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Soundcraftsmen MA5002 Voltage Rails down

M

MNRepair

New Member
Hello!

Looking for some guidance here, I have an MA5002 that I replaced the filter caps, rectifiers, Triac, and Optocoupler.

I am not getting high voltage to the filter caps, and when I measure VAC coming to the transformer from the power supply I get 3 VAC, with correct 120 being supplied. When I received this unit the fault condition it exhibited was that it would have "power pulses" which I assumed were the crowbar circuit firing. Since replacing the mentioned parts, I do not have this fault, but there is no voltage rails, if I put the original optocoupler back in, I can get back to the same fault condition.

Some notes about this unit, the predriver boards were recapped, there appears to be some newer resistors as well, but the darlingtons and output drivers all have the original 4,25 labels on them so assuming original. 12V circuit seems to be functioning as intended.

Any guidance would be greatly appreciated!
 
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Hi, I'm not sure what you mean by "power pulses", but did you also try a new 2900 IC? What is the protection light doing during these issues?
 
Racingh11 said:
Hi, I'm not sure what you mean by "power pulses", but did you also try a new 2900 IC? What is the protection light doing during these issues?
Click to expand...
Yes I did replace the 2900 IC.

Probably not a great descriptor (power pulse), but what happens on, or off a dim bulb tester, theres a large pull of current, the VU meters bounce and light up, the protection light illuminates, then it goes to a state that is similar to right before you hit the power button, and it will repeat that cycle about every 1.5-2 seconds. During these "pulses" I can audibly hear these pulses.
 
Kind of sounds like the original problem might be an output transistor shorted and cycling the protection. The transformers usually make a little noise during the sudden power on. Did you get the correct optocupler? The originals are hard to find, and I have not found any others that work right.

I would be curious if it turned on and powered the caps with the amp boards disconnected.
 
That optocoupler is a triac and uses different pins, besides having a lower current and power rating. The factory H11C4 is an SCR optocoupler.
 
Racingh11 said:
That optocoupler is a triac and uses different pins, besides having a lower current and power rating. The factory H11C4 is an SCR optocoupler.
Click to expand...
I thought I had pulled that part number from another post, probably just a mistake on my end. So you are not aware of a current production replacement?
 
MNRepair said:
I thought I had pulled that part number from another post, probably just a mistake on my end. So you are not aware of a current production replacement?
Click to expand...
I have bought new old stock from ebay. The circuit can be modified for the triac version. It's been done, but not necessary.
 
Racingh11 said:
I have bought new old stock from ebay. The circuit can be modified for the triac version. It's been done, but not necessary.
Click to expand...
Finally got some parts in, I replaced the 4 Darlingtons, and all 12 output drivers. the power cycling is still happening (with original optocoupler). For what its worth, the transformer clicking is much less than before, and the VU meters are only bouncing on the first turn on. So unless theres something else to probe, I'll plan to track down an old stock optocoupler. Thanks!
 
It looks like NTE3091 and 4N40 are equal to the H11C4. I think they are obsolete, but probably some on ebay if you can't find the originals..
 
I ended up trying the zero-crossing triac IC upgrade. It was successful with an IL4117. That's what I had in stock, An MOC3043 would work too. I removed the rectifier diodes and jumpered to the triac IC. The IC got traces cut to pins 5 and 6. 6 got connected to the jumper coming from the 22 ohm resistor R104(R79 MA5002). Pin 5 should be left unconnected. Parts C39, R98 (C34, R94 MA5002), got removed. And pin 4 got connected to the jumper that goes to the triac gate. Be careful, the stock IC socket can have a poor connection to the new IC pins. Pin 2 would not make contact and needed a new socket installed. I chose to use a 6 pin socket instead of the stock 8 pin with two unused.
 
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Racingh11 said:
I have an H11C3 that I just pulled from a working RA7502. Do you want it, or did you order some?
Click to expand...
I bought some new old stock on ebay, H11C4, unit is in the same position of "pulsing". It was a set of 5 so I tried 3 and the same outcome for each.

I am not sure of the impact as I really have not looked into this part, but I noticed the 120V version uses the H11C3, I am using a 120V input, is this part of the problem? I understand that it is more sensitive and can handle a higher voltage peak, but past that I am sure there are nuances I am unaware of.
 
Racingh11 said:
I ended up trying the zero-crossing triac IC upgrade. It was successful with an IL4117. That's what I had in stock, An MOC3043 would work too. I removed the rectifier diodes and jumpered to the triac IC. The IC got traces cut to pins 5 and 6. 6 got connected to the jumper coming from the 22 ohm resistor R104(R79 MA5002). Pin 5 should be left unconnected. Parts C39, R98 (C34, R94 MA5002), got removed. And pin 4 got connected to the jumper that goes to the triac gate. Be careful, the stock IC socket can have a poor connection to the new IC pins. Pin 2 would not make contact and needed a new socket installed. I chose to use a 6 pin socket instead of the stock 8 pin with two unused.
Click to expand...
I will await your feedback on my prior reply before going down this path, but thank you for the information.
 
I have been using H11C4 as a replacement. They are higher voltage and slightly less to trigger. They work well.
 
Racingh11 said:
I have been using H11C4 as a replacement. They are higher voltage and slightly less to trigger. They work well.
Click to expand...
Ok great, I should be set with that part then. Just to be verbose I also swapped the LM2900, but am in the same position as before swapping parts.
 
Hoping to re-ping on this thread. Because this amp is going to be used in a 8+ hour service each day, I took the approach of replace as many components as makes sense. Currently I am in the position where I have essentially no voltage on the rails. I have replaced the following parts:

C32-C35
Rect1/2 - KBPC5004FP
TR1 - Q4015L5TP
SCR - S4055MTP
R90/R91
U1 - LM2900
U2 - old stock H11C4
Q29-Q32 (appropriate small signal diodes)
CR32-CR37 (appropriate diodes)
C21
C30
C22
C31
All output diodes and darlingtons

I know the transformer when isolated is producing 84VAC.

I have also disconnected the predriver boards from the filter cap circuit and am at the same point.

Any help would be greatly appreciated, this thing has been a pain.
 
MNRepair said:
Hoping to re-ping on this thread. Because this amp is going to be used in a 8+ hour service each day, I took the approach of replace as many components as makes sense. Currently I am in the position where I have essentially no voltage on the rails. I have replaced the following parts:

C32-C35
Rect1/2 - KBPC5004FP
TR1 - Q4015L5TP
SCR - S4055MTP
R90/R91
U1 - LM2900
U2 - old stock H11C4
Q29-Q32 (appropriate small signal diodes)
CR32-CR37 (appropriate diodes)
C21
C30
C22
C31
All output diodes and darlingtons

I know the transformer when isolated is producing 84VAC.

I have also disconnected the predriver boards from the filter cap circuit and am at the same point.

Any help would be greatly appreciated, this thing has been a pain.
Click to expand...
Does the protect LED turn on and then off after a few seconds when you turn it on? If so, check voltage at pin 1 of U2. It should go from 0V to about 1.8-2V when it's supposed to turn on the triac.
 
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