Quad 405 problem

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21-Oct-2017 05:11 PM

Willy Van Win

Willy Van Win

Posts: 6

Hello,

just have used the 405 kit from dada with opamp OPA.
after checking all, i tested the channels seperately.
+50v, -50 v and massa on cooling plate.
One channel ok, the second give a smoke on resitor R39.
Service manual says open connection on R20/21.
R20/21 are ok. Replaced R39. Now R2 smokes.
Replaced R2 and checked components around TR2
R12 was around 4K, so replaced it with 3K3. Tested TR2 which was ok. Tested again and no smoke anymore.
But -50V on output! Replaced TR7/8 and TR9/10.
Test again and still -50V on output. Checked some components and found R29 open. Replaced it with 8K2.
Tested again and still -50V on output. Replaced IC1, checked D3/4/5/6 , all ok. Checked for closed connections on the cooling plate, but all ok.
Any help or suggestions are welcome....
Willy
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23-Oct-2017 01:34 AM

EJP

EJP

Posts: 1405

You need to start thinking about small signal transistors. Check the voltages around Tr2,3,4 against the schematic. I would also check that you have C10 the right way round. R39 open-circuit indicates that the amplifier had been oscillating, which suggests the decoupling on the op-amp. Try swapping the op-amps around if you get this far.

EJP
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23-Oct-2017 09:44 PM

Willy Van Win

Willy Van Win

Posts: 6

Have replaced all transistors with the types in the faq.
Have checked all components and found no faulty ones.
Checked again the pcb for possible short circuits.
Tested the amplifier outside with no load.
+51V and -51V and massa to cooling plate, and a wire from cooling plate to ground input.
output voltage=0.01 to 0.08V it varies a little.
Again R39 is becoming very hot!!
Voltages:
IC1: pin4=-13V pin7=15,7V pin6=1,3V
TR1: c=50,6V b=49,6V e=49,1V
TR3: c=2,9V b= 50,6V e=50,1V
TR2: c=1,17V b= 35V e= 0,55V
R30/R31: -26V
TR7: c=1,6V b=50,1V e=50,6V.

because i used the zeners from the upgrade kit it ssem strange pin4 of IC1 = -13V i replaced the opa with the original 301A and tested again.
Now R39 was directly smoking and pin 6 of ic1 was dropped to 0,6V.

I find this one a very diffucult one.....
has checked all components on the board ( C's, L's and R's) and they are all ok. Replaced all transistors also, checked polarity of the elco;s -- all is ok.
Not one voltage around TR1-TR3-TR4-TR5 are correct...
What is wrong here ?

Any idea / help is very welcome.


Willy
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23-Oct-2017 09:50 PM

Willy Van Win

Willy Van Win

Posts: 6

Forgot to mention:
i replaced D3,D4,D5 and D6 with 1N4148 also.
I think this is ok.
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23-Oct-2017 11:02 PM

Willy Van Win

Willy Van Win

Posts: 6

Sorry guys,
not seen on schematic: voltages without E are relative to +rail.
So i replaced again IC1 with new OPA627AP.
R39 becomes very hot after a minute.
Voltages ok around TR1/TR3/TR4/TR5
Collector TR7= 1,6V and must be 6V
R30/R31 = -26V - OK
IC1: pin4= -13,2V pin7= +15,3V pin6= 1,3V
TR2: c= +35,4V b= +1,2V e= +0,6V
looks much better, but still not ok!
what to do ? pin4 should be -15V i think..
also pin 6 and voltages on TR2 are too low...

Willy
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24-Oct-2017 12:34 AM

EJP

EJP

Posts: 1405

To reiterate, R39 burning indicates oscillation, which means you should check the extra decoupling capacitors around the op-amp.

It also seems to me that both Tr2 and Tr3 are faulty.
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24-Oct-2017 01:41 AM

Willy Van Win

Willy Van Win

Posts: 6

What do you mean with extra decoupling capacitors ?
I have removed C3 as indicated in the upgrade manual.
I have not changed the sensitivity, but checked C4 and C1.
Is there any need to add extra capacitors for the OPA chip?
C2 already replaced which was in the kit.
TR2 and TR3 just checked - they are ok.

Willy
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24-Oct-2017 02:38 AM

EJP

EJP

Posts: 1405

You need to add 100nF from each of pins 4 and 7 to ground if you modernize the op-amp in a 405.
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25-Oct-2017 12:02 AM

Ton

Ton

Posts: 18

About decoupling op the opa's, if you follow the instructions in the DIY upgrade guidelines, you decouple D2 (pin 4) with a 2K7 resistor and you decouple D1 (pin 7) with 100nF.


But in fact, if you read Bernd Ludwig's upgrade manual section A1b about the decoupling as being imperative, D2 should also be decoupled with 100 nF. 


The component list in the DIY upgrade guidelines talks about "2x C 100nF decoupling-capacitors for D1" and you could interpret that as decoupling-capacitors for both D1/D2.


So should you use the 2K7 resistor or the 100nF on D2 ? 


So it's a bit confusing or am I missing something thats in the DIY guidelines. 
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25-Oct-2017 01:15 AM

EJP

EJP

Posts: 1405

If you haeve both the 2k7 resistor and another 100nF capacitor, use them both in parallel.

EJP
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25-Oct-2017 02:44 AM

Ton

Ton

Posts: 18

Thanks, that is clear now.  Hopefully it helps the starter of this topic. 

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25-Oct-2017 02:23 PM

Quad 405

Quad 405

Posts: 1

There was an issue with some 405's that the signal input earth should always be connected to the case earth otherwise R2 and R39 would fry. Here is wat Joost said back in 2009.

Hallo Bart,

 

Voor ik commentaar geef op de blog eerst maar even contact.

 

Het was niet mijn bedoeling om je te corrigeren, we zijn veel te blij met reacties.

 

Afhankelijk van de mate van oscillatie en de belasting van de module op dat moment kan R39 van het Zobel netwerkje ook in rook opgaan.

Aangezien ik tot nu toe alles met onze eigen modulen doe, was ik dit probleem glad vergeten. In de tekst bij de dubbele voeding ga ik dat nog aanpassen.

Een oplossing hiervoor is R2 vervangen door een draadbrug. Aan gezien R2 toch kortgesloten wordt door het doorverbinden van de output aarde en de ingang aarde.



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26-Oct-2017 09:31 AM

Joost Plugge

Joost Plugge

Posts: 938

Hmh, it is always bad news if you are confronted with remarks/opinions from the past. The R2/R39 issue in that post hat to do with oscilation that occurs when you are testing certain level 405-1 boards outside the chassis. I don't know if this is the case in this situation.

Joost Plugge

DaDa Engineering

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26-Oct-2017 12:13 PM

Willy Van Win

Willy Van Win

Posts: 6

Problem solved!
have placed two little 100nF directly on pin 4 and pin 7 to ground. After that and with the input ground connected to the cooling block also, all was stable. This was done after reading Bernd Ludwig's modification.
Have also done all standard modifications from his circuit diagram.

Thanks for helping me.
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