Hello,
I've had an old JVC A-X30 amp for about 16 years, it's been incredibly reliable and resilient to have lasted though my teens - running 4 ohm loads flat out and outdoor use on undersized generators without ever missing a beat. The last few years it's been used to run speakers for my TV system and as it has no remote it's been left on a lot even when it's not being used. Recently it stopped working (no power) I replaced blown fuse but the new one blew instantly. Checked a couple of parts and found the D718 output transistor (right channel) shorted.
I removed it and the amplifier powers on now.
I don't know the manufacturer; assumed maybe toshiba as it has a large capital T above the numbers. But the replacement toshiba transistors on eBay have a different logo and while originals are green for the B688 these ones were not. I ended up buying 2 pairs of KEC branded ones ($4) in the hope that a different brand might be ok if I changed left channel also. I installed the new transistors but I could find a repair manual so was unsure how to set the bias and what to set it to. Wired in a 100w light bulb in place of the fuse and decided to hope for the best.
Connected test speaker and powered on. Light bulb lit briefly then dimmed down and I heard speaker protection relay activate. So far so good! Carefully increased volume a little: sound was clear and undistorted. Repeated for the other channel which was also good.
I was very happy as I thought "old faithfull" was fixed. Reinstalled fuse and reassembled.
I connected better speakers and played some music. The sound didn't seem quite right but couldn't place exactly what was wrong. I increased the volume and at the first sign of distortion (just past half way as has always been the case with this amp+speaker combo) there was suddenly a loud crack and a spark followed by a red glow could be seen through the ventilation slots in the case. I quickly switched it off as power was still on.
On closer inspection I found output transistor B688 in the right channel blown.
I have now found a service manual, determined parts I need ARE manufactured by toshiba and now know how to set the Bias. I've got 2 pairs of D718 and B688 ordered and am ready to try again. Apart from obviously setting the Bias is there anything else I should do? Is there any risk installing the working pair of original transistors into the right channel, leave the kec ones in the left and test? This would allow be to determine if anything else has been damaged in the right channel as a result of this incident.
I've had an old JVC A-X30 amp for about 16 years, it's been incredibly reliable and resilient to have lasted though my teens - running 4 ohm loads flat out and outdoor use on undersized generators without ever missing a beat. The last few years it's been used to run speakers for my TV system and as it has no remote it's been left on a lot even when it's not being used. Recently it stopped working (no power) I replaced blown fuse but the new one blew instantly. Checked a couple of parts and found the D718 output transistor (right channel) shorted.
I removed it and the amplifier powers on now.
I don't know the manufacturer; assumed maybe toshiba as it has a large capital T above the numbers. But the replacement toshiba transistors on eBay have a different logo and while originals are green for the B688 these ones were not. I ended up buying 2 pairs of KEC branded ones ($4) in the hope that a different brand might be ok if I changed left channel also. I installed the new transistors but I could find a repair manual so was unsure how to set the bias and what to set it to. Wired in a 100w light bulb in place of the fuse and decided to hope for the best.
Connected test speaker and powered on. Light bulb lit briefly then dimmed down and I heard speaker protection relay activate. So far so good! Carefully increased volume a little: sound was clear and undistorted. Repeated for the other channel which was also good.
I was very happy as I thought "old faithfull" was fixed. Reinstalled fuse and reassembled.
I connected better speakers and played some music. The sound didn't seem quite right but couldn't place exactly what was wrong. I increased the volume and at the first sign of distortion (just past half way as has always been the case with this amp+speaker combo) there was suddenly a loud crack and a spark followed by a red glow could be seen through the ventilation slots in the case. I quickly switched it off as power was still on.
On closer inspection I found output transistor B688 in the right channel blown.
I have now found a service manual, determined parts I need ARE manufactured by toshiba and now know how to set the Bias. I've got 2 pairs of D718 and B688 ordered and am ready to try again. Apart from obviously setting the Bias is there anything else I should do? Is there any risk installing the working pair of original transistors into the right channel, leave the kec ones in the left and test? This would allow be to determine if anything else has been damaged in the right channel as a result of this incident.
Thanks for the quick response. Will certainly check the resistors. And other parts as best as I'm able. So you think that there IS a cause? I had assumed being 35+ years old and left on all the time had killed the original transistor. And incorrect Bias voltage or an incompatibility had blown the replacement..
It's quite likely the right channel's 2SB688 was damaged somehow by the previous fault, and just happened to "let go" at this moment.
The originals are indeed Toshiba transistors. KEC do apparently make clones of them, as do Unisonic. However i'd be careful where I buy them, as there are also Chinese copies of the devices which are undoubtedly garbage.
I'd be inclined to recommend OnSemi MJW21193/4 as replacements.
The originals are indeed Toshiba transistors. KEC do apparently make clones of them, as do Unisonic. However i'd be careful where I buy them, as there are also Chinese copies of the devices which are undoubtedly garbage.
I'd be inclined to recommend OnSemi MJW21193/4 as replacements.
Yes... use a 'bulb tester' when working and diagnosing the fault. It can prevent a lot of damage.
thanks for all the suggestions. Fake transistors... this never even occurred to me but now that i've looked into it, seems like thats almost certainly what I have. And the new so called "Toshiba" ones I'm waiting to receive will be the same.
Jaycee, you recommend OnSemi MJW21193/4. I have 6 pairs of B817 / D1047 from an Onkyo home theatre that i'm scrapping for parts. I don't know what to look for in the data sheet, but would these maybe work?
Jaycee, you recommend OnSemi MJW21193/4. I have 6 pairs of B817 / D1047 from an Onkyo home theatre that i'm scrapping for parts. I don't know what to look for in the data sheet, but would these maybe work?
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