Help! Yamaha MX-1 slow to kick in.

theokalat

Active Member
My mx-1 when powered on each day for the first time usually only took a few seconds for a click from the relay to start playing music.

Lately when i power the mx1 on each day it takes about 1 minute or longer for the relay click and music to be heard.

But once it comes alive it is faultless and plays all day without any issues even when switching off and on again during the day.

The electrolytics in both amp boards have been replaced a few weeks ago, and before reassmbling i checked each board carefully for any cold solder joints.
I reflowed some suspect joints during that process.

I am lost as to what the issue is as this amp has been rock solid till this issue reared its ugly head.

Anyone with any tips on what the problem is and how to fix it would be highly appreciated.
 
My mx-1 when powered on each day for the first time usually only took a few seconds for a click from the relay to start playing music.

Lately when i power the mx1 on each day it takes about 1 minute or longer for the relay click and music to be heard.

But once it comes alive it is faultless and plays all day without any issues even when switching off and on again during the day.

The electrolytics in both amp boards have been replaced a few weeks ago, and before reassmbling i checked each board carefully for any cold solder joints.
I reflowed some suspect joints during that process.

I am lost as to what the issue is as this amp has been rock solid till this issue reared its ugly head.

Anyone with any tips on what the problem is and how to fix it would be highly appreciated.

Most likely a dried up electrolytic cap in the protect circuit.
Try swapping out
C255---220µf / 6.3 volt
C256---10µf / 16 volt
 
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Hi avionic

Thanks for the quick reply.

Have not checked the electrolytics in the protect circuit yet.

Will do this and report back. Most likely i will just replace all the electrolytics in amp that haven't been done yet for peace of mind.

Cheers
 
Is it always or only when the amplifier is cold?
Another cause could be a high DC-offset when the amp its cold which keeps the protection circuit active until the value reaches a normal value after a while when everything has warmed up a bit. Did you check DC-offset?
 
You would need to monitor the offset before the relay .Say a driver board test point. CT and E.. Or TP1 and ground. Reference the service manual for TP1 location. Use the idle current adjustment TP1.
 
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OK guys I will be a little more specific with the problem I have been having with the amp.

It all began a few weeks ago when I brought the mx-1 out of storage for a few months.

When I switched the mx-1 on, it would go into protection mode on and off continuously for a few minutes until it finally stayed in protection mode and no music played.

I proceeded to open her up and see if there is any visible sign of the problem, I found that on the left channel amp board one carbon resistor looked burnt and cracked, and right beside it one electrolytic cap was bulged at the top. Underneath these two components, the circuit looked a mess, burnt, melted and black.

I removed these two components, cleaned up the board and apart from replacing the two burnt out components, I also decided to replace all the electrolytics in both channel amp boards while I was at it.

When all above was done and put back together, I switched her on, and this is when I have been having this problem with the protection been on for a minute or two before she finally comes out of it and music plays.

My multimeter is at my brothers home, soI do not have access to one immediately to check the dc offset.

But in the meantime I will be replacing every other small electrolytic in the amp that has not been replaced yet (only 15 total I think) apart from the 4 big cans).

So we will see how she goes, should be able to get my multimeter back over the next few days and will also check the dc offset.
 
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I managed to get hold of my multimeter this afternoon.

I tested the left and right main boards after switching on and leaving at idle with no load for 30 minutes.

Results as follows:

Left Channel

R218
TP1 - TP2 14.4mv
TP1 - TP3 14.2mv

R234
TP1 - TP2 15.1mv
TP1 - TP3 12.6mv

R233
TP1 - TP2 12.4mv
TP1 - TP3 18.6mv

Right Channel

R224
TP1 - TP2 14.0mv
TP1 - TP3 14.6mv

R248
TP1 - TP2 14.4mv
TP1 - TP3 15.6mv

R247
TP1 - TP2 15.3mv
TP1 - TP3 14.7mv


According to the service manual, tolerance is 14mv +/-1mv and only states to just check only one in each main board and adjust to specs accordingly.

I see that R233 is way out of range and some of the others are out too.

Is this a problem that needs further investigation?
 
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Measure from TP 1 to chassis ground on one of the emitter resistors on each channel( R218 and R224)..The above measurements are for bias(idle current).. We need to know what the DC offset is from a cold start up.
 
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I managed to get hold of my multimeter this afternoon.

I tested the left and right main boards after switching on and leaving at idle with no load for 30 minutes.

Results as follows:

Left Channel

R218
TP1 - TP2 14.4mv
TP1 - TP3 14.2mv

R234
TP1 - TP2 15.1mv
TP1 - TP3 12.6mv

R233
TP1 - TP2 12.4mv
TP1 - TP3 18.6mv

Right Channel

R224
TP1 - TP2 14.0mv
TP1 - TP3 14.6mv

R248
TP1 - TP2 14.4mv
TP1 - TP3 15.6mv

R247
TP1 - TP2 15.3mv
TP1 - TP3 14.7mv


According to the service manual, tolerance is 14mv +/-1mv and only states to just check only one in each main board and adjust to specs accordingly.

I see that R233 is way out of range and some of the others are out too.

Is this a problem that needs further investigation?

Your bias looks fine.
There looking for 26-30mv between TP 2 and TP 3 all of your above measurements are in the ballpark.
 
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Tested the DC Offset with results as follows:


+ve lead of multimeter to TP1 of R218 & R224 and -ve lead to chassis ground.
Tested immediately after switching on, at 2 minutes in, 5 minutes in, 10 minutes in and 30 minutes in with no load and no speakers connected.

