Vta ST-70 one channel out. After some initial trouble shooting I believe the problem is a lm344 on driver board. Checking good channel to bad, voltages are different on legs of device. Power off and measuring resistance leg to leg of lm344s they are different. Anyone else have issues lm344?
3 posters
LM344 possible failure on vta st-70 driver board.
fzapped- Posts : 7
Join date : 2018-05-19
New2Tubez- Posts : 185
Join date : 2018-03-20
Location : NY
- Post n°2
Re: LM344 possible failure on vta st-70 driver board.
by New2Tubez Sat Aug 26, 2023 7:15 pm
Had one go out in my ST120. Whooshing sound and lower volume on one side. Replacement confirmed Bob’s suggestion.
If you do a search of forum threads, you’ll likely find answers.
If you do a search of forum threads, you’ll likely find answers.
Bob Latino- Admin
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Join date : 2008-11-26
Location : Massachusetts
- Post n°3
Re: LM344 possible failure on vta st-70 driver board.
by Bob Latino Sat Aug 26, 2023 7:54 pm
When the LM334 goes out on a VTA driver board the most common symptom is very low volume on that channel. Many times either the driver tube is bad (or going bad) on that channel OR someone wrongly substituted a 12AT7 or 12AX7 driver tube for the normal 12AU7 driver tube that is supposed to be in that socket. NOTE 1 > besides a 12AU7 you can also use the military/industrial versions of a 12AU7. The 5963, 5814 or 6189 tubes are 12AU7 type tubes that were made by RCA, Sylvania and GE. These three mentioned tubes are are very rugged and were made for the US military before the advent of solid state gear. These three tubes are still available on Ebay at reasonable prices. You can also use the Mullard CV4003 and European ECC82 tubes which are also 12AU7 equivalent tubes. NOTE 2 > The early VTA amps and VTA driver boards built before 2012 use a higher gain 12AT7 driver board. You can easily recognize the older boards (pre 2012) because they do not use an LM334 current regulator on each channel. Use a 12AT7 driver tube with the older boards.
Bob
Bob
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fzapped- Posts : 7
Join date : 2018-05-19
Bob Latino wrote:When the LM334 goes out on a VTA driver board the most common symptom is very low volume on that channel. Many times either the driver tube is bad (or going bad) on that channel OR someone wrongly substituted a 12AT7 or 12AX7 driver tube for the normal 12AU7 driver tube that is supposed to be in that socket. NOTE 1 > besides a 12AU7 you can also use the military/industrial versions of a 12AU7. The 5963, 5814 or 6189 tubes are 12AU7 type tubes that were made by RCA, Sylvania and GE. These three mentioned tubes are are very rugged and were made for the US military before the advent of solid state gear. These three tubes are still available on Ebay at reasonable prices. You can also use the Mullard CV4003 and European ECC82 tubes which are also 12AU7 equivalent tubes. NOTE 2 > The early VTA amps and VTA driver boards built before 2012 use a higher gain 12AT7 driver board. You can easily recognize the older boards (pre 2012) because they do not use an LM334 current regulator on each channel. Use a 12AT7 driver tube with the older boards.
Bob
Thanks Bob, Since I had a total channel out I went on the assumption LM334 was good. Tried to find why LM344s would have different voltage readings, found a poor solder joint. Fixed.
Thanks again
Bob Latino- Admin
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Join date : 2008-11-26
Location : Massachusetts
- Post n°5
Re: LM344 possible failure on vta st-70 driver board.
by Bob Latino Sun Aug 27, 2023 5:08 pm
fzapped wrote:
Thanks Bob, Since I had a total channel out I went on the assumption LM334 was good. Tried to find why LM344s would have different voltage readings, found a poor solder joint. Fixed.
Thanks again
The most common cause of a kit built amp not working properly is a poor solder connection. It is NOT usually a bad part and/or a wiring error. The other thing about a bad solder connection is that it may not show up immediately upon first turn on. What happens many times is that the connection is not soldered with enough heat and after multiple ON/OFF cycles (heat up and cool down cycles) the connection goes partial and you lose one or both channels. This may sound like a "shotgun approach" but sometimes just soldering all the connections on the channel that is not working will cure the problem.
Bob
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