crate TX50a limo

[pontiacpete]

I'm working on my limo, tracing a 15mv signal through the circuit and finally find there is a signal on pin 1 of U6 the TDA1562 but no signal on pin 7.
So does this mean I can assume U6 is shot?

[Jazz P Bass]

TDA 1562

What does the Mode pin look like.
Should be High.
I think this IC needs a load.

[pontiacpete]

no signal on the mode pin.
I have the speaker hooked up, also.

[J M Fahey]

Hi PontiacPete.
You won't have an "audio" signal on pin 4 (mode) , but a certain DC control voltage.
What JPBass suggests is to measure the DC voltage there .
You need *more* than about 5V there for the chip to turn on.
That voltage comes from the +12V supply, through R74 (470K), charging C57 (10uF). If not, you *might* have either R74 open, C57 shorted or very leaky or just a bad chip.
Check that the +12V from the battery reach Pins 9 and 10.
I have bought a pair of TDA1562, haven't gotten to design the PCB yet, please comment on the sound, it will be very helpful to me.
Do they feel like "real", solid 50W or more like an "enhanced" , gimmicky 20 W?
Good luck.

[pontiacpete]

Thanks again Jazz P Bass
I really misunderstood, duh
Obviously I don't know this IC. "duh"s come in 2's around here, I was also reading the pins wrong.
Looking at this chip from behind, i.e. the heat sink first then IC, is pin #1 on the right and do they go consecutively from right to left in numerical order 1-17?
If so pin 1 has a sad looking square wave and pin 4 has no voltage. There's no signal on the node between R72 and R73,,
any ideas?

Thanks JM Fahey
for helping me get that straight. I checked R74 and that seems OK, in parallel with the diode in circuit it measures 235ohms, not opened. Haven't tested the cap yet.
As far as the sound of the TDA1562 goes, it's ok. This amp is quite loud,. I've been using these for 10 yrs in my street band , it always cuts thru well in a trio and in a quartet setting. It maintain an clean sound(as well as a battery power transistor amp can get) which suits me fine. I never have to play it above 9 o'clock on the dial. Really I'm hoping to get my battery powered tube up and running to replace, though.
Digikey told me they've discontinued this IC.

pete

[Jazz P Bass]

IC datasheet

Did you happen th read the IC data sheet?
Page 5 shows the pin out diagram.
Page 6 explains pin#4. Mode pin.
Low level input: it is in Mute/Standby Mode.
Do you have 12Vdc at the top of R74?
Pin #4 has to be high or the amp will not output.

[pontiacpete]

"Did you happen th read the IC data sheet?"

I read pg 6 and trying to understand it.

"Do you have 12Vdc at the top of R74?"

yes, there's 12v
Also, another stupid mistake, I had to detach the power supply PCB from the chassis and took the previous readings not realizing the screws and the screw holders provided the ground, duh, again. I clipped them to the chassis and now I have 11v on pin 4.
So it could be working?

Again, I greatly appreciate your help, and your patience P bass

pete

[J M Fahey]

Hi PontiacPete. You can't measure a 470k resistor, in parallel with something else, read 200 something ohms and declare it healthy. You just don't know. You should unsolder at least one end and measure it "in the air".
Even better, replace it and C57 (pay attention to polarity).
If after that you have 12V on one end of R74 and nothing on the side that goes into pin 4, you can declare the power chip dead without further testing.
If you have between 5 and 12 volts there, the chip *should* sound.
Thanks for the "real world" info, that's what really counts.

[pontiacpete]

Changed out C57 and R74(that did turn out to be good). I made yet another mistake and was reading pin 5 instead of pin 4. Pin 5 has the 11vdc on it and there's nothing on pin 4. I guess now I can "declare the power chip dead". Wheew! Could that explain why there is a square wave on pin 1?
I noticed when removing the big PCB which holds that chip that one of the screws had loosened and almost snapped off near the head. Vibration I'd imagine.
thanks guys, I'll be back with the new one in and post the results.

[pontiacpete]

I received the TDA1562 yesterday , put it in and now I have sound, however the mic input board is producing a very high screaming noise out of the Hi-Z input. I put the scope on pin 1 of U2-A and I have normal sinewave but on pin 7 there's a square wave....(no a square wave, I mean a clipped wave, my head is in a fog from this horrible cold I have) , does this mean U2 is suspect?
The instrument input channel seems to be working, it has some static ,, maybe the pots need cleaning.

pete