[CopperFox]

Yes, I've found the caps can leak sometimes especially if they are squeezed into position up against other components so care is needed.

That's another interesting finding about cathode cap capacitance, I assume you mean cathode caps not WCF caps. I imagine that because you have very fast acting MKP film decoupling caps that this interacts with cathode cap capacitance to give a better overall frequency response, especially with the higher capacitance caps.

With the higher capacitance caps, the higher you go the more likely you will need to put back in the small capacitance bypass caps to restore the treble frequencies. The higher you go potentially the better transient response you will get, but at some point there will be a trade off with speed of the charge/discharge cycle.

Yes, meant the cathode bypass caps and not the wcf caps. Edited the post.

 

[CopperFox]

I also had to replace my meters after using a wrong value resistor in the wrong place! I don't expect there will be other damage.. except your pride . The resistors near the power supply need upgrading anyway as they are prone to cooking.... and that will be a good opportunity to do that. You need to use uprated Mills in that area, I will post a picture.

Is your amp still working alright?

Yes there is a resistor hidden inside those wires, I can't remember the value. but it adjusts the voltage to 12v. If I can remember I will let you know, but you can read off the resistance from the other wire.

The amp isn't starting properly. Tubes light up, UV meter lights turn up but the meters didn't move, sound came through to headphones but at a very low volume. After further looking around I noticed that some of the eight power resistors (Millls 5w 330ohm) were also darkened and/or cracked. So I took the board out and replaced those eight with 10W Ohmite Audio Gold 330ohm resistors, which I had lying around. Haven't yet tried again after that.

I noticed that some earlier posts stated that the resistor in the uv meter light wire would be 68 ohm or 0.68 ohm. But it looks like 8 ohm to me.


Edit: added photo of resistor on uv meter light wire:

 

[CopperFox]

These are the main PSU resistors circled to upgrade with 5 watt Mills, read off board values. Also at point 3 in the pic upgrade the Zeners as well as MIlls too as they can be a bottleneck, to make sure you don't run into problems down the line.

Yeah, the two blue resistors in point 3 were the ones that got charred. Will be replacing those with something. - what would be the best here? There are two other components inside that circle too, which are smaller and black/gray and have a letter Z on them. The board has the text "100V 1W" on those spots. What are those components and is there any recommendation for their replacement?

Although the two blue resistors there got literally charred, those two Z components at least look alright. Maybe it would still be good to replace those.


Edit: reading back on the thread, there were advice also to replace the PNP / NPN transistors in cases where those two blue resistors were burned. Where would these transistors be located?

Edit #2: Below that middle hump on the case?

 

[baronbeehive]

The amp isn't starting properly. Tubes light up, UV meter lights turn up but the meters didn't move, sound came through to headphones but at a very low volume. After further looking around I noticed that some of the eight power resistors (Millls 5w 330ohm) were also darkened and/or cracked. So I took the board out and replaced those eight with 10W Ohmite Audio Gold 330ohm resistors, which I had lying around. Haven't yet tried again after that.

I noticed that some earlier posts stated that the resistor in the uv meter light wire would be 68 ohm or 0.68 ohm. But it looks like 8 ohm to me.


Edit: added photo of resistor on uv meter light wire:

It looks from your pic that the resistor bands are: brown, black, gold and red in which case it would be 1R, so probably .68R would be right. You can check resistor values here: https://www.allaboutcircuits.com/tools/resistor-color-code-calculator/

 

[baronbeehive]

Yeah, the two blue resistors in point 3 were the ones that got charred. Will be replacing those with something. - what would be the best here? There are two other components inside that circle too, which are smaller and black/gray and have a letter Z on them. The board has the text "100V 1W" on those spots. What are those components and is there any recommendation for their replacement?

Although the two blue resistors there got literally charred, those two Z components at least look alright. Maybe it would still be good to replace those.


Edit: reading back on the thread, there were advice also to replace the PNP / NPN transistors in cases where those two blue resistors were burned. Where would these transistors be located?

Edit #2: Below that middle hump on the case?

You should replace all those resistors around the PSU area with uprated Mills 5w which will insure against future problems like we have all done. Looks like yours have burnt out with a surge in those zener diodes, which are those black components with a z on them. You should replace these with uprated zeners also. When you replace them make sure you put in the new ones the right way round, check out the line going round one end.

Yes I would definately advise to change the transistors as well as the originals were prone to failure with increased loads. It sounds a lot but once you have done all this, and now would be a good time to do it, you shouldn't have to touch the PCB again, hopefully .

Yes they're under those covers if I remember right, look for 3 wires going into that area from the inside of the top chassis. Make sure you replace PNP with PNP and NPN with NPN and make sure there is no contact with the metal on the chassis, there should be an insulated pad there between the transistors and the chassis!

We mentioned what are the replacement transistors back in the thread, the originals are superseeded now with better ones.

 

[baronbeehive]

We've all been through this BTW .

Have you got a multimeter? It would come in handy if you need to check resistor values to find the fault you have with no sound.

