Hello!
I've been working on a Superscope R-1240 which I posted about in another thread:
Superscope R-1240 (Expert TA 940)
Now I've solved some of the initial issues. But I would like to get some input about another(?) issue which I hope there might be some experience about in the Marantz forum of AK. The Superscope R-1240 has more or less exactly the same main amplifier board as Marantz 1040. And this is where my problem is. Initially I got oscillations after repairing a "burned" main amp board for right channel. When replacing the driver transistors I first used the wrong replacements and got oscillations immediately, which made it impossible to adjust the bias. I wrongly tried to replace the 2SC1509 and 2SA777 with KSC2383Y and KSA1013Y which had gains at 214/215. When I found NOS A777 and C1509 transistors for the drivers with gain around 100 I'm now able to adjust the bias. But, after a while the other channel, the left side, starts to oscillate. Bias trimmer still possible to affect the bias voltage, but not always possible to adjust to 20 mV which is the spec. I have a low hissing noise which will not change with increased volume.
I suspect that it's actually the output transistors that causes this. If I put back the original, not burned but drifted, transistors A837 (139 gain) and C1667 (26 gain), it gets better. But I still get oscillations after a while. The new transistors I used are the in many places recommended MJ21194G/93G. Does anyone have similar experiences?
Below is what I put in the main amp board.
All caps are new, also bias trimmers and some other components that were burned.
Left channel:
H701: 2SA722 (S, T) original hfe 291 --> KSA992 hfe 341
H703: 2SA722 (S, T) original hfe 347 --> KSA992 hfe 341
H705: 2SC1509 (Q, R) original hfe 102 --> NOS 2SC1509 hfe 98
H707: 2SC945 (Q, R) original hfe 259
H709: 2SA733 (Q, R) original hfe 168 --> KSA733G hfe 259
H711: 2SC1509 (P, Q) original hfe 58.5 should be 65-135 according to the spec I found. So a little low, but I don't have anything to replace these drivers with.
H713: 2SA777 (P, Q)) original hfe 71.7 same as above. Don't have anything to upgrade with. It seems these have lost a bit of gain.
Output transistors: MJ21194G hfe 50, MJ21193G hfe 77.
I got these transistors with this gain from Mouser. I have 4 of the 93G, and all are around 80 gain.
Right channel:
H702: 2SA722 (S, T) original hfe 290 --> KSA992 hfe 351
H704: 2SA722 (S, T) original hfe 118 --> KSA992 hfe 351
H706: 2SC1509 (Q, R) original hfe 52 --> NOS 2SC1509 hfe 120
H708: 2SC945 (Q, R) original hfe 263
H710: 2SA733 (Q, R) original was fried --> KSA733G hfe 263
H712: 2SC1509 (P, Q) original hfe was 25. --> took the one from H705 when I did this side. 2SC1509 Hfe 96
H714: 2SA777 (P, Q)) original was split in half. Fried. --> found one NOS 2SA777 hfe 116
Output transistors: MJ21194G hfe 84, MJ21193G hfe 78.
Should I find other output transistors with lower frequency? Should I install a small cap on the legs of the output transistors? Other suggestions?
Thanks!
/Christian
I've been working on a Superscope R-1240 which I posted about in another thread:
Superscope R-1240 (Expert TA 940)
Now I've solved some of the initial issues. But I would like to get some input about another(?) issue which I hope there might be some experience about in the Marantz forum of AK. The Superscope R-1240 has more or less exactly the same main amplifier board as Marantz 1040. And this is where my problem is. Initially I got oscillations after repairing a "burned" main amp board for right channel. When replacing the driver transistors I first used the wrong replacements and got oscillations immediately, which made it impossible to adjust the bias. I wrongly tried to replace the 2SC1509 and 2SA777 with KSC2383Y and KSA1013Y which had gains at 214/215. When I found NOS A777 and C1509 transistors for the drivers with gain around 100 I'm now able to adjust the bias. But, after a while the other channel, the left side, starts to oscillate. Bias trimmer still possible to affect the bias voltage, but not always possible to adjust to 20 mV which is the spec. I have a low hissing noise which will not change with increased volume.
I suspect that it's actually the output transistors that causes this. If I put back the original, not burned but drifted, transistors A837 (139 gain) and C1667 (26 gain), it gets better. But I still get oscillations after a while. The new transistors I used are the in many places recommended MJ21194G/93G. Does anyone have similar experiences?
Below is what I put in the main amp board.
All caps are new, also bias trimmers and some other components that were burned.
Left channel:
H701: 2SA722 (S, T) original hfe 291 --> KSA992 hfe 341
H703: 2SA722 (S, T) original hfe 347 --> KSA992 hfe 341
H705: 2SC1509 (Q, R) original hfe 102 --> NOS 2SC1509 hfe 98
H707: 2SC945 (Q, R) original hfe 259
H709: 2SA733 (Q, R) original hfe 168 --> KSA733G hfe 259
H711: 2SC1509 (P, Q) original hfe 58.5 should be 65-135 according to the spec I found. So a little low, but I don't have anything to replace these drivers with.
H713: 2SA777 (P, Q)) original hfe 71.7 same as above. Don't have anything to upgrade with. It seems these have lost a bit of gain.
Output transistors: MJ21194G hfe 50, MJ21193G hfe 77.
I got these transistors with this gain from Mouser. I have 4 of the 93G, and all are around 80 gain.
Right channel:
H702: 2SA722 (S, T) original hfe 290 --> KSA992 hfe 351
H704: 2SA722 (S, T) original hfe 118 --> KSA992 hfe 351
H706: 2SC1509 (Q, R) original hfe 52 --> NOS 2SC1509 hfe 120
H708: 2SC945 (Q, R) original hfe 263
H710: 2SA733 (Q, R) original was fried --> KSA733G hfe 263
H712: 2SC1509 (P, Q) original hfe was 25. --> took the one from H705 when I did this side. 2SC1509 Hfe 96
H714: 2SA777 (P, Q)) original was split in half. Fried. --> found one NOS 2SA777 hfe 116
Output transistors: MJ21194G hfe 84, MJ21193G hfe 78.
Should I find other output transistors with lower frequency? Should I install a small cap on the legs of the output transistors? Other suggestions?
Thanks!
/Christian
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