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NAD3155 in protection mode

Timggg said:
Diode back in, powered up and still in protection mode as expected.

Checked the schematic briefly for output transistors- too many on the schematic. Do I start with those marked up in the photo & any need to remove them from circuit to test ?
Click to expand...
So the DBT result was a bright bulb?
 
These within the red bubble are the main output devices.
Lookup the pinouts for them (2SD1148 [NPN] and 2SB863 [PNP]) and they can be tested in-circuit looking for shorts but of course un-soldering the leads and testing is best.
Shorts between Collector-Base, or Collector-Emitter or Collector-Emitter are dead failures.


3155.outputs.png
 
About the DBT test...if the bulb is not BRIGHT then you don't have a direct short in the amp.
However because the DBT is a current limiter (aka series resistor) it's dropping the voltage getting into the amplifier.
This lower voltage may not allow the power supply to function and energize the relay. So possibly use a larger wattage bulb for
the DBT or over-ride the DBT; just plug the cord directly into the outlet.
 
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Thanks Jhjove for details on the output transistors. Will test tomorrow.. :boring:soon.

And the 0.53V diode - will look at replacing at a later stage (it’s in the to buy list for now).

I haven’t built a DBT yet so didn’t use one :(. I guess I need to prioritise this as it has been mentioned and queried several times by several members…
 
Timggg said:
Thanks Jhjove for details on the output transistors. Will test tomorrow.. :boring:soon.

And the 0.53V diode - will look at replacing at a later stage (it’s in the to buy list for now).

I haven’t built a BDT yet so didn’t use one :(. I guess I need to prioritise this as it has been mentioned and queried several times by several members…
Click to expand...

No worries, I just really love trying to figure this stuff....maybe get too involved. :)
You probably found some links + suggestions on DBT but I'll share 2 that are ok: here is one and another one.
 
Thanks for the link on DBT Jhjove…the vintage unit is pretty cool but I’ll go the fit for purpose option :)

Tested output transistors in circuit with the following results :

863

  • B-C (about 0.428V on Q710/711/760/761)
  • B-E (about 0.458 on all)

1148
  • B-C (about 0.48V on Q758/759 & 0.42V on Q708/709)
  • B-E (about 0.49V on Q757/759 & 0.45V on Q708/709)
But there are also abnormal readings on both 863/1148 which should be OL - see photos. I have measured the V on the same combination from the same type of transistors. A bit doubtful with the results as they suggested all the 8 output transistors have failed/shorted). I also observed OL on some of these measurements (eg some voltage readings, and it went OL after I sort of moved by sliding the test pin on transistors leg) :(

Is the next step unsolder the legs and test ? And if yes, do I unsolder all legs or just some . Please help.
 
Timggg said:
Is the next step unsolder the legs and test ? And if yes, do I unsolder all legs or just some . Please help.
Click to expand...

These values may be OK, a short would be 0V or nearly 0V on the diode test.

But if you look at the schematic you will see that the NPN pair are sort of connected in parallel (same as the PNP pair).
The collectors are directly connected whereas the base and emitter connections have small value resistors in a series connection to their respective leads.

These small values could "mask" a bad device, so it's best to un-solder the collectors and the emitters then retest.
 
Update :
- before attempting to un-solder the C & E on the transistors for testing, I did some quick continuity test on a few components most are within or closer to the heat affected discolouration pcb area and found some interesting results. Photos show points where there is continuity on Q704/901/904 & IC902.

Do the continuity result mean anything ?

While doing the test, I realised my DMM battery is low so didn’t proceed any further. I have issues yesterday too with the DMM (didn’t realise it was low battery).

Can I bend the C & E legs of the output transistors when I desolder to lift off pcb for the test, to not result in damage ?

Getting new battery tomorrow…
 
Note there is no continuity between any of three legs and between any leg with the heat sink on Q904 & IC902. Fault ?
 
Q704 is a 2SC2690 NPN, pinout is E-C-B.
Your "continuity" test is reporting a short from the Collector to the Base. Were the results the same when you reverse the leads?
The collector of these types of transistors (TO-126) are connected to the mounting plate...and so when attached to a heatsink (without an insulator) the heatsink is connected to the collector.


