DB Drive 1200

[Perry Babin]

I assumed that pin 3 of the LM361 was fed from one of the outputs of the TL072 on the driver board. Follow the traces back from pin 3 of the LM361. Which op-amp drives that pin?

 

[Hectorscaraudio]

It dose trace back to the TL072 pin 3. I have this board out of the main board to find this
trace.

 

[Perry Babin]

Pin 3 isn't an output pin. Does pin 3 of the LM361 also go the the output pin of one of the op-amps?

 

[Hectorscaraudio]

Ok found it after pin 3 of the LM361 smd cap C158 , resistor R108 , cap C177 , and then a trasistor Q171 C3198 then pin 1 of the LT072.

 

[Perry Babin]

If you don't have a generic diagram for this type of amp, email me. The JBL BP1200.1 is (should be) close.

babin_perry@yahoo.com

The transistor comes from the integrator and then to pin 4.

 

[Hectorscaraudio]

LOL thats the first time I used the diagram from your DVD ,man once you know what your looking for it really helps !! Awesome work Perry once again .. Ok yes it's off the other TL072 pin 1 through a 2.2k resistor then to pin 3 of the LM361..

 

[Perry Babin]

Post the DC voltage for the other (newly found) TL072.

 

[Hectorscaraudio]

New TL072 DC voltage
1. -10.00
2. .040
3. 0.00
4. -12.38
5. 0.00
6. 0.008
7. 0.80
8. 11.83

 

[Perry Babin]

Has the voltage on pin 3 of the 361 changed? I don't see any voltage here corresponding to the -4.79v you had previously.

 

[Hectorscaraudio]

Ok I check the LM361 pin 3 did not match voltage after R157 -7.2V . I put a little pressure with the probe on pin 3 and the voltage came to -7.2V triangle wave form. I guess cold solder, because I had changed out the LM361 with another .

PS. previously LM361 pin 3 had +4.79V.

 

[Hectorscaraudio]

Resoldered all pins on LM361 and now pin 3 dose not have a triangle wave form just -7.2V.

 

[Hectorscaraudio]

On LM361 pin 3 after 4-5mins. amp on, triangle wave form came back still -7.2V no rca's in. When I put in rca's LM361 pin 3 triangle wave form bounce's around a little bit, and pin 4 as well -0.77 with triangle wave form no rca's in , rca's in triangle wave form bounce's around a little bit. Pin 9 wants to make a square wave, but goes back to flat line, if I higher up the rca signal square wave stays on but looks ugly (+1.5V no rca's) , pin 11 just stays flat line +2.2V with rca's and with no rca's.

 

[Perry Babin]

Solder a jumper between pin 1 (that goes to the LM361) and pin 2 of the TL072. That should give you 0v on pin 3 and a triangle waveform on pin 4.

 

[Hectorscaraudio]

Ok solder jumper LM361 pin 1 to LT072 pin 2 and everything looks the same ?

 

[Perry Babin]

Solder the jumper between legs 1 and 2 of the op-amp that has pin 1 driving pin 3 of the 361.

 

[Hectorscaraudio]

OK solder pin 1 and 2 TL072 . Yes LM361 pin 3 is now at 0.00V , pin 4 has a triangle waveform ,but it takes 2-3mins for the triangle waveform to show ?

 

[Hectorscaraudio]

Also a clean square waveform on pin 9 LM361.

 

[Perry Babin]

The triangle waveform should be produced within a few seconds.

Is there excessive DC on either pins 3 or 4 of the 361 before the triangle waveform appears?

Do you also have a clean square wave on pin 11 of the 361?

 

[Hectorscaraudio]

Ok pin 4 on the LM361 triangle waveform comes on every time, after I probe the crystal oscillator first? No high current draw on LM361 pins 3 and 4 before triangle waveform comes on after probing crystal oscillator. No square waveform on pin 11only +2.2V LM361.

 

[Perry Babin]

Make sure that the legs on the crystal are not broken. If there is any flux or other contamination around the crystal, clean it thoroughly.

The crystal is very sensitive to loading. The load presented by the scope probe can stop it from oscillating (when it's working properly). When you need to see if it's oscillating, probe the pins on the 16 pin IC next to it.

If pin 9 has a square wave and pin 11 does not, the LM361 is very likely defective. It's possible that they have it disabled but I've never seen an amp with this basic design with one output disabled. With the voltages you posted, it doesn't look like either output is supposed to be disabled.