After an hour or two of running properly, the left channel on my Bob Latino ST120 seems to be dropping out altogether. This is a new thing, as it has been working well for the past couple of months. What could be going on? Any ideas how I might diagnose and fix this?
3 posters
ST120 left channel dropping out - problem solved !
ChetJettison- Posts : 35
Join date : 2021-12-03
- Post n°1
ST120 left channel dropping out - problem solved !
by ChetJettison Sun Dec 05, 2021 11:54 pm
LeGrace- Posts : 390
Join date : 2016-08-07
Location : Ontario, Canada
LM334 on driver board may be seeing too much heat, or require replacing.
ChetJettison- Posts : 35
Join date : 2021-12-03
- Post n°3
Re: ST120 left channel dropping out - problem solved !
by ChetJettison Mon Dec 06, 2021 8:58 am
Just for a little more information: the amp powers up, runs fine for an hour or so, then I’ll notice that the left channel is gone. I switch the amp off, let it cool for a few minutes, then when I turn it back on, the left channel is back.
The glow of the left driver tube seems dimmer than that of the right? Could it be the driver tube?
The glow of the left driver tube seems dimmer than that of the right? Could it be the driver tube?
LeGrace- Posts : 390
Join date : 2016-08-07
Location : Ontario, Canada
- Post n°4
Re: ST120 left channel dropping out - problem solved !
by LeGrace Mon Dec 06, 2021 10:04 am
LM334 is temperature sensitive which can lead to issues like you describe. How is ventilation around amp? A simple test is to point a fan to blow air at the driver board. Also swap the driver tubes around and right channel should now drop if weak tube related. After that others will need to chime in.
ChetJettison- Posts : 35
Join date : 2021-12-03
- Post n°5
Re: ST120 left channel dropping out - problem solved !
by ChetJettison Mon Dec 06, 2021 11:07 am
Thanks for the replies, LeGrace! I'll try swapping left and right driver tubes to start with, but based on some other posts I found I suspect you may be right about the LM334. This is one of the three-leg versions, correct? I'll have to order a replacement.
If indeed it is the LM334, I wonder why it failed. Perhaps just a faulty unit? My amp is stock; it runs hot of course, but I don't expect it runs any hotter than any other ST120. There's plenty of room around the amp on all sides. I haven't covered the ventilation slots on the bottom.
If I replace the LM334, maybe I'll try to install a heat sink on it if there isn't one already. I seem to remember there's a little clip-on heat sink on it, but no thermal compound, so maybe adding thermal compound would help.
Anyway, I'll try the tube swap when I get home tonight.
If indeed it is the LM334, I wonder why it failed. Perhaps just a faulty unit? My amp is stock; it runs hot of course, but I don't expect it runs any hotter than any other ST120. There's plenty of room around the amp on all sides. I haven't covered the ventilation slots on the bottom.
If I replace the LM334, maybe I'll try to install a heat sink on it if there isn't one already. I seem to remember there's a little clip-on heat sink on it, but no thermal compound, so maybe adding thermal compound would help.
Anyway, I'll try the tube swap when I get home tonight.
Bob Latino- Admin
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Join date : 2008-11-26
Location : Massachusetts
- Post n°6
Re: ST120 left channel dropping out - problem solved !
by Bob Latino Mon Dec 06, 2021 12:27 pm
ChetJettison wrote:After an hour or two of running properly, the left channel on my Bob Latino ST120 seems to be dropping out altogether. This is a new thing, as it has been working well for the past couple of months. What could be going on? Any ideas how I might diagnose and fix this?
If the amp "drops out" after an hour, I doubt it is the LM334 current regulator. It is more likely a bad solder connection on the left channel. The amp heats up after an hour and the heat causes expansion and the possibility that a solder connection can go "partial". Check all solder connections on the left channel and resolder any that look suspicious.
Bob
ChetJettison- Posts : 35
Join date : 2021-12-03
- Post n°7
Re: ST120 left channel dropping out - problem solved !
by ChetJettison Mon Dec 06, 2021 12:29 pm
Thanks for the reply, Bob. I'll take a close look. Any particular spots to check? Would the issue just be on the VTA board, or could it be elsewhere?
Bob Latino- Admin
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- Post n°8
Re: ST120 left channel dropping out - problem solved !
by Bob Latino Mon Dec 06, 2021 12:45 pm
ChetJettison wrote:Thanks for the reply, Bob. I'll take a close look. Any particular spots to check? Would the issue just be on the VTA board, or could it be elsewhere?
It could really be anywhere on the left channel ... Check the input jacks and the attenuator connections (if you have an attenuator). It can't be the main power supply because the power supply is common to both channels.
