Laney, schematics and books.

[Lennart]

As my Laney LC30 Mk1 doesn´t want to work anymore, I now want to go into the world of valves. To learn! I have a basic understanding of solid state amplifiers. Now I want to read and learn about the valve amps.
The book by Gerald Weber, All about vacuum amps, is it a good starting point?
The schematic for the LC30 I find in the library of this page is for Mk2. Mine is a Mk1. Will it do for this too?

[stingray_65]

Welcome Lennart!

Do give Merlin Blencowe's sight a looking over, TONS of great information there,

http://www.freewebs.com/valvewizard/

SO,

Your Laney is broken, What are your intentions? repair, replace modify?



 

[Lennart]

Hi,
I want to repair, I love to repair things! But, I need knowledge.
In this case the HT-fuse blows. I tried with four new EL84, problem remains. I will now print out the schematic for the amp and study to see if I will understand anything.

[Shrapnel]

The schematic for the LC30 I find in the library of this page is for Mk2. Mine is a Mk1. Will it do for this too?

It will be similar, but not the same.  (I have the SII schemo as well, and nothing before in the LC30, and I don't know if an LC50 would be close enough.)

[stratele52]

Hi,
I want to repair, I love to repair things! But, I need knowledge.
In this case the HT-fuse blows. I tried with four new EL84, problem remains. I will now print out the schematic for the amp and study to see if I will understand anything.

If you remove your four EL84 did the fuse blow ? If not chek, cathode bias resistor look to be a 56 ohms, 7 watts on some laney schematics

[Lennart]

If you remove your four EL84 did the fuse blow ? If not chek, cathode bias resistor look to be a 56 ohms, 7 watts on some laney schematics

The fuse blows with all four removed. It blows only when standby is turned on.

[stratele52]

Do you know the " gadget " that made by Gerald Weber to save fuse when checking a short like you have in your amp ? I use it and work

It's a 100 watt bulb lamp in serie with the power cord of your amp and plug into the wall outlet. This work in North America .
This bulb is limiting current goes to your amp amp you can do somes test without blowing a fuse.

In other way do you remove all tubes, preamp and power and what's happenend ?

Which schematics is the closer I can use for your amp ?

[Lennart]

Do you know the " gadget " that made by Gerald Weber to save fuse when checking a short like you have in your amp ? I use it and work

It's a 100 watt bulb lamp in serie with the power cord of your amp and plug into the wall outlet. This work in North America .
This bulb is limiting current goes to your amp amp you can do somes test without blowing a fuse.

In other way do you remove all tubes, preamp and power and what's happenend ?

Which schematics is the closer I can use for your amp ?

Hmmm...sounds interesting with the lamp. We have 230 volts in Sweden. And it´s hard to find an old fashion bulb these days, because we can´t buy them anymore. I suppose it´s the bulbs resistance that´s important.
I mailed Laney about the schematic, and it´s the same for S1 and S2. Only the fuses differ. You can find the schematic in the library on this page.
With all valves out the fuse blows. And it´s black in there!

[stratele52]

I know this gadget with old style bulb can be made with 2 X 50 watts  hallogene bulb. In your country you use mostly neon ?

For your amp , the problem is in your power supply this include output transformer . Any modifications before , in the amp ?

You have to isolate the power supply to find where is the defective çomponent.

I found the schematic LC30

[Lennart]

I know this gadget with old style bulb can be made with 2 X 50 watts  hallogene bulb. In your country you use mostly neon ?

This year came a new law in EU wich makes it is illegal to make old style bulbs. Too energy consuming. Instead we these "green" lamps with 1/10 power consuption. 7, 9 and 11 watts.

For your amp , the problem is in your power supply this include output transformer . Any modifications before , in the amp ?

I don´t think so. Bought it second hand. Everything look untouched inside.

You have to isolate the power supply to find where is the defective çomponent.

OK.
Will I have any use for the bulb thing anyway?

[Lennart]

OK I made a very simple bulb "thing". A 25 W lamp, wich I measured to 164 ohms.
Now the fuse doesn´t blow.

[Shrapnel]

Okay,

This is what I understand. You Power up the amp. As soon as you take it off standby, the fuse blows, even with all tubes removed. No immediate evidence of any modifications.

Probably safe to say it is not the power transformer.

For your amp , the problem is in your power supply this include output transformer . Any modifications before , in the amp ?

Now, The FIRST place I would check is the 4 diodes that make up the bridge rectifier. One blown diode (to a short condition, not open) would blow your primary fuse. Check all four and let us know. I presume you have a multimeter.

