AB Systems 410C clip light always on

[stoneattic]

Now that you guys solved my Samson idle and clip light problems I've moved on to the next repair project.

One of the Clip lights on my AB Systems 410C, which you've guys had helped me with before regarding the level pots, is always on.

I swapped inputs so that instead of monitoring channel 2 it now monitors channel 1 and it remained on. So unless I'm missing something, which is a definite possibility, it has to be the indicator PCB and not the amp channel itself. The amp does not sound like it's clipping at all.

Attached are all of the schematics, documentation, etc., that I been able to get. It's quite old but AB International (formerly AB Systems) sent me what they had. From the troubleshooting guide with the schematics and via email with the folks at the current AB International I have been able to determine that the 410C is kind of a cross between the 410B and 900 (not 900A) and 9220 (not 9220A).

My guess is that the Q6 MAC 92-4 Triac is shorted. Before I order a replacement, does anyone have any other ideas?

Indicator PCB Assy.pdf
410 Series S.M (2).pdf
600-900.pdf
410b-man2.pdf
600-900-man2.pdf

[Jazz P Bass]

The second, third & fourth file must be resized & reloaded.
Get Cute PDF Writer & reprint then as a resized pdf file.
They are coming up blank.

As to Q6, does it show as shorted.
Is R12, triac side, at ground potential.
There should not be a voltage drop across R2 in a working circuit.
(no ground reference because of the triac)

Have you diode checked Q5 for C to E short.

[stoneattic]

I fixed the attachments and added an additional one.

I haven't measured anything on the board yet. Last night I was just trying to narrow it down to the Indicator PCB, then I wanted to study the schematic to figure out what to do next.

Honestly I'm not sure what/how to measure the Q5 or Q6. Can I measure them in circuit?

[g1]

The third and last attachments are still not working. (410b-man and 600-900-man). They need to be resized to work.

[stoneattic]

I think all the attachments are working now.

[g1]

410b-man attachment is still not working, sorry

[Enzo]

So Q5 turns on the light, and Q6 turns on Q5. One or the other is shorted or it is not. Howe to test? Put ohm meter leads on emitter and collector and see if they are shorted. There are three legs on the transistor, only three combinations that can be shorted. Same on Q6, see if the two main terminals are shorted, and check the gate to either main terminal.

There is nothing in the circuits around those parts that will make them look not shorted if in fact they are shorted. Think about it. Touch your meter probes together - they read shorted now - what could you connect to them that would make them measure anything else but shorted?


How did you decide Q6 was bad rather than Q5?

[stoneattic]

410b-man attachment is still not working, sorry

I THINK I've got the attachments straightened out, finally!

[stoneattic]

So Q5 turns on the light, and Q6 turns on Q5. One or the other is shorted or it is not. Howe to test? Put ohm meter leads on emitter and collector and see if they are shorted. There are three legs on the transistor, only three combinations that can be shorted. Same on Q6, see if the two main terminals are shorted, and check the gate to either main terminal.

There is nothing in the circuits around those parts that will make them look not shorted if in fact they are shorted. Think about it. Touch your meter probes together - they read shorted now - what could you connect to them that would make them measure anything else but shorted?


How did you decide Q6 was bad rather than Q5?

I know that some components need to be pulled to be tested and I'm not familiar with transistors, especially triacs, so I wasn't sure if they needed to be.

I guessed Q6 mostly because I've come across posts were triacs have gone more than I've seen posts about NPNs going. Not exactly the best diagnostic logic I guess. I was probably going to just replace both to be sure, considering the cost is minimal. I originally posted because I wanted to check that I wasn't missing something obvious, or at least obvious to someone who know what they are doing.

I will test Q5 & Q6 for shorts tomorrow. Hell, I was proud of myself for narrowing it down to the Indicator board!

