Avalon U5 need power LED

[BroMarduk]

BTW, if I was not clear (probably not) the LED works fine if I remove it from the board. I think the low voltages in my unit are the cause for this one not lighting. I haven't done the math yet. My bigger problem is getting the voltages to what @pucho812 recommends they should be.

@Khron, after looking at Kingbright's site I have to agree it was a typo. I've done worse, probably today.

 

[pucho812]

These are jumpers for cracked traces from the secondaries of the power transformer into the psu circuit. Let’s confirm continuity for those traces and let’s measure vac using a dmm The power transformer has 2 x secondaries that output about 20vac each but they are wired in series so you should see about 40vac. The two outer pads will give you 40vac. If you go left to right or right to left, the outer pad and the next pad over should show you 20vac. Next pad and last pad will be the other 20vac. The two inner pads are wired to form the series connection.
You also check for 40vac at the two capacitor pads.
Careful when you test, we don’t want to electrocute ourselves or short something out.

 

[BroMarduk]

I have continuity where both of the jumpers are, so looks like a cracked trace from the transformer is not the issue.

The transformer output pads read from (starting from where the blue wire is connected in the photo as a reference)

Outer L - 19.810 VAC
Inner L - 0.000 VAC
Inner R - 0.000 VAC
Outer R - 19.809 VAC

The capacitor pads measure 19.838 VAC and 19.840 VAC.

I then moved to the input side of the transformer, and it measures 120VAC.

I then checked the primary and secondary outputs for shorts, and they seem OK

Secondary 0.012 Ohms
Primary 3.133 Ohms

Still, looks like an issue with the transformer...

 

[noizetoys]

I'll venture a guess and say the "3" got typed by mistake, within that "230mA"; 20mA is indeed a pretty standard rating for 3-5mm LEDs.

Thanks for catching that. I edited the post bo 20mA.

Here is an internal photo and some voltage measurements of the board. If there are any other places I can check, let me know.

I'll reach out to my friend and ask him to take a look at it. Can't promise anything, but it's worth a try.

 

[Khron]

Still, looks like an issue with the transformer...

So if the transformer outputs what it's supposed to, but the resulting DC is nowhere near what it's supposed to be... I'm now really curious how you figure it's still the transformer.

Not like there's a rectifier bridge (or several) and filter caps, either of which could have failed (at least partially), or voltage regulators, between the transformer and the circuitry it powers...

 

[pucho812]

I have continuity where both of the jumpers are, so looks like a cracked trace from the transformer is not the issue.

The transformer output pads read from (starting from where the blue wire is connected in the photo as a reference)

Outer L - 19.810 VAC
Inner L - 0.000 VAC
Inner R - 0.000 VAC
Outer R - 19.809 VAC

The capacitor pads measure 19.838 VAC and 19.840 VAC.

I then moved to the input side of the transformer, and it measures 120VAC.

I then checked the primary and secondary outputs for shorts, and they seem OK

Secondary 0.012 Ohms
Primary 3.133 Ohms

Still, looks like an issue with the transformer...

how are you measuring the capacitor pads? ideally at the two pads as above, you should see around 40VAC. measuring between the two capacitor points, for VAC you only get 20V? then something is not making a connection. i
While it is very very rare, I did have an instance when the inner two pads of the secondaries were not connecting due to a cracked trace and had to jumper the two pads together with a wire. But again very rare. from what you are telling me so far, you are only getting half of what is need. It's possible the transformer has a fault, but unlikely it has one.

So lets confirm again, on the secondaries which are the pads at the bottom of the transformer, furthest away from the 115/230 mains switch.

Outer L + Inner L should show you 19.810 VAC
Outer R + Inner R should show you 19.810 VAC

Outer L and Outer R should show you 40VAC
if you don't have that, then I would confirm there is continuity between the two inner pads.

It rare for a transformer to give up the ghost but that doesn't eliminate it.

 

[BroMarduk]

Man, it's been a long day. The transformer voltage is fine. I was just measuring wrong as I had a different picture in my head. Boy a schematic would be nice for this unit....anyway, thanks @Khron and @pucho812 for pointing that out.

Since the transformer is transforming correctly any other places to test between the resistor test points and here?

 

[Khron]

Long story short, the power supply section can't be dramatically different from this:

https://sound-au.com/project05b.htm

 

[Ike Zimbel]

I have continuity where both of the jumpers are, so looks like a cracked trace from the transformer is not the issue.

The transformer output pads read from (starting from where the blue wire is connected in the photo as a reference)

Outer L - 19.810 VAC
Inner L - 0.000 VAC
Inner R - 0.000 VAC
Outer R - 19.809 VAC

The capacitor pads measure 19.838 VAC and 19.840 VAC.

I then moved to the input side of the transformer, and it measures 120VAC.

I then checked the primary and secondary outputs for shorts, and they seem OK

Secondary 0.012 Ohms
Primary 3.133 Ohms

Still, looks like an issue with the transformer...

You don't show a measurement for the 40vac across the outer pads, so Pucho's theory that the two inner pads may not be making contact is still very valid, and would explain your issues. I would strap a jumper across them just to rule that out totally.

 

[BroMarduk]

@Ike Zimbel, I did not measure both outer pads at once before. Measuring Outer L and Outer R is 39.177 volts.

 

[Ike Zimbel]

@Ike Zimbel, I did not measure both outer pads at once before. Measuring Outer L and Outer R is 39.177 volts.

Well, that rules out the bad jumper theory. As others have said, that points to the rest of the power supply.

 

[BroMarduk]

@Khron Thanks for that article. It explains a lot and the U5 does have very similar power supply circuitry.

I'll see check the voltage regulator supplies, outputs and caps next I guess.

