Repair of a legend
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Introduction:
This is the first time in my life that I got the chance to lay my hands on a piece of the Holy Grail: A defect Moog Modular 904B voltage controlled high pass filter. I had done much analysis of the Norlin service manual circuit to create my clone of the 904B, the T904B, but it is a different story to have an original on my operation desc!
The module and the modular system it normally resides in is owned by the english record producer Andy Gray-Ling, who contacted me concerning additional modules for his modular and technical questions.
The 904B
When the module arrived, I tested it. It created just a heavy noise and crackling output, no input signal was processed by the filter. The reason was found quite fast, a dead PNP transistor (2N4058), which was part of the input buffer stage of the first filter pole (see above). But for exchanging it I had to disassemble the module totally, meaning in this case to spread the to board sections of the module apart. For me it felt a bit like a kind of "blutaar" (blood eagle) execution method, a sacrilege for this veteran, but was necessary.
The layouts however don't fit to any of the circuits I know, so this is a new circuit variant for me. The boards and the components are quite corroded, but after replacing the 2N4058 the module worked perfectly again, sounds brilliant after all that decades!
The next picture shows a big noise on the output signal which occured after replacing the dead transistor. The reason seems to be a kind of ground loop, wich occurs when the front panel is removed. The noise vanishes when the front panel is mounted again. If I am right, this is poor engineering of Moog to be panel-mount-dependent for grounding!
Anyway, the module works again, see
Please send questions or remarks to:
Carsten Toensmann
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