Toshiba SB-420 Stereo Amplifier

A really beautiful amplifier and the matching tuner is equally nice. I believe these were produced around 1974-77 but I have been able to find little info on them.

 

The ST-420 tuner is a lovely piece of design, I especially like those linear meters for the Signal strength and Tuning centre. The way the tuning scale fills the width of the unit and the angled front panel makes it my favourite tuner to date.



I'm very impressed by the build quality, If these had a Pioneer badge they would fetch silly money but it seems these Toshiba units have slipped through the net. The PCBs are marked Aurex which only appears on Toshiba's high end offerings. Two 10000uf capacitors in the amplifier power supply, no twin transformers here but the transformer is very big for a 40watt amp.



I was given this amp by a friend who said it had a nasty crackle on one channel that was un-effected by operation of the control knobs. When I tested the amp with my headphones, I nearly went deaf from the huge pops and bangs it made. I quickly turned it off and started tracking down the source of the noise.



I am convinced the issue is coming from the RIGHT channel on the tone control board "CCT-AF-C51". The noise is like the wind blowing, with random pops and bangs, I assume this is a transistor at fault?



I've found the service manual via a thread on audiokarma and upon close inspection I can only assume that Tr009, Tr0011 and Tr0013 are the cause of my still ringing ears! Also C031 had exploded which I'm guessing may have caused the fault?

I could really use some help in trying to resolve this issue. I don't know if I'm attacking the right components and if I am, what would be suitable replacements?

A Google search suggests NTE159 PNP for 2SA841-GR and NTE199 NPN for 2SC1000-GR but is this correct for my application and are there others I could use? If you are reading this and could give me some advise I'd be forever grateful.

On the schematic TR009, TR011 are marked 2SA493-GR but appear as 2SA841-GR in the parts list and in real life. Maybe a 2SA493-GR would also work here?

Anyway the sound of the one working channel suggests very decent sound indeed, with very good bass performance and a mid-range bursting with mustard. The top end however is a little foggy but the power amplifier is still waiting for a re-cap.

If you can help, please don't hesitate to get in touch via the comment box. Until the next time guys...