(I was going to edit the last post to include this info, but the time limit expired sometime between clicking edit and pressing save changes, sorry! Since this has gone on a new page I will quote the previous info too)
I got the scope out, set to AC, and measured the rails of the power amp regulator outputs, preamp regulator outputs and bridge inputs for both, and the cap output's for the power amp regulator, but with the regulator disconnected (it's hard to get the probes in with the regulator connected so I didn't risk it).
The bridge inputs look identical as expected. A sine wave. The cap outputs seem fine, it showed as a straight line until I went down to ... somewhere.... between 10-100mv/div (I didn't note what setting I was using sorry, but can do it again if you wish). Here I started to see a very very very tiny thing like this: |\|\|\. Very small in amplitude, and as far as I am aware normal (cap charging and discharging?)
The output of the preamp regulator was normal, again flat lines with a small hint of |\|\|\. The +ve rail on the power amp regulator also showed this behaviour.
The problem rail, the -ve rail on the power amp which is giving -12.6v, I am seeing a really very large version of |\|\|\|\. I didn't note the setting again, but I believe this was visible around 1v/div. I can test again and note the setting if required, but I am off to eat for now!
I got the scope out, set to AC, and measured the rails of the power amp regulator outputs, preamp regulator outputs and bridge inputs for both, and the cap output's for the power amp regulator, but with the regulator disconnected (it's hard to get the probes in with the regulator connected so I didn't risk it).
The bridge inputs look identical as expected. A sine wave. The cap outputs seem fine, it showed as a straight line until I went down to ... somewhere.... between 10-100mv/div (I didn't note what setting I was using sorry, but can do it again if you wish). Here I started to see a very very very tiny thing like this: |\|\|\. Very small in amplitude, and as far as I am aware normal (cap charging and discharging?)
The output of the preamp regulator was normal, again flat lines with a small hint of |\|\|\. The +ve rail on the power amp regulator also showed this behaviour.
The problem rail, the -ve rail on the power amp which is giving -12.6v, I am seeing a really very large version of |\|\|\|\. I didn't note the setting again, but I believe this was visible around 1v/div. I can test again and note the setting if required, but I am off to eat for now!
It seems like the bridges don't quite isolate the supplies. I switched secondaries around on the preamp supply (just noticed I had the preamp negative fed from secondary #2, but power amp negative from secondary #1) and I see similar behaviour to before.
The power amp supply then turns normal +-28v, the preamp positive rail is OK at +13.6V, but the preamp negative rail initially shoots up (down?) to -28v, and then very slowly decreases. It went down to -18v pretty quick, then got really really slow (to the point where it seemed like it had stopped decreasing).
The power amp supply then turns normal +-28v, the preamp positive rail is OK at +13.6V, but the preamp negative rail initially shoots up (down?) to -28v, and then very slowly decreases. It went down to -18v pretty quick, then got really really slow (to the point where it seemed like it had stopped decreasing).
AndrewT said:
Tried this with 10k resistors just now and it didn't seem to make any difference at all. I think 10k might be too high though, that would be ... 2.8mA load? The LM338 datasheet says 3.5mA is typical minimum load. 10k was just the first lot of resistors I picked up.
I can try a lower value if necessary, of course, but individually both supplies show the correct output voltages without any load.
Looking at it I think I can fit the smaller transformer in there just about should this not be possible to do for whatever reasons.
Nuuk said:As most of the other contributors to this thread have recommended a separate PSU for the pre amp, I'm surprised that you need to ask!
Uh.... I am asking because I want it ( or, would like if I am to be "polite"
) all off the same transformer.If they are showing the correct voltages what is the problem?
They're not when all hooked up [together]. That's the point!
So you have made up your mind but are asking for our ideas?
Andrew asked what the voltages were like with a load. Please don't think that I am having a go at you but your posts are somewhat inexpliset. You are asking questions, we are trying to help, but you don't come back with the information that we need to help you.
What we really need to see is a circuit diagram showing exactly how you have this amplifier set up, wiring and all. While Brian came up with an explanation for that 'middle' circuit not working, it's strange that it doesn't work with the extra pair of bridges.
Nuuk said:
So you have made up your mind but are asking for our ideas?
I was just asking if anyone had any further thoughts on any possible solutions to the problem, before I dismantle it and put in a separate supply for the preamp / take out one of the power amp regulators, since I seem to be the only one who can test possible solutions to the problem at this point.
And I have tried this. I posted, it was exactly the same as with no load.
I guess this is connected to the above comment?
I have tried with a load, and I even posted this. (i'm not sure if it was *enough* of a load, and I was hoping for feedback on that). Given it was the same as without a load, I did not post voltages. Saying that "it was the same" should be quite enough, no? There was really no need. I am guessing you just missed this post...?
What other information haven't I come up with that you would like? It is possible I have missed something too.
It's as BWRX's "correct way" diagram. I can draw a diagram / picture / both of my own if you like, but I fear it's going to be identical.
I'm not sure if I should leave the regulated supply '0v' lines to safety ground / the disconnecting network, or leave them 'floating. In Andrew's post he said not to and BWRX's post seemed to imply it didn't really matter?
There is only a small difference from before, in that the preamp negative rail now does very slowly drop down from -28v. I've only left it long enough to drop down to ~ -18v as it seems to drop very slowly indeed from here on. There still seems to be some sort of interaction between the two supplies even when they're on separate bridges.
Hi,
I'm thinking of a dummy load like 500r not 10k which draws a current of less than the manufacturer recommends as minimum.
+-28V into 500r gives +-56mA Pd~=1.6W (three 1500r 600mW in parallel).
+-15V into 500r gives +-30mA Pd~=450mW (one 510r or 470r 600mW).
Fit, measure and post.
Can you take and post those pics yet?
