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Bad output device?

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simonp1100 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 07 April 2018 at 7:39am
Originally posted by Muckerbarnes1 Muckerbarnes1 wrote:

If you have 27V on LS o/p don't put speaker back in circuit. The crackling and hum you describe and assuming your check of o/p devices is correct, suggests to me the bridge rec is duff. As previous, take it out and test it. 

Check as before with your DVM, also check it in high resistance setting for reverse reading, often a diode can read ok on those gadgets, but show it's fault with a resistance in reverse, where there should be none at all. 



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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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 07 April 2018 at 2:41pm
Originally posted by Muckerbarnes1 Muckerbarnes1 wrote:

It's 2x NPN bipolar as in the OP. No FETs. 

The fwd test on the BR of 0.7 and nowt in reverse;
Was this the same on all 4 diodes in the BR? 


Yep, all four.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 07 April 2018 at 5:54pm
Originally posted by AJordan AJordan wrote:

Originally posted by Muckerbarnes1 Muckerbarnes1 wrote:

It's 2x NPN bipolar as in the OP. No FETs. 

The fwd test on the BR of 0.7 and nowt in reverse;
Was this the same on all 4 diodes in the BR? 


Yep, all four.

As I said the problem is not the rectifier.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 09 April 2018 at 4:59pm
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.

1. Have you check transistor isolation between the metal heatsink and the collector (can / case)  of each transistor after you fitted them back in, these should be isolated.

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.


2. The voltage that you are getting on the speaker output, is it a positive reading or a negative reading (with negative of your meter going to the ground) & also what is the voltage across each main power supply cap ???.

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.

3. Check each output transistor emitter resistor (R22 ohms). Check ALL diodes on BOTH boards

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?

4. They is also a +15 & -15 volt supply that is going from the main board to a small daughter board via two 1K wirewound resistors to supply a op-amp, is this voltage there ??? (measure across each zener diode (15V approx) or if not each 1K wirewound resistor on one end should have 15V approx.

15v was present on the preamp board before I disconnected it.

5. What is the chip number ???.

Can’t see that yet.

Sorry for the long rambling answers.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 09 April 2018 at 5:22pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 09 April 2018 at 5:58pm
Originally posted by AJordan AJordan wrote:

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.

1. Have you check transistor isolation between the metal heatsink and the collector (can / case)  of each transistor after you fitted them back in, these should be isolated.

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.


2. The voltage that you are getting on the speaker output, is it a positive reading or a negative reading (with negative of your meter going to the ground) & also what is the voltage across each main power supply cap ???.

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.



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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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 09 April 2018 at 7:51pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2018 at 6:32pm
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 thoughSmile). 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2018 at 6:55pm

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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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2018 at 7:54pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2018 at 8:27pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2018 at 8:32pm
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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