Multitrack cassette recorder repair problem

Start by looking for fuses ( which may not be obvious glass and metal tubes ). Look on the board to see if there are any voltage test points marked, or a list of the pin functions of any connectors, where you might be able to check for supply voltages. If any are missing, or if you can't tell, then look for transistors or monolithic regulators mounted on heatsinks. Transistor regulators should have one pin at a higher voltage than the other two, which will be at a (nearly) similar voltage. For positive rails, if the transistor type starts "C" or "D" , then likely, the middle pin will be at the highest positive voltage, the left pin will be at some lower voltage, and the right pin at a slightly lower voltage than the left pin. For transistors starting "A" or "B", the right pin will be at the highest voltage, and the centre pin at the lowest. For negative rails, reverse the "A/B" and "C/D" descriptions. Monolithic regulators start "78" for positive types, and "79" for negative types. The next two digits specify the output voltage, so a 7805 is a +5v reg, a 7812 is a +12v reg. A 7905 would be a -5v reg and a 7915, is a -15v reg. The pinning for the 78 series is, from the left, tab away from you, IN - GND - OUT. A 79 series is IN - OUT - GND.

If that information doesn't get you on the track, then it is likely that a set of schematics would be necessary to get to the bottom of the problem, or possibly having a more experienced servicer's eye cast over it. Good luck with it.

Arfa

Thanks Arfa. I shall try those things now.

Unfortunately there are no fuses in this unit. However, upon a visual inspection I identified a transistor with visible damage to its ceramic exterior. This transistor, a C1815, is in place but almost half of its ceramic exterior has been chipped off. Quite a bit of copper is showing, but there is no broken connection.

Does anyone know whether it's likely that this sort of damage would stop a transistor from working? I have taken a digital image:

(the chipped transistor is circled in red). Thanks

Was that a shill just to get to the 'two free ipod nanos'? In any case, I didn't wait for the pic to load. I hate popups, but I hate audio popups even more.

However, when I see a component damaged as you describe, I consider it toast and start looking for the cause. It's not the external damage that's the issue. That transistor has literally exploded through some fault; either an internal fault in the component itself--or more likely--a fault in the circuit, which overloaded it.

jak

Agreed. However, it looks like that transistor is in the bias oscillator, rather than the power supply. If it's gone short circuit though, it may well have loaded up the power supply, and done some damage. As far as there being no fuses in there, I bet there are somewhere - but they might well be fusible resistors, and if you don't know what you are looking for, you might struggle to spot them. If the silk screening on the board has circuit symbols at each component location ie a little zig-zag for a resistor, look to see if you can spot any zig-zags that have an extra little 'sine wave' at the end of them, indicating that they are a fuse type. You can usually spot them because they look 'different' from all the normal resistors. They are often, although not always, stood on end, and usually have matt finish bodies, often brown or grey. You will often see two gold bands on them also. The circuit descriptor may be different from other resistors. Sometimes, manufacturers will use, for instance, "PR" for "protector" or "FR" for "fuse resistor". They are likely to be located near to the bridge rectifier(s), smoothing caps, and regulators.

The transistor type is very common. Virtually any general purpose NPN transistor with Japanese pinning ( flat towards you, pins down, E-C-B ) will do. The full type number for that transistor is 2SC1815.

Also, its a bit hard to see for sure, but does that electrolytic cap just below and to the right of your red circle, have a slightly domed top ? If it does, it may have been responsible for the demise of the transistor, and will need to be replaced.

Arfa

Oh I see. I didn't know that.

You have good eyes. Yes, that capacitor is slightly domed and could be the reason why the transistor failed. However, there is a large 4700uF cap next to the power input, which is very domed (fit to bursting it seems), and that could be equally responsible. I think on balance it would be best to trash the unit.

Thanks for your help,

max

Well, before doing that, for the sake of a couple of caps, I'd go ahead and change them. If that transistor is in the bias oscillator, you can just snip it out for now - it ain't gonna stop the unit working. If you still come up dead, then by all means give it up if that's what you want, but you never know, it might just come back up, and you'll feel really satisfied that you've saved an old friend from landfill ...

That transistor can always be replaced later. It would only affect recording / erasing with it snipped out.

Arfa

Greetings, all.

I may be getting into this message chain way late, but the picture you took tells me your transistor is destroyed. Replacement is a minor thing, but the more important questions is "what caused it to fail?". I'm a Senior Electronics Technician and spent way too many years repairing equipment for the music industry. (When I started in electronics, tubes were a mainstay.)

IN FACT, I have several Tascam Porta Ones that I'm working on repairing. It's going a little slow at the moment due to not being able to lay my hands on the service manual & circuit wiring diagrams, but I'll get there. (Yes: it's a cheap plug if someone wants to be a big help here....)

At any rate, please feel free to pursue asking me all the questions you want (advice is free, actual repair work is not). The only caveat is that I don't monitor newsgroups with any frequency, so you may do better trying to email me at (and work to get this because I'd rather not have every bot in cyberspace sending me junk) seniortech AT carolina DOT rr DOT com and I will do what I can to guide you in the right direction.

Thanks and a hat tip.

Andrew....