1176 Power Supply Repair

[corgan4321]

Hello,

I have an Urei 1176 Rev F on the bench that had a burned power supply. I replaced the failed 30V zener with a part I got on eBay but am only getting 26.5V out of the Zener when fitted into the unit. I tested the zener out of circuit with a 1k resistor in series and a 40V DC supply and I”m only getting the Zener to clamp down 24V. Is this a bad part? In circuit, R84 (the 3W resistor) is dropping 11V - about 125mW through there.

Thank you!

 

[pucho812]

what's going on before R84? What voltage do you get?

 

[corgan4321]

what's going on before R84? What voltage do you get?

I’ve got 37.5v at junction of R84 and C25 which seems correct to me. I tested the power transformer out of circuit and it tested fine.

 

[pucho812]

what do you have after R84?

 

[corgan4321]

After R84 is the junction of the Zener diode which is clamping at 26.5v in circuit

 

[Brian Roth]

Hmmmm....sounds like the Zener is out of spec.

Bri

 

[corgan4321]

that’s my suspicion as well… is there a reputable place to get a replacement part? Does it make sense to replace the part with a modern regulator in 2024?

 

[Brian Roth]

Not too many choices from a web search, but one kinda got my attention. Studio Electronics is quite reputable and shows stock. Owner David Kulka is also a long time member of GDIY.

https://studioelectronics.biz/sunshop/index.php?l=product_detail&p=1148

And UREI changed to a 3 terminal regulator design in Rev G. OOPs....I misnamed the schemo shippet below as F!

Bri

 

[gyraf]

Just put a 3.5V zener in series with it?

Note that it would only have to be 3.5/30 = 0.116x the wattage of the whole zener, so you can probably easily get away with a standard 1.3W type

/Jakob E.

 

[corgan4321]

I ended up ordering a diode from Studio Electronics but adding a series Zener is a great backup plan, thanks for the suggestion.

 

[zonkers31]

that’s my suspicion as well… is there a reputable place to get a replacement part? Does it make sense to replace the part with a modern regulator in 2024?

Digikey actualy has them in stock, but someone made a pricing mistake. should be 35$ not 355 if you call them, they would sell it to you for the proper price I would think.

 

[corgan4321]

Digikey actualy has them in stock, but someone made a pricing mistake. should be 35$ not 355 if you call them, they would sell it to you for the proper price I would think.

Hm, even if that were the case it’s more expensive than the Studio Electronics NOS option

 

[corgan4321]

Not too many choices from a web search, but one kinda got my attention. Studio Electronics is quite reputable and shows stock. Owner David Kulka is also a long time member of GDIY.

https://studioelectronics.biz/sunshop/index.php?l=product_detail&p=1148

And UREI changed to a 3 terminal regulator design in Rev G. OOPs....I misnamed the schemo shippet below as F!

Bri

I think at some point in the near future, a small utility PCB designed to replace the 30v Zener rail with the Rev G design would be in order! Seems like that’s a better/safer bet moving forward.

 

[Brian Roth]

I think at some point in the near future, a small utility PCB designed to replace the 30v Zener rail with the Rev G design would be in order! Seems like that’s a better/safer bet moving forward.

One unknown to be determined would be the heat sink required by the 7824. Perhaps someone here has a "G" and can describe the factory heat sink design.

Bri

 

[corgan4321]

I found the attached image of a Rev G. Seems to be minimal heat sinking. Switching the lamps to LED would even further lower consumption on that + rail.

 

[Brian Roth]

The incandescent bulbs in the original design are connected before any of the regulation stuff....IOW they are fed from the "raw" DC voltage.

Bri

 

[corgan4321]

The incandescent bulbs in the original design are connected before any of the regulation stuff.

Bri

Good point

 

[Brian Roth]

Good point

Personally, I have mixed "emotions" about replacing incandescents with LEDs in these applications.

Yes, the LED will last "forever"...no re-lamping in the future. Also less current drain.

Minus side ..the magic color temperature will change. And, the spread of the illumination inside the meter tends to have hot spots.

Plus side...you can have bright blue VU meters <g>.

Bri

 

[Rob Flinn]

Personally, I have mixed "emotions" about replacing incandescents with LEDs in these applications.

Yes, the LED will last "forever"...no re-lamping in the future. Also less current drain.

Minus side ..the magic color temperature will change. And, the spread of the illumination inside the meter tends to have hot spots.

Plus side...you can have bright blue VU meters <g>.

Bri

I sound that using standard yellow, or I also found some Amber let's that looked quite good. I actually had a job removing bright white leds that a previous tech had installed in one of my clients 1178 and 1176 because he hated the light quality. I personally don't understand the led brightness wars that seem to be going on nowadays. I always fit standard old leds. I hate being laser beamed when I look at a led straight on with the modern ones.

 

[Brian Roth]

I sound that using standard yellow, or I also found some Amber let's that looked quite good. I actually had a job removing bright white leds that a previous tech had installed in one of my clients 1178 and 1176 because he hated the light quality. I personally don't understand the led brightness wars that seem to be going on nowadays. I always fit standard old leds. I hate being laser beamed when I look at a led straight on with the modern ones.

Several years ago I found a LED replacement for the famous #47 bayonet bulb and I installed it behind the "jewel" of a friend's Fender (model forgotten) guitar amp. Wow that was bright.

Bri