Common transformer into separate rectifiers into separate regulators will work. I still think there is a mis-wire somewhere.
edit: typed 50r and now changed to 500r.
I'm thinking of a dummy load like 500r not 10k which draws a current of less than the manufacturer recommends as minimum.
+-28V into 500r gives +-56mA Pd~=1.6W (three 1500r 600mW in parallel).
+-15V into 500r gives +-30mA Pd~=450mW (one 510r or 470r 600mW).
Fit, measure and post.
Can you take and post those pics yet?
Common transformer into separate rectifiers into separate regulators will work. I still think there is a mis-wire somewhere.
edit: typed 50r and now changed to 500r.
AndrewT said:
Ok, should be able to do this later thanks. Room is a mess at the minute because we're decorating.
Nope. I think I might need to find out what's going on with the camera situation. They've had it a while. Last time I phoned they said "they were waiting for spare parts". Why they can't just send me a new camera I have no idea!
Hm, if there is I honestly can't see it!
I have drawn a picture. You'll have to forgive the mess of the "wires" though. I tried to label the tabs of the bridge but it's probably quite hard to see in the mess of "wires". It is as follows:
+ AC
AC -
An externally hosted image should be here but it was not working when we last tested it.
Ug. Too early in the morning. The red and black wire from the transformer I drew wrong here. The black should be where the orange is (but between #2 and #4 of course), and the red where the yellow is (but between #2 and #4). Basically, I should have drawn the red wire black and the black wire red
Hi,
that diagram is quite legible. I can follow it easily.
Don't worry about the red/black swap over.
Disconnect the transformer from the mains and from the rectifiers.
Much more important is:-
do the RED/BLACK make a pair of secondary?
do the YELLOW/ORANGE make a pair of secondary?
are the two secondary windings completely separate?
i.e. testing with a continuity meter between
1.) red & black should give near zero ohms.
2.) yellow to orange near zero ohms.
3.) red to orange near infinite ohms (open circuit).
4.) black to orange near infinite ohms.
After you connect the transformer back to mains, check the AC voltage across EACH rectifier. AC to AC should read the nominal AC voltage of each winding.
and just in case also measure the + to - voltage across EACH rectifier.
While you are doing all this testing I assume you are playing safe and still USING the light bulb tester!!!!
that diagram is quite legible. I can follow it easily.
Don't worry about the red/black swap over.
Disconnect the transformer from the mains and from the rectifiers.
Much more important is:-
do the RED/BLACK make a pair of secondary?
do the YELLOW/ORANGE make a pair of secondary?
are the two secondary windings completely separate?
i.e. testing with a continuity meter between
1.) red & black should give near zero ohms.
2.) yellow to orange near zero ohms.
3.) red to orange near infinite ohms (open circuit).
4.) black to orange near infinite ohms.
After you connect the transformer back to mains, check the AC voltage across EACH rectifier. AC to AC should read the nominal AC voltage of each winding.
and just in case also measure the + to - voltage across EACH rectifier.
While you are doing all this testing I assume you are playing safe and still USING the light bulb tester!!!!
Nuuk said:The bit we need to see in detail is the regulator circuit(s)!
Please take your time, rushing anything in this game results in failure - and I am speaking from experience!
Not trying to rush as such
As for the actual regulator circuits, these are from carlosfm.
AndrewT said:
Phew!
Ok. I didn't think it mattered much being AC, but I wanted to point it out incase.
Well, this is how I have wired it I believe. But I have read forward in advance so see next quote!
>>> EDIT: All I know about the transformer is that it is 'dual secondary'. I assume this meant that they're completely separate. Perhaps the tests following will tell?
Yep.
Yep.
Yep. (not sure if you meant red to yellow, both the "+ve's", but this also measures as 'open circuit' / high resistance)
Yep.
Maybe you've thrown a trick question in again and can see what's wrong, with any luck!
AndrewT said:
I looked at the AC to AC on the scope and saw identical waves on each. Can't remember if I measured these with a multimeter but I can do this later too if required.
Should I be checking the voltage between + and - without any caps or anything connected? I know I tested the output of caps, with both regulated supplies connected, and they were reading the correct and expected +36v, even though one regulator -ve rail was still outputting wrong. This is why I wanted to emphasize that individually all regulators (e.g just the one regulated supply on the transformer) output the correct voltage
Nuuk said:
For the preamp I replaced the 2.2k with a 1k. This gives (when working on its own) +-13.6v. I didn't change anything else on the preamp supply.
Earthing is still quite a subject for me. I know it's important, but I am still trying to "get there" with it. So you'll have to forgive me if I am vague or anything here.
I've got safety earth connected to the case (it has been permenantly connected since putting it in the case), but I am not currently taking anything back to the 'disconnecting network' (i.e the regulator 0v's are 'floating' as previously mentioned, I think this is the correct terminology).
The amp boards do have wires for the disconnecting network / earthing, but given I haven't yet put power to these I am probably right in guessing you don't want to know about these
Hi,markiemrboo said:
these results confirm you have a dual secondary tranformer that has no internal shorts at cold temperatures and no loading. A high voltage isolation test is better but needs specialised test gear (MEGGER in UK).
Sorry, I forgot to add that the DC measurement across the rectifiers is with smoothing and either dummy loads (maybe the better option at the moment) or regulators connected. I'm trying to ascertain where the voltages are going wrong. Working forward towards the error we know we have at the output.
AndrewT said:
I assume that it's going to cost quite the bit of money to get it tested like that though? Doesn't sound like something I can realistically do, so I hope this basic test is OK for now...
Ok. Well, we know the regulator output voltages. I will remeasure the voltage from both sets of caps (power amp and pre amp sets) and AC bridge voltages as soon as it is safe for me to do so (when everybody buggers off out of my room! heh)
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