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little dot II + problem: Cant change volume

post #1 of 66
Thread Starter 
edit: the first problem with the amp was that the voltage regulator was busted and had to be replaced
-second problem is that the volume can't be adjusted!! (see post 13)


Hey all, traded for a LD II+ (used, knowing it was broken, but fixable?!?)

Problem:

I just got it second hand and listened to it for about three hours before one of slots for the tubes died. I switched the tubes around and they all work, except when they're in that slot (the rear left) - the tube won't even light up – the result: sound only on the right channel



Possible Solution:

http://www.head-fi.org/forums/showpo...0&postcount=10

-how do I check if that's the problem in the first place before attempting to fix?
-7805 voltage regulators, can any brand be used (or doesnt matter)?
-can another type of voltage regulator be used so the problem doesnt occur again? ie: non-7805 regulators? if so which ones?


Can anyone see any other possible problems?





It seems the transformer is preventing it from turning over. Er, it's 2am right now, but where can one tell where the transformer wires are connected to the PCB according from the picture above?



Last question:

http://www.head-fi.org/forums/showpo...9&postcount=20

-why is it illegal not to have any fuses? sorry if it's trivial~

Thanks in advance!

post #2 of 66
How to check : 1/ just get voltages reading before and after the lm7805 feeding the bad slot with the amp on. Be careful about what you're doing as there will be high voltage running on the pcb. If possible, fix the lead of the DMM with alligator clips then turn the amp on, then read the first value, then turn down the amp, let the caps discharge and do it again for the second value. If you have voltage at the input but not at the output, it's probably the reg. 2/ Otherwise, check for shorts or cut with the amp turned of (you'll have to somehow trace the circuit for that).

The regulator : any brand should be fine

Improving the regulator : if it burned once, it can burn again. See circuit 12 on how to rather simply improve current output of a three pin reg and put the heat on a more rugged device : http://pubpages.unh.edu/~aperkins/pd...ces/LM7805.pdf If the regulator is asked to deliver less than 1A (check the circuit and the heaters requirements), you could perhaps improve the heat transfer in between the reg and the case. It all depends what's already there.

And for your safety it is a very good practice to add a fuse on the live wire (only if you're comfortable working on the main, otherwise, get someone to do it for you). Considering the price of a fuse holder and a fuse, it's completly dumb not to have one.
post #3 of 66
Quote:
Originally Posted by barqy View Post
how do I check if that's the problem in the first place before attempting to fix?
Do you have a multi-meter? If you do you can poke around in there and make sure the voltage and capacitance measurements are similar on both sides of the board. I wouldn't do the voltage measurements with the amp turned on if you're a novice, though, for safety reasons (yes, it is very dangerous).

Quote:
Originally Posted by barqy View Post
-7805 voltage regulators, can any brand be used (or doesnt matter)? -can another type of voltage regulator be used so the problem doesnt occur again? ie: non-7805 regulators? if so which ones?
Brands don't really matter, the voltage regulator's job is to provide power to the the filament on the tubes, as long as it's working it should be fine. 7805 is the part number and that's what you need to get, the 78 means it's a voltage regulator and the 05 means it provides 5 volts.

Quote:
Originally Posted by barqy View Post
It seems the transformer is preventing it from turning over. Er, it's 2am right now, but where can one tell where the transformer wires are connected to the PCB according from the picture above?
It's not the transformer that's preventing you from removing the PCB, it's the 7805. To remove the PCB, you need to desolder the parts on the PCB labeled 7805 (there are two sections of three legs each, you need to get all of them melted simultanuously to get the PCB out, it's gonna be a test of your soldering skills ).
post #4 of 66

^your amp


^LD's publicity photo

Can you spot the difference? No, your 7805 aren't missing, they're put on upside-down (most production LDII+ units are like yours). If you look at the top of your amp, you'll see two black screws coming out behind the power tubes, those are the screws bolting the 7805 upside down to the chasis.
post #5 of 66
Thread Starter 
^

I'll bust out the DMM when I get home (later tonight~)

-As for adding a fuse, I have no clue how to do it it unforunately

-I was told to add a heat sink to the voltage regulators, where abouts could I get somethign small enough (and how would it bolt down?)

I noticed in the pic posted above, the LDII+ already has a heat sink on the voltage regulators (i'll have to check if mine does)

Could I solder the voltage regulators on top of the PCB rather than the bottom (i guess if there is enough room) as in the LD publicity photo?


