Happy New Year!
Having sorted the MA-12's, time to move onto to the USC-1 preamp...
It's designed to be mounted into the front of a cabinet of some description.
They keen-eyed amongst you may notice the absence of mains transformer. The power supply is derived from the MA12 amplifier via a multi-pin lead.
It's a reasonable layout, but the chassis edges are sharp. There's two PCBs, each containing Two EF86's and an ECC83. The EF86's produce two diferent levels of gain, one setting for TAPE1/2, GRAM and MIC, and the second stage conntecting the inputs from RADIO and AUX. There's no real RIAA equalisation on the GRAM but a little bit of loading. The second EF86 feeds a tone stack, which is then amplified by one half of the ECC83, followed by the other. There's also a low pass filter implemented here. It's full of horrible Hunts...
... and a good while spent changing all those nasty caps out...
I chose not to re-stuff the can on this occasion, as there's a nice bit of tag strip that forms the power supply filter, so they could just be mounted there ...
At some stage in the past, the volume control has been replaced, for one without a switch. It's also been wired up backwards. It's now wired up correctly, so as not to deafen the unsuspecting user.
How's it sound? I thought there would be more "blow" due to the EF86's cascaded, but it's reasonably quiet. Obviously the TAPE setting is nothing more than a head amplifier, so not suitable for a line level output from a modern deck. Pity about the RIAA, or lack thereof...* Tone controls are actually very nice, and don't offer too much cut or boost.
The switch-gear is lovely, and has stood the test of time really well.
*upon reading the manual, it transpires the input stage was designed for a Decca FFSS cartridge, the manual does give some suggestions about use with other carts.
So what is special about the Decca FFSS cartridge? Is it perhaps mechanically tuned to apply the RIAA characteristics? (Or just crap at higher frequencies).
ReplyDeleteI'll attempt to find out...
DeleteHi,
ReplyDeleteHelp. I have a Linn Numerik with a SMPS (less than) Brilliant power supply U25W.
My anti-surge fuse at the power in socket popped when I ran up the DAC for the first time in a year (my equipment has been in storage). Linn don't want to know.
It is similar to the Karik you worked on, my son is bollocking me down the phone for even contemplating touching it (he knows colleagues in the US who died touching SPS's).
Yes, it's probably the caps you point out because Linn sourced the unit from another manufacturer and some are low temp units (just web chat, not knowledge) but I don't want to die yet. The fear of Covid is bad enough. The question is - do you do those repairs - if I 'pull' the SMPS and box it up (is it ok to do that) and sent it could you, would you consider repairing it. If so, how much and how long.
PS I'm in France (which doesn't help).
Richard
Anyone looking for a heathkit usc-1?
ReplyDelete