Jamo A-101 Sub Amp

[aussiegreg]

Hi Guys!

Can anyone assist with a schematic / service manual for the Jamo A-101 Sub Amplifier? I've had an issue in the past where the "glue" used to stabilise the larger components on the board has been "cooked" and carbonised causing shorts. Repaired that no problem but now (12 months later) I'm getting really poor and distorted output plus random noise and Pops. I've had a basic look around for a repeat of the "Black Glue" incident but to no avail. Circuit Diagram would save me a lot of time.

Cheers
Greg.

[lexwalker]

Re: Jamo A-101 Sub Amp

Hi Guys!

Can anyone assist with a schematic / service manual for the Jamo A-101 Sub Amplifier? I've had an issue in the past where the "glue" used to stabilise the larger components on the board has been "cooked" and carbonised causing shorts. Repaired that no problem but now (12 months later) I'm getting really poor and distorted output plus random noise and Pops. I've had a basic look around for a repeat of the "Black Glue" incident but to no avail. Circuit Diagram would save me a lot of time.

Cheers
Greg.

Some pictures of the board would help. As for the glue, it does not need to turn black because if it has turned to deep yellowish to brownish in color then better remove those as they are degrading and becoming conductive. If you find any remaining then recommend removing those degraded glue. Also look and check for badcaps...

[aussiegreg]

Re: Jamo A-101 Sub Amp

O.K. Pix attached. Have cleaned off all the glue and done some further testing. I have replaced 2 suspect caps (the large brown 680uF 80V caps at the bottom right in the component side picture. The cap nearest the edge of the board was running quite hot after only a minute or so and I figured new ones couldn't hurt. Problem is that the same fault still exists... Outside cap runs very hot even with no input audio. The original noise and popping seem to be reduced but I dare not run the amp under load with the cap running hot.

Ideas anyone?

Cheers
Greg.

[stj]

Re: Jamo A-101 Sub Amp

lots of cracked solder joints.

[Agent24]

Re: Jamo A-101 Sub Amp

That solder job looks pretty shabby to begin with, too. Yeah, reflow all the joints stj pointed out, plus I'd probably do the rest of the through-hole parts as well for good measure.

I wonder if the crap they have poured all over those SMD components on the right hand side of the board has also gone conductive? Also I wonder if there are cracked joints under that as well?

[stj]

Re: Jamo A-101 Sub Amp

i was tempted to draw a big red circle around the whole board!!!

[lexwalker]

Re: Jamo A-101 Sub Amp

O.K. Pix attached. Have cleaned off all the glue and done some further testing. I have replaced 2 suspect caps (the large brown 680uF 80V caps at the bottom right in the component side picture. The cap nearest the edge of the board was running quite hot after only a minute or so and I figured new ones couldn't hurt. Problem is that the same fault still exists... Outside cap runs very hot even with no input audio. The original noise and popping seem to be reduced but I dare not run the amp under load with the cap running hot.

Ideas anyone?

Cheers
Greg.

Cleaned off? Looking at those pictures, I can still see them in many spots. I can also see dry solder joints (all over the board) and shabby soldering job. Check the images attached...

#1. Check those CrapXons capacitors. Can go bad without bulging. Recommend replacing them whenever possible.

#2. Degraded glue there, thus do remove them. Also check the surrounding capacitors in the box highlighted (as they are very close and possibly touching those two standing TO220 thingies).

#3. More degraded glue traces. Recommend cleaning them up...

#4. Inspect the soldering. Make sure the through holes are not damaged as well. Do not that this is a double sided PCB, thus the through holes have connections to both top and bottom sides.

#5. Check for cracked solder joints. Can use a magnifying glass and bright LED flashlight to find them. If not sure then reflow each suspected solder joint.

#6. Remove that dark goo stuff if possible. Becareful not to break any SMD components there.

#7. Do check the shabby soldering for possible shorts between the pins....

lots of cracked solder joints.

Indeed. Can see hints of them all over the board..

