PA amp dead ? - Page 1
Forum section: electronics / repair

author: dunebuggy
author_rank: Newbie
timestamp: June 09, 2022, 02:01:38 pm
content: Hi,first time poster, not a pro but curiousHave a Jedia Mobile 60 PA amp I'm using the 70 volt tap for speakers.Worked fine for 10 years.Now it's not turning On, no LED.Checked the 2 fuses, one on the AC connector, the other inside, all good.The only weird thing I found is the voltage at the secondary of the power transformer reads 0 volt (?). I've unplugged the 2 wires from the circuit and I'm measuring in ACIf I measure the phase individually to ground it reads 20volt each.Is the transformer dead ?Thanks in advanceEric

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 09, 2022, 02:19:23 pm
content: Do you have a schematic? if so post it here.It is possible that it is the power transformer, they do go bad. They shouldn't really go bad for no reason but I have seen it more than once.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: June 09, 2022, 03:27:43 pm
content: maybe this fitshttps://elektrotanya.com/jedia_l60as_amplifier_sch.pdf/download.html

author: dunebuggy
author_rank: Newbie
timestamp: June 09, 2022, 03:46:51 pm
content: That schematic is for the L60 stereo ampMine is mono.Might have the same power trasfo, Looking at the schematic nowthanks

author: dunebuggy
author_rank: Newbie
timestamp: June 09, 2022, 04:18:52 pm
content: Dont see info on the secondary output except in D6 there is a 24volt.might be out of my league...

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 09, 2022, 04:20:01 pm
content: That schematic does not appear to cover the power supply section.

author: dunebuggy
author_rank: Newbie
timestamp: June 09, 2022, 05:26:43 pm
content: Here is a pix of an intriging sealed part screwed on the bottom of the chassis2 reds from the secondary1 black and 1 red going to B+ connectorWould that be some kind of rectifier ?thanks again

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 09, 2022, 05:28:28 pm
content: pic didn't attach, is it too big?

author: dunebuggy
author_rank: Newbie
timestamp: June 09, 2022, 05:47:04 pm
content: sorrythanks

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 09, 2022, 05:56:47 pm
content: That is your bridge rectifier, Has four diodes in it. Should be two AC connections from transformer and two DC connections out.Usually they are labeled close to the pinsNotice the AC and the + label

author: dunebuggy
author_rank: Newbie
timestamp: June 09, 2022, 06:25:56 pm
content: thanks !, makes senseBut i'm still puzlled by the fact that the secondary reads 0 volt between the 2 wires .AC mode used on the multimeter

author: TheMG
author_rank: Frequent Contributor
timestamp: June 09, 2022, 06:35:59 pm
content: Measure the resistance of the transformer primary, it might be open-circuit. If it is, then it's likely the transformer has a built-in thermal fuse that has gone open.Voltage readings between secondaries and ground are just stray capacitance between primary and secondary winding, doesn't mean anything.

author: dunebuggy
author_rank: Newbie
timestamp: June 09, 2022, 09:01:50 pm
content: hmm , primary is measuring about 8M ohm, I removed the plastic and no hidden thermal fuseuntil now never heard of the thermal protection fuse, thanks MGIs 8M ohm means open (almost...)

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 09, 2022, 09:13:55 pm
content: 8M ohms is certainly open, yes.

author: dunebuggy
author_rank: Newbie
timestamp: June 09, 2022, 10:20:30 pm
content: Thank you for the repliesÉric

author: TheMG
author_rank: Frequent Contributor
timestamp: June 10, 2022, 12:07:23 am
content: The fuse is usually buried deep within the windings, as that is where the temperature is highest. Usually you can see/access the leads of the fuse where they connect (under the plastic insulating tape wraps). Replacing the original fuse is usually not possible, the next best thing you can do other than replacing the original one is to solder a new one with a lower temp rating to the outside of the winding, in parallel with the leads of the burnt out one.

author: dunebuggy
author_rank: Newbie
timestamp: June 11, 2022, 05:06:34 pm
content: It's fixed, thanks again for all the infosThe thermal fuse was blown and buried very deep in the primary.Eric

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 11, 2022, 07:12:53 pm
content: Cool, glad you were able to find it!The other question is if there was a particular reason that the thermal fuse blew, like an overload condition of some sort.

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 11, 2022, 07:36:34 pm
content: In my experience they just fail sometimes, thermal stress, bad connections heating the wrong end, etc. I'm sure they do sometimes actually prevent fires, but I always find them blown with either no signs of heat (discolored magnet wire and whatnot) or they failed on the connection end.