General information

Job Number	0303-76
Equipment type	Integrated Power Audio Amp		Made in:	Spain
Maker/Model/Chassis	Ecler	PAM960
Dates 2003	In	Repaired		Out
	Mon.31.Mar	Sat.05.Apr (my birthday)
Comments

Symptoms observed

1	No sound from one of the channels
2	Unit goes into protection mode

Repair notes

Full repair description and observations
1	When I opened the set up, I noted that one of the channels was badly short circuited, few burned resistors, blown fuse, etc.
2	There were 10 main Mosfet type output transistors, 5 IRFP240 and 5 IRFP9240. One set were +ve and the other –ve driven but I forgot which is which. There were 6 of these which where short circuit, while 4 still gave good diode check results. The diode check was not as straight forward as normal transistors giving 0.7V between b-E and b-c. Sometimes I got 0,5V, and 0,2V and 0.08V readings, but never 0.00. It appears that the following pattern took place: I get 0.55V reading, then I check the other junction and I get 0.2V. If I check again the junction which previously gave 0.55, now it will give 0.08V!! If I check again some other junction, and back again to this junction, I will get back the 0.55V
3	Well I searched in the shops for the transistors and fortunately I found in Malta at LM2. I made an estimation of about LM35 and the owner said OK.
4	I decided to change all other voltage and biasing transistors, which usually they cost few cents. There were 8 or 10 to change and about 2 or 3 where short circuit. The transistors in concern were: BC558b, BC548b, MJE340 (x2), MJE350 (x2) BF871 (x2), BD???.
5	There was also 10 12V zenner and 0.22Ohms/5W resistors at the Mosfets ends to be changed, and some resistors which were burnt out. A 12A fuse was not available in shops so I used a 10Amp one. I also changed 4 of the electrolytic capacitor (those at the output stage)
6	The buying, and desoldering and soldering took lot of time, as also did the checking and rechecking of the components not checked. If I miss a component, everything would burn up again!
7	Some hard work left to do was fixing a bad burn in the PCB which cause some tracks to break up. Fortunately, I could know how things are from the other identical copy channel. There was only one link, which had to be redone.
8	I cleaned the circuit from loose solder and was on the point to switch on. However I was very afraid that something might be wrong and would cause again a severe damage. So after long considerations, I decided to buy a variac (7A, 260V (about 1700VA)). The manual also suggests to power up via a variac.
9	MAPP had a crappy 1000VA made in china Variac, while G&E had a good built 1700VA model called Metrel from Slovenia. I had no other choices. It was very heavy to import from abroad, so I decided to buy that from G&E at LM90.
10	I followed the manual of the amplifier which said that I should supply ac via a variac, starting from 0V and gradually increase the voltage. I have to connect an ammeter along one of the fuses of the channel (the other fuse should be in place). When I see a current overload more from 0.8A I should stop increasing the ac voltage and turn off the amp and check for something wrong. This will prevent severe damage to the channel.
11	I did this, but I made 2 mistakes (see below in the precautions). I started to increase the voltage, ammeter remained always 0Amps, so I continued to increase the voltage. Around 100Vac, I heard a crackling sound, one of the resistors produced smoke, and I switched off quickly the amp. I checked what happened and found out 2 power transistors got short circuit, as well as 2 diodes, resistors and high wattage resistors. Not much damage but still I was very discouraged.
12	Next day (my birthday) I decided to change the faulty components and try again. I realised of two critical mistakes. The ammeter was set to read ac current not dc current and I did not fixed the circuit to the heat sink, so excess heat may have broken the ‘heat fragile’ Mosfets.
13	Everything was desoldered back and I was again trying to increase the voltage with the variac. On 110Vac, the amp was just 0.2A. The critical value (according Ecler manual) was 0.8A so I was on the good track. I increased further the voltage until I finally arrived to 240V ac. And the current was 0.5A approx.
14	The manual continues saying that there should be 0.07A per transistor pair, hence in this case 0.07A x 5 = 0.35A. To adjust this current there was a special preset. I altered the value until I got 0.35A. I found out that the other ‘good’ channel had a current reading of 0.7A. I adjusted it back to 0.35A. Probably the user has tweaked with these pots to get more powerful sound ?!?
15	There where some more settings to do but using an oscilloscope. Since I do not posses one, I could not make these adjustment. I switched on the amp for 5 hours and the current drifted slightly to 0.40A due to heat production, so I compensated and adjusted the pot so that when hot the current reads 0.37A. (When cold, initial current was now 0.33A).
16	The amp was connected to a speaker and a sound input was applied.

Repair summary

CHANNEL SEVERLY SHORT CIRCUIT, VARIOUS COMPONENTS INCLUDING TRANSISTORS NEED TO BE REPLACED

Precautions, fouls and advises
1	When reading current, be sure that you select the correct form between ac and dc. In circuit, the current should be read almost always in dc form (like voltages). By mistake I placed the reading on ac and I always had a 0 reading. So I did not realised about the current build up (and thought that ammeter was faulty!!!)
2	Mosfets, esp. in such power amplifiers are heat sensitive and if not connected to heat sink, even for short testing periods, they can get damaged and develop short circuit. Always connect circuit to heat sink as in the original state before testing.
3	The variac is a very important tool when it comes to power amplifiers, and faults which consists of high current damage. The procedure is like this! You connect a dc ammeter to a fuse (not being mains ac fuse) or in series after a voltage supply resistor. You connect the item under test to the variac and increase the voltage slowly. If you note a build up of current (1g 1A and above) at quite low voltages (eg 110V) then there is something wrong. The faulty component could be easily identified by seeing which component gets very hot!!! J

Other measurements and technical comments

1

N/a