I've been having issues with a JC77. The first was the chorus selector switch which has been dealt with but now its blowin' fuses. Originally there was a lack of volume, before the selector switch repair. I plugged the pre-amp out into a good amp and it still had very little volume, before fixing the switch. I'm thinking the transformer is bad but I'm at a loss. Any ideas and/or help on what might be wrong would be great. Thanks in advance,Greg
PS How do you test a transformer any way?
The 'blowing fuses' may be the transformer.
My first suspect would be the output transistors.
The four bolted to the chassis.
As this is a stereo amplifier, you can check one side to the other.
With the power off & the power supply drained, check each leg from one to the other with an ohmmeter.
If you find a dead short on any of them, there is your fuse problem.
Last edited by Jazz P Bass; 06-14-2012 at 02:11 AM.
Don't I need to take the transistors out of circuit to get a reading?
Not for a quick test for shorts. You may need to remove from circuit for more accurate tests.Originally Posted by TarheelTechinTraining
Which fuse is blowing, the main fuse or one of the fuses on the pc board?
Do you have a light bulb limiter?
I sure do Bill. Its the main fuse thats blowing. I didn't use the light limiter at first because the amp worked before the selector "fix". After it blew the fuse I did use the limiter and it glowed Very bright, a dead short somewhere. I did check the outputs and there is no short. Thanks to all,Greg
Whenever you are working on ANYTHING, and it develops a NEW problem along the way, it is almost always because of something you did. Look closely at your work. Make sure when you fixed your switch or whatever that you did not reconnect cables to the wrong connectors. Make sure anywhere you soldered you did not inadvertantly leave a solder blob shorting two points. Come up with a missing screw or nut? Could be wedged under a board, shorting out the supply.
The transformer was not bad the day before, why would it be now? WHy do you think the transformer is the problem, as opposed to just about everything else in there? Transformers can fail, but they are almost always the last thing on a failure list.
Education is what you're left with after you have forgotten what you have learned.
Well I thought the same thing " worked before but not now what did I do" When I got the amp it had little volume but played, hence maybe the transformer. I know you're right enzo but for the life of me I can't seem to find what is wrong. I've triple checked my wiring with the schematic and I don't have any missing screws.
I'm with Enzo here, you always suspect what you did before the problem developed.
Try pulling the two fuses on the pc board, does the limiter still light up? Have you checked the power supply diodes for shorts?
I been soldering for well over 50 years now, and that STILL is my first thought in these situations, "Oh lord, what did I do NOW?" We all have done it, and it ain;t like it will never happen again.
We look for something we did, but ultimately if we don;t spot anything, then we have to just approach it as a new problem the same as if it just now came in through the door.
You replaced the chorus switch? It is on the cirtcuit board, yes? Had to dismount the circuit board to do it? I see the main rectifier bridge is off the board, so ther are four wires that could have gotten confused. DO what Bill said, pull those two secondary fuses, main still blow? By now I hope we are using a light bulb limiter instead of actually blowing fuses.
The four power transistors are off board, any of them get disconnected? Potential for problem there. Or did you pull the traqnsistors off the chassis at all? ANY chance there is a missing insulator under one? Power off - duh - and go down the row of four, are any of the collectors shorted to chassis?
This is a shortcut to me, but it may be more complicated than you care to make it, but... A quick way to look for shorted power transistors is to measure for continuity from the speaker hot lead to the two DC terminals of the main bridge rectifier. The output transistors are each wired directly to a V+ or V- supply, and those originate at the bridge. SO a shorted output transistor will show as a short from V+ or V- to the speaker jack.
Try all that stuff.
Education is what you're left with after you have forgotten what you have learned.
I couldn't agree more.Plus there aren't two fuses on the pcb , like the schematic, looks like they were never there. Also do you mean the zener diodes or the BRD?
And the lack of volume was there before the "fixed selector switch".
And yes after the first fuse blew I used the limiter.
No continuity from bridge to speaker hot leads, V+ or V- Thanks fellows
Check the main bridge rectifier for shorts.
And the lack of volume was there before the "fixed selector switch".
Looking at the photos, have you completely removed the pc board from the amp? It looks like you have unsoldered the power transformer leads and others as well?
I agree check the bridge for shorts perhaps if you do it this way by unsoldering the +'ve and -'ve pins on the pc board and measuring the DC voltage across them.
The circuit you provided is a JC120 which shows 80 volts DC but the JC77 will probably be less.
That way you can also test the transformer (which I don't think will be faulty) the bridge rectifier and the way the transformer secondary connects to the board.
The pins have red and brown wires wrapped to them.
With everything mounted back in the chassis have you checked what Enzo suggested ?
If the insulating washers under the output transistors are perforated or if there is any connection between the back of the transistor (collector .|. middle pin of transistor) and the heatsink this will blow fuses.Or did you pull the transistors off the chassis at all? ANY chance there is a missing insulator under one?
Power off - duh - and go down the row of four, are any of the collectors shorted to chassis?
SOmehow it completely escaped me that the schematic was not for this amp. Howefver, looking at the photo, I can see two wire jumpers where the fuses would have been. I see the holes for the fuse clips in the board.
