I have this very large German table radio that has a blown fuse. I'm trying to trace out the B+ supply and filter circuits to see if any of the filters are shorted or anything on the main B+ supply line that could have caused the fuse to blow. I am having a hard time seeing if this radio has diodes or a rectifier bridge, since I don't think any of the tubes are rectifiers. Does anyone have a schematic of this model or know of it being common for these to develop shorts in them, funny thing is I see one on Ebay now and they say it has the same problem as mine!

I'm not familiar with the opus 55, but i do have an Opus 6.
I do some work on Grundigs ... The germans used selenium rectifiers, often disgusied as black tubes, that look just like tubes.
I would plan on doing a total recap of the unit. (there will be lots & lots of caps). I would start at the power supply I'm guessing 2 x 50uF @450vdc. These are great radios when redone.. IMO light years ahead of amaerican sets of the same vintage.
Also. check the speakers, My Opus had an open voice coil on a woofer, and plan on rebuilding the electrstatic tweeters, as the foam dries out and then KAPUT>>>

OK now looking closer, I found that selenium rectifier, it is a cylindrical can that resembles a can type of electrolytic cap, so that is what I thought it was, but actually I see it appears to be a bridge rectifier with 2 leads going to the transformer and the other 2 leads being B+ and B-. I tested the radio with a variac and found this rectifier to get quite hot, even though there appeared to be no shorts across the B+ line. I'll replace this rectifier with a modern bridge rectifier or 4 diodes and replace filters and see what happens. I also see there is a 6AB4 phase inverter tube that is not lighting up, will need to replace for proper audio.

You'll need to add some series resistance in the AC input to the replacement diodes - those selenium rectifiers had quite a bit of internal resistance - IMHO put about 100ohms 2watts in each AC leg.

After redoing the rectifier (It shouldn't get 'super' hot) I would recap the power supply.. as I said before. most likely 2 x 50uF . This is important, as if they are shorted, all bets will be off.

Dave,
I don't know how close this is, but here is a schematic for an Opus 52.
http://www.oldradio.de/service/schaltun ... opus52.png

Hello DaveD,

If you PM your e-mail address to me I can send you a schematic for an Opus 55 TS (Euro model, in German).

Regards,

René

Thanks Rene' for offering to send me the schematic, I have PM'ed you my e mail address. Hopefully it is similar enough to my model, since I think mine is the "export" or American version of it. I noticed in some photos of this radio that some models of it have electrstatic tweeters on the sides, but mine doesn't, the sides of the cabinet are solid wood so it can't provide the tweeters. The front speakers seem to consist of 2 woofers and some kind of larger round tweeter, I"m not sure if it is a PM type or electrostatic, but I'm thinking PM since there are only 2 wires connected from the chassis to the speakers. There are 2 or more other wires that appear that they are also supposed to be for speakers, but are not connected anywhere, and are terminated with tape. I'm thinking that those are leads for models that would have the side tweeters.
Yes, I definitely will be taking the advice of replacing at least the main elecrolytic filter caps (2X 50 MFD, 450 V) and then eventually probably all the tubular caps. I was wondering that if I make up a silicon rectifier out of 4 diodes in a bridge or intall a pre-made bridge rectifier if I would have to add some resistance to the AC input to the bridge or else the output DC line to the filter circuits due to the fact that the modern diodes would not have the internal resistace of the old selenium rectifier. I would think that since this is a transformer powered set, the amount of increase in B+ supply voltage should not hurt it, but maybe it would. I was reading 250V AC across the high voltage winding of the transformer with nothing connected to it (but with the filaments connected to the filament winding). I would think that the B+ when rectified and filtered would be about 350V DC which might be OK but maybe slightly high? I guess I could try expeimenting with various values of power resistors in series (maybe starting with about 100 ohms) and see what the B+ voltage ends up being under load. If the schematic says what some of the B+ voltages should be, I could know what to look for when I change the values of the resistor. I hope to get to work on this set later today.
Thanks for all your input, guys, I think this will be an awesome sounding and performing radio once done! Dave

...I just looked up the Opus 55 TS at Radiomuseum. This seems to be the 'parent' model that your Opus 55 Super (which is indeed listed as a US-export model) and at least one other Opus 55 variant is based on, and it's the only Opus 55 variant that Radiomuseum has a schematic for. I've looked at that, and after you replace your selenium rectifier and 50+50µF electrolytic, you'll want to see 280 volts coming off the rectifier (or on the first 50µF cap), and 245 volts on the second 50µF cap. (According to the schematic, a 50kΩ/volt meter was used to measure this.) You'll probably see a higher voltage there with the new rectifier and caps and will need to add a resistor to adjust your B+ accordingly. The schematic shows an operating current of 110 mA (0.11A); that should help you calculate an approximate resistor value (desired voltage drop/0.11) and minimum wattage rating (desired voltage drop x 0.11). It's probably going to wind up being a low-ohms, high-watts resistor and should come right off the + connection of the rectifier.
...This model also uses a filter choke between the two 50µF caps that apparently has a value of approximately 10H and an internal DC resistance of approximately 330 ohms, and apparently has been known to develop shorted turns internally (I found a discussion at Radiomuseum where a few other members were talking about repairing one of these, and one of them mentioned these values when discussing the choke, as well as advising to make sure there aren't any shorted turns in it). If there are shorted turns, the choke will run hot and that 245V B+ might be off somewhat...

In my Telefunken Opus 7, I ended up replacing the input power choke coil, the filter capacitor and two EF89 vacuum tubes. I was unable to ascertain the value of the original choke coil that was across the dual 50uF 350VDC filter capacitor since it measured open. I ultimately installed a Hammond 155H 5H 50mA choke coil that proved to be adequate both in inductance and current handling.