Teac AG980 receiver won't turn on

Thread Starter

sean5302

Joined Jul 26, 2021
21
I've realised how little I know about electronics and would like to learn.
I was recently given a Teac AG980 tuner amp that goes into standby but won't light the front panel display or produce any sound.
I have downloaded the service manual but am struggling to follow anything.
Using a multimeter I find 4V ac in standby and 40V ac on pressing the standby switch on pretty much every component around the board.
Should I be able to find anything much with a multimeter or do I need an oscilloscope.
I'm thinking it's a push-pull amp, with the negative part of the sine wave fed by half of the board and the positive part by the other half. I wouldn't see the components of the sine wave with a multimeter, would I?

I've looked around the board and nothing seems corroded, burnt, blown or physically defective.
There are 2 transformers which both test OK on resistance and input and output voltages.
How can I learn mere, please?
 

twohats

Joined Oct 28, 2015
241
"Is comprehension not one of your skills?"
Read my line 3:

Apologies, I will try harder.
The batteries have been replaced, working on the comprehension.
Any chance of a link to the service manual?
Good luck.......
 

LesJones

Joined Jan 8, 2017
3,472
Monitor the voltage between the ends of D131. With the unit in standby mode the voltage should be zero. When you do whatever is required to take it out of standby the voltage across D131 should be about 6 volts (DC). This is the voltage to the coil of RLY106. The contacts on this relay switch the mains to the primary of the large transformer.(Which provides power to the main part of the unit.) Report the results.

Les.
 

Thread Starter

sean5302

Joined Jul 26, 2021
21
Sorry for the remark.
I've just joined the Forum and don't know whether people are helpful here, or not.
The first response I got wasn't encouraging.
 

twohats

Joined Oct 28, 2015
241
@
sean5302

Thanks for the service manual link.
Are there any electrolytic capacitors with a bulging top or signs of leakage? These are usually kaput.
Good luck...
 

LesJones

Joined Jan 8, 2017
3,472
I must admit when reading your first post I thought what's the point of saying you have the schematic without posting it or at least a link to it. I tend not to respond when people do not give enough information about the problem.

Les.
 

Thread Starter

sean5302

Joined Jul 26, 2021
21
Beginning to see now.
Yes, with the unit in standby there is 0V dc between both sides of D131 (and 110V ac)
At D131 with unit out of standby, ie On, there's 6V dc (and 110V ac)
 
Last edited:

Thread Starter

sean5302

Joined Jul 26, 2021
21
No sign of bulging caps, burn marks, poor solder joints or smells. Everything looks perfect.

I'm in the UK and my mains voltage is 240V ac 50Hz. Surprised to see 110V ac.
 

twohats

Joined Oct 28, 2015
241
My take on this; IC108 via R199 supplies dc volts to D131 cathode and relay.
Q137 switches the relay ON, by taking D131 anode and relay to 0v.
Q137 is triggered from pin 26 of main IC 101.
How do you see it?
Regarding your 110v ac, is you DMM on 0v, not to confused with the mains power supply ground, just a thought.
Good luck, don't give up .
 

MrChips

Joined Oct 2, 2009
24,134
Measure voltages at both legs of R188 with respect to GND, on STANDBY and on ON.
Edit: Do the same at R199, just below and to the left of R188.

TEAC AG980 R188.jpg

TEAC AG980 R188 on schematics.jpg
 

MrChips

Joined Oct 2, 2009
24,134
Just to clue you in with what we have found so far.

3.9VDC comes in at diode D107.
This drops to 3.9V - 0.6V = 3.3V to power the MCU chip.

In STANDBY mode the MCU chip should be sending 0V to R188.
In RUN mode the voltage a R188 should rise to close to 3V in order to turn on Q137 which turns on the relay.
The relay should be powered with DC voltage supplied by R199. We don't yet know what this voltage ought to be.

110VAC seen on the meter is an anomaly. You should ignore this for the moment.
UK line voltage is 230VAC.
 

LesJones

Joined Jan 8, 2017
3,472
From your tests in post #12 it looks like the coil on the relay is being energised when in the ON state (Which is correct.)
First test fuse FS101 for continuity. (This protects the live feed to the large transformer.)
With with the unit in the ON state and your meter set to the 200 volt AC range measure the voltage between pins 1 and 3 of the 3 pin connector CP108. (This is close to the two bridge rectifiers D134 and D135.) I expect a reading of between 60 and 90 volts. These are the two ends of the center tapped secondary of the large transformer.

Les.
 
Last edited:

Thread Starter

sean5302

Joined Jul 26, 2021
21
Sorry, I'm wrong with the last sentence.
R188 has 3.2V dc on one leg and 0.8V dc on the other when unit is ON
0V dc on both legs when unit is on standby.

R199 has 6.8V dc on one leg and 0V dc on the other in ON
8.2V dc one leg and 0V dc in standby.
 
Top