Technics SU-V4 repair, 2SK109 U/S ?? - UK Vintage Radio Repair and Restoration Discussion Forum

captainslog7718 · 19th Jan 2019, 1:45 pm

Hi All, first post.

Trying to fix a Technics SU-V4 amp that ceased to function some years back and has been sat on the shelf.

Superficially an input dual JFET (2SK109) appears to have gone u/s as I am reading a high voltage (8.5VDC) on the source. Diagram attached (hopefully).

I am slightly puzzled by the circuit that shows only a small (3pF) capacitor feedback from the tone control in feedback loop to the gate of the suspect transistor (RHS on diagram). I would have expected a resistor of some sorts down to 0V, as with the other half of the FET.

Can anybody throw any light on this circuit configuration.

Regards Billm

orbanp1 · 19th Jan 2019, 3:03 pm

Hi Bill,

My guess would be that some of the "joints" are missing in the schematics.
See the attached schematics section.
Check this on the PCB and see if it makes sense.

Regards, Peter

captainslog7718 · 19th Jan 2019, 4:11 pm

Cheers orbanp1

I realised there were some shortcomings on the circuit diagrams (chassis ground is discontinuous and others) so I had downloaded multiple diagrams but they were all similarly bad. I did think the diagram might be at fault as otherwise it would have been a poor design. Will have another look at the beast tonight with the right diagram. Much appreciated.

many thanks Billm

captainslog7718 · 24th Jan 2019, 11:45 am

Hi All.
Having found another couple of errors in the on-line aquired circuit I now think I understand how it works (see Attached 1).
Now the problem. At turn-on the inverting input of IC303 (called a DC Servo), the AN6552 dual Op Amp rises quickly (1 second) to 2V. The integrator output climbs to +10V in same time, putting that large input into the input FET pair Q302 which in turn turns off the following transistors and puts an offset on the Amp output. This offset feeds back into the integrator via the non inverting input and over the next 10 to 15 seconds it brings the integrator output down to zero, the biasing transistors start to function and the amp offset goes to zero. Of course by this time the protection has kicked in.

My understanding is that the integrator inverting input should be a virtual earth and not jump in potential at power up. This suggests a dodgy feedback capacitor although checking with a DMM it seemed OK. Can anyone confirm that the inverting input should be a VE. Attachment 2 is a more pictorial circuit of the intergrator. Regards Billm

19th Jan 2019, 1:45 pm	#1
captainslog7718 Retired Dormant Member Join Date: Jan 2019 Location: Preston, Lancashire, UK. Posts: 3	Technics SU-V4 repair, 2SK109 U/S ?? Hi All, first post. Trying to fix a Technics SU-V4 amp that ceased to function some years back and has been sat on the shelf. Superficially an input dual JFET (2SK109) appears to have gone u/s as I am reading a high voltage (8.5VDC) on the source. Diagram attached (hopefully). I am slightly puzzled by the circuit that shows only a small (3pF) capacitor feedback from the tone control in feedback loop to the gate of the suspect transistor (RHS on diagram). I would have expected a resistor of some sorts down to 0V, as with the other half of the FET. Can anybody throw any light on this circuit configuration. Regards Billm Attached Thumbnails

19th Jan 2019, 4:11 pm	#3
captainslog7718 Retired Dormant Member Join Date: Jan 2019 Location: Preston, Lancashire, UK. Posts: 3	Re: Technics SU-V4 repair, 2SK109 U/S ?? Cheers orbanp1 I realised there were some shortcomings on the circuit diagrams (chassis ground is discontinuous and others) so I had downloaded multiple diagrams but they were all similarly bad. I did think the diagram might be at fault as otherwise it would have been a poor design. Will have another look at the beast tonight with the right diagram. Much appreciated. many thanks Billm

24th Jan 2019, 11:45 am	#4
captainslog7718 Retired Dormant Member Join Date: Jan 2019 Location: Preston, Lancashire, UK. Posts: 3	Hi All. Having found another couple of errors in the on-line aquired circuit I now think I understand how it works (see Attached 1). Now the problem. At turn-on the inverting input of IC303 (called a DC Servo), the AN6552 dual Op Amp rises quickly (1 second) to 2V. The integrator output climbs to +10V in same time, putting that large input into the input FET pair Q302 which in turn turns off the following transistors and puts an offset on the Amp output. This offset feeds back into the integrator via the non inverting input and over the next 10 to 15 seconds it brings the integrator output down to zero, the biasing transistors start to function and the amp offset goes to zero. Of course by this time the protection has kicked in. My understanding is that the integrator inverting input should be a virtual earth and not jump in potential at power up. This suggests a dodgy feedback capacitor although checking with a DMM it seemed OK. Can anyone confirm that the inverting input should be a VE. Attachment 2 is a more pictorial circuit of the intergrator. Regards Billm Attached Thumbnails