Orange Terror Bass SMPS 6V issue

[Mars Amp Repair]

Hello folks,
So glad to have the forum back. I read on The Amp Garage that this forum is actually on a server in the administrator's home. I was fearful that all these years of invaluable information might be gone.

Anyway, I have a Terror Bass smps in which the 6V filament supply is dragged down by pretty much any load presented to it. If only the fan, the supply reads about 4V and if presented with the 2 12AX7's, it bogs down to about 2V.
I have replaced the main filter cap of 3300uf @ 25V as well as the smaller electrolytics at the 6V supply, installed a 5V 7805 series reg ic, as I did not have a 7806 6 volter with similar results.

It appears that this SMPS has no feedback from the secondary, so it seems it isn't expected to attempt to regulate the secondary with a heavier load. The minisule load the 6V supply presents to the primary shouldn't be an issue anyway.
I have loaded the HV output just to see if the secondary is capable of supplying current, and the HV does not droop very much and supplies plenty of current.
The Ultra Fast rectifier diodes all meter & tracer check good. I have new ones on order as at this point, I'm really grabbing at straws.

Does anyone have any ideas about what could be wrong causing the 6Vdc supply from supplying enough current to drive a relatively low current?
The schemo I have is just a bit different than the SMPS in this amp, but it is very close.
One component in the 6V supply that is not in this SMPS is the diode to ground from the #2 pin on the 7806 regulator, which I'm supposing is to offset the 6V reg output by the diode drop of 0.7v to make up for the 6.3v the supply is supposed to be. There are probably other differences.
BTW, I have also replaced the smaller electrolytic caps around the IR2153 IC as a matter of course as I've had start-up issues with those.
Thanx, Glen

[g1]

Did you see the 470uF C27 on pg.1 (upper right) ?
What happens to the HV rail when the 6V is dropping out?

[Mars Amp Repair]

Did you see the 470uF C27 on pg.1 (upper right) ?
What happens to the HV rail when the 6V is dropping out?

Hello G1,
I did not notice that cap! It would seem weird that the fan that plugs directly into the power supply, would rely on a cap that was mounted on another board, but I'll certainly check it out.
I did observe on the scope that when a load is presented to the supply, the otherwise clean DC at the output of the bridge, becomes a kind of sinewave as it is loaded. At that point, I actually added 1000uf to the 3300uf as an act of desperation, just to see if anything changed. No difference. I don't recall if I subbed in C120 470n. At 64Khz, I guess that could make a difference, too.

I have disconnected the 6V plug that goes to the amp, and provided a load just by clipping in a 12AX7 directly to the SMPS 6V plug, and get the same results, except the load is 1/2 of 2 12AX7's and the drop of the power supply is 4V instead of the 2V when loaded by both tubes. As mentioned, just connecting the fan reduces the 6V drive to about 4v.

I'll tack in a 470uf next week & pray .
Thanx, Glen

[g1]

I'll tack in a 470uf next week

My concern is more about an existing C27 cap being leaky and adding load to the supply, rather than just not doing it's filtering.
Another possibility could be some other part loading the 6.3VDC line.
The fan seems to have it's own cap at C127.
Measuring current through F4 fuseholder, and voltage across R114 (to fan) may tell you if there is a loading issue or the supply is just weak.

[Mars Amp Repair]

My concern is more about an existing C27 cap being leaky and adding load to the supply, rather than just not doing it's filtering.
Another possibility could be some other part loading the 6.3VDC line.
The fan seems to have it's own cap at C127.
Measuring current through F4 fuse holder, and voltage across R114 (to fan) may tell you if there is a loading issue or the supply is just weak.

I hear ya G1. Thanx for the response.
But, I have isolated all those possibilities by unplugging the fan CN2 & the filament supply to the main board CN3, then merely clipping in one dangling 12AX7 (Pins 4&5 + Pin9) then seeing the voltage drop to around 4V.
As mentioned before, with all the loads removed the regulator puts out 6V and the input to the regulator is at about 9V. Under load, the input to the regulator drops to somewhere about 3v or 4V above the output.
Loaded, a 10v sinewave at the operating freq of 64KHz appears at the output of the bridge, and the resultant DC voltage at the input of the regulator dips to 4V clean DC. So, it's just like the secondary output just doesn't have enough poop to drive the 6V supply w/load.
As mentioned, I'm ordering the Ultra Fast diodes in the bridge in a 'rit of fellous jage'...oops, act of desparation . Then, what the hell is left? The transformer? Anyone ever experience anything with a SMPS xformer like that? Seems really unlikely & I most likely wouldn't be able to source one anyway.
I'm actually quite gabberflasted at this point
Ya know, typically when I get to this point of silliness with anything, I keep thinking in the back of my mind that I'm missing something reeeeeally simple... AND as we all know, the mind IS a terrible thing...
I'll keep ya all posted.
Thanx again, Glen

[Mars Amp Repair]

Well, I received the Ultra Fast UF4004 rectifier diodes for the 6V supply & it turned out to be the problem . Since I couldn't test the diodes at 64Khz as the meter is a static test and the junction tester in conjunction with the scope is at 120Hz, just proves at higher frequencies they just apparently weren't doing the job.
The diodes were under the heatsink for the switching xistors & other diodes. So, I was able to access the diodes & remove them without removing the heatsink, and installed the new diodes on the bottom of the board.
Most likely only one of the diodes was defective given with no load it was putting out a clean 6V.
There ya have it. g

[Jazz P Bass]

Thank you, Glenn.
SMPS can be a bear.