Thorens TD 320 (316, 318, 320 and 321) Motor not turning - Page 1
Forum section: electronics / repair

author: Sören_Marodören
author_rank: Contributor
timestamp: September 13, 2025, 07:11:35 pm
content: Hi,I just helped a friend who have a Thorens TD 320 turntable that didn't work. The turntable did not rotate.Or to be true, sometimes it worked, but mostly not.A closer examination showed that the output to the motor was a bit weak so the motor was just shaking. The voltage was at 1.4-1.5 Vrms. Should be around 3 Vrms.The PCA looked ok. No bad solder joints, oxide or dirt. I measured the electrolytic capacitors in circuit and they all looked good.One strange thing was that this particular unit had 2 x 4 of 22 ohm resistors above and below the output transistors (four pairs of BC547 and BC557) for the motor.I was not able to find any schematic that had these. Could this unit be a bastard made of parts from other units?...After testing all transistors on my Curve Tracer I did some measurements on a 4.7 V reference voltage.It was at 4.35 V and fluctuated a bit up and down. And I could see that the output to the motor fluctuated the same way.Hmmmm... what could cause this?I then decided to replace the 100 µF capacitor (C105) that filters the 4.7 V, and voilà, it helped.Once again I measured the capacitor (on my TH2830 LCR meter) and it showed no error. F=100 µF and ESR=0.7 ohm.So, I took my Keithley 2400 and set it to sweep the voltage from 0 V to 20 V with 1 mA.I connected it to the capacitor via a 10 kohm resistor and connected my oscilloscope across the capacitor.(I tested the setup on a new 100 µF capacitor first to make it all work before I test the "faulty" cap.)To my surprise I could see bump in the sweep at around 12 V. See attached picture.In the next sweep there was a smaller bump at a higher voltage. The new cap did not have this behavior.The last test I did was to supply the "Faulty" cap with 25 V and measure the leakage current.After 30 minutes it still leaked 1.8 µA. The new cap was at just a few minutes at 35 nA. 50 times lower.So, with this said, always take an extra look at the Electrolytic Capacitors. They can look ok but still be faulty.And in this particular case, the design is very sensitive to leaky capacitors.

author: Swake
author_rank: Frequent Contributor
timestamp: September 23, 2025, 09:17:53 am
content: Interesting.Also your stubbornness about finding what went wrong isI have to admit that I would not have had the idea.This repair also shows that even a good LCR meter is not always enough. Another test protocol must be used. Maybe we need something that can test caps at higher voltages _and_ measure very low leakage currents.

author: floobydust
author_rank: Super Contributor
timestamp: September 23, 2025, 06:27:11 pm
content: The repair hypothesis doesn't make sense to me.C105 is filtering the unregulated 1/2Vcc point, around 8.5-11.5VDC for the motor sine-driver stage. It's a couple 10k resistors, so leakage current can be quite high with no problems.This node having a problem cannot affect the 4.7V reference- which is only for the speed controller circuit.Old turntables I will outright replace all electrolytic and tantalum capacitors. It's less hassle than letting them die out one at a time. Only 4 I can see:C101 1,000uF 25VC105 10uF 25VC109 10uF 6V (would use 16-25V)C111 1uF 25V (tant, would use a film cap)