Bad output device? |
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simonp1100
Young Croc Joined: 01 September 2008 Location: Bristol Status: Offline Points: 1103 |
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It cannot be the bride rectifier faulty due to the fact the fuses would still be blowing when he removed the output devices (assuming the bridge rectifier had a short in one of the diodes) as the fuses are in the primary side BEFORE the rectifier. The problem is more likely to be a driver transistor or a resistor gone high. If the 27 volts he is getting on the output is a negative one, then this would give an ideal of a fault in the negative driver part of the circuit and if the voltage is a positive one, then this would give an ideal of a fault in the positive driver part of the circuit. |
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AJordan
Young Croc Joined: 06 July 2007 Status: Offline Points: 1013 |
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Yep, all four. |
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simonp1100
Young Croc Joined: 01 September 2008 Location: Bristol Status: Offline Points: 1103 |
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As I said the problem is not the rectifier.
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AJordan
Young Croc Joined: 06 July 2007 Status: Offline Points: 1013 |
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Sorry for the delay getting back, had a busy weekend.
Try the below things: Firstly, I don’t know if I measured it wrong the first time.
But since I put the b/r back in I am now getting 42V across the speaker
outputs. OK, so I am getting continuity between the collector and heatsink of both devices but when I remove the screw that connects the collector to the circuit the continuity is broken, but I still get continuity between the circuit and heatsink so I suspected the short is somewhere else. However, with the screws out I powered up and checked the voltages. One device was reading -42v on base and emitter and 0v on collector (disconnected from circuit). Other device was reading +41v on base and +40 on emitter AND on collector (disconnected from circuit). Could this device be shorted after all? These tests were all with negative probe on ground.
Relative to ground I am getting +40V on the terminal that
was connected to the red speaker lead and 0V on the other. +42V across each
cap. This bit is confusing me. I assumed you mean the two big green R22J? one of those is connected to the emitter of one device while the other is connected to the collector of the second device. The emitter of the second device is connected to a very small resistor whose value I can’t read. I googled the R22J and it seems to be 0.22 ohm. I’m not sure I trust my meter with values that small since on its lowest setting it reads 0.7 with the probes together. Anyway I tried it and each one reads 0.9 so around 0.2 ohms I guess. I have disconnected the preamp board for the moment but have
checked all diodes on the main board and they all test in circuit as OK except
for the one which is across the collector and emitter of the device that I
suspect is faulty. I guess though that the short I am reading could be in
either the diode OR the device so I should take out the diode and test it out
of circuit? 15v was present on the preamp board before I disconnected
it. Can’t see that yet. Sorry for the long rambling answers. |
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Earplug
Old Croc Joined: 03 January 2012 Location: Europe Status: Offline Points: 7199 |
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I think that you need to go one stage back in the circuit, ie the 2 driver transistors with the small finned heatsinks - and the other 2 there which look similar. Desolder the legs from the PCB and check continuity in Diode mode.
The left-most leg will be the Base and should give around 600 ohms between it and the other 2 legs. The polarity will depend if the device is PNP or NPN, but one way will be open, the other give you a reading. Then check between the other 2 (right hand side) legs. If the reading on one is less than around 3/400 ohms, or a short, that is the problem. |
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Earplugs Are For Wimps!
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simonp1100
Young Croc Joined: 01 September 2008 Location: Bristol Status: Offline Points: 1103 |
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If you high voltage on the base of one of the output devices then this is far to high, so i would take out BOTH driver transistors (ones on small heatsinks) and test them, if faulty replace. The fact that you are getting +40 volts on the output indicates the + amplifier stage has a fault. Edited by simonp1100 - 09 April 2018 at 6:01pm |
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AJordan
Young Croc Joined: 06 July 2007 Status: Offline Points: 1013 |
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OK guys, I'll take em out tomorrow and test but before I do. Does having 42V on all three legs of all of the four small transistors on the board point to something else? Further back in the circuit maybe?
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AJordan
Young Croc Joined: 06 July 2007 Status: Offline Points: 1013 |
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OK, still plugging away on this one evry time I have half an hour or so to spare. Very much treating it as a learning experience and I do feel like I am learning stuff(still a long way to go though). I had a look at various output stages on the net and compared them to what I could see on the board and I believe this is known as a Quasi-complementary output stage. Feel free to correct me if I am wrong.
I took out the output transistors again and also the drivers and the transistor from the bias servo circuit and another that I am not sure what it does but seems to be linked to the bias servo. All test good. Anyway, first thing monday I did what I should have done from the start and emailed Laney to see if they had a schematic. Five mins later one dropped into my inbox so big up the Laney service dept. for that one. Link: http://www.dropbox.com/s/9ti3mwiuv24plzb/Laney%20TM200%20schematic.pdf?dl=0 I am now checking voltages with the above mentioned transistors out of the board(so no chance that a short through any of those could be affecting the results) to see if that will shed some light. All seems correct apart from around the bias servo circuit. I am reading +43V on the base, collector and emitter pads of TS3 and -42V on the base pad for TS4 +43V on the collector pad and -43 on the emitter pad although that would appear to be correct acording to the schematic. BTW, the schematic shows +/- 40 on the rails but my meter says +/- 43V. +43 on the base of TS3 is the big puzzle for me, I can't see where it could be coming from. R25 and R26 read ok but I am not sure about P1. It should be 4K7 but I only get it as 1K4 although I am not sure I am testing it correctly? Is there a way that I can test C18 and 19 in cicuit for shorts? Would that cause the problem if they were shorted? Any suggestions gratefully recieved! |
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APW
Young Croc Joined: 13 November 2012 Location: Kent, UK Status: Offline Points: 1173 |
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TS4 isn’t conducting i.e. switched off, the base of TS4 should be 0.6 to 0.7 volts more positive that the emitter, check you have 15volts across D12, if not check R19 hasn’t gone high in value or open circuit, if its OK check that D12 isn't short circuit. |
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AJordan
Young Croc Joined: 06 July 2007 Status: Offline Points: 1013 |
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TS4 isn't connected, I took it out. All the measurements are being taken from the board without the transistors in.
I will check those things. Edit: forgot to say, I have +0.6V on the emitter of TS 1&2 so I guess R19 is ok? But I will check anyway. Edited by AJordan - 18 April 2018 at 8:05pm |
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APW
Young Croc Joined: 13 November 2012 Location: Kent, UK Status: Offline Points: 1173 |
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Yes,
it looks like R19 and D12 probably
are ok, best to chech them anyway.... however without TS4 in the circuit the output
voltage will swing hard to the positive rail.
Edited by APW - 18 April 2018 at 8:32pm |
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AJordan
Young Croc Joined: 06 July 2007 Status: Offline Points: 1013 |
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I see, although it was hard to the + rail before I took it out. What did you think about my readings for TS3 or do you think I'm barking up the wrong tree with that one?
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