C17
and still if I measure at the release pot where it connects onto the PCB basically at R7/C22 I get a DC voltage swing between having the release pot CW or CCW.
(caps replaced so far, C1, C28+C817,C2,C5,C600+C816, C22, C3 and C6)
when Release is full CW I get 0.222 VDC at the gate of Q1
when Release is fully CCW I get 0.1543 VDC at the gate of Q1.
why is this DC getting through C3 and C6 first of and secondly where is it coming from? I have replaced C3 and C6 with ceramics (that is all I had) but even if I remove them it's still happening. if I remove C22 I get pretty much no DC swing.
so it looks like is coming from the Release pot attack pot combo?

any ideas/pointers/previous experience?

many thanks

So I had a very similar issue and after lots of head scratching and testing I ended up having to rebuild the noise reduction side of the circuit off board which fixed the swing when moving the release pot. Don't know if this will also help for you or not. From memory I believe I lifted the gate from Q1, the leg of C9 that's connected to R6. And the leg of R7 connected to the gate for Q1. Then essentially had R6/R7 C3/6/9/22 and Q1 connections floating in the air above the PCB but kept rigid with how it was arranged and soldered. I think maybe in the end there might have been a leak to ground on one of the traces around C9 as I measured a few Mohm resistance between a free trace ground after I had removed the components.

I would suggest trying to play around with isolating some off that area off the board. This part of the circuit likes having R7/C6/C22 close to the gate of Q1.

It's a very sensitive part of the circuit that area and probing around that area isn't much use unfortunately because it effects the part of the circuit you're testing so much. It took me many hours to solve my issue in the end!