Urei LA22 Repair Questions

[Siegfried Meier]

Hey guys,

I'm repairing a broken LA22.  Apparently some screws fell onto the PCB while it was on, and the top was panel was removed (it wasn't me, trust me...).  A few parts burned out and I replaced them, thinking this was all there was to it.  However, I can't get any voltage into the unit at all, and it appears that there's nothing getting past the toroid trafo.  Is it possible that something like this could have burnt out the transformer?  I checked the fuse, and it appears to be ok.  It's a funny fuse holder where you have to line up which voltage you want the unit set to, with a small arrow to line up.  I do believe I've got it set correctly.

Just wondering if anyone has any advice on what to look for, and if in fact a toroid can blow from something like this - I have never seen something like that, so just curious.

Thanks!
Sig

 

[nielsk]

A good place to begin is to measure the AC voltage on the primary and secondary of the power transformer. If you have voltage at the primary and not the secondary, disconnect the secondary and see if it reads correct voltage. If so the issue is probably past the transformer. If not you have a bad transformer.

 

[Siegfried Meier]

You're talking the output of the trafo, correct?  I get nothing across any of the leads.  I'm wondering if the power switch was fried then.  Have to pull more of the boards apart, it's an annoyingly put together piece of gear...

 

[Biasrocks]

Thermal fuse in the torroid?

Regards,
Mark

 

[Siegfried Meier]

Thermal fuse?  I've never heard of this...

Here's a pic.  How would I know if it has this, and can it be replaced?  Is it possible that something as above mentioned could cause this?

Thanks,
Sig

 

[desol]

It's not too tough man...test the 120v coming into the unit through the power switch into the transformer. If it's good up to and including the primary of the transformer...and there's nothing at the secondary...your transformers toast.

 

[Biasrocks]

Thermal fuse?  I've never heard of this...

As others have said, check for 120 on the primary side, disconnect the transformer from the circuit board and look for proper voltages on the secondary.

From http://sound.westhost.com/xfmr.htm#s6-safety

Many power transformers are fitted with an internal "once only" thermal fuse that will become open circuit in the event that a preset temperature is exceeded. This temperature is chosen to be the maximum safe temperature of the windings before the insulation melts or breaks down, so in the event of a fault, the thermal fuse will open before the insulation is damaged and the component becomes potentially dangerous. It also helps to prevent the risk of fire (and no, this is not intended to be humorous - a friend of mine had his house burned to the ground because of a faulty power transformer in a video recorder - as determined by the fire investigators. True story!). See Figure 6.1 (below) as an example of how bad things can get if the transformer is not protected.

Once the thermal fuse opens, the transformer must be discarded, as it is usually not possible to gain access to the fuse for replacement. This is not as silly as it may sound, since the thermal effects on the insulation cannot be predicted, and the transformer may be unsafe if it were still able to be used.

Regards,
Mark

 

[Siegfried Meier]

Toroid appears to be just fine.  There are test points marked U+ and U- and I get nothing there.  I've pulled the 2200uF caps and they appear to be fine - resistance slowly increases when metered out of circuit.

I haven't pulled the regulators, but they appear to look fine and I've checked across pins and get resistance, so I'm assuming those are ok as well.

Where else can I look?  This thing is driving me nuts...  :-\

 

[PRR]

Rectifiers.

 

[Siegfried Meier]

Thanks PRR.  You're referring to the four 4003 diodes in the bridge then, correct?  I wondered about these but they "look" fine, and I see some resistance when I measure across them in circuit.  I've usually been able to see that they're blown, so I'm just curious.  Perhaps I will just pull them anyway then?

Thanks,
Sig

 

[Siegfried Meier]

Removed the bridge rectifier diodes, they checked out fine with a DMM, but I replaced them anyway.  Absolutely nothing.

However, there's an F1 and an F2 in there.  Small little yellow things that I believe are soldered in fuses.  I've seen these before in a Dorrough meter - blew it when the DC adapter was plugged in reverse (the older ones allow that).  I have a feeling these are the same deal here. 

So, the questions is, how do I figure out what the value for these are?  I don't see any mention of them in the manual...at least that I can tell.

EDIT - BUSS - 2A.  That stands for Bussman, which is now (or might have always been) Cooper Bussman.  I found some different looking 2A Bussman fuses at Mouser, they should work fine I assume.

Thanks!

 

[PRR]

> Bussman, which is now (or might have always been) Cooper Bussman

They were different companies, once, but as you say everybody has merged.

 

[Siegfried Meier]

Just a quick question, how often do these types of fuses show up in gear?  I've never built a piece of gear that has them, and out of all the things I've repaired, I've only seen them twice now.  What is the purpose exactly, that can't be controlled with a regular IEC inlet fuse?  I mean I suppose double fusing is a good thing, and these are before the bridge rectifier in the circuit, so they're the first thing post transformer, meaning they're handling AC voltage at that point.  Are they protecting the circuit should extra voltage somehow get out of the trafo?  Or are they protecting the trafo in cases such as this, when something shorts out the circuit and it backfeeds somehow?  Seems kinda pointless I suppose, so I'm just curious here.

Thanks,
Sig

 

[BYacey]

Usually those pico fuses would be found after the rectifiers and filter caps, or even downstream from the regulators. If they measure 0 ohms, don't waste a lot of time looking there.  If you don't measure any AC voltage at the secondary windings of the transformer, you've getting ahead of yourself checking caps, secondary fuses, etc.

Unplug the unit from the AC power,  set the power switch to the on position, and measure the resistance between the two blades on the AC line cord.  If it reads infinite, start checking your main fuse, fuse socket, power switch or the primary winding on the transformer. If all is good up to this point, it should read somewhere between 1 and  100 ohms depending on the power rating of the transformer.

 

[Siegfried Meier]

According to the schematic, the fuses (F1, F2) are before the bridge rectifier and just after the power transformer.

The fuses have no continuity, so my guess is they're burnt out.  Are you suggesting something different with your post?

 

[kazper]

Sounds like the fuse popped.  Replace and retest, if your unsure of the value size to the VA of the transformer.

 

[Siegfried Meier]

So, I replaced the 2 small yellow solder in fuses.  Unit worked great for a few minutes.  Then the left side started to distort (the one I replaced the burnt out components on) - channel 2 sounded great.  I noticed the transistors were getting hot on chn 1, so I shut it down to inspect.  Now it won't turn on again.  Checked the solder fuses, again burnt out.

WTF?  Is it possible that the newer replacement transistors I put in just won't work in the circuit?  What would make them overheat like that?  This unit is seriously giving me heartburn.  It's about time that it gets retired to a junk pile...

Any help much appreciated before I give up on this.
Thanks,
Sig

 

[1954U1]

If you have 1 ch working and 1 ch not working, thats not bad at all.

I'd start to measure onboard every single component of the 2 channels,
and writing all the differences.

 

[Siegfried Meier]

Ok, so an update and some theory questions for y'all.

I socketed those 2 fuses, and bought new 2A Slo Blo fuses.  Tested the unit again, no transistor overheating, solid sounding compression for over 2 hours running now... 

So, can someone tell me why the other, regular 2A fuses did not work, and caused the transistors on the LEFT channel only to overheat.  Had one blown, and that was the reason?  I don't think they were Slo Blo, but I don't understand either way what caused this...

I'm happy the unit is now fully working again, but confused and baffled all the same...

Thanks,
Sig