[NZaudio]

well,well stoneattic. It is always best to start there where the cause occurs.
To test transistors:
switch your multimeter to diode test
see the pinout for your transistor to be measured and start measuring as follows:
1-red lead on base, black lead on emitter = should measure open circuit or a 1 on your DMM
2-red lead on base, black lead on collector = should measure open circuit or a 1 on your DMM
3-black lead on base, red lead on emitter = should measure anything from 500 to 900
4-black lead on base, red lead on collector = should measure anything from 500 to 900
5-red lead on emitter, black lead on collector = should measure open circuit or a 1 on your DMM
6-black lead on emitter, red lead on collector = should measure open circuit or a 1 on your DMM
The above 6 measurements steps are for a PNP transistor.
For an NPN transistor just change lead colours and results.
These 6 steps are necessary for all transistors
I think this is a good way to start learning, if you have any of your results different to the wanted measurement results as above, replace the item. If you measure approx what I wrote down, your item is still OK
Hope that helps

[NZaudio]

Another way for you would be the A and B measurement. As you have 2 identical channels, measure all points and find out where the difference is.

[stoneattic]

So far this is what I've found measuring unpowered:

a) The schematic is not correct, or more likely it is for a different model. It is similar though. The cathode of the LED goes to ground and there is a resistor from the anode to a MPSL01 (Q5) as the schematic shows. However that resistor (R13 on the schematic) measures 10K, not 5.1k as the schematic shows.

b) Here are the measurements I get on the MPSL01 (Q5 on the schematic). Note that according to the datasheet attached it is an NPN so I followed NZaudio's recommended test procedure and got the following:

black on B to red on E = 100k
black of B to red on C = open
red on B to black on E = 100k
red on B to black on C = open
black on E to red on C = open
red on E to black on C = open

This does not match NZaudios, but the other channel's MPSL01 (Q5) gives the same readings. My guess is Q5 is good and perhaps the package is different than the datasheet I found?

c) The schematic shows a different value for the resistor (R11) between ground and the base of Q5 (100k instead of 51K) and a different value for R12 (100K instead of 152k) between the base of Q5 and one leg (MT1 or MT2?) of Q6

d) I have not been able to find a datasheet for the MAC 92-4 triac, but found the CQ92N listed as a replacement (data sheet attached). I'm not sure how to test this but measured between the the pin that is connected to R12 to either of the other pins and get open. If it were shorted I would have expected to have seen the short between that connection and ground, but it appears open. Between the other two pins (Gate and MT1 or MT2?) gives continuity.

e) The PCB is not labeled which is a hassle, particularly since the board has the 4 LEDs (2 "Clip", 2 "On") that go through the front panel and are attached somehow that I can not figure out. Since the board is on stand offs the LEDs legs are pretty much full length. Anyhow, the component side of the board is hard to see. If I replace anything on the board I will have to figure out how to get them out.

f) All of the above measurements match between the two channels.

g) I've never seen the other "Clip" light come on, but I'm not surprised. It would be painful to get that kind of volume. I did diode test and and it tested fine. That was until I did the quick touch it with the 9V and never saw it light then tried the 9V on the one that's always on and then realized I just killed them both. One died open and the other shorted. On the bright side the "Clip" light isn't stuck on, haha.

I am at a loss here. I need to replace the LEDs which is a hassle because I can't figure out the mounting. I will probably just clip them all off, drill them out and replace then with bezel mounts. It looks like the LEDs might actually have mounted from the front and where then solder to the already installed PCB, which looks like a PITA as well as being very repair unfriendly.

Any other ideas?

MPSL01.pdf
cq92b (MAC 92-4).pdf

[g1]

It's hard to tell what kind of meter you are using. NZaudio gave instructions for checking transistors with a DMM set to DIODE function.
You gave measurements in ohms so maybe you are using an ohm meter or a DMM that does not have a diode test function?
Anyway, some meters read "backwards" when checking transistors on ohm function. So reverse your red and black probes and check again.

[Jazz P Bass]

And next time use a 1K resistor from the battery to the LED.
Current limiting.

[stoneattic]

And next time use a 1K resistor from the battery to the LED.
Current limiting.

Yeah, I know better, but I've done it really quickly plenty of times without a problem and was in a rush.