Build Thread:MS PQ76

[Gustav]

Hi too all DIYer's

I've just finished my own MSPQ76 but I have some issues:

All the voltage are good from toroid to main board.

I made all the mods from the "Errata" book.
But when I turn the unit on is like my gain pots are reversed... And I have a very little sound even if I push them max
In Bypass everything nice.

I changed the gain pots by 22k lin as you said, maybe I wiring them bad, could you help me please?
Thanks

Reverse pin 1 and 3 on the gain pots from what you have now.

Make sure you changed the resistor values as well as the gain pot in accordance with the (non-critical) errata.

Also, check your filter board wiring (1 goes to 1, 2 to 2, etc).

Some people try to go by colors on the wires and make it out from the pictures - this is NOT recommend, and it seems to be the main cause of errors.

Gustav

 

[kustohm]

Hi too all DIYer's

I've just finished my own MSPQ76 but I have some issues:

All the voltage are good from toroid to main board.

I made all the mods from the "Errata" book.
But when I turn the unit on is like my gain pots are reversed... And I have a very little sound even if I push them max
In Bypass everything nice.

I changed the gain pots by 22k lin as you said, maybe I wiring them bad, could you help me please?
Thanks

Reverse pin 1 and 3 on the gain pots from what you have now.

Make sure you changed the resistor values as well as the gain pot in accordance with the (non-critical) errata.

Also, check your filter board wiring (1 goes to 1, 2 to 2, etc).

Some people try to go by colors on the wires and make it out from the pictures - this is NOT recommend, and it seems to be the main cause of errors.

Gustav

Yes Gustav, that's what I thought to do, just asking the creator to be sure.
Thank you!

 

[kustohm]

Reverse pin 1 and 3 on the gain pots from what you have now.

Make sure you changed the resistor values as well as the gain pot in accordance with the (non-critical) errata.

Also, check your filter board wiring (1 goes to 1, 2 to 2, etc).

Some people try to go by colors on the wires and make it out from the pictures - this is NOT recommend, and it seems to be the main cause of errors.

Gustav



Yes Gustav, that's what I thought to do, just asking the creator to be sure.
Thank you!

I reversed in good order the gain pots last time.
I think there's other issues because the sound (with every pots on 1= like bypass) is weird as the bandwith is not complete... I check the filter board wiring: is good
Can you help me please?

 

[gyancey]

HI Gustav - should the 50K gain pots be linear or audio taper?  BOM doesn't specify taper.

 

[T.LO]

I sucessfully built my MS PQ76. It was a lot of work in selecting parts and values. I took it very serious. It was fun anyway.  In this thread you guys wrote about an Errata to this project. It wasn´t included in the downloaded file. Would someone send it to me? I want to fine tune the unit.

So my questions

Do I have to change the 50k LIN pot to a 22k LIN to have more the gain?

In case I change the THAT1206 to THAT1200. Do I have to change the 20k resistor to 27k resistor too?  I think they influence the NE5534.

The M/S Mode is a little louder than L/R mode. Is it the normal case?

T.LO

 

[TwentyTrees]

Congratulations on your build!  On your specific questions, I’m having trouble putting my hands on the errata but if memory serves I think there were  a couple of resistor changes on the main PCB which, together with the change of pot values from 50K to 22K, improve the range of gain controls. If you reach out to Gustav I’m sure he will help you out.

M/S is indeed louder than L/R in this unit - that’s inherent in how the maths works, so just needs compensating with the gain controls.

Not sure about the THAT chip question - maybe Gustav can help with that one too. Why are you looking to use the 1200, do you want to drive the output stage harder?

 

[T.LO]

I changed the That1206 to That1200 and 20k Resistors to 27k.  I kept 50k LIN potentiometer for now.

There ´s is an improvement in the handling, when I compared the EQ signal to the bypassed signal. Before the bypass mode was always louder. Now it´s better for my taste.

I want to substitute 50k LIN pots with stepped attenuators.  Before I need to be sure about the 22k LIN change.

Furthermore, I want to troubleshoot / improve this "warm up" ramp. There was a dicussion in this thread, but I wasn´t able to figure out a solution for it.