[aussiegreg]

Re: Jamo A-101 Sub Amp

Thanks for the advice... Amazing how this shows up in a pic!... I examined the board carefully under a Maggy Lamp both before and after sending the pics and these defects weren't visible... After looking at the pic I was stunned at how easily the fractures could be seen!

Long story short, Reflowed every through hole joint on the board and all is well... I guess fractured solder joints are to be expected when you place a board with big heavy components in an environment that's designed specifically to vibrate intensly at low frequencies!

Thanks for the assist... Another lesson learned!

Cheers
Greg.

[nissam72]

Re: Jamo A-101 Sub Amp

Hi Guys,

I have similar one. But it has another problem, Fuse blown out, I have replaced MOSFETS, (Power supply side) and disconnected from main amplifier power line, even though fuse is blowing,

Kindly help me

[Agent24]

Re: Jamo A-101 Sub Amp

Is this the main 240v fuse that's blowing? I assume it has one, I can't see one in the photos already uploaded. Can you upload photos of your own board?

What did you disconnect? I am not sure what you mean.

Does your board have the glue all under the MOSFETs like the ones in the previous photos? Did you remove that glue? Possibly you have a bad PWM controller too.

You need to give more details, there hasn't been much information already provided about the circuit by anyone else. What model number are the MOSFETs, the PWM IC etc?

[nissam72]

Re: Jamo A-101 Sub Amp

Is this the main 240v fuse that's blowing? I assume it has one, I can't see one in the photos already uploaded. Can you upload photos of your own board?

What did you disconnect? I am not sure what you mean.

Does your board have the glue all under the MOSFETs like the ones in the previous photos? Did you remove that glue? Possibly you have a bad PWM controller too.

You need to give more details, there hasn't been much information already provided about the circuit by anyone else. What model number are the MOSFETs, the PWM IC etc?

Thank you for immediate reply,

Mosfets are IRF740, MOSFETS are partially glue covered, I did not remove glue, there is two glued areas, both are having one small chip each, I try to disconnect power from SMPS to Amplifier, because I doubt about that amplifier side any short circuit?

Thank you

[nissam72]

Re: Jamo A-101 Sub Amp

Is this the main 240v fuse that's blowing? I assume it has one, I can't see one in the photos already uploaded. Can you upload photos of your own board?

What did you disconnect? I am not sure what you mean.

Does your board have the glue all under the MOSFETs like the ones in the previous photos? Did you remove that glue? Possibly you have a bad PWM controller too.

You need to give more details, there hasn't been much information already provided about the circuit by anyone else. What model number are the MOSFETs, the PWM IC etc?

Thank you for immediate reply,

Yes main fuse is blowing, MOSFETS are IRF740, MOSFETS are partially glue covered, I did not remove glue, there is two glued areas, both are having one small chip each, I try to disconnect power from SMPS to Amplifier, because I doubt about that amplifier side any short circuit? I dont understand what is PWM IC?

Best Regards,

Nissam

[Agent24]

Re: Jamo A-101 Sub Amp

What I called the PWM IC is the controller that drives the MOSFETs. It's probably under one of those glued areas.

Just to clarify, did the old MOSFETs actually blow? Because in your photos it looks like the MOSFETs are the original ones, but they do not look blown or anything.

Have you checked things like the bridge rectifier, mains filter capacitor etc for shorts also?



The switching transistors in an SMPS don't switch by themselves - they are driven with a controller IC that usually uses PWM to account for variations in the load current. This is likely the IC under the glue. If this IC is faulty, the switching transistors may not be switching on and off - rather just full ON. In this case they would look like a short across the mains input.

It's one possibility, probably unlikely, but don't rule it out yet...

[nissam72]

Re: Jamo A-101 Sub Amp

What I called the PWM IC is the controller that drives the MOSFETs. It's probably under one of those glued areas.

Just to clarify, did the old MOSFETs actually blow? Because in your photos it looks like the MOSFETs are the original ones, but they do not look blown or anything.

