Build Log: A Physician hooked on Crack (My very first DIY project)

[Grainger49]

Looking at your meter it is a digital that needs the scale selected, not autoranging.

Whether the reading rises might be a function of your meter.  First try directly across one of the capacitors.  The reading typically rises across a capacitor.  A good one to try is the first one across terminals 20 and 21.  This is a good one to test for a rising resistance.  Then swap the leads the resistance should show a negative value dropping.

 

[Laudanum]

Just noticed that you've typed 5 amp fuse.  You may have edited your post while I was typing my response yesterday or I may have just misread.  Reagrdless, the manual states a .5 amp (0.5, 1/2 amp fuse)  NOT 5 amp.  Atleast my manual does.   Again, Doc B recommended a 1 amp slo-blow fuse in a post on this board.   But dont use a 5 or 10 amp.  Make sure it's .5 or 1 amp fuse ... preferably 1 amp slo-blow.  

 

[Grainger49]

Ok, going back to the start of the resistance checkouts:

Terminals 7 and 9 (B3 & B6) are no longer zero ohms to ground, right?  The solder joint should have fixed it.

Your last readings indicate that terminal 13 is 500 ohms to ground, which you read.  So terminals 1 and 5 wired back to terminal 13 through the 22k1 plate resistors check all the way to terminal 13 (22k1 +500= 22k6).  Let's say that the lack of a climbing resistance reading is a function of you meter.  At this point every reading you have that is not right is a "climbing" reading. 

It may be the time for the smoke test (stock fuse if you don't have a 1 amp slow blow).  I know you don't have spare parts available close to where you are, but at some point you are going to have to turn the Crack on.  Cross your fingers and turn it on and off very quickly, as fast as you can.  Trust me this has a function. 

Then turn it on with it upside down (best to have it on and plug it in), have one of your meter leads clipped so you can start taking voltage readings.

Pray for no smoke.  Post back.

 

[sanadsaad]

Tried the terminal 20 and 21 resistance check. Reading is 0. Tried to start the crack twice with a 1A regular fuse(cant find slo-blo in the market). Blew up twice. I found the following replacements in the market:

-the uf4007s
-270 ohm 5W wirewound resistors
-3K ohm 10W wirewound resistors
--All the electrolytic caps

Cant find the metal film resistors anywhere though.

 

[Grainger49]

Are you saying that the whole power supply blew up?

Everything you list will work just like the original.  The 10W resistors will be larger than the ones you have, believe it or not.  How did you determine that those needed replacing.  They are not in the power supply.  Maybe they got tied to the power supply somehow?

We have to solve the problem with the power supply first.  There was a short in the high voltage to take all this out.

I don't know if you need film resistors yet.

 

[sanadsaad]

Oh no no. The fuse blew up. I went to a shop to get the fuse and checked if they have any replacement parts in case they blow up later. The shops here close for 8 days on account of Eid so I wouldnt be able to get anything if I needed em. So lets start the troubleshooting. Im going over all the wires again according to the manual to see if I missed anything.

 

[Paul Joppa]

While you are checking things, make sure of the orientation of the diodes and capacitors. This is fairly often the cause of blown fuses. Check the orientation of the sockets as well - the octal has a slot for the pin on the tube bass, and the sockets have been known to get installed 180 degrees out. Finally, make sure wires go to the correct terminal - that's another common mistake, and especially hard to see after you make the initial mistake.

 

[Grainger49]

Tried the terminal 20 and 21 resistance check. Reading is 0.  .  .  .  

This is wrong, the capacitor should be a high resistance to ground, terminal 20.  Does terminal 20 read near zero resistance to the top plate?  If the capacitor is internally shorted you get zero ohms and the fuse blows.  But I don't think this is the problem because you got a zero reading before applying the power.  A bad capacitor often, not always, has a domed end.  Ends of these power supply caps should be flat.  If not, they are bad.  

But my next suggestion is a little harder.  Remove one cap from the power supply and measure it.  Regardless of whose meter it is you should get a high reading not a low reading.

 

[sanadsaad]

The caps are all flat. One thing I should mention again. There was a revision in my manual advising me to connect a black wire from terminal 4 to terminal 14U instead of 22U. Im desoldering the terminal 20-21 cap right now. will post soon as tht's done.

 

[Laudanum]

That revision to connect from transformer terminal 4 to 14U instead of 22L is a possible solution for quieter operation of the amp.  It isnt your problem unless ... the solder joints are bad.

