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Old 20th Apr 2008, 12:31 pm   #1
howard
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Default 1960 Toshiba 8TM-300SB MW/SW 8 transistor pocket radio

Hello again,

I recently bought this early Japanese Toshiba (on the back marked Tokyo Shibaura Electric Co. Ltd) transistor pocket set complete with leather carry case from eBay for £5.50. It was quite grubby but only needed a quick clean with toothbrush and upholstery cleaner and cosmetically it came up really nice, no dents or scratches, just one little spot on its gold grille. It was totally dead though which is unusual for a Japanese transistor set even of this age, so I took it over to Ron Bryan to take a look at. It was attached to a 9 volt power supply and still nothing, but gradually it came back to life ! It sounded awful at first but slowly but surely it improved and eventually was working quite well on SW and was working from 200 - 300 metres on MW but then it would fade away. So I started a search for service data for it.

This is a very rare set, I only found one brief mention of it on the web, but I did find some info on a similar model, the 8TM-300S, which is identical in appearance and which initially I thought was just a US variant. No sites offer service data for this model either but I eventually found that it was featured in Sams Transistor Service Manual (TSM) volume 7 published in 1960 in the USA. So I emailed a couple of US websites who keep Sams manuals but didn't get a reply. So next I placed a request on the forum, not expecting anything and surprise surprise, within a few minutes petervk2mlg posted a circuit diagram on the forum from an Australian manual. This circuit diagram for the 8TM-300S model has a 4.5 volt battery so in the hope of still finding the 9 volt circuit diagram, I sent a PM to batterymaker on the forum here to see if he had Sams TSM volume 7 and by chance he did and very kindly emailed scans to me of the relevant pages. The Sams TSM service data with its 'CircuiTrace' is excellent. Ron checked to see how these two circuits matched up with my set, both are for the 4.5 volt set and are essentially the same circuit but capacitors and resistors differ in values.

Ron spent some time on this set which is really fiddly to work on. The first problem found was a blue plastic cased electrolytic which had liquid oozing out the top of it so it was replaced but that made no difference to the performance. Another of these which was accessible was also replaced, as a precaution. The 390pf feedback capacitor was checked but was fine so that was put back in. So then the 2SA92 separate oscillator transistor was replaced with the closest European equivalent, an AF124, a very fiddly job as this transistor is deeply recessed and had to be lowered into position on the end of a piece of tubing, but that made no difference either. The 5000pf ceramic disc feedback capacitor was then tested and that was found to have dropped down to 2800pf so a temporary substitute capacitor was fitted and that at long last brought all of MW back to life. We then tried to find a suitable replacement 5000pf ceramic disc capacitor, and we both keep them, but every one we tested had the same problem. Eventually I found a 5600pf orange dip 630v cap around the right size and that was fitted. The original Toshiba 2SA92 transistor was then reinstated (which worked better than the AF124) and the quiescent current was increased slightly by soldering a small 22k ohm resistor on the back of the PCB in parallel with the 3K9 bias feed resistor. I then reassembled the set.

This is the fourth Toshiba radio in my collection and I've found them to be surprisingly well built. This one and my mid 1960s 8L-688L which I restored a couple of years ago both have robust metal tuning capacitors where one would expect to find tiny Mitsumi units in Japanese transistor sets of their age. Its sound quality is typically early Japanese transistor though, a bit bright and tinny, but it's exceptionally sensitive and picks up everything on SW and MW, a remarkable achievement for a little transistor radio which is almost 50 years old. My thanks to Ron, Peter and Bill.

Howard
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Last edited by howard; 20th Apr 2008 at 12:36 pm.
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Old 5th May 2008, 6:21 pm   #2
itactics
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Default Re: 1960 Toshiba 8TM-300SB MW/SW 8 transistor pocket radio

Thats a very interesting account of bringing a classic transistor radio back from the brink of death! Toshiba is revered amongst collectors of pocket transistor radios, more so for their original and unique cabinet designs than for their performance. Well done!
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