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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 8th Mar 2019, 4:51 pm   #1
Bomber county
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Default Tesla 525a

Just bought this from the local car boot much to the disapproval of wife....Are these rare in the UK? (bought off one of our Eastern European cousins). I didn't know Tesla even made radios, had it been a Bush or something run of the mill I'd have left it. It seems to have FM, 2 short wave bands and and aux input so quite useful in working order. All looks original apart from the codged up volume pot someone has stuck through the near mint speaker grill cloth Most of the caps and the valves have Tesla branding on them.
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Old 8th Mar 2019, 6:04 pm   #2
mark_in_manc
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Default Re: Tesla 525a

Tesla were the Czechoslovak electronics combine, so that's which part of Eastern Europe it hails from. Bits are still going, at least some under other names - 'JJ' valves use machinery sourced from a Tesla plant. I drove past Tesla Stropkov a few years back in Slovakia, which still has a web presence and appears to be making things.
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Old 12th Mar 2019, 10:57 am   #3
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Default Re: Tesla 525a

"Chocolate block" capacitors? I can't find any reference to them online. I have replaced the 3 that were obviously toasted but all is still not well (HT drops after a few seconds). Should I replace all of them regardless. What type are they? Also what is the thing on the mains transformer in photo two that has been tied up with fuse wire? Is it some kind of fuse or thermal cutout?
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Old 12th Mar 2019, 2:16 pm   #4
60 oldjohn
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Default Re: Tesla 525a

Looks like a thermal cut-out wired in the mains lead to protect the transformer marked PO1. The circuit is available top right of this page for 1.99, unfortunately it is not in English.
C72 should change, (Audio coupling capacitor) if this is one you have changed the next suspect part is U1 the bridge rectifier if a modern part is used you will need an extra resistor.

John.

Last edited by 60 oldjohn; 12th Mar 2019 at 2:24 pm. Reason: Added last sentence
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Old 12th Mar 2019, 6:12 pm   #5
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Default Re: Tesla 525a

I found a copy of the schematic for it but it looks to be in Polish. Not too much of a problem but they seem to write the component values differently to we do. All the resistors are the same red oxide red colour with no colour code bands as well. Weirdly I have an audio output but the tuner side is dead at present and the magic eye isn't lighting up, I presume due to the 250v not maintaining (it comes up then drops off to 150v).
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Old 12th Mar 2019, 7:34 pm   #6
60 oldjohn
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Default Re: Tesla 525a

Regarding the resistors, are the values marked on in ink ? Most of the capacitors are marked in PF, C72 is marked as 10K, 10,000 pf or 10nf.


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Old 12th Mar 2019, 8:52 pm   #7
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Default Re: Tesla 525a

Being completely off-topic. Being born in the last years of the USSR I can say that Tesla was a well remembered good brand from these days. They manufactured everything from : amplifiers, cassette decks, tv's, radios, radiograms, r2r's, speakers, turntables,cd players, oscilloscopes,laboratory equipment, ultra sound generators..... you name it . There was a well knows Tesla 'Talisman' radio which came along with a beautiful 'Sonet-Duo' reel to reel, one that my grandparents owned, beautifully decorated, just as a jewel. We even had a CM130 reel to reel deck which was made in 1986... I think? ... The original factory collapsed in 1990. As I'm aware, the factory operated from the 40's till the split of CzechoSlovakia into what we know it as today
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Old 12th Mar 2019, 10:25 pm   #8
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Default Re: Tesla 525a

Quote:
Originally Posted by 60 oldjohn View Post
Regarding the resistors, are the values marked on in ink ? Most of the capacitors are marked in PF, C72 is marked as 10K, 10,000 pf or 10nf.
It was the 10K etc marks on capacitors that threw me a bit, having never come across that before. The resistors do have some pen markings on them but some are absent, I tested a few of them with a meter and the values are spot on. Better than brand new Chinese ones. I was scratching my head regarding the rectifier as I couldn't see anything in there that I would normally identify as a rectifier. Its a flat oblong slab like thing tucked away down the front of the chassis. Do they have a reputation for going bad? its a B250C100.
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Old 13th Mar 2019, 12:51 pm   #9
60 oldjohn
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Default Re: Tesla 525a

If it is a Selenium Rectifier they are usually past their best 50+ years on. A modern Silicon rectifier will need a resistor to keep the voltages down a bit. Try ~100 ohms to get the correct 210v. The rectifier needs to be on the chassis it act as a heat sink. Also the 100R resistor needs to be maybe 5w on a heat sink, a bit of experimentation needed, unless someone can do the maths.


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Old 13th Mar 2019, 2:10 pm   #10
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Default Re: Tesla 525a

The Sobell 511W I fairly recently reworked had a Selenium rectifier (the first I've ever encountered).

Started out with a 1N4007, a small selection of 3 Watt metal film resistors & monitored the HT as I wound up the Variac. Final value for full mains with correct HT throughout, was 220 ohms, which has around 2 watts to contend with at high volume. After some 4 hours of merriment, the resistor remained tepid.

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Old 13th Mar 2019, 7:21 pm   #11
60 oldjohn
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Default Re: Tesla 525a

This set uses a Bridge rectifier. If you have a Variac proceed as above. If you go by guessing the voltage, it will rise considerably while warming up then fall.


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