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    Latino ST120 Problem - bad bias resistor

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    guentherj2

    Posts : 47
    Join date : 2013-06-22
    Location : Bellingham, WA

    Latino ST120 Problem - bad bias resistor

    Post by guentherj2 on Fri Aug 16, 2013 9:17 pm




    Good Day All,

    So I'm listening to music and I hear a quick pop. I didn't immediately hear any change in the sound coming out of the speakers - I was listening at a low level.

    I powered off my CD player and preamp to check my bias on the ST120. The left rear bias was high - about 23 VDC I think. The other three bias points were OK at about 0.550 VDC.

    Then I saw a quick light show in the SOVTEK GZ34 rectifier tube. I powered off the amp to investigate further.

    I found that the 10 ohm resistor on pin 1 of the rear left KT88 tube was physically chipped and the resistance was open so that obviously needs replaced.

    What might have caused this and might it have damaged my left rear KT88???

    I'm going to test the GZ34 and have a spare if needed. I'll have to look around to find a tube tester to test the KT88.

    Assuming I have all good tubes - should I just replace the resistor, plug everything back in and fire it up and see if all is well???

    Any suggested methodolgy I might consider so I don't damage anything further?

    Thanks,

    John
    .



    Elrick

    Posts : 71
    Join date : 2012-09-02
    Location : Bellingham WA

    Re: Latino ST120 Problem - bad bias resistor

    Post by Elrick on Fri Aug 16, 2013 9:44 pm

    Since you are in my fair city I'll post a reply and get the ball rolling. Did you see the light show in the GZ34 after you noticed that you couldn't set the BIAS or was that after the light show? If the KT88 test out OK then I would just replace that 10 Ohm 2 watter and fire it up. Someone else can explain why that resistor opened up in the first place.  I'm assuming nothing else looked wrong under the chassis?

    Do you run your amp through line power, or reduce the incoming voltage with an autotransformer? Just curious, I have a ST-70 that's been fully refurbished including a beefier PS transformer and I still run it through a variac at 115 V just to add a measure of safety since these amps are pushed pretty hard.

    guentherj2

    Posts : 47
    Join date : 2013-06-22
    Location : Bellingham, WA

    Re: Latino ST120 Problem - bad bias resistor

    Post by guentherj2 on Fri Aug 16, 2013 10:04 pm

    The rectifier tube tested bad - no suprise. Fortunately, the KT88 tested good. Now I have to find a 10 ohm 2 watt resistor.

    I first noticed the high bias - probably due to the bad resistor - then the rectifier flashed.

    peterh

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Latino ST120 Problem - bad bias resistor

    Post by peterh on Sat Aug 17, 2013 4:35 am

    guentherj2 wrote:


    Good Day All,

    So I'm listening to music and I hear a quick pop. I didn't immediately hear any change in the sound coming out of the speakers - I was listening at a low level.

    I powered off my CD player and preamp to check my bias on the ST120. The left rear bias was high - about 23 VDC I think. The other three bias points were OK at about 0.550 VDC.

    Then I saw a quick light show in the SOVTEK GZ34 rectifier tube. I powered off the amp to investigate further.

    I found that the 10 ohm resistor on pin 1 of the rear left KT88 tube was physically chipped and the resistance was open so that obviously needs replaced.

    What might have caused this and might it have damaged my left rear KT88???

    I'm going to test the GZ34 and have a spare if needed. I'll have to look around to find a tube tester to test the KT88.

    Assuming I have all good tubes  - should I just replace the resistor, plug everything back in and fire it up and see if all is well???

    Any suggested methodolgy I might consider so I don't damage anything further?

    Thanks,

    John
    .


    i'd consider the kt88 bad and have it replaced before i continue.
    The schenario is typical for a flashing powertube. Your gz34 might have survived.
    The resistor is to be replaced with one that is matched to the remaining one, they need to be matched.

    Bob Latino
    Admin

    Posts : 2373
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Latino ST120 Problem - bad bias resistor

    Post by Bob Latino on Sat Aug 17, 2013 8:14 am

    Hi John,

    That 10 ohm resistor is of course bad if it checks out as open on a multimeter resistance scale. That resistor is a 10 ohm 2 watt resistor and your local Radio Shack does not carry a 10 ohm resistor in a 2 watt size. I will send you out another 10 ohm 2 watt resistor later today. Just cut out the old resistor that goes from pin #8 to chassis ground and solder in the new resistor .. NOTE - If that bias resistor blows again on that SAME tube socket then that output tube is bad. If it does happen again > Send that output tube back to me and I will send you a new KT88.

    Bob

    guentherj2

    Posts : 47
    Join date : 2013-06-22
    Location : Bellingham, WA

    Re: Latino ST120 Problem - bad bias resistor

    Post by guentherj2 on Sat Aug 17, 2013 10:51 am

    Thanks Bob. Yes, those 2 watt resistors are not readily available. I look forward to getting and replacing the new resistor and hope that takes care of it. Lets hope the KT88 tube is OK - it checked out on a tube tester but the old tube tester I used is not 100% reliable so we'll see.

    John

    guentherj2

    Posts : 47
    Join date : 2013-06-22
    Location : Bellingham, WA

    Re: Latino ST120 Problem - bad bias resistor

    Post by guentherj2 on Thu Aug 29, 2013 10:53 am

    Bob quickly sent me a couple of 10 ohm 2 watt resistors. I replaced the bad bias resistor and GZ34 rectifier and am happily back up and running.

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