Crest CPX1500

[5inc]

I have this dead crest amp, a friend gave it to me to play around years ago. Apparently it got blasted by a surge in a party and the big caps popped, a few resistors burned up, some diodes died and a pair of power transistors got shorted.

I begun experimenting on it before I knew about the use of variacs or low-voltage operation for repair, and plugged it straight into mains, big mistake. That tought me a valuable lesson in it's day, but also made the repair go 2steps back. Now I'm back on the move, a bit more experienced and I want to get to the bottom.

Allthough after replacing everything I suspected was failing, I still face a shorted amplifier.

I don't have a variac yet, so I built 2plugs in series to plug in a 2000w light bulb and the amp for testing purposes. The bulb lights up shy and then shines brightly. Protect lights stay on, no fan.
Tried isolating the problem by disconnecting everything possible but it narrows down to power supply, at the least. And I fear the transformer may be shorted.

Could I test that by simply disconnecting and isolating the secondary leads and plugging it in to check for short?

thanks!

[Khron]

Re: Crest CPX1500

Is that a linear (50Hz, big iron) transformer?

--- EDIT ---

Yes, it is.

Step one: search for service manual -
Step two: yes, disconnecting the secondary of the transformer from the rest of the circuit should be enough to see if it puts out the voltage it's supposed to.

Given that the CPX 1500 has 100V main caps, i'd expect something along the lines of maybe 80V DC across each, which would require 2x 55-60VAC at the input of the bridge rectifier (ie. measured between TX13 and TX12/TX14, and then TX16 and TX15/TX17 respectively), or 110-120VAC (between TX12 and TX14 / TX15 and TX17).

Obviously, you'll want to measure that with alligator clips connected to the transformer leads (and NOT holding them with your hands )

You'll wanna take a look at that crowbar(?) protection circuit that's between the speaker output and ground. There's a good possibility that's (also) toast, and what's keeping the protection tripped.

[Longbow]

Re: Shorted Crest CPX1500 with protect leds

Could I test that by simply disconnecting and isolating the secondary leads and plugging it in to check for short?

Yes. But probably not necessary. Check your diode bridges in the secondary.

[5inc]

Re: Crest CPX1500

My original question was aimed towards getting a burnt transformer diagnosis, why?
because I believe I get a short circuit, since the 2000w lamps shines bright. It has burned some component in front of my nose before, so I'm not so comfortable leaving it on for longer periods, can't read much in that time.

I just checked the the bridge rectifiers and diode bridge and they all seem ok.
First I'll have a look at the schematics again to see that crowbar protection circuit you mentioned Khron, but again, my gut tells me it might be the transformer (really hope not), if I don't see anything there I'll go ahead and disconnect it.

[budm]

Re: Crest CPX1500

Is there anyway you can disconnect the secondary side winding from the the load so all you will have is the primary winding to test and see if the lamp will still light up brightly?
Pictures of what we are dealing with?

[5inc]

Re: Crest CPX1500

okay, we have a picture of the amp in question. I'm in the process of disconnecting the secondary windings.

https://s27.postimg.org/kd8h9kiyb/20170126_231902.jpg

yes budm, I think what you mean is exactly what I was asking in the begginning.

[5inc]

Re: Crest CPX1500

hurray, just disconnected secondary leads, plugged the amp in and the bulb doesn't shine, at all. Tested for voltage on the back of the mains socket and I get a normal 260vac, that makes me really happy.

Since I have no idea where to attack now, I will connect one of the 3 secondary windings, and check for short untill I hit the defective one. If it shows up.

But that will have to be tomorrow, I have a hard day ahead.

thanks lads

[5inc]

Re: Crest CPX1500

I will also check the voltages before I start connecting windings too.

[Khron]

Re: Crest CPX1500

Next step would be, with or without the transformer reconnected, to check for a short-circuit between the speaker output posts.

[5inc]

Re: Crest CPX1500

ok people, I just checked voltages on secondary and I have a steady 30-0-30vac through oranges-white, and 80-0-80vac on blues-white and reds-yellow.

No short between speaker outputs, so I will now start by connecting an 80v line, then the other and then the 30v one to see which ones are shorted.

[5inc]

Re: Crest CPX1500

I isolated the problem down to the 30v line.
Gonna have a look around for shorts and where it goes.

testing was made with only colour power leads connected, no link connectors connected to main boards.

[5inc]

Re: Crest CPX1500

okay khron, I didn't knew what the hell you were talking about with the crowbar protection circuit, googled it and I have found this CR320 in the schematic, read resistance through it and I get 5.3ohms, and CR321 reads 1.2ohm. Are those zener diodes?

https://s30.postimg.org/83ola6kgh/20170127_231739.jpg

thanks!

[petehall347]

Re: Crest CPX1500

if you remove the wires that come from the +-15v power supply and also the 28v take off that connects to c401 you can fire it up to test that regulated supply is working .. if it is you could then check for short circuit from ground to each of the wires .
wires should go to c403 c404 .
cr320 is a 22v 1/2w zener
c321 is a 30v 1/2w zener .

you could also pull the 28v take off first ... this is power supply for fans etc and where those diodes reside

[5inc]

Re: Crest CPX1500

right... i think I get it. I have one thing to say though, there is no wire that connects to c401 or c403 or c404, it's all on lines the board.

but taking r330 and r338 out might take down the 28v input... I'll try that

[petehall347]

Re: Crest CPX1500

yes it should work .. plus it should keep at least some of the thermal circuits intact if i am looking at it correctly .

[5inc]

Re: Crest CPX1500

no luck still get a short, if I also disconnect one lead of c401 +28v dies?

[5inc]

Re: Crest CPX1500

sorry, I meant c402 -15v dies?

[petehall347]

Re: Crest CPX1500

disconnecting c402 is no use .. same with c401 ..
+28v is tapped off before the +-15v regulators . so short is before or after the regulators ..
test rectifier diodes cr400 401 402 403 .
also the other diodes in that circuit .

[petehall347]

Re: Crest CPX1500

i suppose you could check input and output sides of the regulators for short to ground .
u400 u401 ..
or if its easy to remove them do that and see what happens .

[5inc]

Re: Crest CPX1500

ok, I took the regulators out, plugged in and no short. While I was reconnecting them i got a spark between 2legs of +15v regulator(I luckilly have a couple lying around so I just replaced it), from the main capacitors i guess.

When I plug everything back on I still get the short. That's probably because I still haven't removed the bad zeners, so I'll get down to that now. I don't know if I should test it without anything in there. Should I?