A possible way to find the connection is to turn the amp upside down on a double layer of towels, take off the bottom cover and let the amp play until the left channel goes out. Next > Use an insulated object like a plastic pen and tap on every connection on the LEFT channel and see if the left channel comes on even for a split second. If it comes back on momentarily, you may have found the connection. Resolder that connection.
Bob
ChetJettison- Posts : 35
Join date : 2021-12-03
- Post n°9
Re: ST120 left channel dropping out - problem solved !
by ChetJettison Mon Dec 06, 2021 1:04 pm
Bob,
I don't have an attenuator, so I'll start with the input jacks and move around from there. I'll try your pen tapping technique too.
There's no risk to the tubes, running them upside down during the diagnostic?
- Jeff
I don't have an attenuator, so I'll start with the input jacks and move around from there. I'll try your pen tapping technique too.
There's no risk to the tubes, running them upside down during the diagnostic?
- Jeff
Bob Latino- Admin
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- Post n°10
Re: ST120 left channel dropping out - problem solved !
by Bob Latino Mon Dec 06, 2021 1:10 pm
ChetJettison wrote:Bob,
I don't have an attenuator, so I'll start with the input jacks and move around from there. I'll try your pen tapping technique too.
There's no risk to the tubes, running them upside down during the diagnostic?
- Jeff
No - You can run the amp upside down with no problem. Just place the amp straight DOWN on the towel(s) on a table. Don't move the amp sideways on the towels or you can bend the pins on the output tubes.
Bob
ChetJettison- Posts : 35
Join date : 2021-12-03
- Post n°11
Re: ST120 left channel dropping out - problem solved !
by ChetJettison Mon Dec 06, 2021 1:12 pm
OK. Last question for now: is there a particular area that might be affected by a cold solder joint (or some other malady) which would result in the left driver tube not glowing as bright as the right driver tube? There's a palpable difference in brightness between the two.
Bob Latino- Admin
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Location : Massachusetts
- Post n°12
Re: ST120 left channel dropping out - problem solved !
by Bob Latino Mon Dec 06, 2021 1:17 pm
ChetJettison wrote:OK. Last question for now: is there a particular area that might be affected by a cold solder joint (or some other malady) which would result in the left driver tube not glowing as bright as the right driver tube? There's a palpable difference in brightness between the two.
You could swap the left and right driver tube to see if the problem moves to the other channel ?
Bob
ChetJettison- Posts : 35
Join date : 2021-12-03
- Post n°13
Re: ST120 left channel dropping out - problem solved !
by ChetJettison Mon Dec 06, 2021 1:20 pm
OK, I'll do that first thing when I get home this evening.
ChetJettison- Posts : 35
Join date : 2021-12-03
- Post n°14
Re: ST120 left channel dropping out - problem solved !
by ChetJettison Mon Dec 06, 2021 7:47 pm
Just got home to try the tube swap. This time, right after warmup, the left channel was at about 20% volume, and distorted. I'll check the solder connections next. But does the low and distorted left channel indicate anything?
EDIT: And after turning it back on for further testing, the left channel is somehow back on and sounding normal. I'll let it run for a while and see what happens.
EDIT 2: The longer I have it running, the more the left channel seems to be slowly fading out. It wouldn't be a solder joint if it's slowly fading away, would it?
EDIT: And after turning it back on for further testing, the left channel is somehow back on and sounding normal. I'll let it run for a while and see what happens.
EDIT 2: The longer I have it running, the more the left channel seems to be slowly fading out. It wouldn't be a solder joint if it's slowly fading away, would it?
ChetJettison- Posts : 35
Join date : 2021-12-03
- Post n°15
Re: ST120 left channel dropping out - problem solved !
by ChetJettison Mon Dec 06, 2021 10:47 pm
Friends, after all my worry over my amp, it seems the fault lies with my Full-Monte Dynaco PAT-4 preamp. Time to send an email over to Dan at UpdateMyDynaco.
Bob Latino- Admin
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- Post n°16
Re: ST120 left channel dropping out - problem solved !
by Bob Latino Wed Dec 08, 2021 9:29 am
ChetJettison wrote:Friends, after all my worry over my amp, it seems the fault lies with my Full-Monte Dynaco PAT-4 preamp. Time to send an email over to Dan at UpdateMyDynaco.
Glad to hear that you found the issue. The first rule of troubleshooting "an amp with a problem" is to eliminate all other possible causes of of the problem. Signal source, preamp, interconnects, speaker wire and, speakers. If you had just swapped the interconnects going from the preamp to the amp, the problem left channel "low volume issue" would have moved over to the right channel and the PREAMP (or the signal source) would have been exposed as the cause of the problem.
Bob
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