I think we can safely eliminate the output transformer, at least with a simple test anyway. Besides, if that 315mA HT fuse hasn't blown, I wouldn't look past the first place I mentioned as I find it doubtful that the output transformer is the problem. There is a fast way to make sure though and it won't cost you anything but time. Test all the primary leads of the output transformer for a short to ground. If those leads (typically red, blue, and brown on newer amps at least) show a path to ground, then it is possible for it to be a bad output transformer.

[ EDIT ]
Be careful of any high voltage left inside of the amp. Disconnect the wire from C400 where it connects to the diodes and short it to ground through a resistor. Just remember to hook it back up when you're done.

[Lennart]

Now, The FIRST place I would check is the 4 diodes that make up the bridge rectifier. One blown diode (to a short condition, not open) would blow your primary fuse. Check all four and let us know. I presume you have a multimeter.

All diods OK.

There is a fast way to make sure though and it won't cost you anything but time. Test all the primary leads of the output transformer for a short to ground. If those leads (typically red, blue, and brown on newer amps at least) show a path to ground, then it is possible for it to be a bad output transformer.

All the leads of the secondary side of the transformer seems to be shorted to ground. No good, eh?
Look at that page wich is called Service Diagram, drawing No. 2910. Up to left there is PCB 9088, with the speaker outputs. P1 and P2 is zero ohms to ground. Am I in the right place?

[VMS]

Here's some great info on debugging an amp:

http://www.geofex.com/ampdbug/fuseblow.htm

[stratele52]

Shrapnel , if is the HT fuse is blowing , as Lennart says , the problem is AFTER the fuse. Can't be diodes or power transformer.

[Lennart]

Shrapnel , if is the HT fuse is blowing , as Lennart says , the problem is AFTER the fuse. Can't be diodes or power transformer.

But from what I answered Shrapnel, what do say?

[stratele52]

Shrapnel suggest to you to test PRIMARY lead to ground, NOT  SECONDARY !!

Secondary have always very low resistance to ground.

I agree with Shrapnel

[Lennart]

Shrapnel suggest to you to test PRIMARY lead to ground, NOT  SECONDARY !!

Secondary have always very low resistance to ground.

I agree with Shrapnel

OK. On primary side no shorts.

[Lennart]

Disconnected the three leads from the primary side of the output transformer. Measured values in ohms are 25; 23 and 48. Seems OK? And with no short circuits to ground the output tranformer seems to be OK?

[Shrapnel]

Shrapnel , if is the HT fuse is blowing , as Lennart says , the problem is AFTER the fuse. Can't be diodes or power transformer.

Okay, My bad, I mis-read which fuse... I was thinking the fuse on the power line (FS103), and  it is the HT 315mA fuse blowing (FS104).


@ Lennart,

As to the Output Transformer: Those readings look as if they are probably good on the primary. The secondary side will always have a ground reference IN CIRCUIT, and a very low resistance on the windings.

NOW, knowing that the diodes of the bridge rectifier are good, and the PRIMARY windings of the output transformer are not shorted to ground. My first suspects would be C400, the 100µF 500V capacitor, followed by C40, 15µF capacitor. It *might* be time for a cap job.

[Lennart]

NOW, knowing that the diodes of the bridge rectifier are good, and the PRIMARY windings of the output transformer are not shorted to ground. My first suspects would be C400, the 100µF 500V capacitor, followed by C40, 15µF capacitor. It *might* be time for a cap job.

None of the two caps are shorted. None of the caps on the board is shorted as far as I can measure. And none of the resistors...

[Shrapnel]

None of the two caps are shorted. None of the caps on the board is shorted as far as I can measure. And none of the resistors...

There has to be a short somewhere. Check pin 4 of the EL84 sockets to make sure they are not shorted to ground somehow. Pin 9 should only have resistance to the B+ rail, the only thing you should see from pin 9 to ground is the fluctuation of the filter caps charging to the ohm-meter's testing voltage, and practically nothing else but.

We already determined the output transformer wires  on the primary side are not shorting to ground. I presume your first test on the primary wires of the output transformer  was in-circuit... IF not, make sure pin 7, where the transformer wires hooked up are not shorted to ground. As long as R59 is around 10K, if there was a short after that, it should not draw enough current to blow a fuse with the tubes removed given the voltages listed inside. Actually, the only pins on the EL84 sockets that should read anything to ground is pin 3 with about 56Ω. and pin 2 with about 221k5Ω. (1k5Ω grid resistor (R101, R102) + 220kΩ resistors bias reference resistors (R55, R56))

I still suspect the capacitors though.

