Hi, and thanks for your reply. Luckily I have a Variac, and a bench power supply.
The burned up R17 made me suspect power transistors Q6, Q8 and q& (BC184) but they all read fine on a Peak tester though it is possible there is a failure under load. C8 and C9 were fine but are now replaced. R13 and R15 are in spec.
Clearly something is drawing too much current. Measured in circuit, I am seeing 2K3 ohms across each motor coil, 4K6 across the pair. I read somewhere that I should be seeing 5K across each one. Can anyone confirm?
Finally, tant C19 measures fine but I will change it for film as you suggest.
The burned up R17 made me suspect power transistors Q6, Q8 and q& (BC184) but they all read fine on a Peak tester though it is possible there is a failure under load. C8 and C9 were fine but are now replaced. R13 and R15 are in spec.
Clearly something is drawing too much current. Measured in circuit, I am seeing 2K3 ohms across each motor coil, 4K6 across the pair. I read somewhere that I should be seeing 5K across each one. Can anyone confirm?
Finally, tant C19 measures fine but I will change it for film as you suggest.
Mouser shows quite a few 2-3W 43K resistors (at least in the US):
https://www.mouser.com/c/passive-co...ough-hole-resistors|~Resistance|~Power Rating
Hi, thanks, Mouser UK scores where all the others fail.
Interestingly every resistor I pulled and checked was in spec after 34 years. AFAIK without reinstalling it, normal function is now restored to the board and the R17 failure was down to it being underspecced in the first place. I may go for a 3W replacement if there is room.
I have put some thermal paste behind the power transistors, which according to the datasheet improves heat transfer even through the plastic body.
Interestingly every resistor I pulled and checked was in spec after 34 years. AFAIK without reinstalling it, normal function is now restored to the board and the R17 failure was down to it being underspecced in the first place. I may go for a 3W replacement if there is room.
I have put some thermal paste behind the power transistors, which according to the datasheet improves heat transfer even through the plastic body.
Hi, I’m hoping someone can help me with a fault on the PSU.
I have acquired a Linn Axis turntable which has already had a previous attempt at fixing it, the fault when I got it was it wouldn’t rotate or would rotate backwards.
The speed change switch also didn’t work, on inspection I found most of the cap’s had been recently replaced. After finding this forum I checked the rest of the electro caps and found 2 bad ones which I replaced I also found the common wire for the speed change switch had broken under the sealant it was held down by. Unfortunately it still didn’t start by itself but the speed change switch worked after much testing found R20 was high resistance once replaced with 390K its starts up and you can change speed perfect but now it wont stop. The led goes out when you press the button and the voltage reduces on the motor but it will continue to rotate. If you stop the platter the heat sink on the back heats up so somethings not right.
Any ideas would be greatly appreciated
I have acquired a Linn Axis turntable which has already had a previous attempt at fixing it, the fault when I got it was it wouldn’t rotate or would rotate backwards.
The speed change switch also didn’t work, on inspection I found most of the cap’s had been recently replaced. After finding this forum I checked the rest of the electro caps and found 2 bad ones which I replaced I also found the common wire for the speed change switch had broken under the sealant it was held down by. Unfortunately it still didn’t start by itself but the speed change switch worked after much testing found R20 was high resistance once replaced with 390K its starts up and you can change speed perfect but now it wont stop. The led goes out when you press the button and the voltage reduces on the motor but it will continue to rotate. If you stop the platter the heat sink on the back heats up so somethings not right.
Any ideas would be greatly appreciated
here I assume an accumulation of several faulty parts.
At first step in such situations I replace all caps are and all resistors, where are voltages measures above 250V (use two or three resistors in series for better voltage splitting and enhancing the reliability) and check all transistors in the power amps for motor coils (replace all transistors and quad op amp from the front end of power amps, if one of the BUX85F is faulty). Check also current sensor resistor in series to the motor GND connections.
After doing this you can start usual troubleshooting steps - first step here is checking DC conditions in "Motor Off" position without connected motor.
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CMOS ICs U1 and U2 (CD4013) often faulty and U6/U7 (4066) sometimes.
Are the voltage of 13V5 present (C, Q2/E, Q1) ?
Destroy of the CMOS parts happens due the fact, that most users don't switch-off completely the Linn AXIS motor control unit, so that overshooting and over-voltage effects on the mains still present in the st-by mode.
I recommend a hard shutdown (external switch-off in the mains wire).
Furthermore, I would buy a second control board cheaply in a defective condition, repair it and provide it with sockets for all ICs.
This makes it easier to carry out the troubleshooting procedure in the event of later failures.
Having experienced the electronic trouble of the PSU circuit, I have identified two problems with the motor
- first: the significant radial play of the motor axis was the main reason of the regulation troubles encountered with the original card
despite its replacement with the Hercules II ,the motor fault mechanical diagnostic is rarely highlighted within the analyses
- second: the motor is sticked on the card and I do not know the right way to remove it in avoiding to broke the circuit , if somebody knows a dedicated procedure, please let us know.
Raymond
- first: the significant radial play of the motor axis was the main reason of the regulation troubles encountered with the original card
despite its replacement with the Hercules II ,the motor fault mechanical diagnostic is rarely highlighted within the analyses
- second: the motor is sticked on the card and I do not know the right way to remove it in avoiding to broke the circuit , if somebody knows a dedicated procedure, please let us know.
Raymond
I'm getting ready to replace the caps and a few other parts on my ca. 1990 Axis and, being a neurotic retired graphic designer I made myself a notated photo layout of the back of the board to help, particularly in the desoldering. Thought someone else might find it useful. If anyone spots any mistakes please report!
Be extremely careful when removing the old caps. The board is a 2 layer PCB and it's VERY easy to lift the traces and also pull the collets out of the holes when getting the caps out. Be patient and you should be OK though.
Also V1,V2 and D1 rarely need replacing. I've done plenty of Axis boards and have never changed those parts.
Also V1,V2 and D1 rarely need replacing. I've done plenty of Axis boards and have never changed those parts.
Here is URL to site with many turntable PSU schematics: http://www.turntablepsu.com
Hope you will find it useful.
Regards,
Hope you will find it useful.
Regards,
Hello,
I fully agree!
At the end, the motor is the main concern regarding the PSU...
First of all is the dismounting task using two screw drivers since you are wondering what the limit is before to broke it...
I had to cope with a motor which seemed electrically ok,( wiring) but the "radial play"of the motor axis was the source of the different PSU failure!
I read many post on line reporting the similar diagnostic!.
Do the test in taking the axis with two fingers, if the play looks effecitve, you have 50% of chance to get a motor out of mechanical limit....I can assure you!
Regards
Raymond