Toshiba SB-M55... repair or return?

[eurekaiv]

I've been looking for a decent smallish amp to use in our office to play records on so I stopped by the local goodwill which happened to have a Toshiba SB series integrated for $30. Not the deal of the century by any means but not a rip off. Unfortunately, the power section is pretty much doa. Upon powering it up it played but with a lot of noise and static in the controls and so I de-ox'd it and went back at it but the right channel seems completely dead now. It does sound pretty darn good through headphones (and the de-oxit did wonders on the noisy controls) so I know the problem is definitely in the power amp somewhere. I'm well versed in troubleshooting electronics so I'm confident I could repair it if I had a schematic and some time but I'm just not sure its worth it. It does have a nice feature set (parametric EQ w/ defeat mainly) and its exactly what I need in terms of size and features but I think I may end up returning it rather than throwing time and money after a $50 amp... :scratch2:

(old eBay photo below)

 

[Oilmaster]

If it plays well on headphones, then it would normally mean the amp stage is in good working order. Most amplifier designs allow the headphones to be driven by the driver stage only, but if the output transistors/stage would be faulty, then the amp would probably not be working at all, or at least a bad sound through headphones. As such, a 'quick' conclusion would be that the amp works normal.

The headphones are normally driven directly from the amp output, with a couple of resistors to lower power to headphones, hence no intermediate switch or relay.

So I would deeply investigate the section between amp output and speaker terminals; a couple of configurations are possible:
- there is a physical speaker selection switch (off/on/A/B), and some contact surfaces are badly oxidised (worst case: one or more wiper points are broken off)
- there is a protection relay in the middle, which has either bad contact surfaces or is plain faulty (edit: or the circuit driving the protection relay is faulty)
- low-cost hifi: the headphone jack connector has switch-kind contacts which automatically cut off the signal to speaker terminals when the headset jack is plugged in. These switch contacts can be badly oxidised (open to air), or deformed and lacking contact when the headset jack is removed again. Sometimes the headset jack connector is made of poor quality plastic and cracked (seen it all...)

EDIT: or the volume pot went completely sour, that happens as well sometimes.

Hope this helps you

 

[eurekaiv]

Thanks for the reply... I sorta assumed there must be a small headamp driver stage after the pre but looking at the thing this morning I can clearly see the two resisters mentioned on the little PCB connected to the headphone jack.

I also have good news to report. I fired it up this morning and it was purring like a kitten. I think the de-oxit worked its magic on whatever excess corrosion was in this thing while it sat overnight. I'll replace the large filter caps, clean it up a bit and call it mine. it really is quite a nice little amp I think. I see only one IC in the thing so it appears to be a discrete design which is quite cool, especially given the time period from which it comes. It's clearly not up to the build quality of my Luxman and has about 100 more electrolytics in the circuit, but for the money, I think it was a solid buy—now that its working of course.