Have you checked things like the bridge rectifier, mains filter capacitor etc for shorts also?



The switching transistors in an SMPS don't switch by themselves - they are driven with a controller IC that usually uses PWM to account for variations in the load current. This is likely the IC under the glue. If this IC is faulty, the switching transistors may not be switching on and off - rather just full ON. In this case they would look like a short across the mains input.

It's one possibility, probably unlikely, but don't rule it out yet...

Good Morning,

Yes, old MOSFETS are working good. Bridge rectifier and capacitors also good. Fuse is not blowing when any MOSFETS is out of the PCB. Any possibility to replace PWM IC?

Thanks,

Nissam

[nissam72]

Re: Jamo A-101 Sub Amp

What I called the PWM IC is the controller that drives the MOSFETs. It's probably under one of those glued areas.

Just to clarify, did the old MOSFETs actually blow? Because in your photos it looks like the MOSFETs are the original ones, but they do not look blown or anything.

Have you checked things like the bridge rectifier, mains filter capacitor etc for shorts also?



The switching transistors in an SMPS don't switch by themselves - they are driven with a controller IC that usually uses PWM to account for variations in the load current. This is likely the IC under the glue. If this IC is faulty, the switching transistors may not be switching on and off - rather just full ON. In this case they would look like a short across the mains input.

It's one possibility, probably unlikely, but don't rule it out yet...

Good Morning,

Yes, old MOSFETS are working good. Bridge rectifier and capacitors also good. Fuse is not blowing when any MOSFETS is out of the PCB. Any possibility to replace PWM IC?

Thanks,

Nissam

[Agent24]

Re: Jamo A-101 Sub Amp

If the old MOSFETs are not bad and the new ones also cause the fuse to blow, then I guess they are not the problem.

At first it seemed odd to me that removing either MOSFET would stop the fuse blowing, but I looked again at the photos and it actually looks like the MOSFETs are in series, so I guess that makes sense. Though why they did that, I am not sure...


Anyway, before you look at the controller IC, I would try some other things.

1) Check that the insulating washers\pads on the MOSFETs are intact and not degraded. If they are shorting to the heatsink, you're probably going to get problems.

2) If the PSU is cheap (or badly designed) enough, shorted secondary rectifier diodes or other components could plausibly upset the controller enough to blow the fuse. Check all the outputs for shorts.

3) Check feedback components (like opto-isolator) for failures. A fault here could also do weird things to the controller.

4) Check main transformer for shorted turns. You will need a ring tester circuit.


If nothing is bad there, you will have to carefully remove that black glue to get at the controller IC as it is likely underneath. If it's really well stuck on there, maybe try dissolving it with solvent, or heating it with hot-air\hairdryer to maybe melt it and make it easier to pull off.

[diydidi]

Re: Jamo A-101 Sub Amp

Hi
I'm also repairing one. What is the value of the large resistor next to C30?

[budm]

Re: Jamo A-101 Sub Amp

"At first it seemed odd to me that removing either MOSFET would stop the fuse blowing, but I looked again at the photos and it actually looks like the MOSFETs are in series, so I guess that makes sense. Though why they did that, I am not sure..."
The two power MOSFETs are setup as a half-bridge to drive the power transformer AS1040.
The GATES are driven by transformer TXL2 (AS1042) so it is not direct drive by the SMPS IC so if the IC is bad, it will not cause the GATE to stuck high due to transformer isolation.
You need to remove those glues on the bottom, they may be conductive and caused the MOSFETs to be on.

[Agent24]

Re: Jamo A-101 Sub Amp

The two power MOSFETs are setup as a half-bridge to drive the power transformer AS1040.

Ahh, it seems I just learned something new. Been looking at too many flyback designs I guess...

[japlytic]

Re: Jamo A-101 Sub Amp

I have handled a Jamo SUB250 unit and it has a variation of an "old school" computer power supply circuit (self oscillating) - the A-101 should be of the same variant mentioned thereof.