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Thread: Urei 6500 power amp fan not working

  1. #1
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    Urei 6500 power amp fan not working

    Hey guys,

    I'm working on an Urei 6500 power amplifier. It's got two amplifier modules that slide into the unit which contains the power supplies. One of the module's fan isn't working, and then the unit overheats. The fan itself works. Looking at the schematic, the fan is wired into the secondary of the power transformer, and then a network containing two transistors, a few diodes, and a thermistor. Q9 and Q10 test out okay, as do the diodes. I'm getting 59k at the thermistor. I'm a bit confused about the schematic. The fan is wired in series off of the white leg of the transformer, and then connects with the transistor/thermistor network to control the speed. What would i be missing here to have the fan not operate?

    (The working module's fan engages at full speed when first powering up and then reduces it's speed.)
    Attached Files Attached Files

  2. #2
    g1
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    The good side thermistor measures same?
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

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    The good side measures 50k, so i'm assuming 59k is in the ballpark enough

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    1) Short C to E of the TIP47. If the fan does not come on then one or more diode is bad, there is no power across wht-wht or the fan is bad
    2) Connect a 4.7K across the thermistor. If the fan didn't come on then it could be the transistors or R60 is open

    So, since you checked the diodes, the fan and the transistors my money's on R60
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    So shorting the collector to the emitter turns the fan on. Connecting the 4.7k across the thermistor didn't turn it on, and R60 reads 5k right on the dot. Double checked CR21 and CR22. R58 is right at 3k.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by stereomonostereo View Post
    So shorting the collector to the emitter turns the fan on. Connecting the 4.7k across the thermistor didn't turn it on, and R60 reads 5k right on the dot. Double checked CR21 and CR22. R58 is right at 3k.
    I missed the zener - you should get pretty close to 10V across it. If not it's probably shorted.
    Experience is something you get, just after you really needed it.

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    The zener tested out well, and i swapped in a replacement just to be sure. Q10 in both modules is a MJE340, which they used before the TIP47, but it's working fine in the functioning module

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    Supporting Member Jazz P Bass's Avatar
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    How about some voltage measurements?

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    g1
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    Where that resistor is circled (R58?) is that a 3K or 10K there?
    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

  10. #10
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by stereomonostereo View Post
    The zener tested out well, and i swapped in a replacement just to be sure. Q10 in both modules is a MJE340, which they used before the TIP47, but it's working fine in the functioning module
    Did you measure the voltage across it?
    Experience is something you get, just after you really needed it.

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    There's no good way to measure voltages while this thing is powered up. The chassis is one solid piece except for the top, and the modules slide into a slot with a plug that goes to the power supply. All of the measurable components are on the bottom of the modules.

    That being said, i went back and wired the 4.7k across the thermistor and the fan worked. Initially i disconnected a leg of the thermistor when i put the resistor in and that's why i was getting nothing. So it looks like a bad thermistor. Anyone have an idea of how to spec a replacement?

  12. #12
    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by stereomonostereo View Post
    So it looks like a bad thermistor. Anyone have an idea of how to spec a replacement?
    How about a pic of the thermistor?

    Are there any markings on it?

    There are two types of thermistors.
    Negative Temperature Coefficient.
    And Positive TC.

    More than likely, they used an NTC.
    Measure the good one & hit it with a heat gun.
    If the resistance goes Down, it is an NTC.

    http://ijirts.org/volume4issue4/IJIRTSV4I4002.pdf

    NTC temp vs resistance chart: http://www.weissinstruments.com/sg_u.../NTC_Chart.pdf
    Last edited by Jazz P Bass; 02-17-2017 at 11:51 PM.

  13. #13
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by stereomonostereo View Post
    There's no good way to measure voltages while this thing is powered up. The chassis is one solid piece except for the top, and the modules slide into a slot with a plug that goes to the power supply. All of the measurable components are on the bottom of the modules.

    That being said, i went back and wired the 4.7k across the thermistor and the fan worked. Initially i disconnected a leg of the thermistor when i put the resistor in and that's why i was getting nothing. So it looks like a bad thermistor. Anyone have an idea of how to spec a replacement?
    What Jazz said +

    Without any useful part numbers, I would do it by measuring the old one. Measure the resistance at 25C and again at 100C (dunk in boiling water) and then search the datasheets for nearest match. For NTC types, which this is, the nominal resistance at 25C is given i.e a "10k" thermistor is 10K at 25C.

    PS: The aim of doing this is to determine the "B" parameter. Once you have B, the resistance at 25C and the physical size it should be possible to find something at Digikey or Mouser although thru hole parts are getting rarer.
    Last edited by nickb; 02-17-2017 at 06:30 PM.
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    image1.jpg

    It's a small beige piece with no markings anywhere. Measures 59k where the working one measures 50k

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    g1
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    I get the impression that if you swap the therms you will find it is not the issue.
    Putting a 5K across it to make it work is quite a change to the circuit.
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    "there's another kind of party lights that I can't stand to see,
    when there's a man in that patrol car and he don't wanna party with me"

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    It's definitely the thermistor. I just swapped them and the problem follows the one that's suspect
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    Excellent. Fairly simple fix then.
    Those hand drawn changes on the schem. made me think it may be a mod to correct a circuit design problem. I was thinking back to a similar circuit that got re-designed many times then scrapped. In that model, changing the thermistor often corrected the problem for awhile, then it would come back. That was some scr or triac circuit though.
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    when there's a man in that patrol car and he don't wanna party with me"

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    We're starting at 50k on the functioning one, and then dropping to 10k when in boiling water. So i believe i'm looking for a 50k with a 10k B parameter then?

  19. #19
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by stereomonostereo View Post
    We're starting at 50k on the functioning one, and then dropping to 10k when in boiling water. So i believe i'm looking for a 50k with a 10k B parameter then?
    Not quite so easy

    B= ln(R/R0)/(1/T -1/T0); R= 10k, T=100C (=373K), R0=50K, T0=25C (=298) plugging the numbers in gives B= 2385K. This a an unusually low number, something like 4000K is more common. It could be really be that but my guess is that thermistor is starting to fail too.

    So looking at Digikey this could be a candidate B57861S503F40 EPCOS (TDK) | Sensors, Transducers | DigiKey. The B is much higher this means the fan will kick in early which is better than the other way around. Keep looking and maybe you will find one cheaper and better.
    Experience is something you get, just after you really needed it.

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    Quote Originally Posted by nickb View Post
    Not quite so easy
    I find it never is

    I think i'm going to go with that as a replacement and do the other one while i'm at it. Thanks for all the help!
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