Here’s my take on the M2/8Ohm/Out3. Circuit and specifications according to John's post #1 in this thread. I borrowed part of Gene’s layout in post #2207 (p 111), to have R3-R8 placed in the same order as the “descending” switches.
The bottom of the “box” is made from 1,5 mm thick, perforated aluminum sheet (5 mm holes). Hope that will be sufficient heat sink. Resistors mounted flat to the sheet, with thermal compound in between. The sheet is reinforced with 12x12x2 mm aluminum profiles. Coil strapped with plastic cable ties to a piece of wood. Top cover is a black aluminum mesh (for covering vent holes, etc.). Ventilation should be sufficient. Dimensions are 300x140x80 mm.
I did the following tests on the unit (test report enclosed), if anyone’s interested. I found a bad solder joint!
I got problems with the “Cliff UK” speaker jacks. For two of them I get consistent readings when measuring on the solder lugs and measuring with a plug. However, for the other two I get OL (overload) for the plug, but reasonable reading at the lugs. For one of the bad jacks I can get consistent value for the plug, if I press the plug sideways. However, once I release the pressure reading goes back to OL.
I cannot trust these jacks, and will replace all four. Consequently, I have not yet played with this unit. Found the following two types instead, and will try to source them (in stereo/TRS configuration). Any comments on speaker jacks? Is it possible to use “switched” jacks?
https://www.parts-express.com/Switchcraft-Z15J-1-4-High-Power-Speaker-Jack-093-1056
https://www.parts-express.com/Neutrik-NMJ6HC-S-1-4-Stereo-Phone-Jack-Switched-090-976
I hope that @JohnH and Marshall Forum consider this fair use of names. I want to give credit. Finally, thanks to John and Gene for all work and support!
The bottom of the “box” is made from 1,5 mm thick, perforated aluminum sheet (5 mm holes). Hope that will be sufficient heat sink. Resistors mounted flat to the sheet, with thermal compound in between. The sheet is reinforced with 12x12x2 mm aluminum profiles. Coil strapped with plastic cable ties to a piece of wood. Top cover is a black aluminum mesh (for covering vent holes, etc.). Ventilation should be sufficient. Dimensions are 300x140x80 mm.
I did the following tests on the unit (test report enclosed), if anyone’s interested. I found a bad solder joint!
- No connection between chassis or any part of the circuit
- Continuity within each sub-circuit
- Resistance between different lugs on in- and output. Interesting thing: Resistance increases as more attenuation is added, until the last step when it decreases slightly.
I got problems with the “Cliff UK” speaker jacks. For two of them I get consistent readings when measuring on the solder lugs and measuring with a plug. However, for the other two I get OL (overload) for the plug, but reasonable reading at the lugs. For one of the bad jacks I can get consistent value for the plug, if I press the plug sideways. However, once I release the pressure reading goes back to OL.
I cannot trust these jacks, and will replace all four. Consequently, I have not yet played with this unit. Found the following two types instead, and will try to source them (in stereo/TRS configuration). Any comments on speaker jacks? Is it possible to use “switched” jacks?
https://www.parts-express.com/Switchcraft-Z15J-1-4-High-Power-Speaker-Jack-093-1056
https://www.parts-express.com/Neutrik-NMJ6HC-S-1-4-Stereo-Phone-Jack-Switched-090-976
I hope that @JohnH and Marshall Forum consider this fair use of names. I want to give credit. Finally, thanks to John and Gene for all work and support!
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