Identification of blown marine radio power supply transistor - Page 1
Forum section: electronics / beginners

author: highcap
author_rank: Contributor
timestamp: June 01, 2023, 07:55:16 pm
content: Hey all, I'm glad to be part of the forum!I have two shorted transistors (emitter and collector) on a marine radio dc dc converter, that probably setps up voltage from the 12v input.The problem is markings are washed off, so I don't know what transistors to place in.If someone can give some hint or a general-use transistor that would be of great help, as I will greatly enjoy to see this old device working.Thank you!

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 01, 2023, 09:45:07 pm
content: Make and model # of the unit would help. Maybe there's a manual out there.The other thing to do would be to trace out the circuit, and knowing the power requirements, make an educated guess.

author: highcap
author_rank: Contributor
timestamp: June 01, 2023, 09:57:30 pm
content: It was the first thing I tried, searching by the model to find the schematics, sadly didn't end up with anything.International Marine Radio Company Limited - ENGLANDType: RALUTracing out a circuit is out of my expertise sadly...

author: Andy Watson
author_rank: Super Contributor
timestamp: June 01, 2023, 10:18:32 pm
content: To me, the power supply appears to have been a home-made contruction. Is it part of the radio set, or is a unit that "somebody" has cobbled together? I think your only option here will be to trace-out the circuit and find the power-requirements of the radio.

author: Benta
author_rank: Super Contributor
timestamp: June 01, 2023, 10:25:05 pm
content: Quote from: Andy Watson on June 01, 2023, 10:18:32 pmI agree.Either DIY or "British automotive standard".

author: Fraser
author_rank: Super Contributor
timestamp: June 01, 2023, 11:01:07 pm
content: The transistor that you have shown appears to be an OC36 Germanium PNP type. The other transistor may be the same, a complimentary npn type or something completely different ! Please supply detailed images of the transistors top so that I can see whether I can help.The power supply has the appearance of a “home brew” design and the Germanium type of transistor fits with the era of components used in that design. Germanium transistors are still available via eBay but some are expensive and “New Old Stock” that may have failed over the years due purely to age.https://www.web-bcs.com/transistor/tc/oa/OC36.php?lan=Fraser

author: highcap
author_rank: Contributor
timestamp: June 01, 2023, 11:08:31 pm
content: Quote from: Fraser on June 01, 2023, 11:01:07 pmI will get back with more pictures soon, thank you!Quote from: Andy Watson on June 01, 2023, 10:18:32 pmI can't tell... for a 1960 transceiver everything looks homemade inside. The power supply sits at the bottom of the wooden box, and above there's the rf oscillator and amplifier with a couple of valves. Alu-separated compartments, similar as the power supply cover.

author: highcap
author_rank: Contributor
timestamp: June 01, 2023, 11:26:30 pm
content: Back with the pictures and a surprise!As photographing one of the transistors from an angle, with the flash on, markings magically appeared (good tip for others with same issue). Likely a Mullard OC28 2S4. Note that at normal eye view, from any angle, the markings looks about 85% gone. That's the unscrewed one which I measured out of the circuit, and as this one is removed, there's no more short at the DC input. (Both transistors having E and C shorted)The transistor in my first post is the one still screwed into place. This is the remaining to be guessed now, thank you for your help again!

author: Fraser
author_rank: Super Contributor
timestamp: June 01, 2023, 11:58:00 pm
content: In your first post I can read the transistor as an OC36

author: highcap
author_rank: Contributor
timestamp: June 02, 2023, 12:15:03 am
content: Quote from: Fraser on June 01, 2023, 11:58:00 pmRealized I have wrongly flipped the image vertically (after the photo was with upside-down markings), which can give a false impression of the reading.This is the proper perspective:

author: Fraser
author_rank: Super Contributor
timestamp: June 02, 2023, 01:03:12 am
content: Interesting, with the image in the correct orientation I can see OC 2 and then an unclear number that could be an 8. If that is the case, then both of the power transistors are OC28

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 02, 2023, 02:32:18 am
content: Yea, there're probably the same. Classic DC to DC converter circuit. Probably something like this, but with more taps, rectification, and filtering for tube level DC voltages: