LA-4 Help Thread!

[Rybow]

Should be OK. Throw it in there and see what happens. If it starts to bulge, then replace it with a 63V cap.

 

[dirtyhanfri]

Thanks, actually, I did it

Also I finished all the drilling in the case, the wiring and even run it once...

Bad smell... smoke...., One of the 15R (1/2W Metal Film) resistors near to one of the BD139 gets burned. I´m almost sure it`s because a bad orientation of the 139 transistor, I`ll try to swap it tomorrow, replace the burned resistor and then see what happens.

Could the transistor get burned? It was really hot, I touched the IC´s and they didn´t look so hot, so I hope they`re ok.

I´ll post some photos tomorrow

Thanks to LunyTune and everyone sharing info on this project

 

[dustbro]

.., One of the 15R (1/2W Metal Film) resistors near to one of the BD139 gets burned. I´m almost sure it`s because a bad orientation of the 139 transistor, I`ll try to swap it tomorrow, replace the burned resistor and then see what happens.

Could the transistor get burned?

Those resistors fry pretty easily, so I'm almost positive you should replace it.

 

[dirtyhanfri]

Hi

It`s me again...

I replaced the burned resistors and changed orientation of the transistors, then plug it again and.... No Smoke, good! But some issues.

Yesterday I had correct voltages on +-18V Rails, and I was using one of them (+ and 0) to feed the light in my meter bridge (Not led) and the other for the compressor, smoke, yes, but with light working ok.

Then, today, after changing the transistors (BD139 & 140) for new ones, looking for right orientation, when I plugged it, the light was not working, I tried to disconnect the compressor and then the light was ok, changing the rails has the same effects. So I measured again and had  unstable voltage on the - rails, changing between 3 & 7 Volts. The + Rails was ok. Then I touched the 7818 and it was really HOT (my middle finger knows it  :-[) The 7918 was not so hot, a bit too, but not as the 78.

So, I changed it and then try again... same results, I can´t change the 7918 because I have no more, shops closed for two days... sh*t!

I oriented them whit the metal plate corresponding with the big white line in the pcb draw, I´ll look for some photos in the thread and check for it, but I don´t know what could be wrong, the caps and the rectifier in the power supply doesn´t get hot.

I´m a bit lost, I´m almost sure the issue is in the regulators, maybe changing both of them will make it work, don´t know if a bad regulator could damage the other.

What do you guys think? What could be useful to check?

Thanks

 

[gary o]

Hmmm .....Im no expert but first disconnect the light for now....I had problems , I disconnected PSU section from the compressor & measured the PSU voltages looking at the schematics....in my case 1 reg didnt seem to be doing its job was about to replace it but then I found it was bad solder joint on that reg...

Once your sure your PSU is ok have a look for shorts maybe messy solder in yr compressor section if looks good you can re connect it hoping not to make smoke...

good luck

 

[Rybow]

I had a lot of issues with my meter lamp. I had it connected to the + rail through a 220Ω resistor, and then to the star ground. The current draw was too high and was overheating the regs causing them to go into thermal shutdown. Just to get the unit working, I replaced the 220Ω resistor with a 360Ω resistor. No thermal shutdown, but the meters barely light up. I am going to try dropping the resistance again, and adding heat sinks to the regs. You might want to try running your lamps straight to the star ground instead of 0. Do you have a current limiting resistor on your lamps?

Agreed with gary. Disconnect your power supply, confirm that you have +/- 18VDC coming out. Then hook up your circuit, re check voltages, then reconnect your lamps. Your regs are oriented correctly as well.

 

[dirtyhanfri]

I´m almost sure I´ve cheked voltages with the light unplugged of the power supply, because I remember measuring both of them without change anything, but not 100% sure, I´ll do it again, anyway I know I had bad voltages in the -18 rails.

For the meter, I don´t know too much about the light, it comes into the meter with his own pins is this one:
http://es.farnell.com/jsp/search/productdetail.jsp?id=7758227&Ntt=7758227

Really I don´t care too much for the meter light, I could use the "Power led" to feed one Led and put it under the meter or something.

So, I´ll measure again voltages with nothing plugged in the power supply and check for shorts, if the voltages are ok I´ll plug the compressor and then check voltages going in the IC sockets before placing them.

If the voltages are bad (as I think) I´ll look for shorts or something bad, If I don´t found nothing will replace the regulators and measure again.

I Thought to put a little plate of aluminium in each regulator, will it be useful? anyway, I think it can´t be good for the regulators get that hot...

More news soon, thanks for all  the support

 

[Rybow]

If your PS voltages are good without the comp plugged in then your regs are fine. A piece of aluminum on the back could help dissipate some of the heat.

From what I gather about your meter, there is no current limiting resistor on the lamp, and there doesn't seem to be any info on the bulb rating. To get these to work without overheating the regs, you must put a current limiting resistor in series with the lamp. Since it's not an LED, you should use at least a 1W resistor as it will get very hot. If you know the wattage and voltage of the lamp you can calculate the resistor value using ohms law, or just put a potentiometer in series, turn it until you get the desired brightness, and then measure it to get the resistance value.

I have wondered lately about the advantages of bolstering or even twinning the power supply for a 2 channel unit. Probably aren't many.

 

[dirtyhanfri]

Ok, thanks for the info

I have measured the voltages with nothing plugged in the power supply. The + rails looks ok, +17.9V.
But the - rails gives me only around 7V.

I looked to the toroid and I think it was ok, showing me around 18V AC in the secondarys (I´ve plugged the two "0" wires of the secondary together)

Then, I think there´s something wrong with the 7918 regulator, I measure the output voltage and I see that 7.2V DC.

