Thorens TD 280 II - Repair

[Childof60]

So, a friend received from his Father's estate a Thorens TD 280 that is in need of repair. Basically it acts as if the motor logic is not correctly energizing the motor windings as it "stutters" at a very low speed on 33 and runs slow on 45. I checked the service manual and did the easy things including capacitor replacement (simple) and substitution of the IC's (sockets are wonderful). Both changes have not resolved the problem. I did try to adjust the 33 rpm adjustment pot and was able to get it to run at 35 rpm. at one end of the range. Backing off the limit stop slightly causes the unit to revert to stuttering. The test point voltages seem universaly low even though the power supply checks out fine.

At this point I am guessing it is the zener diode used for speed reference (D106). Has anyone seen this problem and come up with a fix?

 

[The Slider]

So, a friend received from his Father's estate a Thorens TD 280 that is in need of repair. Basically it acts as if the motor logic is not correctly energizing the motor windings as it "stutters" at a very low speed on 33 and runs slow on 45. I checked the service manual and did the easy things including capacitor replacement (simple) and substitution of the IC's (sockets are wonderful). Both changes have not resolved the problem. I did try to adjust the 33 rpm adjustment pot and was able to get it to run at 35 rpm. at one end of the range. Backing off the limit stop slightly causes the unit to revert to stuttering. The test point voltages seem universaly low even though the power supply checks out fine.

At this point I am guessing it is the zener diode used for speed reference (D106). Has anyone seen this problem and come up with a fix?

Looking at the schematic in service manual, D106 is used as the reference voltage in the power supply regulator, not really anything to do with speed unless the regulator is unstable. Some versions moved to separate collector resistors on the motor drive transistors for better stability, the 2-phase motor drive circuit on these units isn't very robust, it's like a little power amplifier but without good stability margins. Make sure the switch contacts on the speed selector are clean, it switches some high impedance parts of the circuit, and it is an RC oscillator circuit, so all the impedances are fairly critical to maintain stable oscillation frequencies. And clean or replace the two little speed adjust pots too.

 

[Childof60]

Looking at the schematic in service manual, D106 is used as the reference voltage in the power supply regulator, not really anything to do with speed unless the regulator is unstable. Some versions moved to separate collector resistors on the motor drive transistors for better stability, the 2-phase motor drive circuit on these units isn't very robust, it's like a little power amplifier but without good stability margins. Make sure the switch contacts on the speed selector are clean, it switches some high impedance parts of the circuit, and it is an RC oscillator circuit, so all the impedances are fairly critical to maintain stable oscillation frequencies. And clean or replace the two little speed adjust pots too.

Thx for the insight. In reading the service manual and the circuit I completely missed the nature of the design. Deoxit has definitely helped the situation. It is a pain to adjust the speed.

 

[The Slider]

Thx for the insight. In reading the service manual and the circuit I completely missed the nature of the design. Deoxit has definitely helped the situation. It is a pain to adjust the speed.

Yea, I have a TD 320 and moved all the electronics to a separate box with a new power supply, along with some other mods, a lot easier to work on

 

[Childof60]

Yea, I have a TD 320 and moved all the electronics to a separate box with a new power supply, along with some other mods, a lot easier to work on

That is a great idea and a beautiful setup.

 

[Childof60]

I just realized this is a MK1 not a MK2 and the circuit layout is different. If anyone has a link to a MK1 service manual it would be great news.

 

[Childof60]

So, I continue down the rabbit hole with this Thorens 280 MK1. As no circuit diagram is available online (at least per Google) I started checking precision components in what seems to be the motor control section. I found the two precision (1%) capacitors were out of spec and replaced them. Tested all the 1% resistors - nothing out of spec. At this point only two possibilities seem remaining: 1) the motor is defective (end of the line); or 2) one of the complimentary BC547/557 transistors have gone bad. The "extra bonus points" challenge is the circuit clearly changed (a lot) between the MK1 and the MK2. At least I have it running at this point - just at 35 RPM and 47 RPM - with the speed pots turned to the maximum position. Backing off the smallest fraction causes the motor to stop.

I really don't like this random replacement of parts strategy but given the lack of service information it seems the only way at the moment. Thoughts and suggestions are appreciated.

 

[Childof60]

Continuing saga. I pulled all the complementary transistors and found there were in all over the map in hFe. I suspect they deteriotsyrf over the years and due to the nature of the design have caused speed regulation problem. New replacements are on order and I will gain match them. If this fails then it is time to move onto parting this very (cosmetically) nice turntable out.

 

[Childof60]

And the solution...

The TD280 was sort of end of the era turntable for Thorens. While there are service manuals online for the MK2-4 I have not found after extensive searching the manual for MK1. This is problematic for trouble shooting a defective turntable as Thorens changed the test point voltages and circuit board layout - with several design changes - for the MK2 and above versions. This is a informational post for future AK'ers that may have to trouble shoot this turntable.
So, for a friend I started to work on a 280 MK1 that would not run at 33 - just "stutter". Adjusting the P2 33.3 pot had no effect. I then recaped, replaced the three IC's, and the eight pair gain transistors. This got the turntable running at 35.5 RPM at one extreme of the adjustment pot - anything else caused the "stuttering" to start again.

The problem (after a lot more trouble shooting) turned out to be the speed adjustment 100K pot had failed so that at one extreme the resistance was zero and the slightest turn caused it to jump to 1 mega ohm! Incidentally I did (multiple times) de-oxit the pot.

Not fun to figure this out with no schematic or service guide. In the end it has been worth the effort. My friend will be getting back a fully functional Thorens owned by his father. The lesson learned is pots can fail.