Unicode UB252 Solid State Bass amp help

[DrewMeyer]

So I bought this old Unicode (Univox?) Bass amp head which doesn't power on. I opened it up and there are some wires from the board which are not connected to anything and neither the power cable now the power switch are  wired to anything. Unfortunately there is little to no information on these amps so I'm looking to see if anyone can help me figure out where all the wires go to get it working again. Can provide pictures and any other information as necessary. Thanks!

[Enzo]

ANy amp must have mains wiring to operate.  The power cord plugs into the wall, and mains power comes up it to the amp chassis.  That is wired to the power switch and fuse and to the primary of the power transformer.   All that will have to be established.   You will have to determine which wires are which on the power transformer.

We will want to test the transformer before going too far into this.  A shorted transformer may put the brakes on in your project.

One might want to think about all the wires were disconnected for a reason.

http://reverb.com/item/10206-univox-ub252-1973-black
Is it that?

Before doing mu reconstruction, I would check the output transistors for shorted condition, and same at the main rectifers.


Here:

[DrewMeyer]

ANy amp must have mains wiring to operate.  The power cord plugs into the wall, and mains power comes up it to the amp chassis.  That is wired to the power switch and fuse and to the primary of the power transformer.   All that will have to be established.   You will have to determine which wires are which on the power transformer.

We will want to test the transformer before going too far into this.  A shorted transformer may put the brakes on in your project.

One might want to think about all the wires were disconnected for a reason.

http://reverb.com/item/10206-univox-ub252-1973-black
Is it that?

Before doing mu reconstruction, I would check the output transistors for shorted condition, and same at the main rectifers.


Here:

Wow man, thanks! That's the amp alright. How would I check the transformer, with a voltmeter? And by rectifier are your talking about the diodes or something else?

[Enzo]

The mains wiring is pretty simple, the secondary side of the transformer feeds the bridge rectifier diodes, yes, D2,3,4,5.


I would use an ohm meter to see if the primary and secondary are shorted together.   It is usually not easy to use resistance to detect shorted turns.   Then you can make a simple tester.  Go to this web site, and under tech tips find the transformer shorted turns tester, and read about it.

http://www.geofex.com/

[DrewMeyer]

Ok, cool. I'll try that soon. One thing to note, I was comparing the schematic to my amp and noticed my amp does not have Q5 or Q6. I'm thinking the schematic was a different version of the amp? But anyways, how can I go forth if I'm trying to repair my amp with a schematic that's not entirely accurate.
EDIT: Also, just curious, but my amp doesn't have a fuse that I can find. Should it have one?

[Roly]

$300 is a lot for a cab and "the head doesn't power on".   

Like most of the other techs here I consider having a circuit to be a bit of a luxury.  It can certainly be useful, just like a map might not be an entirely accurate representation of on-the-ground reality, so a circuit is a general idea.

There's a good chance that what you have inherited here is an amp that has had an output stage blow up in the hands of somebody who didn't know how to fix it, and who couldn't afford to have it fixed (or couldn't be bothered finding a true quote).  If this is the situation there is a good chance of being able to put this back in working order.

Some crisp internal pics will tell us a lot more.   :tu:

[Enzo]

Um... Q5 Q6 are the output transistors, if you don;t have those, you don;t have an amplifier.   It is possible they have been removed, and might have been mounted on the chassis.   But the amp must have them.

[J M Fahey]

Agree and add: it's a nice simple amplifier, I would treat it as an "amp kit", with some parts already mounted, and where I have to complete the rest.
We need those pictures

[DrewMeyer]

Um... Q5 Q6 are the output transistors, if you don;t have those, you don;t have an amplifier.   It is possible they have been removed, and might have been mounted on the chassis.   But the amp must have them.

That's what's weird. There's no spot on the board for them. I guess like you're saying they could have been mounted elsewhere, but there just isn't any place I can see where they would have been. I'll definitely put up some pictures ASAP. What kinds of things should I get pictures of? Top and bottom of the board and maybe the mains wiring?

[J M Fahey]

Start with a full chassis one, taken perpendicular (straight from above) , and then we'll ask "please a close up from here or there"
The power transistors might have been bolted straight to the chassis, so all you'd find would be a few "unused" holes.

[DrewMeyer]

Start with a full chassis one, taken perpendicular (straight from above) , and then we'll ask "please a close up from here or there"
The power transistors might have been bolted straight to the chassis, so all you'd find would be a few "unused" holes.

Ok great, I'll do that. And no, no unused holes other than two which allow the nuts holding the handle on to fit into the chassis.