Immediately after switching on
R218 5.1mv
R224 12.5mv

After 2 minutes
R218 1.1mv
R224 11.5mv

After 5 minutes
R218 1.1mv
R224 11.2mv

After 10 minutes
R218 1.1mv
R224 11.8mv

After 30 minutes
R218 0.6mv
R224 11.9mv

I need to make a correction to a previous post where I stated the left channel that had a bulging electrolytic cap and burnt and cracked resistor. It was actually the right channel amp board. The components in question are C250 and R294. These two components are right next to each other and the circuit track underneath was melted and black as if it was burnt.

The higher than normal dc offset on the right channel may or may not be linked to the above component failure.

I also noticed that on the left channel, D221 and D226 have no insulating pads between them and the heatsink, whereas D227 and D232 on right channel have insulating pads between them and heatsink.

The previous owner did state to me he did some restoration on amp so I am worried that he stuffed up something here.

These four diodes according to s/m are called a diode array FMG-22SX2 10A.

If anyone knows whether they should have insulating pad between them and heatsink or not, please advise so I can rectify this asap.

Cheers
 
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Its not a DC offset issue..Its a protect circuit or power supply issue.
 
If anyone knows whether they should have insulating pad between them and heatsink or not, please advise so I can rectify this asap.
I'd have to see a photo of the heatsink side of the diode arrays.If there is no metallic surface on the heatsink side of the diodes - no insulator is required.. If there is a metallic surface.You can check for continuity between the backing and the pins. Most likely the backing is internally isolated from the pins.
 
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Thanks Avionic for your help so far

The diodes have a plastic backing and no metallic surface, so I assume no need for insulator pads.
 
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How can I go about testing power supply and circuit protect boards as the next step in diagnosis. Is there anything specific i should be testing or looking out for visually?

I am just waiting to receive the rest of the electrolytics and replace so in the meantime i think i will also test all diodes and transistors throughout and see if there are any bad ones and report back.
Probably do a quick check on all resistors and poly caps with multimeter to make sure all are within spec.

Apart from that, i don't have any other ideas for further diagnosis
 
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....
I proceeded to open her up and see if there is any visible sign of the problem, I found that on the left channel amp board one carbon resistor looked burnt and cracked, and right beside it one electrolytic cap was bulged at the top. Underneath these two components, the circuit looked a mess, burnt, melted and black.

I removed these two components, cleaned up the board and apart from replacing the two burnt out components, I also decided to replace all the electrolytics in both channel amp boards while I was at it.
....

Hi,
I just got MX-1 too; It's not bad as your case, the music came out but when I open the top cover (for cleaning inside) the first thing I saw is small dead gecko body :( then I found that right channel board R298, R297 carbon resistor looked burnt and cracked, and right beside it one electrolytic cap C244 was leaked and fractured...and more.. It's realy shock me!

Here is what I done:
- replaced all 4 electrolytic cap C244, C250, C243, C237 - 100uF/50v (M) by new 470uF/50v (M) at both channel board
- replace burned resistor near by C244 cap - R298, R297 by other same value (100K)
- replace bulged resistor R294 by 3K (I can't find out 3K3 value in my part)

Turn it on again and I feel LF sound has a bit improvement than before but Left channel gain seem a litter higher than the right channel; especially the left heatsink much hotter than the right one.

My understood is MX-1 normally run in class A at small input signal --> so it must be hot; means the left channel are working correct and there are something else at the right channel?

Any comment that what am I wrong or missing somethings?

BR
 
Just a wild stab, but any time you're finding burnt resistors and dead caps, you better also be checking all the transistors in those circuits. Just because they aren't dead short doesn't mean they're still providing necessary gain.
 
Just a wild stab, but any time you're finding burnt resistors and dead caps, you better also be checking all the transistors in those circuits. Just because they aren't dead short doesn't mean they're still providing necessary gain.

Thanks for your good point, I didn't know that.
I still have trouble with its APS on left channel: +33v at 2SC3856 collector same as manual but B(-)=-75V at 2SA1492 collector :pity:
 
How can I go about testing power supply and circuit protect boards as the next step in diagnosis. Is there anything specific i should be testing or looking out for visually?

I am just waiting to receive the rest of the electrolytics and replace so in the meantime i think i will also test all diodes and transistors throughout and see if there are any bad ones and report back.
Probably do a quick check on all resistors and poly caps with multimeter to make sure all are within spec.

Apart from that, i don't have any other ideas for further diagnosis

Did you get a chance to look at C256 (10µf/16vdc) and C255 (220µf/6.3vdc) in the protect circuit?
 
Did you get a chance to look at C256 (10µf/16vdc) and C255 (220µf/6.3vdc) in the protect circuit?

Hi Avionic,

I was away on holidays in Greece forthe last 3 months, just came back and am back to diagnosing the problem.

Swapped c256 and c255 with new caps, but no chance to test the amp with new caps due to my carelesness in blowing her up.

Before I went on holidays I was using my multimeter to test TP points on right channel amp board, and accidently touched some other metal part on the amp and there was a large spark and lots of smoke from the right channel board.

All the fuses blew, and after using my DMM to check each component in the right channel,I found 4 transistors are shot, along with some burnt resistors(replaced).

All other components on right channel seem to be ok going by the readings from the DMM.

So all that remains is just the 4 transistors that I am now trying to find suitable substitutes for.

I have started a new thread in the Yamaha section for the replacement transistors I am after.
 
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