The first thing I think is the 7918 is bad, but, I think it wasn`t overheated or damaged by me (I`ve been careful) And I don´t trust in just changing it (I´ll try it, but I think it will be the same).

I´ve looked for shorts, I found nothing...

I´ve noticed that the regulators don´t get so hot with nothing unplugged, the 7918 a bit maybe. but as soon as I plug the comp it gets really hot, especially 7818, also the 7918, but less

Also I saw, in the compressor +-18VDC Input, with no IC placed in IC2 Socket I have continuity between - and 0, it goes away as I put the IC in his socket, is it normal?

Thanks

 

[Rybow]

Just to confirm your toroid wiring real quick, I've put a couple of JPEG's in this post.

Not sure about the continuity question. To check out that regulator, measure the voltage between the input pin and the ground pin. Then you can see what voltage is actually going into the regulator, and eliminate it as a suspect. If it did overheat, it should have shut itself down as a fail safe before being damaged. These regulators can be touchy, and they won't get hot until current is drawn through them. Unless there is a short of some kind.

 

[dirtyhanfri]

Well, my toroid has that pinout, but it says 18, not -18, is this one:

http://es.farnell.com/jsp/search/productdetail.jsp?SKU=9531688

I´ve wired it that way already, but thanks for the tip.

I´ll check the regulator tomorrow, but I´m almost sure it´s bad, not so much things before it on the power supply, just the rectifier, and the +18 rail is ok, so I think the rectifier should be ok, no?

Thanks

 

[Rybow]

Cool. Just double checking. Did you measure the voltage on the input pin of the regulator? That will confirm if it's bad. If there is something wrong with the bridge rectifier, then the voltage on the input pin of the reg will be wrong. After the bridge rectifier, you should read 25V after the bridge rectifier if it's good.

You may as well just switch in that other regulator. They don't cost much.

 

[dirtyhanfri]

I´ve already bought some of them (and also some 7818 and TL074) today.

I find totally frustating can´t keep working on one project because i don´t have a 0.30€ piece, so each time I go to get something like it, I get 3-4 of each.

Maybe I´ll go on with a pair of this more, will see later... First make this one work, but I have some kind of good vibe about this project.

 

[Rybow]

You'll be smashing stuff in no time, and yes, you probably will crave another channel when your done. I started off just doing a mono unit, but it was long before I convinced myself that I might as well build another!

Tell you the truth, I completely ruined my first PCB when I started this project, so you are already one up on me.

 

[dirtyhanfri]

YTell you the truth, I completely ruined my first PCB when I started this project, so you are already one up on me.

Well, today, replacing the 7918 one of the solder pads went out, so I wired it point to point and attached it to one stand-off under the PCB, not the best way, but I think it´s functional.

Well I replaced it for fisrt time and it gave me 21V  , so I replaced it again, 21V, exactly the same than the other, the voltage between input and ground is the same in the 2 regulators, aprox 30V.

Ok, then, thinking and thinking I had a posible solution, I put a 1M, 1/2 Watt resistor in series in the -18V rail, and then dropped it to 19.8 V, so I put another one, and then I had 17.8V. Is this too bad?

I built a wire with the two resistors in the - wire, I plugged it into the comp and the power supply, run it, and no smoke, it´s ok, but I noticed a hum coming from the toroid (the input and output aren´t connected to anything), and it get really, really hot, I don´t know if it´s usual, as the noise.

Also I´ve seen that the needle in the meter doesn´t keeps in 0 with no audio passing in GR mode, but it goes to -20 in the other 2 positions.

I couldn´t pass audio, i´ll try it after, but I´m a bit worried about the toroid

 

[Rybow]

Did you measure 21V without the compressor circuit attached to the power supply? The voltage on the PS will be higher without a load connected, so you don't need those resistors in there. Connect your circuit and recheck.

Your meter lamps are rated at 8V/50mA. A 200Ω resistor in series is what you want put in to get those lamps lit properly. Might want to even go 220Ω or even 250Ω to control the brightness.

Have you calibrated your meters? Try the meter zero adjust trimmer. It's the 500Ω trimmer that's beside the 250K trimmer.

The circuit will probably be pretty noisy until you mount it in a chassis.

 

[goldenGeek]

I have finally managed to blow life into my LA4 clone, but I'm having some irritating 50Hz noise (ground). If i desolder the power ground from the chassis the noise disappears (well, duh, since its not grounded anymore ;-) ). Do anyone know what might cause this ground noise? What is the best places to start checking? Replace the toroid? What is common places for making unwanted ground loops on this project?

Any help appreciated

-J-

 

[Davo]

try rotating your toroid

 

[dirtyhanfri]

Hi, thanks for the tips

Really, the meter lamp looks so nice, I´d like to keep it, I´ll try with a 250R resistor.

I´m afraid of the toroid, it gets REALLY hot, so f*cking hot, It´s hot enough to heat the enclosure (the toroid is protected by a plastic case), also I can hear a Hum, I´ll chek tomorrow for current derivations to the chassis.

I noticed the 7818 don´t get too hot with the compressor conected, I think it´s ok.

I´ll check tomorrow the trimmers, try to adjust it, and start testing with today I couldn´t try anything (work, work, and more work).

But, really, I´m worried about the toroid...

Thanks for all the support!

 

[goldenGeek]

try rotating your toroid

I'll try that. I also tried to disconnect the toroid from the PCBs yesterday and the noise was still there, so I guess it's my toroid in some way or another... I think I have another transformer around to try.