Hi frickecello,
Basing an experience with a poster about his Quested
active speaker he finally solved the problem of fuse blowing when he
removed and properly install the output transistors again making sure
there were no shorts between heatsink and the other set of outputs.
Please check yours and the integrety of the 1/4 inch mono jacks to
see if they are not short. singa
Basing an experience with a poster about his Quested
active speaker he finally solved the problem of fuse blowing when he
removed and properly install the output transistors again making sure
there were no shorts between heatsink and the other set of outputs.
Please check yours and the integrety of the 1/4 inch mono jacks to
see if they are not short. singa
Hi Singa,
Thanks, I will check for shorts in jacks and between OTs and heatsink and I'll tell you what happens.
Output transistors out!
Hi guys,
I did another test but failed... I disconnected the preamp (whole unit preamp) board from the power amp board just to make sure that was not the problem... But I forgot to re-solder D1 from the measurements I did and guess what? the fuse blew since the protection circuit was not working (without D1) and again just Q24 fried. I wonder why Q24 fries (with the protection circuit defeated) and not any other OTs or component. Obviously this was not intended to happen.. 😛
OK, following your advice I desoldered all output transistors, and just wanted to make sure if it safe to power the amp on this way and why should I defeat the protection circuit to test the power amp preamp (diff. amp, drivers, etc.)? Why can't I just test the power amp preamp with the protection circuit on?
Thanks for all your help!
Hi guys,
I did another test but failed... I disconnected the preamp (whole unit preamp) board from the power amp board just to make sure that was not the problem... But I forgot to re-solder D1 from the measurements I did and guess what? the fuse blew since the protection circuit was not working (without D1) and again just Q24 fried. I wonder why Q24 fries (with the protection circuit defeated) and not any other OTs or component. Obviously this was not intended to happen.. 😛
OK, following your advice I desoldered all output transistors, and just wanted to make sure if it safe to power the amp on this way and why should I defeat the protection circuit to test the power amp preamp (diff. amp, drivers, etc.)? Why can't I just test the power amp preamp with the protection circuit on?
Thanks for all your help!
Hi,
If the amplifier goes on protect mode when is powered will disable the ground for Q3/Q4 this will disable the differential input amplifier and the amplifier will not be operational. You can try it but if it goes in protect mode then you will not be able to read the voltages and compare them with the one shown in the schematic.
If the amplifier goes on protect mode when is powered will disable the ground for Q3/Q4 this will disable the differential input amplifier and the amplifier will not be operational. You can try it but if it goes in protect mode then you will not be able to read the voltages and compare them with the one shown in the schematic.
New findings...
Fortunately it didn't go into protect mode with the output transistors out.
I checked test voltages and here they are (All rails are OK):
Across R17 2.54V (R17 is OK)
Across R15 0V (R15 is OK)
Across R27 1.88V
Across R31 1.16V
At Q18's base 63.5V
Across R4 0.55V (OK)
Across R13 -0.78V
Across R7 0V (R7 is OK)
Across R20 0V (R20 is OK)
At Q14's base 61.6V
I'm not sure what the offending components might be, I already checked for CE shorts but didn't find any. Any ideas?
Thanks in advance!
Fortunately it didn't go into protect mode with the output transistors out.
I checked test voltages and here they are (All rails are OK):
Across R17 2.54V (R17 is OK)
Across R15 0V (R15 is OK)
Across R27 1.88V
Across R31 1.16V
At Q18's base 63.5V
Across R4 0.55V (OK)
Across R13 -0.78V
Across R7 0V (R7 is OK)
Across R20 0V (R20 is OK)
At Q14's base 61.6V
I'm not sure what the offending components might be, I already checked for CE shorts but didn't find any. Any ideas?
Thanks in advance!
Hi frikecello,
It should be obvious to you that the voltage readings do not
tally with the schematic.R13,15,7 should not be 0 volts it could mean
R33.34 is open? (10 ohm fuse).
The most obvious is that the base of driver transistor Q14,18
should be around 1 volt respectively and you measured full rail voltage of
63V?Is it with respect to circuit common or ground? If I remember reading my basic electronic book that says if a transistor measures full rail voltage that means one of the junctions is open. Test with reverse probes als😵ne should conduct and the other "high resistance".