 

[sanadsaad]

Cap removed from 21/22. Reading climbs from 0 to infinity. Cap still off. didnt resolder it. Without the cap, the resistance b/w ground and terminal 13 climbs from 0 to infinity as well.

 

[sanadsaad]

Aaaaaaaahahahaha(crazy laugh when one gets something right)!
It was the capacitor at 20/21.
New resistance readings:

Terminal 1,2,4,5 and 13: 0 climbing to infinity

Terminal 20 and 21: 0 climbing to infinity now when previously it was giving zero.

Turns out the cap terminals werent soldered properly. Actually they were soldered well but I desoldered, tested the cap as suggested by Grainger and then tightened the mechanical connection of cap, wire and terminal and then tested resistance. Then I soldered and rechecked. Onto the third fuse then?

 

[sanadsaad]

This is odd. I left for a coffee break, came back and re-did the resistance check and now im back to the same old readings I was getting before.

 

[sanadsaad]

Its definitely a soldering issue. I find that if I remove a little solder from the joint with a wick, I get proper resistance numbers. Finally fixed it and did the voltage checks. Theyre all perfect! As it's 240VAC here in Oman, my voltage numbers are more or less double what's in the chart. The only question is regarding A9. It should be 0 but im getting 5V. Other than that, the voltage numbers are perfect! I could smell something when I was voltage testing but it was probably the tubes heating up. There was absolutely no smoke and the fuse didnt blow. Thanks guys! You're awesome. Big thanks to you Grainger for pointing the 20/21 thing out. This is one hell of a forum! Ok, now should I solder the black wire to the headphone terminals and ground em to fix the headphone issue or should I leave it at that? Cant wait to connect it to source. Another question. I have a lot of HD tracks on my laptop. Can I burn em to ordinary discs? Never done that. Anyone who has, I'd appreciate the help.

 

[Grainger49]

I have a schematic marked with terminal numbers.  According to that schematic nothing should be connected to T14 except the cap and two resistors.  The power to the 12AU7 plate should come from T13 to T4.

T22 is a ground, T4 is connected to a plate load resistor.  They shouldn't be connected, ever.  I don't see a black wire from T4 to anything in the manual I have. 

The only thing I find in the manual using the search function for Terminal 4 is a red jumper to Terminal 2.

I will look further in the morning.

 

[sanadsaad]

The headphone terminal votage shoots to 31 for less than a second on startup then its all zero from there. Should I ground it as advised or start listening without that? I read Dave's thread where he damaged his headphones and wanted to avoid it.
Also, everywhere in the voltage checking, it's said not to touch the live amplifier with both hands. After fitting it in the wooden board and puttin it on my desktop connected to the CD player, can the top plate be touched or would it conduct current?

 

[Noskipallwd]

Hello again, this is a great forum isn't it, great to have guys like Grainger and Desmond sharing their knowledge and experience. As long as you don't plug your phones in until you turn the amp on you should be safe, I used mine that way for some time. The top plate does not carry current, so you are fine. If you use two hands when measuring voltage you make a nice circuit between your hands, if you happen to receive a shock it would travel right across your heart. Electrical current takes the path of least resistance. The amp is safe during operation, just don't reach under it when it is on, and when you take it from the base for any reason turn it off and let it sit a few minutes. Hope this helps.

Cheers,
Shawn

 

[Paul Joppa]

... As it's 240VAC here in Oman, my voltage numbers are more or less double what's in the chart....

If true, that's very bad. There is a version of the power transformer designed for 240v input; all the circuit voltages will then be the same. If you have the 120v transformer, don't put 240v on it. The circuit is designed for +/-10% variations, but can't handle 100% variation!

 

[sanadsaad]

Thanks Shawn! Yea I think i'll get the headphone grounding done while I have all the equipment out on the dining table(makeshift workplace).
Paul, it's a 240V transformer! I mentioned it to Doc when I placed the order. And if the readings are for 119VAC as mentioned in the manual, shouldnt the ones for 240 be more or less double?

 

[Noskipallwd]

As it's 240VAC here in Oman, my voltage numbers are more or less double what's in the chart.

I completely missed the double part in your post, the circuit voltage readings should be the same, as Paul said. The 240v and the 120v transformer will produce nearly identical voltage and current readings on their respective output taps. I would verify what transformer you received before putting power to it again. I am not sure how to do that. I think you use resistance readings, I am sure Paul, Grainger, or Desmond will let you know what you need to do.

Cheers,
Shawn