Amp circuit problem

[lzidaric]

Hello guys,

My turntable stopped working and I got another one, so i thought i could turn the turntable's inbuilt amp to something useful. I've stripped it of its housing and extended the volume and the tone knobs's wiring, so it would be more flexible to fit the new housing i made. After i made the necesary extensions i turned it on to test if it still works. It did for about 20 seconds, then the power ON Led diode went out and with it the entire assembly. After further examination i found out that two fuses went out. They were low amp 0,315's. I really don't want to scrap the amp, it may be old, but still it's useful. I attached four pictures, other three in detail.
What causes fried fuses that haven't occured before? I checked my new wiring and my tampering hasn't changed anything, I simply removed the potentiometers from the circuit boards and put wires in and soldered the wires on knobs in exact order. I havent attached the grounding wire to the housing when the circuit was stripped, but i was running the turntable once also ungrounded because i was testing something.

In the first picture the preamp is on the left, the pcb with the metal frame. On the top is the pcb with the balancers and the tone controls, it also powers the vu meters, which are visible below. The pontentiometers are in the middle with the extended wiring and the powerboard is on the right along with the transformer. Just above the transformer were five fuses of which 2 were fried. I also noticed one other thing, the bulb that illuminates the vu meters still works when i turn it on, and it is the only thing that actually works so I dont think that any components on the pcbs got fried beacuse the fuses actually saved them, so i reckon i won’t need any multimeter testing.

 

[Nigel Goodwin]

In the first picture the preamp is on the left, the pcb with the metal frame.

That's the power amp, not the preamp - the preamp is the board the tone controls attach to.

As fuses are blown I would suggest it's VERY likely that the power amplifier has blown, and this would normally take the fuses out.

Fuses don't really 'save' things, at least not the way you're thinking of - the fuses blow AFTER the amp has died, in order to prevent the mains transformer getting frying, and being a potential fire hazard.

It 'could' be the bridge rectifier? (or something else), but most likely cause (by a long way) is the power amp.

 

[lzidaric]

Is there any way of saving it? The bridge rectifier is located on the powering board i presume? Someone said is should test the components with a multimeter but i really don't know what to look for. I haven't much experience. If i replace the fuses and turn it on it will be the same right? has it got anything in connection with the ground not being connected?
Thanks for the help

 

[Nigel Goodwin]

Try the fuses and see if they blow again - they probably will.

What value are the fuses?, for feeding a small power amp I would expect them to be in the 1A to 3A range?.

 

[lzidaric]

The fuses are 0,315 amp

 

[Nigel Goodwin]

The fuses are 0,315 amp

Those are pretty small, what values are the other fuses?.

 

[lzidaric]

As seen on the powerboard they are from R to L : 0,63, 0,315, 0,315, 2,0, 2,0.
there is another fuse on the poweramp board, it reads 1,6.

 

[Menticol]

Hey Luka! Nice to see you here

Here's a tutorial about how to use the multimeter. Check if there's voltage coming from the power supply
https://afrotechmods.com/tutorials/2011/11/27/multimeter-tutorial/

If is not, you should check the bridge rectifier as Nigel said.

Here's an explanation about what are the diodes and how they work.

 

[Nigel Goodwin]

As seen on the powerboard they are from R to L : 0,63, 0,315, 0,315, 2,0, 2,0.
there is another fuse on the poweramp board, it reads 1,6.

OK, I take it the two white ceramic ones are the 2A ones?, and look like they feed the large bridge rectifier, and the two large electrolytics, and will feed the power amp.

The two missing fuses in the picture appear to feed the small round bridge rectifier?, which is presumably the supply for the preamp, so a likely cause of blowing the fuse will be the small round bridge rectifier, with one or more diodes in it S/C.

 

[unclejed613]

judging by the pictures, this is of late 60's/early 70's vintage, built in europe, and somebody has made modifications to the wiring harness using telephone wire. as fot the fuses that blew, replace the small round bridge rectifier and the electrolytic cap that the bridge feeds.instead of fuses use 47 ohm 1 watt resistors. if everything is ok, it will work with the resistors in place of the fuses. if there's still a short, the resistors will get hot but provide just enough current and voltage to troubleshoot the circuit, this, of course after replacing the bridge and cap....

 

[lzidaric]

If this works you're a genius but there could still be a problem. I went with my gut on it as i always do and placed 2amp fuses instead of the fried ones to see what could be burning ( i know, stupid, but i'm always too eager). What's the chance i messed it up even more?

 

[Menticol]

To test your amplifier connect it with a bulb in series, as shown on the link. If something is shorted the bulb will glow brightly, but the amplifier will not blow up.

https://diyaudioprojects.com/Forum/viewtopic.php?f=5&t=1051