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...

Pro-Ject Head-Box - The Amazing Headphone Amplifier

Not really vintage but an interesting little box nevertheless. I have little experience with headphone amplifiers, even headphones in general but this unit was very surprising indeed.



I picked this up at a boot fair a few years back and its just sat around in a cardboard box ever since. However the other day I was refurbishing my cherished Cambridge CD1 and felt the need to go hunting for op-amps. This lead me to the Pro-Ject Head Box but sadly it used dual op-amps, not what I was looking for. But upon opening the unit I was so impressed with its design I felt I should take a closer look.



I came across a thread where someone had also been impressed with it and modified theirs with great results. I could feel me figures twitching for the soldering iron, I wanted to get stuck in, it was Lampizator all over again!

I plugged the unit in for the first time and was shocked, it sounded better than my Sansui Ca-2000 on headphones! How can this be, my world has been crushed. Cheap and cheerful op-amps never sounded so good!


So lets gets started!

First off, new caps
470uf   35V   x2  main power supply, after ac-dc conversion
330uf   16V   x2  main power after regulator
10uf     63V   x2  op-amps (no need for such high voltage surely?)
4.7uf    100v  x4  decoupling (by volume pots)

Main power supply caps can be increased but I kept original values as space is at a premium and very little current drain in this amplifier. I used Sanyo oscon on op-amp and 1uf polyester for decoupling. The polyester are by no means the best thing to use but its all I had. Polypropylene should be used instead, wima ect.

Second, op-amp sockets for easy swapping!



Above we have the finished article, or so I thought. After plugging my phones in I had too much mid-range, very crisp and transparent but it was hurting my ears! Hmmm I felt the polyesters may have given this effect but I wasn't sure. So now I had to get those op-amps out and start swapping!

The 4580 came out and the LM833 went in, not much difference on paper but the 833 is used in a lot of the good old TDA1541 Philips players.

Headphones went in and BAM! there it was, perfect. The LM833 sounded lush, shimmery mid with very realistic vocals and as a bonus it had less distortion at higher levels. Anyone who says op-amps have no impact on sound is talking rubbish!

I had to play the whole London Grammer album, start to finish, I was helpless. So this is me signing off in a state of bliss, until the next time folks!

Speaker Cabinet Restoration, Oil or Varnish?

I've been asked by a couple of people whether they should Oil or Varnish their speaker cabs.

Well the answer depends on what state your cabs are already in but ultimately you want Oil not Varnish. Varnish is a quick and cheap way to give a hard wearing protective surface but over long periods of time can flake and crack. It also doesn't feed the wood, the veneer is left underneath getting dryer and dryer. This can result in the veneer itself flaking off. Oil will feed the veneer, help prevent flaking and enrich the colour tone and depth. However oil will require more work and regular polishing to keep it at its best.

If your speakers are already varnished but in good condition with no flakes coming off, then there is little to be done. You can oil them, they will go shiny but then dull as the oil drys. The oil will sit on top of the varnish and wont feed the wood. You can varnish them again but in my opinion the best option here is to leave them alone, a regular polish will keep them nice.

If you have cabs with old varnish flaking off then you can strip the old varnish, rub them down (very lightly) and then build up layers of oil. When stripping I tend to go less paint stripper and more elbow grease. You can find elbow grease at any local hardware store... If that sounds like too much work for you then adding oil to the flakes and cracks will help the general appearance and the use of a good bees wax polish after will do wonders.

The speakers below had slight cracking in the varnish but the oil and bees wax helped a lot.

If your cabs have no varnish on them, the fun can begin straight away. A quick rub down is still advised as this will freshen the surface ready for the oil to soak in. It can also help hide any light marks and scratches.

But which oil should I use? I hear you cry.

Teak oil: Its good stuff but tends to be a bit expensive and is quite thin.

Danish oil: Another good one, very poplar, again, can be expensive but buying in bulk helps.

Linseed oil - Double Boiled: The oil of school workshops, its thick, its good value, its safe.

Personally I use Linseed Oil, it must be the double boiled stuff for the best results though. Its the thickest oil I've found and really gives a good finish. After oil the veneer has more depth, 3 dimensional glistening and improved colour tone. It also leaves a film over the whole speaker which helps protect and continues to feed the veneer. Any of the above oils, and others, will work well but after a few coats have been applied and set we must move to the next stage.

Beeswax Polish.

This is were we receive the fruits of our labor. Apply a small coating over the speaker and leave it for 10 minutes. Now buff away the residue to reveal a glorious shine, buff in the order you waxed. The longer you leave the wax on the more elbow grease you will need, hopefully you have some left from earlier.

So there you have it, bye bye varnish and hello oil. You cant beat oiling speakers on a hot sunny day!