 

[baronbeehive]

Edit: reading back on the thread, there were advice also to replace the PNP / NPN transistors in cases where those two blue resistors were burned. Where would these transistors be located?

We like you.. you actually read the thread, most people think it's too boring lol!

 

[CopperFox]

It looks from your pic that the resistor bands are: brown, black, gold and red in which case it would be 1R, so probably .68R would be right. You can check resistor values here: https://www.allaboutcircuits.com/tools/resistor-color-code-calculator/

Hmm. To me that looks clearly gray, black, gold and brown which would be 8R. That's what I put there and the meter did light up. Looked more bright than the other side though, could be from having a newer light. Or something else.

 

[CopperFox]

We've all been through this BTW .

Have you got a multimeter? It would come in handy if you need to check resistor values to find the fault you have with no sound.

No, but those are not expensive. Think I haven't used a multimeter since elementary school

 

[baronbeehive]

Hmm. To me that looks clearly gray, black, gold and brown which would be 8R. That's what I put there and the meter did light up. Looked more bright than the other side though, could be from having a newer light. Or something else.

You can always check with the other side, if I can find out I will let you know.

If it's the old meter it might need replacing.

 

[baronbeehive]

No, but those are not expensive. Think I haven't used a multimeter since elementary school

It's very useful to have around, if you need to fault find. Really you should check the circuit after every time you work on it on the components you have worked on, before you switch back on. It might save a lot of grief later .

 

[baronbeehive]

I documented my mods fairly exhaustively, but so much has happened along the way I can't remember it all unfortunately .

These are the transistors to get if you decide to replace them:

1 x MJ15024G (NPN) for B+
1 x MJ15025G (PNP) for B-

You're right the resistor in the ammeter wire is 68 Ohm, I think it was measured at each end of the wire so the wire's resistance will be taken into account also. There is a strong possibility that you have fried that ammeter.. or at least partially cooked it.. that's another good reason to have a multimeter lying around, you could have checked it on the terminals to see if the resistance matches the other one.

Re: the lack of sound, the current surge could have triggered the headphone protection circuit.

 

[Maxx134]

sound came through to headphones but at a very low volume.

That's the driver stage your hearing and a good sign only your output stage is off. Most likely cause you already pointed out is the PSU.
No prob, It needed upgrading anyways.

Re: the lack of sound, the current surge could have triggered the headphone protection circuit

For sure the output stage is not working, and most likely due to the power supply failure.

The protection circuit can also be enabled because of this.

Yes I would definately advise to change the transistors as well as the originals were prone to failure with increased loads

Yes and as I recall, the diodes die first, causing the transistors to die next.
Hopefully they didn't blow if he caught in time.

Edit: reading back on the thread, there were advice also to replace the PNP / NPN transistors in cases where those two blue resistors were burned

I was going to replace mine, but I haven't, as they didn't blow up on me yet.
They blow up easier and are obsolete and mouser parts have the updated version which will not easily die.
I would advise to first change the resistors and especially the diodes, because the transistors will die again if those cheap diodes are dead, as usually the case.

For the diodes, get higher rated wattage, but same voltage. I believe they were 100v, and you can even add two in parallel for better heat dissipate and better current.
So regardless of the burnt looking resistors, which most probably went up in value, you must change the diodes which are first to blow and cause the transistors to go right after.
If your lucky the transistors are ok but would need to check on a multi meter with a transistor tester built in.
That's allot of trouble to take out, so try the diodes first, and if the resistors start to cook, your know it's the transistors.
You must buy as a set of two (pnp& non)

Another BIG issue is that upon moving the board, it is booby-trapped with garbage tin wires that run from the board to the transistors (located on unit body ).
TAKE PICS to remember and replaced these short wires as it is essential. They break off easily and then sparks fly!
Even when hit or board closed, they can just call off thru vibration. They crumble because they're brittle, like stranded tin. Just replace them.

 

[Maxx134]

Last advice is that you must be careful with the board trace when pulling out the diodes.
If I recall correctly, there is a bottom trace in that area that goes from one side to the other, so very important not to pull the pads of the board there.
You could do a search in thread for that.

 

[baronbeehive]

Another BIG issue is that upon moving the board, it is booby-trapped with garbage tin wires that run from the board to the transistors (located on unit body ).
TAKE PICS to remember and replaced these short wires as it is essential. They break off easily and then sparks fly!
Even when hit or board closed, they can just call off thru vibration. They crumble because they're brittle, like stranded tin. Just replace them.

Yes those wires are probably THE most fragile of all the other very fragile wires! I think he would benefit from changing the other wire runs as well especially the output wires. ATM I have OCC silver to the outputs and OCC copper to the headphone socket, and at some point will replace the whole run to the headphone socket with silver. I want to maximise the treble textures and so this is the last thing I will probably do when I can face opening it up again .

I will also change a couple of resistors in the PSU area and check circuits at a couple of points I noticed last time I opened it up.

@CopperFox you would benefit from this in improved SQ definately. The existing wires are crap.