Double check the capacitors again, based on this checklist as well. You will need a known good filter cap that can handle the voltages:

There are a limited number of ways for the power supply filter capacitors to be bad. All of the tests on power filter capacitors must be considered hazardous since they may store deadly amounts of voltage and charge even with the amplifier unplugged.

 Any time you suspect power filter capacitors, do the following: With the amplifier unplugged and the chassis open, connect one end of a clip lead to the metallic chassis. Clip the other end of the lead to a 10K 1/2W or larger resistor. Holding the resistor with an insulating piece of material, touch the free end of the resistor to each section of the power filter capacitors for at least 30 seconds. Then:

• Visually inspect the capacitor(s) for any signs of bulging, leaking, dents and other mechanical damage. If you have any of these, replace the capacitor. Also note the condition of any series dropping resistors connected to the capacitors to see if they have been damaged by heat. Replace them if they have.
  
• Use an ohmmeter to measure the resistance from the (+) terminal of each capacitor to the (-). This should be over 15K ohms, preferably much over that. If you get less than that on any capacitor, unsolder that capacitor and remeasure just the capacitor. Under 15K indicates a dead or dying capacitor; replace it. If the resistance is now much higher with the cap unsoldered, there is a low resistance load pulling current, not a faulty capacitor. Always check all of the power filter capacitors while you're in there. If one is bad, consider replacing them all (see "Cap Job" in the Tube Amp FAQ)
  
• If there is no obvious mechanical problem and the resistance seems high enough, temporarily solder a new, known good capacitor of at least as high a capacitance and voltage across the suspected capacitor or section., then plug in and try the amplifier again. If this fixes the problem, turn the amplifier off, unplug it, drain the filter capacitors again, and replace at least the bad section if not all of the filter capacitors.
  

If you are replacing a multisection can, get a replacement can with multiple sections matching the original before you remove the original capacitor. Once you get it, make yourself a note of the symbol on each terminal of the old capacitor, such as square=1uF/450V, triangle=20uF 450V, etc. and then clip the old terminal with the symbol off the old can. Remove the old can, mount the new one, and use the symbol chart and lugs still on the leads to make sure you connect the right sections up in the new capactor.

[Lennart]

There are two pins on the sockets that measure 0 ohm to ground. Counting clockwise from the valve side of the sockets, pins 5 and 6.

[sluckey]

There are two pins on the sockets that measure 0 ohm to ground. Counting clockwise from the valve side of the sockets, pins 5 and 6.

You count counter clockwise when on the valve side of the socket. Those pins are really 4 and 5 and it's very common to read low or close to zero ohms to chassis if the filament winding has a center tap connected to ground.

[Lennart]

There are two pins on the sockets that measure 0 ohm to ground. Counting clockwise from the valve side of the sockets, pins 5 and 6.

You count counter clockwise when on the valve side of the socket. Those pins are really 4 and 5 and it's very common to read low or close to zero ohms to chassis if the filament winding has a center tap connected to ground.

I find it strange that on pin 4 and 5 is almost zero (0.1 ohm) and on R60 47 ohms. How is that possible?
With the amp connected with the bulb gadget, it´s possible to measure some voltage values. Would that be something?

[sluckey]

I find it strange that on pin 4 and 5 is almost zero (0.1 ohm) and on R60 47 ohms. How is that possible?

It's possible because of the reason I gave above. The filament winding is big wire, low voltage/high current. Big wire is very low resistance.

R60 is the common cathode resistor. It has nothing to do with pins 4 and 5.
 

[Lennart]

R60 is the common cathode resistor. It has nothing to do with pins 4 and 5.
 

OK, I trust you. But in the schematics it looks to me as the R60 and C39 are connected to pins 4 and 5. But I´m not so skilled in schematics reading.

[sluckey]

...

[Lennart]

Gosh! Nice sluckey! What is the next step?

[Lennart]

On a Swedish forum someone suggested me to disconnect the C400 and see what happens. And the fuse don´t blows. The amp works but is humming. So the answer is that C400 is defect?

[Shrapnel]

On a Swedish forum someone suggested me to disconnect the C400 and see what happens. And the fuse don´t blows. The amp works but is humming. So the answer is that C400 is defect?

It sure sounds like C400 is shot to me. REPLACE it with a new one that has the same voltage rating and capacitance value If the original is bad, you will know because the new capacitor won't blow the fuse.

[Lennart]

With a new cap the fuse don´t blow. 
Thank you guys for all your interest and help!