JC-120 in need of a lot of love

[laurie]

Next step is to install and connect the new power transistors. They live on an external aluminium bracket and have flying leads back to the amplifier boards.

The old mica washers for the transistors are unusable. Another part I don't have - ordered, should be here tomorrow.
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Early JC-120 power transistors.jpg (200.1 KiB) Viewed 12771 times

[laurie]

The more I look at this amp the more convinced I become that it was left outside for years.

The serial label on the heatsink bracket has the look of water damage, plus it has deteriorated much more than other equivalent labels I've seen on other Roland equipment.

It is crumbling but I wanted to preserve it, so what I've done is cut a piece of archival grade plastic sheet (not sticky) the size of the label and then used a piece of sticky clear plastic "contact" to hold the archival plastic over the label. A bit hard to see in the pic, but the label is protected and physically held in place ... I think it should work.
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Early JC-120 label preservation.jpg (319.46 KiB) Viewed 12734 times

[Pepe]

Nice detail!

[laurie]

Heatsink bracket before and after cleaning...
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JC-120 heatsink bracket before-after.jpg (287.54 KiB) Viewed 12728 times

[laurie]

Continuing the reassembly. Heatsink bracket reattached and power transistor wiring connected to the amplifier boards.

Next step (once the power transistor insulators have arrived) is to carefully power up the amplifier boards and check they work, then trim the output bias using the trimpot.
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JC-120_7.jpg (332.48 KiB) Viewed 12724 times

[laurie]

Power transistors mounted.

Replacement transistors are MJ15001:

• Vceo 140V

• Ic 15A

• Pc 200W

• hfe 25-150

Original transistors were 2SC1079:

• Vceo 150V

• Ic 12A

• Pc 100W

• hfe approx 25 to 140

So the replacements are a pin-for-pin replacement, and are pretty close to the originals. As mentioned earlier - there are sources for the 2SC1079 transistors, however, research shows the devices are often "fake" (I don't know how - probably something common like a 2N3055 relabeled?).

Compared to the non-standard speakers, the non-standard power transistors won't have a lot of affect.

I'll carefully apply power to the amplifier boards tomorrow and hopefully they will work with no issues.
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JC-120_9.jpg (163.43 KiB) Viewed 12148 times

[Pepe]

I have never seen those transistors before picking up the old Roland SH-1000 synthesizer, which is still lying in pieces like a puzzle on our attic. Is there anything that I should know about these? Two of them (Sanken SI 3150E) are mounted to a circuit board. I already suspected them being transistors or rectifiers in some way.

I have never heard of two pin transistors. I always thought that transistors have at least three pins (B, C and E).

[laurie]

I have never seen those transistors before picking up the old Roland SH-1000 synthesizer, which is still lying in pieces like a puzzle on our attic. Is there anything that I should know about these? Two of them (Sanken SI 3150E) are mounted to a circuit board. I already suspected them being transistors or rectifiers in some way.

I have never heard of two pin transistors. I always thought that transistors have at least three pins (B, C and E).

The devices on this amp are 3 pin. The case is the third pin. Package is called TO-3.

When the time comes, if you need to replace your devices, I can help.

[Pepe]

I hope that I won't need to replace anything, since the previous owner said that the power had worked properly (well, who knows for sure?).

So the case is the third connection - interesting! So all the four transistors are sharing the connection of the case for the same purpose?

[laurie]

I hope that I won't need to replace anything, since the previous owner said that the power had worked properly (well, who knows for sure?).

So the case is the third connection - interesting! So all the four transistors are sharing the connection of the case for the same purpose?

No. The transistors are mounted on Mica washers (the part that I was waiting for this week). They are completely isolated from the case and the CB&E all connect back to the PCB.

https://www.mouser.ca/ProductDetail/Aav ... Z2FQ%3D%3D

See the "CBE" pins here:
viewtopic.php?f=14&t=954&start=60#p10373