-Sigh, i'm not exactly sure how to read the circuit diagram 00940. So i guess it's not as simple as replacing the current regular with another type (higher/lower) to prevent it from over heating again?



thanks for the patience.
post #6 of 66
Quote:
Originally Posted by barqy
-Sigh, i'm not exactly sure how to read the circuit diagram 00940. So i guess it's not as simple as replacing the current regular with another type (higher/lower) to prevent it from over heating again?
You might try the L78S05.
post #7 of 66
You willmost likely find that the 2 7805 regulators are actually bolted to the chassis of the amp so that the body of the amp is serving as the heatsink. This is common enough practice and I would say an improvement over the situation shown in the publicity photo shown above. In that case heat dissipation would be less because there would be less heat transfer.

If you can use a DMM you need to check the voltage across pins 1 and 7 - you can work it out, pins 1 an d8 are marked on teh board. You should be ideally reading 4.2VDC, but more likely 5VDC. If you aren't getting any voltage there, you need to track back and see if that votlage reg for that side is gone. Then its simple enough to replace the voltage reg.

For what its worth, the heater on that tube is rated at 4.2V. If its running at 5V all the time from the 7805, I wonder if that the reason these amps "arced".

Seems likely to me and easy enough to fix too by dropping in a resistor to drop the voltage a bit.


Fran
post #8 of 66
Quote:
Originally Posted by fran View Post
You willmost likely find that the 2 7805 regulators are actually bolted to the chassis of the amp so that the body of the amp is serving as the heatsink. This is common enough practice and I would say an improvement over the situation shown in the publicity photo shown above. In that case heat dissipation would be less because there would be less heat transfer.
It is.

Quote:
Originally Posted by fran View Post
For what its worth, the heater on that tube is rated at 4.2V. If its running at 5V all the time from the 7805, I wonder if that the reason these amps "arced".
It's been suggested that that's the reason why the LDII+ arc, that the tubes are being run too tightly.

Quote:
Originally Posted by fran View Post
Seems likely to me and easy enough to fix too by dropping in a resistor to drop the voltage a bit.
Where would you suggest for the resistor to go? I suspect the root of the arcing problem is deeper than that, however, since if the fix is easy to implement then LD would've done it in the first place.
post #9 of 66
Thread Starter 
hey all,

well i drove by my 'local' electronics store and picked up 2x 7805 voltage regulators, 2x heat sinks for it, and some misc. stuff to bolt on the heat sink.


haven't had a chance to de-solder the original (or test out the voltages yet), but will do that when I get home tomorrow.


thanks for the tips so far
post #10 of 66
Quote:
Originally Posted by barqy View Post
hey all,

well i drove by my 'local' electronics store and picked up 2x 7805 voltage regulators, 2x heat sinks for it, and some misc. stuff to bolt on the heat sink.


haven't had a chance to de-solder the original (or test out the voltages yet), but will do that when I get home tomorrow.


thanks for the tips so far
Good luck, take some pics while you're at it, it'll help many people in the future I'm sure.
post #11 of 66
Thread Starter 
Well, turns out it was the voltage regulators that were broke.


I de-soldered the 2 old ones, and put in 2 new ones.

The previously broken channel works now!

Unfortunately, during the de-soldering process, I managed to bust up the PCB on the OTHER channel. (the part where you insert the legs into the PCB, the metallic circle part ripped out, so the PCB at that point can't hold any solder)

Also, the underside of the PCB has a piece of copper strip lifted up and away from the PCB where the red circles are located.

I tried to solder a wire from where the broken PCB hole is from the voltage leg to another point that ran along the same copper strip, but it doesn't seem to do anything...should it?



Result: tube doesnt light up

Here's a pic.





Ugh, can this be fixed?



I'm a bit angry at myself for being forceful during the de-soldering process..

hopefully it can be fixed? crude or not...


Cliffs:
-replaced both voltage regulators, previously broken channel works now!
-during de-solder process ripped out the metalic circular part on the PCB of the other channel (previously working)
-result: tube doesnt light up
post #12 of 66
Looks like you melted part of the 104 film cap on the right side as well...

As far as damages to the PCB traces, you should be able to check continuity with your multimeter. You might want to make sure that you don't just have a short on the channel that's not working right now as well.
post #13 of 66
Thread Starter 
^ wow I got it working

turns out i soldered the wire to the wrong joint :\


now ANOTHER problem.

the amp plays (been sitting for about 10mins, gonna see if it over heats), but the volume can't be adjusted!!

arg, is the volume pot busted? or something else?

Or did I solder the input/output wires to the RCA jacks incorrectly (well I doubt that would have an affect on the volume pot)...


thanks guys
post #14 of 66
Is it really loud?
post #15 of 66
Thread Starter 
^ it's fairly loud?


Loud enough that i wouldn't want to listen to it for more than a minute or 2....


edit: if i turn the pot to the loudest setting, tehre seems to be a huge increase in noise (not volume)...\

edit 2: also the body of the amp is quite warm (not hot) but warm...normal??
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