TEAC stereo receiver won't turn on

[sean5302]

A friend has given me 2 Teac AG980 receivers that are only 4 years old.
I have a link to the schematics:
https://www.manualslib.com/manual/1741324/Teac-Ag-980.html

The fault is that 1 seems completely dead. The other goes into standby but does nothing else. No front display etc.
I've taken the covers off and checked all power transistors on the outputs. All OK.
Mains power goes through the main on/off switch but the large transformer doesn't switch on.
There is capacitor C162 which the manual calls 472M. I'm taking this as a film cap 4700pF or 4.7nF if I have my measurements right. It only measures 3.8nF in circuit, but this is around 20% low. Is that significant?
Fuses are OK.
Solder joints all look fine

I'm a recent newcomer to the world of electronics and am keen to practice.
Any ideas as to where I should be looking?

I'm also interested to know if you guys ever went to classes / night school or whatever. I'd like to learn fault finding, at which I've always been OK at (vehicles, Home DIY etc).

Many thanks

 

[sean5302]

Having removed most of the film caps, the only one that seems amiss is the one I mentioned above.
Out of circuit it measures 3.8nF / 3800pF.
The spec is 4.7nF so how significant is this, please?
I've ordered a couple of new ones but am curious how close to spec these caps need to be. I haven't found anything else wrong, yet.

 

[shrtrnd]

I'm curious about the fact that you say the transformer doesn't switch-on.
Without a transformer output, you're spinning your wheels looking at the small components.
What's the history on these TEACs?
Could they be 120VAC units that were plugged into 240VAC during some ownership transfer between countries?

 

[sean5302]

The units both came from a woman who runs a gym in a massive house / barn.
It's likely that they've been in use for 16 hours every day.

I'm fractionally further on, but not much.
I bought new 4.7nF capacitors, which actually measure 3.9nF, so pretty much the same as the one I suspected.

What is supposed to happen is power goes through a fuse to a small transformer putting the unit into standby.
If the "on" button is pressed, it should then power the front display and send current to the large transformer.
I've checked the small, standby transformer which is OK.
The large, power transformer is also OK.

Mains voltage settings are fine. There's no facility to alter that as these are UK-only designs 240V 50Hz.
I'm going to look with my power supply now and introduce lower voltage from start to finish. What I'm lacking is any sort of experience in electronic fault-finding.

 

[davenn]

Mains power goes through the main on/off switch but the large transformer doesn't switch on.
There is capacitor C162 which the manual calls 472M. I'm taking this as a film cap 4700pF or 4.7nF if I have my measurements right. It only measures 3.8nF in circuit, but this is around 20% low. Is that significant?
Fuses are OK.

'm curious about the fact that you say the transformer doesn't switch-on.
Without a transformer output, you're spinning your wheels looking at the small components.

I too am confused as to why you are replacing all these caps ? .... please put them all back in and anything else you have removed

You havent stated if you have checked the other obvious things like the various DC power rails
You need to establish where you do and dont have AC and DC power
Do you know how to follow a circuit diagram ... do you know the various circuit component symbols ?

 

[sean5302]

I can follow a circuit diagram and I do know what the components are.
Let's start from the beginning.
I checked the mains power leads and plug fuses. There was 240V at the circuit board input. I checked the power switch on the front panel, which is OK.
I checked both transformers but by measuring resistance across primary and secondary windings only. They are OK.
I checked all power transistors, which are fine.

My logic was that I'd test and replace what I thought might fail with heavy use. Caps are supposed to be unreliable according to many Youtube videos I watched.

I'm grateful for any help you can give me, hopefully to follow a methodical plan.

 

[davenn]

I can follow a circuit diagram and I do know what the components are.
Let's start from the beginning.
I checked the mains power leads and plug fuses. There was 240V at the circuit board input. I checked the power switch on the front panel, which is OK.
I checked both transformers but by measuring resistance across primary and secondary windings only. They are OK.
I checked all power transistors, which are fine.

OK much better
Now you need to start checking out the DC voltage rails .. look at each bridge rectifier set and see what DC voltage is on their output

 

[Bluejets]

In that schematic you supplied, the power to the transformer is controlled by a relay.
Could be some type of standby arrangement, I didn't go into it much when I saw that.
Could test T101 as that seems to be the standby supply tranny.

Shouldn't be difficult to bypass there with a link from CP509 pin 1 and 2 for the main tranny.
If that doesn't work, try a link from CP509 pin 1 to CP110 pin 2 BUT might pay to drop an inline fuse in the bypass link on this one as you are bypassing the inbuilt fuse.

 

[sean5302]

I'm going to wait a while because it's currently 36C in my conservatory. I work in there because it's so much easier to see.
I'll come back here tomorrow.
Many thanks for helping so far.

 

[sean5302]

I'm struggling.
I have 240V ac on the main power input.
On cp509 which goes to the power switch on the front panel there is 66V ac. I note the presence of resistor R204 between neutral and ground but that 66V seems strange.
Putting 5V dc from my power supply across relay RLY106 causes it to click and close.
Immediately after that relay is diode D131 which appears shorted, unless I'm getting measurements from elsewhere in the circuit.
Changing multimeter leads from + to - each way makes no difference (with meter on diode mode) I get the continuous short circuit beep each way round.

You guys are probably laughing at me, as it will be basics for you. Not for me though.
Thanks in anticipation.

 

[Bluejets]

Not a good idea to start injecting outside voltages into a circuit to test things.

Diode on the relay is a freewheeling diode to stop line noise as the relay opens and closes.

Most likely when testing devices such as this is to unsolder one leg out of circuit otherwise you will get wrong measurements on many occasions.

Note that you should also put your meter on diode test when checking semiconductors such as this.
You don't say what range you had the meter on.

