Thorens TTA-2000 DC offset

[zoldi211]

Re: Thorens TTA-2000 DC offset

I have an another problem. On RCA terminals I measured high DC voltage - 15-18 V on both R/L RCA inputs !!! What is this? This is normal on this type of power amplifier?

[zoldi211]

Re: Thorens TTA-2000 DC offset

Hi, I replaced Q1 Darlington transistor TP147, but DC raising with 2-3V on RCA terminals. Zener diode? Resistors are ok. Please help !!!

[R_J]

Re: Thorens TTA-2000 DC offset

The input is isolated by a capacitor, the capacitor does not pass DC it is just a phantom voltage your meter is picking up. Once you connect a source the voltage should disappear. or add a resistor across the input, try a 100k. Some amps have a 1Meg across the input and then a 330Ω in series before the input cap

[zoldi211]

Re: Thorens TTA-2000 DC offset

Thanks for your answer. My Yamaha RX-V1800 have preout output and protection turned it off with DC overload, if Thorens are connected.

[budwich]

Re: Thorens TTA-2000 DC offset

what "rail" voltage are you reading in the circuit as shown. what is the impedance reading for the inputs (unpowered)? check the cap on the input?

[zoldi211]

Re: Thorens TTA-2000 DC offset

The voltage is set to 50V. If I increase the voltage, the DC voltage on the RCAs is also grows. How can I measure the impedance? All of the caps are replaced.

[budwich]

Re: Thorens TTA-2000 DC offset

just use a DMM on resistance measurement... watch what happens during the measurement.

[petehall347]

Re: Thorens TTA-2000 DC offset

check the grounds

[zoldi211]

Re: Thorens TTA-2000 DC offset

The impedance beginning on 20k and raising up to 1M within seconds. There is a 33pF capacitor across RCA terminal.

[zoldi211]

Re: Thorens TTA-2000 DC offset

check the grounds

The grounds looks fine....

[budwich]

Re: Thorens TTA-2000 DC offset

The impedance beginning on 20k and raising up to 1M within seconds. There is a 33pF capacitor across RCA terminal.

the circuit diagram that you posted doesn't show that.... it shows it as a "dc block" on the input. The impedance seems as expected. check the cap.

[zoldi211]

Re: Thorens TTA-2000 DC offset

the circuit diagram that you posted doesn't show that.... it shows it as a "dc block" on the input. The impedance seems as expected. check the cap.

All caps are ok, I replaced them all.

[budwich]

Re: Thorens TTA-2000 DC offset

check the polarity

[zoldi211]

Re: Thorens TTA-2000 DC offset

The polarity is ok too. I did not find any problem.

[budwich]

Re: Thorens TTA-2000 DC offset

confirm the cap across the input.... where is that in the circuit diagram?

[zoldi211]

Re: Thorens TTA-2000 DC offset

confirm the cap across the input.... where is that in the circuit diagram?

There is not, I know. I dont have full circuit diagram or service manual.

[budwich]

Re: Thorens TTA-2000 DC offset

you might consider disconnecting the cap on the input and do your impedance check again. Then with no input, do your voltage checks. Go from the result. My limited experience with a DC300A... there is no cap across the input. Is the cap circuit board mounted or soldered across the terminals?

[zoldi211]

Re: Thorens TTA-2000 DC offset

Ok, I disconnect this 33pF caps and measured some impedance 18k-1M.

New detections:

- if I measuring impedance in one channel (R or L), then no voltage on that channel RCA, only 0,1V, what is raising slowly. On second channel (no measuring) 15-18V every time. After turning off and on again, voltage are present on both channels.

- if amplifier is turned off, without power cable - I measured negative voltage on RCA (-15V and slowly descendig). That voltage are present on the top of the input caps, on others are measuring about -1V.

[Niekv]

Re: Thorens TTA-2000 DC offset

Link to the service manual:
https://www.audioservicemanuals.com/...service-manual