Please check the input and driver section carefully.But the puzzleling
thing is that you report fuse blowing so what is short or drawing that much
current.I am confused myself.😕 singa.
It should be obvious to you that the voltage readings do not
tally with the schematic.R13,15,7 should not be 0 volts it could mean
R33.34 is open? (10 ohm fuse).
The most obvious is that the base of driver transistor Q14,18
should be around 1 volt respectively and you measured full rail voltage of
63V?Is it with respect to circuit common or ground? If I remember reading my basic electronic book that says if a transistor measures full rail voltage that means one of the junctions is open. Test with reverse probes als😵ne should conduct and the other "high resistance".
Please check the input and driver section carefully.But the puzzleling
thing is that you report fuse blowing so what is short or drawing that much
current.I am confused myself.😕 singa.
Hi,
I agreed with singa the bases of Q14 & Q18 should be around 1.2 volts. Normally the way I troubleshoot a solid state amplifier is by checking the base voltage and compare it with the collector. Both should read close. R17 should be 60 volt if you read the base of Q9. Read the base voltage of Q9. Check Q17 because is the one that feed Q18. Normally the fault circuit is looking at the output transistor for high voltage. If any one is leaking it will trigger the fault circuit. If you have 63 volts at the base of Q18 it will send the output transistors to 60 volts and this will trigger the fault circuit. You most find out why 60 volts is at the base of Q18.
I agreed with singa the bases of Q14 & Q18 should be around 1.2 volts. Normally the way I troubleshoot a solid state amplifier is by checking the base voltage and compare it with the collector. Both should read close. R17 should be 60 volt if you read the base of Q9. Read the base voltage of Q9. Check Q17 because is the one that feed Q18. Normally the fault circuit is looking at the output transistor for high voltage. If any one is leaking it will trigger the fault circuit. If you have 63 volts at the base of Q18 it will send the output transistors to 60 volts and this will trigger the fault circuit. You most find out why 60 volts is at the base of Q18.
Hi,
I was looking at your reading closely and I think the base of the Q14 should be negative. Do you have negative voltage at R33. It looks like Q15 should be enable so the base of Q18 & Q14 can be balance each other. R20 should read negative voltage. Check to see if you have voltage at the negative rail.
I was looking at your reading closely and I think the base of the Q14 should be negative. Do you have negative voltage at R33. It looks like Q15 should be enable so the base of Q18 & Q14 can be balance each other. R20 should read negative voltage. Check to see if you have voltage at the negative rail.
I will Singa. I don't know what causes Q14 and Q18 to have close to full positive rail voltage at their bases.
Regarding fuse blowing, it only happened with the output transistors in the circuit when I defeated the protection circuit, with the protection circuit enabled nothing happened, right now it is not blowing fuses.
Hi tauro0221,
Yes, the voltage at Q14's base is +60V I rechecked, and +62V at Q18's base. I think that explains why just Q24 was fried when I defeated the protection circuit. Those voltages would enable Q18 but not Q14.
Measuring voltage from one extreme of R33 to ground shows the negative rail voltage at it should. The voltage across R20 is 0V.
Hi frikecello,
I get the picture or I think I do😀 If one end of R33 reads full negative 62V and the other 0V then R33 (10 ohm) could be open or R20 (47.5 ohm) or R7 ( 1K ohm),all 3 resistors are "fuses".
Q6,15 may be open/short.D4 may be short.Anyway something
is not working in that input/driver section.Keep looking,good luck.
singa🙂
MB150
Hi!
I wonder if you still have this unit or if you remember the Power Circuit?.
I want to convert a brand new MB150s from 120V to 220V and according to schematics this may be possibe.
can you help? do you have pictures to share showing if junpers are available in circuit board?
Thanks !
Hi!
I wonder if you still have this unit or if you remember the Power Circuit?.
I want to convert a brand new MB150s from 120V to 220V and according to schematics this may be possibe.
can you help? do you have pictures to share showing if junpers are available in circuit board?
Thanks !
Sorry - jerk reviving thread about GK MB150E-III
I am a a bass player just wondering if there is a fuse I can replace.
I was using the amp - fine. The sound reinforcement guy was patching the weirdest configuration of bedroom equipment and low end audio and my connection to his was through the XLR connector in back of my amp.