Where did you test the "so called" 66vac, i.e. where did you place the probes and on what range.
Note that you are playing a dangerous game with live testing as most can be done without power and use low ohm range after any power supply is completely removed and power capacitors have a chance to discharge.

Sounds like you need someone local with a little more experience to help you track through both the circuit and the hardware.

 

[sean5302]

Thank you.
I had my multimeter in diode mode. The diode I'm concerned about is tiny but it will be possible to unsolder one leg to test it. I'll try that today.
I found the 66V ac on the line going from connector CP509 to and from the power switch on the front panel. With my multimeter on V ac 200V range, the leads measure 66V ac between either of them and the chassis ground of the amp.

I'm using this as a learning exercise. I am a chartered mechanical engineer and have never done anything much with electronics.

 

[Bluejets]

I found the 66V ac on the line going from connector CP509 to and from the power switch on the front panel. With my multimeter on V ac 200V range, the leads measure 66V ac between either of them and the chassis ground of the amp.

Not much point measuring with reference to the chassis as the incoming supply never connects to there.
Reference point will need to be the neutral of the incoming mains ( Terminal 1 ....CP111 or Terminal 1 CP110) for the transformer primary.
Pretty clear in the diagram you linked to and I copied to here #8 above.

As stated above, many of these measurements can be made with power completely isolated and checking continuity from point to point well after and power supply capacitors have discharged.

Stating any reference to an engineer doesn't cut it with me sad to say, best just stick to the facts at hand.

 

[sean5302]

Great, thank you.
By the way, I did test the standby small transformer with my meter set to Ohms. It shows continuity on the input leads and again on the output leads with no connection directly between input and output or to ground.
I'll have a proper look again shortly.

 

[sean5302]

You have been most helpful, so far.
With respect to the neutral mains terminal I'm finding the following:
There is actually 240V ac on the main front power switch
There is 20V ac around most of the board
The standby relay is working fine now (inexplicably so far)
I'm seeing 18V ac @diode D131 and 20V ac at cap C151
240V ac at entry to standby transformer, 19V ac at exit terminal 4 (V.out) and 4V ac at terminal 5 (V.in)
4V ac on terminal 4 changes to 19V ac on activating out of standby

Going right across the board shows 19V ac almost everywhere even at C123 far RHS of board but no dc voltage.

Looking at diode bridge rectifiers D134 and D135 shows 21V ac on every one of the terminals, whether marked + or - or ~
Checking bridge points J188, J183, J189 and J191 shows 20V ac. Nowhere is there a dc voltage.

Does this probably mean that D134 and or D135 are u/s? They are KBPC604-6A units.

Edited this now, because I've been round them with my meter in diode mode.
There is a dead short between + and lower left ~ when putting red lead on + and black lead on ~. When I reverse leads it reads 0.447 (D135)

If I do D134 I see (red lead denoted R, black lead B) Imagine a square box
R+ o/l B~ and B+ 0.447 R~
o/l 0.443 0.445 o/l
B~ o/l R- R~ o/l B-

But D135 has and
R+ o/l B~ B+ 0.447 R~
short 0.451 0.447 o/l
B~ 0.451 R- R~ o/l B-

Many thanks.

 

[Bluejets]

240V ac at entry to standby transformer, 19V ac at exit terminal 4 (V.out) and 4V ac at terminal 5 (V.in)
4V ac on terminal 4 changes to 19V ac on activating out of standby

Unable to follow any of your explanation of this or most of what comes after.
When referencing a point ( say terminal 5) you will need to be specific as there are many terminal 5.

Establish the basic input power initially.
i.e. power to and from the main transformer and stick to that initially instead of wondering about all over the board.
If the relay RLY106 is energising for example, not much point measuring any of it's associated components.("cause it's working...why look for a fault there)
When RLY106 energises, it supplies mains power to the main transformer so go to that section.
e.g. is fuse FS101 ok?
Is power getting to the main transformer?
Are there secondary AC voltages coming out of the transformer, and if so what are they?
etc. etc.

While the neutral point is the reference for any mains input measurements, not so for any low voltage outputs from transformers etc..

 

[Martaine2005]

I am a chartered mechanical engineer and have never done anything much with electronics.

Stating any reference to an engineer doesn't cut it with me sad to say, best just stick to the facts at hand.

A little harsh considering the OP gave the facts. "Never done anything much with electronics".
I bet he's a whizz with mechanical design. Credit is given where credit is due.

Martin

 

[sean5302]

Thank you.
I have established that I have faulty diode bridge rectifiers D134 D135 because I'm only seeing ac alternating current voltages across the board. I have seen no dc direct current voltage anywhere. 20V ac seems present across the whole board.

D135 has a dead short (verified with my multimeter in diode mode) between the red multimeter lead placed on the + terminal and the black multimeter lead placed on the ~ ac terminal. I have removed both to check out of circuit and the problem (a problem) exists there.
I will replace these diode bridges and suspect that heavy use has fried one of them. It may be that something has shorted downstream and caused the problem but I won't know until I see dc voltages.

I'm retired but was Group Engineering director for the world's largest vehicle maker so I'm a little bit more confident with my engines.
Sorry if my ignorance in electronics is winding up the more knowledgeable. I'll try harder to be clear.

 

[Martaine2005]

Sorry if my ignorance in electronics is winding up the more knowledgeable. I'll try harder to be clear.

No need to be sorry, you're doing fine and in good hands with @Bluejets. He's just grumpy and only smiles on Sundays

Martin

 

[sean5302]

I have spent my life working with gifted people who can occasionally be cantankerous.
It's me who's wanting to learn and you guys have great knowledge.
Thanks for your help, all of you so far.