In Need of Soldering Tips

I saw those two orangish looking wires near the bottom-center of the pic and cringed... Those need some heat shrink or some electrical tape or something non-conductive over them to keep them apart.

 

My first question would be is there a light indicating the unit is turned on, and is it lit. Next is there a line fuse and is it blown, even if it looks right. Then I would look for DC voltage across the two resistors on top of the capacitors in your picture when then unit is turned on. Figure those out first.

 

Well, it hasn't caused a problem for 29 years, ha! It is a Croft valve power amplifier made around 1988/89. Hard wired throughout, no printed circuit boards!

JG

 

No, there is no neon light when turned on. The tubes just light up.

At the on/off switch there are 3 fuses, the one directly under the on/off blew (see pic). As soon as I turned it on it blew the fuse (5 amp fast blow). I have tried about 5 fuses and they have blow every time. The guy said to first change the fuse to a slow blow 5 amp fuse. I have a couple of slow blow 5 amp fuses but haven't tried them yet as I don't want to waste them all.



He also said.... It could be a diode on one of the ht supplies that has gone short circuit.

I'm praying the transformer hasn't gone down as the amplifier is also 30 years old, probably wouldn't be easy to replace the transformer.

I really appreciate all the responses so far and the advice being offered!

JG

 

If you feel up to the task, the amp would benefit from having all four diodes replaced with UF4007 diodes, which are functionally equivalent to the 1N4007s but significantly quieter and quite cheap to purchase.

 

By HT he means High Tension (= High Voltage). Usually in a tube amp there will be different levels of power supply. The tube heaters are on one winding, that they are lighting up is good news and makes it less likely (though not impossible) the transformer is shot. There should be two diode/capacitor structures like the ones in your picture early in the thread. On one the capacitors will be high voltage and low value, (like 300 uFd/2000 V), the other one will be opposite (2000 uFd / 50V). Find the one with high voltage. Set your meter to measure resistance, some meters have a setting especially meant to check silicon junctions marked with a diode, use it if it is there. Practice with one of your loose diodes, you should see when you put the leads one way you will have much higher resistance than when you put them on the opposite ends. Check the four diodes near the high voltage caps in circuit to see if any shows zero resistance. If you find one or more, bingo.

Odd the suggestion you replace the fast blow fuse with a slow-blow one. It will still blow.

 

Sorry, my bad wording.... No, there is no neon light when turned on. The tubes just light up.

What I meant was that there is no neon light on the amplifier at all. The tubes just light when it is turned on normally. When I turn it on now, the fuse just blows and the amplifier is dead! I sent a photo of the underneath of the amplifier showing all the wiring, etc. to the guy who made it and he marked the 1N4007 diodes as a 'likely' problem.

I bought the 1N4007 diodes and I have a 40W Soldering Iron which one member says is too powerful for that delicate kind of work and suggested 25W. At this stage with the amplifier being 30 years old I suppose I have nothing to lose, there is nowhere over here where I can bring it to a professional to have it checked it.

JG

 

You can test the diodes as described above. These are older, tough parts and with care you can use the 40W iron but practice first on scrap wire.
One other test you can make if you feel motivated is to pull all the tubes out, replace the fuse and see if it powers up without blowing. You may have a bad tube. Do they all look OK?

 

Thanks for all the input, I really appreciate it. I have replaced some diodes in the amplifier before, but they were pretty straightforward.

It is the fact that the 1N4007 diodes are connected along with two other cables that make it difficult.

Regarding the 40W Soldering Iron, max temp is 480°C, most 25W are rated at 385°C, is that much of a difference?

JG

 

Boy, diodes wired up without physical attachment or separation between leads like that should go in a textbook of how not do do things!

It is better to have more soldering iron power than not enough. The longer you have to heat a joint to melt the solder, the longer the heat is allowed to sink into nearby components, like the capacitors and their liquid electrolyte. This can potentially cause damage, and can plain make it hard to complete the work, especially when dealing with components that are soldered into a large heat-conductive ground plane.

You should have no fear of "disconnecting" the other wires with your soldering. Instead the opposite; often these components leads are fed through the holes of a ring terminal and bent or twisted, keeping them connected before the soldering was done - which can make them hard to disassemble without sucking or wicking away all the solder to allow untying the wires. Just reattach like your picture if they do come loose - but using heat shrink tubing etc. I would replace them all with a mounted bridge rectifier module myself.

