STV-3H-A Replacement? Yamaha JX20 SS Amplifier - Page 1
Forum section: electronics / beginners

author: Supernaut
author_rank: Contributor
timestamp: May 20, 2025, 10:54:53 pm
content: Hi everyone,I just bought an old 1980's Yamaha JX20 solid state guitar amplifier as spares or repair, on first inspection i can see the output transistor looks like its been subject to some sort of moisture damage, ive checked it with the meter and confirmed its dead, the fuse on the PCB is blown too but the rear panel fuse is intact and the transformer windings seem good too. Looks like it should be a straight forward repair although ive never come across these STV-3H-A components before.I read online that it can be substituted with three 1N4148 diodes in series but im honestly unsure, i just do this at a hobby level and ive never dealt with one of these before, im not 100% sure mines is cooked yet but i wanted to find out more about them regardless, i cant seem to find anything available and im not even sure what its doing?Ive included a picture for reference, any help would be appreciated.Edit: added schematic screenshot, this schematic is for a JX30 amplifier buts its similar enough to the JX20 to use it as reference. STV-3H-A is highlighted in the picture (or at last i think that's it)

author: CaptDon
author_rank: Super Contributor
timestamp: May 21, 2025, 04:51:37 pm
content: Those series diodes are a must for comp-sym amplifiers. ECG/NTE used to list them in the diode section of their cross reference catalog. I keep some on hand removed from junk amplifiers that weren't worth repair. You can tell how many series diodes are inside the glass/epoxy by passing 10ma. of current through the device and measuring the voltage drop. Apprx .7vdc per junction of course. Those diodes are usually clamped to the output transitors or the heatsink. They need to sense the same temperature as the transistors whose bias they control! B.T.W., your corrosion looks like the typical "Some numb nuts dropped a beer on my amplifier at the gig last night" style of amplifier demise!!

author: Supernaut
author_rank: Contributor
timestamp: May 21, 2025, 07:17:39 pm
content: Thank you very much for that information, I had no idea it had to be on the same heatsink as the transistors. I had assumed I could maybe make a little PCB and stand offs to add some diodes in series if the component is blown. What would be the best way to test it with a multimeter? I only have a 240v 2A variac so unfortunately i cant power the component manually in a safe way, should i just desolder it from the PCB and test it with the diode function on my meter?Your 100% correct in your diagnosis, i first pulled the amp chassis out and seen the condition of the transistor, i assumed moisture damage but when i stripped the cabinet housing down to clean it up i noticed evidence of spillage due to the staining of the upper MDF back panel (its a little semi open back combo amp, the upper half of the wood has moisture staining on the inside of it but the lower one doesn't.)I also found a ticket in the amp chassis for The Pogues playing Manchester Apollo Theatre in 1988Thanks once again

author: CaptDon
author_rank: Super Contributor
timestamp: May 22, 2025, 12:37:31 am
content: Most multimeters can't properly test multi-element diodes because their open circuit test voltage on the ohms scale isn't high enough. Each diode is around .65 to .7 volts of drop. Many meters use a 1ma. test current and a single silicone diode will measure '645' up to '720' on the diode scale. Germanium diodes and Shottky diodes tend to measure '170' up to around '320' on the diode scale of many digital multimeters (DMM). You could use a 9 volt battery and a 1K ohm resistor in series to apply a test voltage to the diode (Out of circuit) and observe to forward bias voltage. A dual element diode would measure around 1.3 to 1.45 volts and a triple element like the one in your schematic would be 1.7 to 2.1 volts. Then measure in reverse bias and hope to measure the full 9vdc battery voltage. This should help with your testing. B.T.W., I had a rack of amplifiers pushed back against an outdoor set of porch steps at an outdoor pig roast gig. Some drunk fell down the steps and 16 ounces of beer went right into my amp rack!! Amazingly they didn't blow up!! I didn't know if I wanted to turn them off and end the gig or just hope for the best! The worst part of the event was that I had some mikes stolen by the band and never got paid for the gig. Land owners said the band was supposed to pay me but the band who booked me said "The party organizer would pay me". I pulled in a few favors from my local sound engineer buddies making sure the band would NEVER book a sound contract again!! The band agreed to meet me twice to 'pay up' but no one ever showed!!! About a month later the guy who booked me ended up in jail for passing bad checks!! Don't f--k with sound engineers!!! We stick together!!

author: Supernaut
author_rank: Contributor
timestamp: May 24, 2025, 04:33:54 pm
content: Thanks again for the input, my new output transistors turned up today which I managed to install, although as I expected, I lifted three solder pads. Ive found that Yamaha and Panasonic PCB's of the late 70's early 80's seem to dislike having components removed, they seem to be extremely sensitive to heat, even if your trying to be careful about the process.Not a huge deal as the pads live on the mini PCB's that attach the transistors to the wires from the main PCB, il just ditch the two little mini PCB's and go direct with a 3-pin connector to the transistors. I still need to switch the wiring from two conductor to three and it could maybe do with a re-cap although my soldering iron just broke so il need to leave it for now, at least I know the amp is working.I didn't even get round to testing that little diode package since my iron broke, I just fired the amp up with the variac after I installed the transistors and hoped for the best, my iron broke while soldering the last leg of the transistor so I was lucky to get that far. No magic smoke, no heat, no arcing, nothing weird from what I can see, so it will live to fight another day.Almost all of my gear is bought as spares or repair, I cant actually think of many amps ive bought that were sold as working. Same with all of my pedals, mostly sold as spares or repair. Ive got about fourteen amplifiers and sixteen pedals just now (although I have twenty five pedals stuck at customs that I imported from Japan), many rack mounted effects too. My equipment doesn't leave the house so when its fixed it stays with me and generally no one else touches it.