 

[pucho812]

I have more test point. I will get them later when I get in

 

[pucho812]

o.k. lets measure the top and bottom of r15 and r9 for VDC. on one side you should see around +/-32VDC and at the other side of the resistors will see about your +/-15VDC. if you see around _+/-30VDC you are o.k. lets also confirm you are getting about 6V across the zeiner diodes zd1 and zd3. You can also check for 15VDC across zener diodes zd2 and zd4. from there we can start recapping the PSU. the 2200/63V filter caps usually don't go, but the 100uF and 10uF vishay's can go.
in the PSU section these are the following lytics. c8, c9, c6,c7,c14,c11,c12,c13 c31, and c45. c12 and c7 are 10uF/63V, the rest are 100uF/50V. do put in the correct values and voltages and pay attention to polarity.

 

[BroMarduk]

@pucho812 Thanks for the test points. C6-C9 and C11-C14 were apparent, but not C31 and C45.

R15 is -14.8 VDC on the high side and -3.9 VDC on the low side
R9 is 21.18 VDC on the high side and 10.17 VDC on the low side

Across the diodes:

ZD1 6.2511 VDC
ZD2 10.177 VDC
ZD3 0.0053 VDC <----
ZD4 3.9772 VDC

If I need to recap, would you use the same 100uF and 10uf Vishay caps or can I use some similar Würth caps that I have on hand or would a different cap affect the sound profile of the U5 in some way?

 

[pucho812]

O.k. so we know the PSU transformer is o.k. we tested and measured within spec. 39.xxVAC is pretty good when your looking for 40VAC. From the looks of things you have a few possible problems. You may have some shorted zeners. You also have some shorted caps. For the PSU specs and what the caps are doing you are fine to use what you have as long as the voltages and capacitance is the same.. I fixed a u5 to have as a sample unit for the shop recently. I redid all the 100uF and 10uF with panasonics. They worked real well except their spacing on their legs is a little wider then the vishay's, no biggie there, it's not enough to be a problem.

the zd2 and zd4 are 15V zeners part number 1n4744A
zd1 and z3 are 6.2 Zeners part number 1N753A

As for c31 and c45, without looking at a PCB in front of me, they should be to the left of the power section. So if you were to run a line down the middle on the unit and say the power transformer is on the upper right side, below that is the PSU section, and below that is where you run the wiring for the tone bank and so forth. you are looking to the left of that middle line, I want to say at the top/rear of the unit of the PCB. But I'll confirm that tonight.

 

[BroMarduk]

Tested the Zener's out of circuit and they tested fine so then I went ahead and replaced the 100µF and 10µF capacitors with the Würth caps I had for the 10 power side capacitors (C6, C7, C8, C9, C11, C12, C13, C14, C31, and C45) and THAT brought the voltages back to where they should be at all the resistor and IC test points. After I got the voltages back to normal, I replaced the rest of the caps since a bunch of the ones on the power supply side were toast, so it seemed the safer bet.

The power LED is on, distortion/noise is gone, the boost works, and tone seems back to the way it sounded before, so I'm calling this done.

Thanks @pucho812 for the advice and test points (and all the other advice from others). I even learned something.

Capacitors used:

2x Würth WCAP-AT65 860020772010 10µF 63V
18x Würth WCAP-ATLI 860080775013 100µF 63V


Photo is the recapped U5.

 

[amplexus]

Tested the Zener's out of circuit and they tested fine so then I went ahead and replaced the 100µF and 10µF capacitors with the Würth caps I had for the 10 power side capacitors (C6, C7, C8, C9, C11, C12, C13, C14, C31, and C45) and THAT brought the voltages back to where they should be at all the resistor and IC test points. After I got the voltages back to normal, I replaced the rest of the caps since a bunch of the ones on the power supply side were toast, so it seemed the safer bet.

The power LED is on, distortion/noise is gone, the boost works, and tone seems back to the way it sounded before, so I'm calling this done.

Thanks @pucho812 for the advice and test points (and all the other advice from others). I even learned something.

Capacitors used:

2x Würth WCAP-AT65 860020772010 10µF 63V
18x Würth WCAP-ATLI 860080775013 100µF 63V

View attachment 86891
Photo is the recapped U5.

Nice work!

This thread has got me thinkin it’s time I should pull mine from the studio floor and recap it!

 

[pucho812]

Tested the Zener's out of circuit and they tested fine so then I went ahead and replaced the 100µF and 10µF capacitors with the Würth caps I had for the 10 power side capacitors (C6, C7, C8, C9, C11, C12, C13, C14, C31, and C45) and THAT brought the voltages back to where they should be at all the resistor and IC test points. After I got the voltages back to normal, I replaced the rest of the caps since a bunch of the ones on the power supply side were toast, so it seemed the safer bet.

The power LED is on, distortion/noise is gone, the boost works, and tone seems back to the way it sounded before, so I'm calling this done.

Thanks @pucho812 for the advice and test points (and all the other advice from others). I even learned something.

Capacitors used:

2x Würth WCAP-AT65 860020772010 10µF 63V
18x Würth WCAP-ATLI 860080775013 100µF 63V

View attachment 86891
Photo is the recapped U5.

Nice… great job

 

[suadevices]

Hi! I just wanted to share with you my experience repairing this piece of equipment. From one day to another it stopped working. I am in EU so the mains switch was set at 230V. I switched it to 115V and conected it to a 115V autotransformer and it worked! So I guessed that one of the primary windings was open. Once I put a new transformer, the unit didn't turn on. Well, some audio was passing trough but the power led wasn't lit. So, as Pucho812 said:

transformer weight can crack traces and cause connection issues from the secondaries into the psu and recaps

That was it. Once I secured the connections, the unit was working again!