Please help with AC/DC WWII era Blaupunkt US4 repair and saftey - Page 1
Forum section: electronics / repair

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: September 26, 2019, 04:59:08 pm
content: I came across an old tube radio from around 1949 that I would like to restore and give a second live to.It is a Blaupunkt US4 in very good optical condition.I was thinking of the following:Repurpose UCH 11 since I will not be needing the oscillator / AM radio part.Add a suitable transformer like Elma TT IZ 63 (230V primary, 2x115V -> 230V secondary, 65VA).Add a Raspberry Pi to act as media player with line input + WLAN / Bluetooth / Airplay connectivity.Apart from of course replacing all the caps, cleaning the sockets, checking all parts would you rather:A: Install an isolation transformer + suitable PSU for the RasPi and decouple the audio signal with a separate audio transformer.B: Redo the whole power section, add a suitable transformer with multiple voltages for heaters, grids, anodes etc.C:Apart from a first visual inspection nothing else was done. I do not know of the state the NTC and power regulator (UB10000 / U2410PL) are in, but let’s assume at least the NTC is done for after all those years.I will not be doing these modifications on my own. There is an electrician involved, but I would like to prepare everything as thorough as possible and learn something along the way.I have worked on a bunch of tube amplifiers before, but these were all guitar amps from a more recent period that only needed caps / resistors replaced, a couple of new tubes and some biasing or some modifications to the preamplifier section.Sure, one could just recap this thing, not add any external accessories, keep it as close to original as possible and make sure it is plugged in the right way - but that is not what I am looking for.Thank you and kind regards!

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: September 27, 2019, 06:46:07 pm
content: just some quick observations:being a 1949 built, your radio expects 220VAC as supply. 230 VAC will overheat the valves' heaters and shorten their lifetime. (actually, the schematic states 220VAC, you probably just overlooked it. so you have to select your isolation transformer accordingly.also note that you can't simply pull the UCH11, because that would open the heater daisy-chain (that's what the "U" in UCH11 stands for: daisy-chained heaters that may be directly connected to mains, as opposed to "E" type heaters that do require constant 6.3VAC)

author: AnneRanch
author_rank: Contributor
timestamp: September 27, 2019, 09:15:53 pm
content: Yes, safety first!Chassis is OFTEN "hot" - connected to mains.These "U-niversal" tubes are usually problem / dead.Start with checking the continuity of the "heaters / filament " ( with NO mains connected !) and associated serially connected devices - you have what appears dial "light bulb" in series with filaments. The filament wires are easy to identify - generally larger gauge then "signal wiring ".If possible , do NOT unplug the tubes , the cement holding the glass gets brittle over years.Most of the time the power supply electrolytic caps , assuming they are liquid type ( 1949 !) will need "forming".Check with somebody who done that! Usually you need to start with LOW voltage and gradually increase it.More than "isolation transformer" you MAY need adjustable ( variac ) transformer to do that.NEVER , ever reverse polarity to electrolytic caps ! They WILL explode!Be also careful with "audio" transformer - it is also "hot !".Good luck.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 27, 2019, 09:34:45 pm
content: When powering up very old equipment, it always pays to use a Variac and start increasing the voltage s-l-o-w-l-y while monitoring the input current consumption.

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: September 28, 2019, 11:16:21 am
content: Thank you for all the suggestions.Yes, I considered using an isolation transformer with a 10:9 ratio or something along that lines. I do not plan on powering this thing on in the state it is in, but rather do a complete rework. Mainly use whatever is there in terms of component layout and replace more or less everything with known working parts.I know about capacitor reforming, but since I plan to have this radio up and running again on a daily basis I think it is best to just treat all the parts as "needs replacement".Regarding the U type tubes: After some reading up on the subject I think these will see replacement by their E type counterparts.After your suggestions and some more reading through various articles I think this is what I am going to plan for in the next steps:- add a new power supply section (transformer with multiple secondaries), bridge rectifier (possibly based on a GZ34, since I have these already)- rebuild the electronics more or less to specs from the original circuit but substituting the universal tubes- remove the AM receiver parts to gain some space for the transformer and RasPiUCH11 would not be removed from the circuit, but only used as a mixing stage (replaced by ECH11)Any more suggestions?

