MEQ5 3PDT 'In' Toggle Wiring and Troubleshooting Help

[amazingalf]

I've been building a MEQ5 using the analogvibes chassis and wiring diagram and I'm having difficulty getting the 3PDT 'In' toggle to work correctly. The unit also exhibits a few other behaviors that seem incorrect. I've attached the wiring diagram for reference. I'm hoping that someone will be able to point me in the correct direction based on the following:

• All controls on the unit function regardless of whether or not the 'In' toggle is set to 'In' or 'Out'. There is no difference sonically or functionally between the two positions. The unit sounds good aside from some audible hum.
  • My understanding is that this switch should toggle between filtered and unfiltered. Is this correct?
    • What is wrong with my wiring or the schematic? How do I get this switch to function properly.
• It seems like the unit is louder than it should be by about 6db. I needed to adjust the send signal on my DAW by -6db in order to prevent the unit from clipping.

 

[Script]

Your switch and the switch on the wiring guide look not the same pinout to me. You checked connectivity of switch against schematics?

 

[tony hunt]

I spent too long looking at your switch photos this morning. The switch is built differently to the drawing / diagram, but I think you have identified the poles correctly and seem to have wired the poles to the same Red/Orange/Green traffic light system as the diagram. Check where the other wires go.

Tracing a fault usually requires a systematic "divide and conquer" approach. Take one building block after the other. Maybe physically bypass two of the filter banks at a time to localise the issue on the other. Hope that makes sense.

Questions about the hum: was that with the case closed, or with the cover off? Does it remain the same, even if you change the EQ settings? Asking yourself those kind of questions can give you a clue.

 

[Script]

6dB difference. You said unit starts to 'clip'!? Is it your DAW clipping or the unit saturating ?

I'd check in this order
- product description and schematics (are where?)
- interfacing (using wrong cable? balanced / unbalanced connection & also patchbay)
- wrong hookup on XLRs somewhere (hot, cold, ground)
- transformer ratio (parallel/series if applicable. Wrong hookup? You have different switch, also different xfmrs?)

 

[amazingalf]

My description around the clipping wasn't very accurate. The return signal is so loud it causes my DAW to clip. My understanding is that the make up gain of the unit would result in unity between input and output.

Unfortunately the schematics for the MEQ are scattered. I have attached the filter section and line amp section schematics I have been able to find. Analogvibes provides the wiring diagram but nothing else.

I am using a balanced cable. The cables are properly wired and do not exhibit problems on any of my other rack gear.

My xfmers are not the originals but are recommended replacements from Sowter. This could definitely cause some differences.

I've replaced the switch with a new 3DPT and the unit appears to be functioning more in line with expectation. The bypass still seems to be hitting the filters. Its like switching between full eq (In) and less eq (Bypass). I'm beginning to wonder how the original units functioned. Based on the searches I have done I can't get a clear answer on what exactly the In toggle (bypass) actually sounds like in practice.

 

[amazingalf]

So I figured out what was going on after a few nights of staring and thinking. I actually had a ton of problems and I'm surprised it functioned at all given the amount of issues. Here are the issues I discovered:

• Power Transformer:
  • For other people building this: The example in the wiring diagram is for the Hammond transformer.
  • Secondaries were incorrectly wired. I'm using the Sowter PT and I had the pilot light and heater secondaries switched.
  • Grounding for the HV secondaries was soldered to the wrong location.
  • Electrostatic shield was not properly hooked up.
  • Remedying these issues reduced the hum by quite a bit. Voltages were also more in line with expectation.
• Switch:
  • The switch I had was broken. I replaced this with a 3PDT Carling Toggle On-On switch.
• Peak Controls:
  • P1 and P2 was switched on both peak controls. After swapping these the bypass functioned properly.
• Input Transformer:
  • I used the Sowter 3603 and had the wiring completely wrong. Sowter has a nice set of diagrams on their site which helped me understand what was going on here.

Overall this was quite the learning experience. The unit is now functioning as expected.

There is however still a bit of hum I would like to eradicate. I think this has something to do with the XLR input and output jack. Historically the XLR jacks I have used do not have a shell. This pair does.

Do I need to wire Pin1 of each jack to its respective shell? Or does the shell get wired directly to chassis ground?

 

[HerbertR]

Hi, my MEQ5 Builds is different to yours but what I can say is that there were a few things necessary to get all hum and noise 100% away off this sensitive MEQ5 circiuts.... 1. All connections replaced to rotaryswitches and Potentiometers with screened cabels! 2.Power IEC with a EMI/RFI Filter!!! - works like a wonder, no parasitic Powersupply frequencies - no hum and noise anymore.... (SCHAFFNER FN9222-1-06).
I also outsourced the toroid transformers used in my case into external boxes. But Block-Power transformers should make less troubles with interpersing of magnetical fields in other components as far as I know. Well done man! You will love this piece....

 

[amazingalf]

Thanks for the advice Herbert! I'll give the screened cables and Power IEC a try!

 

[amazingalf]

Hi, my MEQ5 Builds is different to yours but what I can say is that there were a few things necessary to get all hum and noise 100% away off this sensitive MEQ5 circiuts.... 1. All connections replaced to rotaryswitches and Potentiometers with screened cabels! 2.Power IEC with a EMI/RFI Filter!!! - works like a wonder, no parasitic Powersupply frequencies - no hum and noise anymore.... (SCHAFFNER FN9222-1-06).
I also outsourced the toroid transformers used in my case into external boxes. But Block-Power transformers should make less troubles with interpersing of magnetical fields in other components as far as I know. Well done man! You will love this piece....

When using screened cables connected to pots and rotary switches, where do you connect the screen?

 

[HerbertR]

When using screened cables connected to pots and rotary switches, where do you connect the screen?

I ve connected them directly to the ground. I ve used screened 3 Pole and 12 Pole Alpha Wires (3312 und 3303) from Farnell. They are expensive ( about 5 Euro and 9 Euro per Meter)