VOX VT50 grief

[malachi1313]

Well initially found this thread from googling issues with this amp. A similar thread was posted here concerning a VT30. Amps are close but I believe the power ic's are different. I re flowed some solder points, cleaned input jacks and from watching a utube vid of someone with same issue, I think it might be the power adjustment pot on the back of the amp. Ordered the part and just waiting for it to come in. Basically I get no sound out of the main speaker but a hum and a pulsing that is adjustable by turning the pot in the back. It sounds great out of the headphone jack. If I put pressure on the pot to the side it stops the problem. None of the voltage adjustments like the Gain and Master affect the sound at all. I also think the pulsing could be one of the 15v voltage regulators has gone out. The part numbers are MC7815 and MC7915. The mouser website has the part but there are so many variations I don't know which one to order, so if someone could help me there it would be appreciated. Got the amp for $20 off of craiglist so there is no heartache if I cant fix it. It does sound good through the headphones but I would like to get it fixed because I could also use it for a vocal amp or keyboard amp. I really hope it isn't the power IC because of the lack of availability of the part. Here is the utube link of similar problem as mine: http://wwa.you2repeat.com/watch/?v=dKPEHtabkCw (OOPS guess it ain't utube) and here is a link to the NTE part number for the voltage regulators: https://www.newark.com/nte-electroni...-to/dp/29C4841 Was wondering if that sight was reputable. Sorry about the huge long post and thanx!!!

[nosaj]

Well initially found this thread from googling issues with this amp. A similar thread was posted here concerning a VT30. Amps are close but I believe the power ic's are different. I re flowed some solder points, cleaned input jacks and from watching a utube vid of someone with same issue, I think it might be the power adjustment pot on the back of the amp. Ordered the part and just waiting for it to come in. Basically I get no sound out of the main speaker but a hum and a pulsing that is adjustable by turning the pot in the back. It sounds great out of the headphone jack. If I put pressure on the pot to the side it stops the problem. None of the voltage adjustments like the Gain and Master affect the sound at all. I also think the pulsing could be one of the 15v voltage regulators has gone out. The part numbers are MC7815 and MC7915. The mouser website has the part but there are so many variations I don't know which one to order, so if someone could help me there it would be appreciated. Got the amp for $20 off of craiglist so there is no heartache if I cant fix it. It does sound good through the headphones but I would like to get it fixed because I could also use it for a vocal amp or keyboard amp. I really hope it isn't the power IC because of the lack of availability of the part. Here is the utube link of similar problem as mine: http://wwa.you2repeat.com/watch/?v=dKPEHtabkCw (OOPS guess it ain't utube) and here is a link to the NTE part number for the voltage regulators: https://www.newark.com/nte-electroni...-to/dp/29C4841 Was wondering if that sight was reputable. Sorry about the huge long post and thanx!!!

Well if you have a meter you could check the voltage coming from the regulators. Check the following datasheets for the pins. For ground(black Lead) use the chassis, red lead probe the input then the output pin. And report findings, reason I say use chassis instead of the ground pin, is because of slippage. I have shorted out transistors before from slipping.


7915 datasheethttp://www.hep.upenn.edu/SNO/daq/parts/lm7915.pdf
7815 datasheethttps://www.jameco.com/Jameco/Produc...dDS/889348.pdf
Good luck and welcome to the Best forum in the World!

nosaj

[malachi1313]

Man that is a scary spot to take a reading with the power applied. Speaker connections are one thing but this one scares me. Thank you very much for the response and info.

[malachi1313]

Probably wont be able to get back with those readings until tomorrow, but I will update thread when I can.

[jondoe]

I'll assume you've googled and found the schematic?

If you want an easy place to measure the 15v rails use CN11, pins 1 and 4 carry the +15v and -15v rails.

[malachi1313]

Yeah I have the schematic and thanx for the tip!!