author: Cyberdragon
author_rank: Super Contributor
timestamp: September 28, 2019, 10:14:41 pm
content: You don't need a whole Pi for an audio source. You can get tiny mono bluetooth recievers for a few bucks from China, which is the most common mod for old radio recievers, and a direct line input is pretty simple.

author: Yetanother_HAMenthusiest
author_rank: Newbie
timestamp: September 30, 2019, 09:45:06 pm
content: No disrespect to Dave, but if you haven't already done so, check Paul Carlson (Mr. Carlson's Lab)'s videos for safety instructions when dealing with tube equipment. As others have pointed out, a lot of the chassis were designed without anything resembling safety and can easily be either directly connected to mains, and/or have a filter capacitor fail as a short to chassis.

author: pbarton
author_rank: Frequent Contributor
timestamp: September 30, 2019, 10:36:44 pm
content: https://www.amazon.co.uk/s?k=GPO+Winchester&i=electronics&ref=nb_sb_noss

author: vindoline
author_rank: Unknown
timestamp: October 01, 2019, 11:59:16 am
content: I've restored quite a few older German tube radios and several times I've tried your idea of "modernizing" it. FWIW, I've never been very happy with a hybrid solution, trying to reuse the old amplifier with a modern signal source. There usually isn't enough room to make things fit nicely and the project just drags on tryouts ng to bodge solutions. For example, I have no idea where you would fit a transformer! From the one photo it looks like the chassis is in excellent condition. If it were me, I would completely remove the chassis and put it somewhere safe in case you want to sell it someday to a collector. I'd then add the RPi media server/amplifier and reuse the speaker and volume knob and possibly add some nice LED lighting for the dial glass. Everything could be powered from an external supply. If you do it carefully and thoughtfully, someone could reverse your mods later if they want to restore it to original condition.

author: AnneRanch
author_rank: Contributor
timestamp: October 01, 2019, 02:31:52 pm
content: Regarding the U type tubes: After some reading up on the subject I think these will see replacement by their E type counterparts.Of course you are free to restore as you see fit.It is not "just replace the U - tubes' - besides adding filament transformer you will have to rewire the entire filament circuit from serial to parallel.Depending on how the wiring was done originally, the filament wires may have been installed first...

author: TimFox
author_rank: Super Contributor
timestamp: October 01, 2019, 11:05:25 pm
content: If you need to reduce the line voltage by 10 volts (230 to 220 V), the cheapest method might be a “buck” transformer with a 230 V primary and 10 V (or 12 V?) secondary. Carefully connect the secondary to reduce the voltage (the other connection will increase it). The secondary current rating must be sufficient for the load current. Such a transformer does not isolate the load, but is smaller and cheaper than a two-winding isolation transformer.

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: October 03, 2019, 12:10:47 pm
content: Alright, so now I am torn.Actually I do not really care much about the "retro" aspect, though I very well like the look of this radio of course.My thinking was, that this thing would not see any real use if restored to the original specs. I can not think of anyone making use of an old AM radio, though my dad seemed to be somehow interested in having it around.I was thinking of doing a proper rebuild to make this a daily player and removing the AM receiver parts should free up enough space to add a suitable transformer, ditch the universal tubes for E types and supply the heater filaments with 6,3VAC from the transformer.Regarding those simple bluetooth modules: I have a bunch of those lying around, though if I was going for the full overhaul I would want a dedicated media player with network streaming ability and a small web server running to make it accessible via a web interface.I will leave this sitting for a while and meditate on where to go with it. You might be correct in that it is probably worth something to a collector in the current state with it all being original (minus the missing back cover unfortunately).Decisions, decisions.Anyway, thank you all for pitching in with your thoughts and suggestions!I’ll keep you posted.

author: TimFox
author_rank: Super Contributor
timestamp: October 03, 2019, 01:15:34 pm
content: The main problem with restoring an old radio is that you cannot listen to the old programs anymore.