PSU Problems

[spaceludwig]

After hooking up the channel strips I recapped and whose xfrmr's I swapped, I fired up the console to make sure everything was in working order. For now, each channel seems to work. However, I seem to be having a power supply issue, which I`ll describe below.

Around 10 minutes after turning the unit on, the headphones started to hum. I unplugged them and stuck them into each (left and right) +4 unbalanced 1/4 inch output since the program out is separate from the headphone out. Same results, sound was distorted and crapping out. I measured the outputs of the power supply and discovered that the negative half of the 15 and 22 Volt rails were operating at half the voltage it's supposed to, i.e. -7.5 and -10.something volts. I turned the unit off, let it cool off, then powered it back on. Since everything was playing properly without distortion I immediately measured the PSU output and the negative and positive rails were operating at their rated voltages. As soon as the distortion came back, the negative rails again measured at about half of what they were supposed to be putting out. I noticed that I could reduce the distortion by unplugging individual channel strips from the main ribbon cable. The volume from the headphone amp got progressively louder as I unplugged  more units. Eventually, the distortion was eliminated. Both heatsinks on the power supply were very hot.

So my question is, what causes the power supply to get weaker. Is it as the power transistors heat up? The console ran fine at some point, could run the whole night. Are the opamps I substituted (OPA604 for the stock TA7130P) demanding more current? If this is the case, how can I upgrade the PSU, with transistors that have a higher power dissipation or current rating?  Is it possible that the stock power transistors (2SD526 & 2SB596) are burnt or kaput?

In the datasheet of the 2SB596 there is a power derating graph

Does this mean that as the transistor temperature rises - i.e. the less efficient the heat sink -  the less power it has?

I can't figure out how to proceed from here. I don't know if the stock power transistors are no longer functional or if I need to upgrade the power supply. Any help would be much appreciated.


Power supply schematics :

Cheers

 

[abbey road d enfer]

I googled the TA7130 and it returned an IF strip, so there must be a mistake there...
Anyway, the increased current consumption is very likely to be the cause of your problem.
Upping the current capability of a PSU is not only a matter of changing a couple of transistors.
The main element that governs the power capability is the transformer. If you want to increase the power, you need to replace the mains xfmr with a bigger one, then you'd need to increase the reservoir caps, and put bigger transistors, and bigger heatsinks, and change the fuse ratings...

 

[spaceludwig]

Hi, Abbey...

link to TA7130 spec sheet here http://dl.dropbox.com/u/18935229/PM700/ta7130p.pdf

Which line tells me what nominal current the opamp pulls? is it the SUPPLY CURRENT?

If that's the case, then is the original opamp not consuming more current then the ones I replaced them with?  (link to OPA604 datasheet http://www.ti.com/lit/ds/symlink/opa604.pdf)

 

[keefaz]

Are you sure the opamps ref number is not TA7136P rather than TA7130P ?

 

[spaceludwig]

Are you sure the opamps ref number is not TA7136P rather than TA7130P ?

You are absolutely correct, my apologies.

 

[keefaz]

From the Datasheet, it seems they consume slightly less curent (3.1 to 4.2 mA vs 5.1 to 7 ma for the 604)
I see it is not the same package, did you make an adaptor ? No error in wiring ?

 

[spaceludwig]

I made my own adapters and there does not appear to be any error in wiring. When there were, they usually resulted in smoke and blown fuses. The units were tried individually and they all work. 

Do you know if the output power transistors lose power as they get hotter? Will putting better heat-sink potentially solve the problem?

 

[spaceludwig]

If the new opamps consume 4 mA more, then 12 channels  X 2 opamps/channel X 4 mA/opamp = The extra current draw should be aproximately 100 mA,  correct?  Would that be enough to crap out the power supply? And if so, why would only the negative rails take a hit? The positive side stays consistent.

 

[keefaz]

I made my own adapters and there does not appear to be any error in wiring. When there were, they usually resulted in smoke and blown fuses. The units were tried individually and they all work. 

Do you know if the output power transistors lose power as they get hotter? Will putting better heat-sink potentially solve the problem?

Have you changed some passive components around the opamp to suit better the OPA604, or did you change only the opamp ? I don't know the original module circuit around the TA7136P opamp, but maybe some change have to be made, a bad behaviour does not always show smoke or make fuse blow, but maybe draw too much current or oscillate

 

[spaceludwig]

I have removed the the compensation capacitors which the OPA604 don't need.

In the POWER DERATING graph I posted above, would I be interpreting it correctly if I said that when the transistor reaches a temperature of 150 degrees celcius it can no longer produce an output?