Carver CM-1090 - Power relay clicking on & off, then fuse blown after 20-30 seconds of power cycling.

[beetle1303]

I gotten a Carver CM-1090 amplifier from a friend who has kept it in storage for decades. Opened it up, thick layer of dust cumulated everywhere. Vacuumed it semi-cleaned, and blown off as much as possible the remaining dust. This is a 220V version.

On initial powered up with nothing connected - it works with all the lights and button indicator lit up as pressed, etc. I then powered off, connected a CD player into the CD RCA input and powered it on. It seemed to work for a short while and when I turned the volume up and down to see the the VUs works, the power started to clicked off and on rapidly. Sometimes it would stayed off a few 5-10 seconds, then powered on, then repeating the clicking off and on cycles. I powered it down, wait for a minute or so, then powered it up again - this cycles went on for the 20-30 seconds, then a quick flashlight on the fuse, and everything stopped. Checked the fuse and it is opened. When I power down, then on again with the blown fuse, it clicked but nothing else happened.

Quick visual check find nothing obviously burnt, nor any burning smell. But noticed the the big capacitors are slightly bulging.

Any clue on what was happening? Could it be the CD input or these capacitors be the cause of the power clicking off and on?
And what would be the next steps with the diagnostics?
Thank you very much.

[stj]

looking at the rust and oxidising on the power section,
the first thing i would do is put contact cleaner in the voltage selection switches and work it in!
then consider doing it with every other switch and pot - maybe even the relay if it's not sealed

[beetle1303]

looking at the rust and oxidising on the power section,
the first thing i would do is put contact cleaner in the voltage selection switches and work it in!
then consider doing it with every other switch and pot - maybe even the relay if it's not sealed

OK, will start doing them ongoing basis.
While waiting for replacement fuse and capacitors, I wanted to put in a 5A (original is 4A) fuse along with a dim bulb tester to power up and see if the power relay still clicking off and on without the CD player connected.

Is it safe to do?

[beetle1303]

I have recently gotten some new parts to replace the 2x biggest capacitors on the mainboard and the 3 small ones on the power-board, and added the 4A fuse. I have also sprayed contact cleaner onto the 2 voltage switches, and worked them a bit by sliding the switches left and right. The relay is seal so nothing done there.
I added a series bulb on the live-cable into the power supply in.
On switching on the power supply, I measure some voltages on the "control board" and seems the have the voltages there. When I press the power button on the front panel - the relay started clicking on of off continuously, and so did the series bulk flickering on and off correspondingly.

Presumably a short-circuit somewhere. I've taken a quick continuity check on most of the diodes, resistors, transistors and capacitors, but nothing obviously shorted.

What/where would be the most likely components to further more thorough check?

[petehall347]

http://www.hifiengine.com/manual_lib.../cm-1090.shtml

[petehall347]

try a higher wattage bulb in your dbt limiter

[beetle1303]

try a higher wattage bulb in your dbt limiter

Will do - is 100W bulb sufficient? If not, how high must it be?

Meanwhile - I done 1 more round of checking and noticed the power regulator, C78M10, has really loose joint, all 3 pins. So I reflowed them, and all the other 3 ICs in the same row. Now, the power relay (and bulb) just clicked once then off, instead of continuously.

Does it mean anything??

[beetle1303]

try a higher wattage bulb in your dbt limiter

Wow - knowledge and/or experience is truly power - after changed to a 100w bulb, it powers right up!!
I presumed the bulb lighted up at around 10-20% wattage level at unloaded condition is normal, and safe to remove the DBT?

I did some googling and reckon that some amplifier has protection circuitry to shutdown in under/over current/voltage conditions - presumably this is the case?

And to petehall347 THANK YOU VERY MUCH!!

[petehall347]

i might use an ammeter and plug it into the mains do a search for plug in power monitor

[beetle1303]

i might use an ammeter and plug it into the mains do a search for plug in power monitor

You meant something like this - it draws 0.2A, so quite safe to plug directly to the wall without the DBT?

[petehall347]

give it a go with clamp meter and into full power

[beetle1303]

give it a go with clamp meter and into full power

Sound and Safe into full power - 0.71A.

Just curious how the math works - 100w / 240v = 0.417A, but it shown 0.2A with 100w bulb, and 0.71A without it?