Rotel Caps Questions and help request.

[LITEZONER]

I am trying to repair an amplifier that keeps blowing the mains fuses, So far i have identified 7 Bad main transistors and 10 Bad audio power transistors, the really confusing part is that it blows both side fuses and they seem independent of each other, so I am having a hard time determining what they share and I am starting with the main caps. They are Rotel 8200uF 50VDC, I do not own an esr meter yet so I pulled them out of the loop, all have tested the same, but I'm not sure good or bad, I set my ohm meter to 20K ohms and it starts out low and rises to 1980 or so and then goes to OPEN, So I tried 200K and get the exact same results, almost to 20 then OPEN CIRCUIT, IS This NORMAL? I know it is a rookie question, but I'm a rookie so it's okay, If this is normal My nest step is to check the two bridge rectifiers, I checked them in circuit and they seem fine, Did not pull them out to check, BTW Since I found bad main and audio transistors I have replaced them all. Any advice or help is very much appreciated, Thank you

[LITEZONER]

Re: Rotel Caps Questions and help request.

It somehow dawned on mr to double check the transformer, it is marked 32V 0V 32V when I take readings I get 37.4v 0V 37.4V Could this be the cause?

[selldoor]

Re: Rotel Caps Questions and help request.

can you post a pic of the back of the board - are the brown semicircles old
glue or have the caps leaked. There are 4 fuses does it blow all 4?
If you have replaced the transistors were you sure to use isolated ones to replace isolated ones?

[selldoor]

Re: Rotel Caps Questions and help request.

It somehow dawned on mr to double check the transformer, it is marked 32V 0V 32V when I take readings I get 37.4v 0V 37.4V Could this be the cause?

That is probably ok with no load

[budm]

Re: Rotel Caps Questions and help request.

The DC voltages on the caps should be about +45 VDC on the Positive filter cap, and _45VDC on the Negative filter caps (it is bi-polar power supply). No load AC reading will be higher on the transformer out put, it should be 32VAC at full load current spec.

[LITEZONER]

Re: Rotel Caps Questions and help request.

Yes all are isolated from the heat sink if that is what you mean selldoor, also yes it is old glue, I should have mentioned that, sorry. and it only blows the two mains not the output fuses. and I don't know what a bipolar transformer is, and I cannot measure the voltages on the caps in circuit before the fuse blows immediately. would you like a picture of the back of the board still?

[LITEZONER]

Re: Rotel Caps Questions and help request.

I have checked, and double checked for any shorts between welds, any isolation issues. etc..

[LITEZONER]

Re: Rotel Caps Questions and help request.

I guess I am going to pull out the rectifiers to check those and the ceramic caps under them, what do you guys think?

[budm]

Re: Rotel Caps Questions and help request.

You need to make this so you will not keep blowing up your new parts.
https://www.badcaps.net/forum/showpo...3&postcount=12
use 75Watts lamp, if it lights up and stay fully lit, you still have shorted circuit. This will help you from blowing the fuses.
What is the make and model of this power amp?
This direct couple amplfiers, when one transistor failed, it will take out chain of transistors with it.

[LITEZONER]

Re: Rotel Caps Questions and help request.

Rotel RB970BX... NICE! I can build one of those.

[LITEZONER]

Re: Rotel Caps Questions and help request.

so if I have this right the output from the transformer goes into one outlet, then out of the other outlet to the board, is this correct?

[selldoor]

Re: Rotel Caps Questions and help request.

In the middle pic on the last block of three top right looks like a blob of solder? is it and has it bridged the two traces?
Seem to have been a bit generous with the solder,have you checked the transistor legs for shorts?

[ben7]

Re: Rotel Caps Questions and help request.

so if I have this right the output from the transformer goes into one outlet, then out of the other outlet to the board, is this correct?

not quite,

You should have the ballast lamp on the primary side.
Like this: plug -> ballast lamp -> transformer -> circuitboard

[budm]

Re: Rotel Caps Questions and help request.

You plug your amplifier AC plug into the DUPLEX of the box I show, the power cord of the box is plugged into an AC OUTLET. What the box does is putting the lamp in series with the primary winding of the power amp transformer.

[LITEZONER]

Re: Rotel Caps Questions and help request.

Okay, light bulb in the primary circuit, Thank you for that tip. I think I did short out a few more IC's %$@# very frustrating, but I have some time each week, i will have to desolder again and check out all the PNP and NPN again, So what are the chances I took out my new transistors as well? My horse sense tell me if the fuse blew it was to save the stuff downstream

[LITEZONER]

Re: Rotel Caps Questions and help request.

so this thing i am going to build, since this is my newest freaking addiction, it acts kind of like an AC resistor or limiter? is that correct? what happens instead of the fuse blowing? I think I understand it would take some currant out of the primary, does this also have the same effect on the secondary, like a Variac?

[selldoor]

Re: Rotel Caps Questions and help request.

"My horse sense tell me if the fuse blew it was to save the stuff downstream"

Tends not to work like that - fuse is to prevent faulty machine from damaging mains wiring.
Whilst you said you had checked for shorts but now may not be so sure perhaps you can have a look at the joints I have marked on your three pics.

[LITEZONER]

Re: Rotel Caps Questions and help request.

I think I just answered my own question, anyway I think I will take a break for the weekend, I think I'll go find some bare wires and a mud puddle to play in.

[LITEZONER]

Re: Rotel Caps Questions and help request.

thanks selldoor, Ill have a close look.

[LITEZONER]

Re: Rotel Caps Questions and help request.

hey what if i were to put a dimmer in line with the light bulb ballast, would it be a more useful tool when it comes to bringing up older stuff slowly?