24-01-2022, 12:10 PM
(23-01-2022, 10:43 AM)Radio Fixer Wrote: Cant see how to PM Craig?
It depends on which theme you're using. Start by finding him in the Member List, and there will be a link to PM him there. If you're using the MyBB Default theme, that link is obvious, but if you're still using Vienna, you have to click his name (above his avatar) to bring up a menu with the contact options.
Alternatively, you can go to your PMs, click on "compose", and start typing his name - it will auto-suggest as you type.
Even easier, this direct link should get you started:
https://golbornevintageradio.co.uk/forum...d&uid=1296
Hopefully he'll be happy to post the schematics in this thread? Though it wouldn't be difficult to reverse-engineer them. But schematics probably aren't necessary.
It's one of the later models, and does look to be fairly straightforward - an op-amp based phono pre-amp, and no line pre-amp. So the line level sources go straight into the power amp via the input switching and volume/balance pots. The schematics posted earlier are for the earlier models, so we can disregard those.
Yes, I think it's worth changing the electrolytics, given the age, and the fact it's in pieces on your bench now. After all, there's not too many of them. The reddy/brown round ones don't have the best reputation, although I can't say they've given me too many problems over the years. The schematic would be helpful to determine (a) if they're polarised or not, and (b) if they actually have to be. Here is an example replacement in non-polarised form:
https://uk.rs-online.com/web/p/aluminium...rs/1763786
Shame they come in packs of 20. I'm sure other suppliers would sell in smaller quantities. But there's a good chance it might be in the lower "limb" of the negative feedback network - if so, it shouldn't have much DC bias on it (should be well under a volt - perhaps only 10s of millivolts), so a polarised one might be suitable. But that needs verifying before proceeding.
The pots look to be fairly standard types that should be available today (although there's a chance the balance control could be a special type). The input selector switch might be harder to find today. Musical Fidelity used them in the A1 and related amps back in the day, and they were quite unreliable thanks to a poor pre-amp design that put a small DC current through both the switches and the volume control. A replacement today involves a bit of metalwork - not pretty, but perfectly safe and reliable. See the bottom of this page: https://www.markhennessy.co.uk/mf_a1/mods.htm - something similar could probably be done here if necessary, but hopefully this switch is OK.
Don't overlook the Tape switch. If it's a "tape monitor" switch - as was the norm at this time - then it interrupts the signal from the rotary switch. So when in the out position, the signal from the rotary switch is passing through the NC contacts. In the MF amp, these always need replacing because of the DC issue...
Even if it's behaving for you now, I'd say that it's definitely worth doing a bit of preventative maintenance to keep it alive for another 40 years
