Tony L
Administrator
Thanks to a very kind site donation recently this landed on the doormat...
I was aware of Magnum from back in the ‘80s and the connection to IncaTech and Colin Wonfor (@CJ14 here on pfm), but I knew nothing at all about this preamp. It describes itself on the back panel as a “Mains-Battery-Powered Stereo Preamp”, which kind of makes sense when one sees the huge capacitor banks inside. It also appears to have both a MM and MC stage on separate inputs, so a potentially useful bit of kit.
Here’s an inside pic:
At present it isn’t working, not so much as a light on the front, but I’ve had a chat with Colin W via email and have a few ideas as to what to check. From my own diagnostics power is getting in, the bridge rectifier seems to be doing its thing, but the relay isn’t clicking open to send power to the rest of the preamp, i.e. the problem looks to be very close to the start. There is no sign of damage, the big caps all look good and in-circuit testing suggests very low ESR etc.
My Retro Chip Tester Pro (thread here) can test the 4000 series logic, and the 4027 near the relay passes without issue (as do the two 4052s elsewhere). The chips are all in nice posh turn-pin sockets. I also have some 555s from when I was fixing an Apple II, so I swapped a new one in and again no change. It’s not the chips! After exchanging emails with Colin it looks like the issue could well be the transistor, or diode next to the relay. I’ve got the transistor in order. I think the diode may be ok, but I’ll need to pop it out to test it properly.
Anyway, potentially an interesting preamp…
PS A Nytech CA252 came in with the same donation. That will be cleaned-up and moved on, probably as a restoration project for someone else at some point in the future. I’m going to focus on this one for a while.
I was aware of Magnum from back in the ‘80s and the connection to IncaTech and Colin Wonfor (@CJ14 here on pfm), but I knew nothing at all about this preamp. It describes itself on the back panel as a “Mains-Battery-Powered Stereo Preamp”, which kind of makes sense when one sees the huge capacitor banks inside. It also appears to have both a MM and MC stage on separate inputs, so a potentially useful bit of kit.
Here’s an inside pic:
At present it isn’t working, not so much as a light on the front, but I’ve had a chat with Colin W via email and have a few ideas as to what to check. From my own diagnostics power is getting in, the bridge rectifier seems to be doing its thing, but the relay isn’t clicking open to send power to the rest of the preamp, i.e. the problem looks to be very close to the start. There is no sign of damage, the big caps all look good and in-circuit testing suggests very low ESR etc.
My Retro Chip Tester Pro (thread here) can test the 4000 series logic, and the 4027 near the relay passes without issue (as do the two 4052s elsewhere). The chips are all in nice posh turn-pin sockets. I also have some 555s from when I was fixing an Apple II, so I swapped a new one in and again no change. It’s not the chips! After exchanging emails with Colin it looks like the issue could well be the transistor, or diode next to the relay. I’ve got the transistor in order. I think the diode may be ok, but I’ll need to pop it out to test it properly.
Anyway, potentially an interesting preamp…
PS A Nytech CA252 came in with the same donation. That will be cleaned-up and moved on, probably as a restoration project for someone else at some point in the future. I’m going to focus on this one for a while.