The lack of volume was there before the switch was changed, but the blowing fuses was not. I could be wrong, but I bet the two problems are not related.
Zeners? No, I am more concerned for the main bridge rectifier. The four heavy wires at the bottom of the photo go to it.
No V rails shorted to output is good, but still need to check if either V+ or V- is shorted to chassis.
You have unwired the transformer? OK, before connecting it back up, see the three terminals? Center is CT/ground, the other two are AC. Make sure the two AC ones are not shorted together. And are you sure which wire goes where of the three?
Education is what you're left with after you have forgotten what you have learned.
Yeah enzo not the correct schematic but when I searched for the 77 thats what I got . Most of the componets are the same, or close enough. The brd tested perfect and its mounted to the chassis from the factory. I'm positive about the transformer to pcb connections, I drew a picture don't laugh and I took pixs plus on the schematic it shows the connection numbers for the three wires. I do have continuity between the ac wires, both orange wires, at the transformer unhooked from the pcb. Thanks so much for everybodys patience with me ,Greg
V+ and V- are shorted to chassis !!
OK, so the transformer is OK. Not that it matters now, but when I asked about continuity between the transformer terminals, I meant the empty ones on the board. Of course the transformer would have continuity.
If the V rails are shorted to ground, then the first four suspects are those output transistors. Dismount them, leave them wired, and see if the short remains.
Education is what you're left with after you have forgotten what you have learned.
Yep short is still there
Also there is no continuity between the empty terminals for AC
Well if you have a dead short to ground from both rails, it ought to be fairly straightforward to locate where.
Education is what you're left with after you have forgotten what you have learned.
Well all 4 outputs test fine, out of circuit. But I'm getting continuity from V+ to 34,32,17,and 15 as well as V- to the same points. 34 & 15 are emitters and 32 & 17 are collectors. so the problem is in the power section?
Let me get this straight. Both V+ and V- are shorted to chassis? And so V+ is also shorted to V-, even if just through the chassis? But did we not already test the main bridge rectifier for shorted condition? Since the V+ is wired to the bridge directly, as is the V-, even if the bridge is not causing it, its terminals should read as shorted together if the two Vs are shorted.
Education is what you're left with after you have forgotten what you have learned.
ok so I removed the bridge and tested to the outputs, still shorted. removed the filter caps and tested to output , still shorted. Checked again for bad soldering and nothing. and V+ to V- terminals no continuity. Please be patient with me this is killing me. Also the pcb is out of the chassis
I'm sorry V+ is shorted to 34 & 17 , collectors. V- is shorted to 32 & 15 , emitters. My bad enzo cranial overload,lol.
Well, I expect V+ is shorted to 34,17, since it is wired to them.
But are both of them still shorted to ground?
Education is what you're left with after you have forgotten what you have learned.
no now only the V+ is shorting.
So I now have the correct schematic,thanks to this forum. Where should I start looking after the "usual" suspects , BRD filter caps and so on?
All the advice you have been getting is good, just apply it to the correct schematic. There are only so many ways a V+ can get shorted to ground.
Education is what you're left with after you have forgotten what you have learned.
Ok I'm pretty sure I've got a bad transformer. With the transformer unhooked from pcb both Vac are shorted to ground. Am I thinking correct here and if so where might I find a replacement?
So to understand what you have done here, you have disconnected the three wires of the transformer secondary winding from the pc board. Then you have read the resistance from one of the secondary windings wires to the chassis and you have gotten a low or zero ohm reading. Is this correct?
No I tested between secondary windings and the ground on the transformer.
1.Do you mean with the three wires disconnected from the terminals on the edge of the pcb the wires are measured to be shorted to the frame of the transformer which in turn is connected (bolted) to the chassis of the amp which is ground ?
2.Or do you mean the two ac wires that are connected to the outside terminals on the edge of the pcb are shorted to the wire that goes to the centre terminal on the edge of the pcb which is also ground.?
Please answer 1 or 2.
If "2" the secondary wires have a very low resistance between them it looks like a short circuit ..the resistance between the centre tap (one that goes to ground) and either of the outside connections is the same and measures half the resistance between the two outside ones.
You can check this by setting your meter to AC volts and measure the voltage at those wires.
It shouldn't matter if the amp is still plugged through the light bulb limiter.
Did you try this before ?
Unsolder the +'ve and -'ve pins on the pc board and measuring the DC voltage across them.The pins have red and brown wires wrapped to them.
Assuming the ac voltage is ok you will have to reconnect them to the terminals on the edge of the pcb making sure the centretapp
one goes to the um.. centre connection which is ground.
Now have we got Ac and Dc ??
Last edited by oc disorder; 06-20-2012 at 01:03 AM.
#1. When I measure AC , secondaries, its about 25v without the limiter. Thanks for everyones help !!
When I test the secondaries ,plugged into the wall ,unhooked from pcb ,the voltages are 25 or less from center tap to Vac and about 50 Vac to Vac. All three wires are off the pcb
Ok thats good the JC120 was about 80v so it makes sense that the JC77 is 50vac.
Now working through ..if you can unsolder those pins (i thought it would be neater than unwinding the wire wrap on them)
you should be able to check the DC volts and make sure the bridge is ok.
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