More importantly though, these diodes are unlikely to blow a fuse. If they shorted internally, they would no longer exist. The voltage drop of each can be measured in-circuit with a diode test on a free-with-coupon Harbor Freight multimeter.

 

I don't think they have Harbor Freight stores in Ireland...

 

Canadian Tire, then.

 

That is what I thought as well, the quicker it heats the easier it will be to do!

Well the actual guy who made the amplifier suggested that!

JG

 

I wouldn't try a slo-blow fuse. The fuse is doing its job to protect your amp. The amp has a major fault somewhere, probably a short or defective component that causes high current flow from the power supply!

 

I thought that myself, the fuse is an F5A250V (fast blow) and he suggested a T5A250V (slow blow). Anyway as soon as I get a chance I will replace one set of the 1N4007 diodes, try it and if it blows again I will replace the other set.

Whatever the outcome, I really appreciate all the input from forum members. I am in awe of the knowledge ye possess!

JG

 

Is this an amplifier with valve front end and transistor outputs?

 

As already mentioned, it is important to have a better quality iron/solder station. Typical Weller units are 70w & will get plenty hot enough for this job. Hotter is better since your touching the spot for less time & therefore less heat will spread to other components. Obviously there are reasonable limits & it depends on the type of solder & the tip your using.
From your description, I highly recommend a small alligator clip attached between the resistor & the pink wire. a 2nd one between the cap & resistor/pink lead connection. They will absorb the heat & minimize it's transfer into the resistor & cap.

This shouldn't be too bad & I've seen worse. Try repairing a pinball machine where the parts are under the playfield & you're looking up at it from underneath.
A little skill & patience & you should be fine.

 

This is a strange one, earlier on in this thread a forum member said that my Weller 40W wasn't suitable and that I should only use a 25W. Personally your advice seems to make more sense to me. Hot is quicker and less chance of heat doing damage.

As soon as I can get a clear run at it, I will replace a diode at a time and then test! I would hate for it to be the last diode and then for it still not to work. Thanks everybody for contributing!

JG

 

That's why it's been suggested you test first and replace only what needs replacing.

 

Firstly, thanks for the advice on the soldering iron power. Within 3 seconds I was able to pull away the required diode without disturbing the other wires. Also managed to replace all the diodes without disturbing the existing wires attached to the joint.

I changed two diodes at a time (8 in total) and I had a suspicion that the diode connected to the same joint as the mains earth lead was the problem, hence the fuse blowing every time I replaced it.

I got them all soldered and with trepidation I turned on the amplifier and wallah! the fuse didn't blow and all the tubes came on. So it was one of the diodes that needed replacing. That dioded had the mains earth wire connected to the same joint. The guy who built the amplifier all of 30 years ago, has a good memory and was very helpful to me.

I have left it on for a couple of hours and everything is fine. Thank God it wasn't the mains transformer.

THANK YOU TO ALL THOSE WHO WENT TO THE BOTHER OF POSTING INFORMATION ON THIS THREAD!

JG

 

Just another quick question!

Does anybody know how wires are connected to a ceramic top cap for a PL519 tube? See attached...

TOP CAP CERAMIC VALVE TUBE HOLDER FOR EL38 12E1 2PCS | eBay

Anode Cap 1/4 Inch - www.thetubestore.com

JG

 

Looks like you pull the metal insert out of the ceramic, feed the anode wire through the hole and crimp the clip terminal metal onto the wire, and then re-insert into the cover.


Otherwise, TO-99 style transistor heatsinks with a wire jammed in.


Don't know what you are working on, but it probably has about 1000+ volts anode voltage in capacitors...

 

It worked out exactly as you said, I just put the soldering iron on the wire to be desoldered for about 2-3 seconds, whipped it away and the solder hardened again in 2-3 seconds. When I was connecting new diodes I did exactly the same put the soldering iron on the joint 2-3 seconds where new diode was to be soldered and it worked a treat.

The amplifier is now working again. Thanks!

JG

 

I just noticed that one of the caps is corroding on the PL519 tube and I got the relevant information that the above cap is the correct size to use when replacing the old one. Just not sure whether it is one wire to be connected to new cap.

JG

 

Here is a pic of what i want to change... the caps on the PL519 are corroding, I want to change them and i was advised thdt the ceramic caps are the right type and fit. So the wire in the pic supplied is a single wire?

JG