A friend who knows Technics very well and has a large collection, helped me with my Technics SU-V8X. It can not set bias, at the outputs, it had high voltage across E CB and my friend said something is wrong on the amp board.
I took out all 24 transistors ( that included the 4 MosFets). The transistors seem to test fine, had some issues with C on Q315/6, however i think that was the micro clips, not making contact, so after 2 more readings C was fine under 800.
I then moved to the MosFets and followed a video online, and i never got OL( open on my meter is 1) readings, just low numbers like 148 158 on my 3.5 digit meter, so i assume they are defective?
Since 2SK117 is discontinued, by chance i went through my spares, and found my spare V4X phono pcb and found 4 2SK170 mosfets (which i hope are fine, as ive stored them in a box over the years).
Specs are very similar , voltage is lower with the 170s, and it actually appears lower noise? I tempted to try them, however will check some resistors first in the path to see if there is any defective. Only difference is VFDS -50v vs - 40v, otherwise pretty close
This might be useful thread for a reference for the parts as well, i hope.
2SK117 Product details
Low Noise Audio Amplifier Applications
• High |Yfs|: |Yfs| = 15 mS (typ.) (VDS= 10 V, VGS= 0)
• High breakdown voltage: VGDS= −50 V
• Low noise: NF = 1.0dB (typ.)
(VDS= 10 V, ID= 0.5 mA, f = 1 kHz, RG= 1 kΩ)
• High input impedance: IGSS= −1 nA (max) (VGS= −30 V)
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2SK170 Product details
Low Noise Audio Amplifier Applications
• Recommended for first stages of EQ and M.C. head amplifiers.
• High |Yfs|: |Yfs| = 22 mS (typ.) (VDS = 10 V, VGS = 0, IDSS = 3 mA)
• High breakdown voltage: VGDS = −40 V
• Low noise: En = 0.95 nV/Hz1/2 (typ.) (VDS = 10 V, ID = 1 mA, f = 1 kHz)
• High input impedance: IGSS = −1 nA (max) (VGS = −30 V)
Fet differential amps Q301-304.
(if Mods think it suits the part section - please move it there, as i was not sure which of the two this belonged thanks Sam )
I took out all 24 transistors ( that included the 4 MosFets). The transistors seem to test fine, had some issues with C on Q315/6, however i think that was the micro clips, not making contact, so after 2 more readings C was fine under 800.
I then moved to the MosFets and followed a video online, and i never got OL( open on my meter is 1) readings, just low numbers like 148 158 on my 3.5 digit meter, so i assume they are defective?
Since 2SK117 is discontinued, by chance i went through my spares, and found my spare V4X phono pcb and found 4 2SK170 mosfets (which i hope are fine, as ive stored them in a box over the years).
Specs are very similar , voltage is lower with the 170s, and it actually appears lower noise? I tempted to try them, however will check some resistors first in the path to see if there is any defective. Only difference is VFDS -50v vs - 40v, otherwise pretty close
This might be useful thread for a reference for the parts as well, i hope.
2SK117 Product details
Low Noise Audio Amplifier Applications
• High |Yfs|: |Yfs| = 15 mS (typ.) (VDS= 10 V, VGS= 0)
• High breakdown voltage: VGDS= −50 V
• Low noise: NF = 1.0dB (typ.)
(VDS= 10 V, ID= 0.5 mA, f = 1 kHz, RG= 1 kΩ)
• High input impedance: IGSS= −1 nA (max) (VGS= −30 V)
------------------------------
2SK170 Product details
Low Noise Audio Amplifier Applications
• Recommended for first stages of EQ and M.C. head amplifiers.
• High |Yfs|: |Yfs| = 22 mS (typ.) (VDS = 10 V, VGS = 0, IDSS = 3 mA)
• High breakdown voltage: VGDS = −40 V
• Low noise: En = 0.95 nV/Hz1/2 (typ.) (VDS = 10 V, ID = 1 mA, f = 1 kHz)
• High input impedance: IGSS = −1 nA (max) (VGS = −30 V)
Fet differential amps Q301-304.
(if Mods think it suits the part section - please move it there, as i was not sure which of the two this belonged thanks Sam )
Hi, the K170 has higher capacitances and that will degrade stability a bit, but might work.
You could look for K184 which I think is equivalent to the 117, just less dissipation possible - should be ok. But I can't tell without the schematic. And you should use parts that are close in Idss to the original ones.
You could look for K184 which I think is equivalent to the 117, just less dissipation possible - should be ok. But I can't tell without the schematic. And you should use parts that are close in Idss to the original ones.
I think you mean the four input JFETS
These are not mosfets, and are not tested like mosfets. So chances are that you are testing them the wrong way.
Hi Alexandre , Thank you. I found a test on youtube. The guy has 1.7K likes so hopefully it's accurate. I followed his steps i got 777 and 760 and 105.6 ohm on my Q304. I will test the remaining 3. So it appears it is good, so the issue is not this JFET. Hopefully if they all test fine and resoldering the components back it may fix the issue of the amplifier with its high voltage defect (may have been a hidden cold joint). You saved me throwing out a good Jfet., Thank you again for the help
I find this place is great to get free schematic, you may know it.
https://elektrotanya.com/technics_su-v8x_sm.pdf/download.html
The youtube video, its good as he tests other types of transistors
edit update - appears all Jfets are fine if going by the above testing procedure
Q301-304 -ve G +D and S = 1(OL)
Q304 105.6 ohm short GS to D
Q303 101.8 ohm short GS to D
Q302 128 ohm and 106 for Q301.
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