Q901 is a 2SD414 (NPN - ECB) showing the collector+heatsink connection. OK
Q904 is a 2SB548 (PNP - ECB) the collector should be connected to the backplane...to the heatsink via the screw, I'd check this again.

IC902 is a 78M24 (24V regulator - VIN-G-VOUT)
pin1 is Vin, pin2 Ground and pin3 is Vout....I think the heat-sink tab is isolated from the external connections.

So far any power-on you've done hasn't smoked anything and just puts the amp into protection, correct?
This area we are discussing is providing the power for the amp.

It is worthwhile to check the values of the +/- 46V supply and the +/- 24V supply. These are DC values in reference to chassis ground.

See schematic below:
RED arrows for the +/-46V
Light BLUE arrows for the +/-24V

On the DMM VDC setting, connect the black probe (COM) to the chassis metal, probe carefully with the red probe to the ARROWS indicated.
If you want to be Extra careful use spring-loaded test leads to hook on the component leads then power-on the amp. Get the reading, power-off, move the lead to the next component then power-on....
If you only have pin-type probes just use care to not let the probe slip.

Example for measuring the voltages:
  • RED probe to emitter on Q901 should read out an approximate +46V DC value.
  • RED probe to emitter on Q902 should read out an approximate -46V DC value
  • RED probe to pin3 on IC902 should read out an approximate +24V DC value
  • RED probe to emitter Q904 should read out an approximate -24V DC value.
3155.PS.edit.png
 
jhjove said:
Q704 is a 2SC2690 NPN, pinout is E-C-B.
Your "continuity" test is reporting a short from the Collector to the Base. Were the results the same when you reverse the leads?
Click to expand...
Yes, positive continuity with either direction on the leads.

Does this mean Q704 will need a definite replacement ? If yes, it’s another item on the to-buy list
 
Last edited:
jhjove said:
So far any power-on you've done hasn't smoked anything and just puts the amp into protection, correct?
Click to expand...

Correct - no smoking. As soon as power is switched on, amp went into protection mode. No other abnormalities other than slight faint noise (from the transformer I think)
 
Timggg said:
Yes, positive continuity with either direction on the leads.

Does this mean Q704 will need a definite replacement ? If yes, it’s another item on the to-buy list
Click to expand...

Well it would best to remove Q704 from the circuit and check with the diode function on your DMM to verify good or bad.
 
Q704 removed and tested to be functioning ok. Pretty difficult to un-solder the 3 legs as can be seen in the photo.

Positive continuity between C & heat sink.

Diode test (NPN) confirmed ok with about 0.56V measured between B-E & B-C. OL for other measurements per diagram.

Will solder Q704 back.

46 V & 26V supply check next but will get test lead with alligator clips (earliest on Boxing Day).

Merry Christmas to all (definitely no snow but an abnormal & extremely hot Christmas here of 43 oC for the next 3 days - probably the hottest ever o_O)
 
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Timggg said:
46 V & 26V supply check next but will get test lead with alligator clips (earliest on Boxing Day).

Merry Christmas to all (definitely no snow but an abnormal & extremely hot Christmas here of 43 oC for the next 3 days - probably the hottest ever o_O)
Click to expand...

To un-solder (I don't know your expertise) I use a combo of a mechanical solder-sucker and then solder-wick to remove the solder. Usually the part then just falls off the board.
If you are worried about over-heating a part use an alligator-clip or small hemostats to lock onto the component's lead (it's draw the heat away getting to the heart of the component)

This type of mini-grab probes work well for snagging onto component leads. IMO, they provide a safer connection than a "hooded-insulated" alligator clip.
TestLeads.mini.grab.banana.jack.png

Have a good Christmas as well!
Weather is crazy any more.
Here in my corner of Northeast Pennsylvania we received a light 1.5" snow over-night. Not enough to drag the power equipment out but enough to push a shovel.
I think I got my 10,000 steps in this morning :thumbsup:
 
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