[BUILD] fripholm's TG1 Zener Limiter boards - support thread

[Potato Cakes]

How hot are your regulators getting? Are you using big enough heat sinks?

Thanks!

Paul

 

[fripholm]

Does this only happen with the positive rail? Could you try and measure the DC current draw of the positive rail when this happens?

However, from what I'm reading, the PSU seems to be okay.

To further delimit the possible fault you could try and lift one side of either R31 and R75 - one at a time. The latter is directly between +28V from the PSU and the output stage. If that one is lifted and the voltage drop goes away, the problem most likely is within the output stage. This stage obviously can't work with the lifted resistor but it helps pinpoint the error.

R31 is part of another discrete on-board regulator that provides a +20V rail (derived from the PSU's +28V rail), that the entire compressor stage including sidechain runs on.

There's nothing else that is directly connected to the 28 Volts rail (except for maybe R66 and R64, which I suppose are 100k and 10k respectively?) which would be able to cause a voltage drop like you describe.

 

[brewbacca]

Thanks for the help guys.

There must be a problem with the output circuit because the voltage drop doesn't occur when R75 is lifted (it looks like I already tried this some time ago, but I must have thought there was some other problem). Leaving R75 in the circuit and lifting R31 still resulted in the drop.

R75 is a little scratched up, probably from my previous troubleshooting attempts, so it might be good to replace that regardless.

pictures of output transistors and R75

 

[fripholm]

Firstly, what's the current draw with and without R75 in the circuit?

I see, you're using 3055/2955 transistors for your output stage. Have you tried the circuit with BD139/140?

I've put the additional footprints for 3055 and 2955 on the board to have options, but have never actually tried it with those nor do I know of anyone who has (ANYONE??). They're pretty similar though and theoretically should work but it's worth a try and change these to BD139/140 - if you happen to have them at hand. Or maybe one of them is faulty...

 

[Potato Cakes]

I've put the additional footprints for 3055 and 2955 on the board to have options, but have never actually tried it with those nor do I know of anyone who has (ANYONE??). They're pretty similar though and theoretically should work but it's worth a try and change these to BD139/140 - if you happen to have them at hand. Or maybe one of them is faulty...

My stereo unit that I wrote about earlier in this thread is using 3055's and 2955's with no problems. I double checked to make sure I was using the correct footprints before soldering them in place.

Thanks!

Paul

 

[brewbacca]

I'll check the current draw when I get home from work, but I initially used BD139/140 and switched thinking that it might be the issue (it wasn't lol).

 

[brewbacca]

Silly question: how do I measure the current draw of the circuit?

 

[Ilya]

Silly question: how do I measure the current draw of the circuit?

You can put a DMM in series with either voltage rail right after PSU. DMM will show you current consumption on that rail. Make sure to put your DMM in current mode.

 

[brewbacca]

That makes sense, thanks!

So with R75 the DC current on the +positive rail is around 0.04 amps (although it starts around 0.07 amps and then drops quickly).
Without R75 the DC current is around 0.02 amps.

In both cases the negative rail is around 0.03 amps.

 

[scott2000]

Those bands on the resistors are hard to decipher from that one pic.... maybe test the values of them to be sure if you haven't already?

 

[Ilya]

That makes sense, thanks!

So with R75 the DC current on the +positive rail is around 0.04 amps (although it starts around 0.07 amps and then drops quickly).
Without R75 the DC current is around 0.02 amps.

In both cases the negative rail is around 0.03 amps.

The current seems too low. As if the output stage is not working.

 

[fripholm]

So with R75 the DC current on the +positive rail is around 0.04 amps (although it starts around 0.07 amps and then drops quickly).
Without R75 the DC current is around 0.02 amps.

In both cases the negative rail is around 0.03 amps.

On my stereo unit each channel draws about 60mA from the positive rail (starting from 70mA then dropping quickly). The negative rail is ~30mA - just like yours.

The output stage in a properly working unit draws ~40mA from the positive rail, which means that without R75 your reading of the remaining 20mA is correct.

All of this doesn't explain the voltage drop you're getting. I was expecting excessive current draw - but nope  ???

What are the DC voltages at either end of resistor R3 on the PSU board with R75 in circuit?

 

[brewbacca]

I'm not using the same PSU circuit at this point and there doesn't seem to be a corresponding resistor (the copy of your circuit I perfboarded  is in another piece of gear :/), but checking the positive voltage before it hits the regulator and after the diode bridge is ~36v and I'm getting ~ -44v on the negative rail before the regulator.  I'm getting around 60vac from the transformer.

 

[scott2000]

Got a schematic of the supply your using??

 

[brewbacca]

I don't have a schematic handy, but I landed on a Chinese kit off of ebay.

https://www.ebay.com/p/PCB-Board-for-Lm317-Lm337-Adjustable-Power-Supply-Voltage-Regulator-Bare-Rf4/6006632706?iid=172179466048

 

[fripholm]

Looks pretty standard to me - without the extra filter resistor (which R3 is). 36 Volts on the positive rail should also give enough margin to properly regulate down to 28 at low currents like you've measured. But it's still quite a lot lower than the negative rail... weird 

What's the power rating of your transformer (in VA)?

I'm getting around 60vac from the transformer.

Is this between both AC wires or between AC and center tap/GND ? If it's the latter, it seems quite high...

What happens, if you temporarily adjust the PSU down to like 24 Volts on each rail to give the regulator even more margin? Does it still drop?

 

[brewbacca]

Transformer is 50VA (antek as-0528)

60v is between the two ends (transformer is wired in series) so the former

Adjusting down to 24v it still drops to around 16v.

 

[scott2000]

I'm guessing the reg heat sinks are good enough ???seems like a bit of voltage to drop ....IDK..... you said the original supply acted the same???so it must be something else???

What does R75 measure??? looks like it's 59 ohm from pic???

 

[brewbacca]

R75 is 56 ohms on both boards.

 

[fripholm]

Looking at the photo it seems you are using metal locking washers for the standoffs. Have you made sure they are not piercing any traces on the top and/or bottom layer? The positive rail runs in close proximity to one of the holes.