Not sure whether is was some weird signal that came from his PA that had the weirdest Y splitter patch cables etc. Or if the AC was of poor quality. But second set the amp didn't work. no sound. Lights on but no action.
I seem t recall a sharp cracking sound somewhere on stage on the break and that could have been when the amp failed. Could have been someone stepping on my bass guitar cable but whatever.
When I got it home - the amp works with the internal speaker only if an external speaker is plugged in. I can mute the internal speaker and use the external. Or both. But the internal speaker alone will not work. obviously the beauty of this amp is the size and portability.
Of course I will get it repaired but if there is a speaker protection circuit that I can click a breaker or replace a fuse - I would like to do that.
If the moderator thinks I am a jerk - I respect - just let me know and I will take the steps. THis just seemed to e a good thread with people knowledgeable about this amp.
Cheers
I am in toronto
jim N
I am a a bass player just wondering if there is a fuse I can replace.
I was using the amp - fine. The sound reinforcement guy was patching the weirdest configuration of bedroom equipment and low end audio and my connection to his was through the XLR connector in back of my amp.
Not sure whether is was some weird signal that came from his PA that had the weirdest Y splitter patch cables etc. Or if the AC was of poor quality. But second set the amp didn't work. no sound. Lights on but no action.
I seem t recall a sharp cracking sound somewhere on stage on the break and that could have been when the amp failed. Could have been someone stepping on my bass guitar cable but whatever.
When I got it home - the amp works with the internal speaker only if an external speaker is plugged in. I can mute the internal speaker and use the external. Or both. But the internal speaker alone will not work. obviously the beauty of this amp is the size and portability.
Of course I will get it repaired but if there is a speaker protection circuit that I can click a breaker or replace a fuse - I would like to do that.
If the moderator thinks I am a jerk - I respect - just let me know and I will take the steps. THis just seemed to e a good thread with people knowledgeable about this amp.
Cheers
I am in toronto
jim N
While this is an old thread it is musical instrument (bass) amp related and is most appropriately moved to the instruments & amps forum.
1) no, no breaker/fuse/speaker protector (relay?)
2) weird, the external speaker jacks are *always* connected, not affected either by the internal spaker switch or anything else.
No visible reason they would affect the main speaker in any setting.
3) a couple doubts:
* are you *sure* the internal speaker works?
* if so I suspect wiring from speaker rail out (right end of L1) to actual speaker terminal and anything in between, including the switch and any connectors and wires along the way, and any and all connection between speaker negative leg and ground.
You can check it by using a short piece of wire between L1 and speaker + , and another from speaker - to power supply ground.
Somwhere along the path you have an interruption or poor contact.
**maybe** plugging an external speaker flexes or twists the board and contact is resumed, there is no *electrical* reason for that but there might be a *mechanical* one.
I don't know what L1 is
I really appreciate your application of logic to my problem. The internal DI was plugged into a very sketchy system and I wonder whether something bad came back from his crappy PA and allowed signal to back up the system. Because I also think (in the aftermath) that I am not sure the output is the same either.
I think I am bringing it into the shop tomorrow. Should I be looking for a schematic somewhere?
Thanks a bunch JMFahey!
I really appreciate your application of logic to my problem. The internal DI was plugged into a very sketchy system and I wonder whether something bad came back from his crappy PA and allowed signal to back up the system. Because I also think (in the aftermath) that I am not sure the output is the same either.
I think I am bringing it into the shop tomorrow. Should I be looking for a schematic somewhere?
Thanks a bunch JMFahey!
Thanks 🙂
The schematic is in the very first post of this thread.
Open it in a separate window and compare it side by side with the real flesh and bone amp on your PCB.
Where the schematic shows a straight wire connecting PCB to speaker you may actually have a couple connectors, crimped wires, etc. in the path, each a *possible* poor contact.
Or cracked tracks (sometimes hair thin cracks) or poor/cracked solder.
The schematic is in the very first post of this thread.
Open it in a separate window and compare it side by side with the real flesh and bone amp on your PCB.
Where the schematic shows a straight wire connecting PCB to speaker you may actually have a couple connectors, crimped wires, etc. in the path, each a *possible* poor contact.
Or cracked tracks (sometimes hair thin cracks) or poor/cracked solder.
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