HR732 Blowing Fuses

Bought an HR732 new in '92 and have been using it ever since. I recently moved it from whole house duty to 2 channel in my theater room. It started cutting out while playing a CD last nite, and then the display went off. I pulled the cover to find the fuse blown. 16 years, I figured it was due. I replaced the fuse and it played a few seconds and blew again. Removed the CD player and replaced the fuse again, it blew when I shut off the receiver. Went thru 2 more fuses, they wouldn't last for more than a minute or so. Any ideas, before considering a repair shop?

Bought in '92 means its 16 years old. That means your overdue for the electrolytic capacitors to wear out.
Hints can sometimes come from how the fuse blows. If fuses blow immediately, maybe flash and cover the inside of the glass tube with silvery metal, then you've got a substantial short. Likely to be the rectifier stage having bad diodes in that case. Likely to find this after a lightning storm for instance. When fuses blow slower, like in your case, then its harder to say for sure what your failures likely to be. If nothing else much anything has gone on, like no parties, then it's a pretty good bet for aged components, very often the electrolytics. Especially if its been in a hot environment.
In your case if your comfortable with an iron, then I'd look to replace the power supply filter capacitors. If it's a switch mode supply, then forget it, but if its a more convential supply then thats a good starting point. If the largest filter caps are bulging at the top, that's a significant sign of their failure. And I wouldn't plug it in again until repaired.
Hope that gives you a bit to start with. If not comfortable with opening and repairing, then be sure to go to a pro!!
Good luck,
Mark

Hey radioeng2, great little tidbit about the bulging caps. I just know that this is going to clue a lot of people in as a good sign that it is time to have their amps looked at BEFORE they die.

Thanks!!!

Thanks for the info, I'll check for bulging caps tonite. I wasn't looking, but nothing looked "funny" while I was in there. The fuse would blow after a minute or so of being on or upon shutting off the unit. It looked like a flash bulb, and the fuses themselves were pretty clean on the inside. I had a hard time telling whether the first fuse was bad or not. (Thin filament, and no marks on the glass) The fuses after the first, had a very small black mark on one end or the other.

What is a 'switch mode supply'? I was thinking it may have been the on/off button going bad, since it did blow twice upon shutdown. The first new fuse blew after I hit play on the CD player, and forgot the volume was up. As soon as a the music started it blew. The others all blew with no components or speakers connected.

I guess I'll need a schematic if I want to replace anything. Any suggestions on a good resource?

Thanks for all the info.

I pulled the top off at lunch. The large black caps under the ACCD board looked new. No bulging or discoloring. The tranformer, on the other hand, has some yellowy, almost glue like, substance on the side facing the rear of the unit. (about the size of a thumb print) The only other thing I noticed was one of the red wires that goes from the rear convenience plugs to the back of the switch has a black mark on it. It's hard to tell if it's a heat mark? I took a few pics and then my camera died. I'll get it charged and post a pic.

..........The tranformer, on the other hand, has some yellowy, almost glue like, substance on the side facing the rear of the unit. (about the size of a thumb print).. .

Probably residual insulating lacquer from manufacture...no biggie

Is this fuse an internal fuse or the main power fuse? What's the rating? Is the circuitry covered with an insulating blanket of dust? If so, and it's a small rating fuse, dust may be trapping enough heat to cause the circuitry to draw more current

OK...your follow up comments makes me wonder if you for sure put the same type of fuse back in. Just to make sure of the simpliest stuff first!!

You need to know the rated value and make sure about whether its fast or slow blow! Usually it'll be marked what it should be right next to it. Failing that, it might take consulting a schematic.

If you have the original fuse, there should be some letters along with the value. AGC for instance, is fast blow. MDL is slow-blow. Makes a big difference.

A switch mode supply is referring to the typical modern style that modulates the line voltage up to a super high frequency and then chops it back down. Traditional PS used BIG transformer, recifier stage and then big filter capacitors. Switch mode supply is a little board, little components, dozens and dozens of them and fail right and left for little reason.

Gotta go...

radioeng2 - I think your on to something. I, luckily, recovered the original fuse out of my jacket that I wore to Radio Shack. I looked at the blister pack the fuses came in, and they were fast fuses. Anyway, the original was 125v-4a, NEWGGC. The RS was 125v-4a BussGMA. I'm going to look for the match to the originals. Maybe I'm lucky and it is this simple. Thanks for the help.

Hi 70chevelle,
Look at the solder joints of all the transistors that mount to the heat sink. I'm going to bet you find some broken. Often the bias transistor will be intermittent and can cause this fault. It can also cause the outputs to fail short circuited.

This was a very common fault under warranty, we did tons of them.

-Chris

anatech - I'll take a look this evening. I did get some slow blow 4a fuses and tried at lunch time. It didn't blow the fuse, but I could hear the receiver going in and out of protect mode. (A clicking noise on the right side) It's probably going to be off to the repair shop.

Radioeng2 - It must be switch mode. It has a decent sized transformer, but no large caps.

70chevelle,

I have to agree with Anatech that this is most likely a bias issue. When I emailed you on Sunday I thought the bias was a possibility only because this happened on another 732 that came across my shop. The fact that Anatech has seen this on other 732's makes me really think this is the problem. The good news is the 732 has an access panel below that curcuit board so you can check the solder joints easily. Look at the joints around the C3419 transistor that is mounted next to the output transistors.
I believe the original fuse is a 4 amp fast blow, the slo-blo may damage the output transistors if you have a bias problem and the fuse outlasts the SOA on the outputs (it may also damage the outputs even with the correct fuse).
The power supply is a conventional transformer type in this receiver, switchmode power supply is not used in this model.

I see Anatech just joined the Carver Forum, that guy ran the biggest Carver repair center in Canada and prolly repaired a few thousand Carver products himself.

CL- I found the Transistors (A1265/C3182), the D812 Diode, and the 2 8200uf caps. Nothing is bulged, cracked or burnt, as far as I can tell. I pulled off the bottom access panel and found no broken solder joints. Here are a few pics, if they can help.

Transistor between heatsink & main board (Fuse holder below white tag in front of transformer, only states 4a 125v)


8200uf Caps & D812 Diode


Solder joints of Caps & diode



How would I go about testing these?

Thanks again for the info.

What was the result?
I have a 'pre-on' thing going on. It got progressively worse, till now it does not start up at all anymore...
Thanks for any help. Jordao

OK, now I have taken the bottom access off and noticed 'blackened areas' in a few spots on the circuit board. top side, several half cilyndrical units and some other 'fried' looking components around...
methinks this puppy needs a good resting place...?