Congratulations on your build!  On your specific questions, I’m having trouble putting my hands on the errata but if memory serves I think there were  a couple of resistor changes on the main PCB which, together with the change of pot values from 50K to 22K, improve the range of gain controls. If you reach out to Gustav I’m sure he will help you out.

Thank you for the congratulions. I´ll contact Gustav for the errata book.

M/S is indeed louder than L/R in this unit - that’s inherent in how the maths works, so just needs compensating with the gain controls.

I see. I wasn´t sure about it. Thank you

 

[Gustav]

I changed the That1206 to That1200 and 20k Resistors to 27k.  I kept 50k LIN potentiometer for now.

There ´s is an improvement in the handling, when I compared the EQ signal to the bypassed signal. Before the bypass mode was always louder. Now it´s better for my taste.

I want to substitute 50k LIN pots with stepped attenuators.  Before I need to be sure about the 22k LIN change.

Furthermore, I want to troubleshoot / improve this "warm up" ramp. There was a dicussion in this thread, but I wasn´t able to figure out a solution for it.

The range is better with the change in resistors/pots, but your inability to get unity gain before the switch is odd. It may have to do with using the 1200 instead of the 1206. If not, possible build error could be at play.

I would not recommend the swap in input chip - the only "advantage" is less headroom.

If by "warm up ramp", you mean shortening the time of inrush current, you can put a resistor to ground from the decoupling cap at the output stage. That should speed it up a little.

Glad the build has worked out!

 

[T.LO]

Hi there

thank you Gustav for all the helpful advices.  I returned  to the 1206´s and I changed the resistors near the NE5534.  (20k to 15k and 10k to 5k1) I changed the 50k Potentiometer to 22k stepped switch.

If by "warm up ramp", you mean shortening the time of inrush current, you can put a resistor to ground from the decoupling cap at the output stage. That should speed it up a little.

Yes I meant it. It works. Thanks again.

 

[T.LO]

at least the front view.

 

[erquere]

Hi ! I have a problem with my PSPQ76, big him coming and increasing some seconds after I switch th unit on, I changed the BC transistors, some electrolytic pots, checked voltages and the PSU pcb, everythink OK, except the both 100R resistors that gets brown (heat) and that hum noise. Any idea about its origin ?

PS. No hum when bypassed

 

[Gustav]

Hi ! I have a problem with my PSPQ76, big him coming and increasing some seconds after I switch th unit on, I changed the BC transistors, some electrolytic pots, checked voltages and the PSU pcb, everythink OK, except the both 100R resistors that gets brown (heat) and that hum noise. Any idea about its origin ?

PS. No hum when bypassed

The 100R on the inputs?

Check for faulty orientation on protection diodes, or shorts in the input as a start...Those resistors shouldnt see anything but your audio signal, unless there is an error.

Gustav

 

[erquere]

No, the 100R near the BD139 (see picture attached). The BC560 are both boiling when on, and huge hum out.

I have changed the N4001, no more hum but some crackling, then I changed the 100R, the BC107B, the BC560, the BD 139 and BD140, and former elcos with nichicon golden ones (audio special), and i ordered some new elcos to change the ones who are not golden ones yet. All are 63V. Now no more crackling but the hum (noise ?) is back and huge. So I don't understant what's happening.

Should I change again the N4001, the BDs and ceramic condos ?

 

[Gustav]

No, the 100R near the BD139 (see picture attached). The BC560 are both boiling when on, and huge hum out.

I have changed the N4001, no more hum but some crackling, then I changed the 100R, the BC107B, the BC560, the BD 139 and BD140, and former elcos with nichicon golden ones (audio special), and i ordered some new elcos to change the ones who are not golden ones yet. All are 63V. Now no more crackling but the hum (noise ?) is back and huge. So I don't understant what's happening.

Should I change again the N4001, the BDs and ceramic condos ?

Its really hard to tell, because im only catching a (maybe) glimpse of the backside,  but it looks like the bottom BD139 on your picture is mounted backwards.

Check orientation of those and the BD140 and report back!

Gustav

 

[erquere]

You have good eyes ! That was the problem (i 've been confused by ST datasheet...) Thanks a lot Gustav !!