trouble sourcing restistor

[Suburbanite]

Did a very dumb thing last night and got my valves round the wrong way,
4 el84 where the 4 12ax7 should be and visa versa, needless to say, Bang one cement resistor in pieces doh,

The resistor in question was an "IRC 9427 pw5a 470 10%" I can only seem to find a 470ohm 5w 5% from rs supplies, any one know if this is fine?

its number r52 on the schematic

Also while looking for a spare I came accross an artical about these cement resistors causing intemitent issues when they're legs are bent as soon as they leave the block, this might actually explain my own amps issues (loss of volume when amp heats up)

any clues?

[Enzo]

I doubt the bend is the issue.

IRC 9427 pw5a 470 10%" I can only seem to find a 470ohm 5w 5%

OK, let's go through it.

IRC is the brand - any brand will do

9427 is the date code - the part was made in the 27th week of 1994. It does not matter when your new one was made.

pw5a - 5 watt resistor - that is what you want.

470 - it's value it 470 ohms, yes.

10% - this is hte tolerance. It means the part will be within 10% of its nominal value, in this case +/-47 ohms, so a good 10% 470 ohm resistor will measure between 423 ohms and 517 ohms. This is close enough for many applications, including this one.

You found 5% parts. That means the value of the part will be within 5% of its nominal value - in other words it is more accurate than the original. Nothing wrong with that.

Before powering up again, make sure R48,49,50,51 are all OK - none went open I hope.

[Suburbanite]

Thanks for the reply, the other resistors tested out ok (phew) and I'm picking up a new resistor on the w.e, only found one copmpany who stock in london not even Maplins have it,

this is the quote about the resistor

"Another problem is how the resistors are prepped. Look at all the other resistors in your amp. The leads come straight out of the component, bend at a 90 degree angle somewhere on the lead, and then are soldered onto the PCB. The 5W resistor's leads are bent as soon as they come out of the body. You should never bend leads like this. This puts stress on the inside of the component and causes it to crack. Remember, some resistors could be burned up, cracked, and totally disfunctional and you'd never know from looking at the outside. The only way to test it is with a multimeter."

sourced from http://studentweb.eku.edu/justin_hol...resistors.html

[Enzo]

I understand what they are saying, but in my experience, I have not seen that as a factor in component failure.

REmember, the only reason your resistor failed was from the excess current caused by improper tube going into the socket. Your resistor never cracked or otherwise failed from the bent leads.

[Suburbanite]

I know, I'm an idiot forgot that the el84's and 12axl's have the same pin configoration, was working on a el34 amp before

[Enzo]

Not picking on you, we all make mistakes. About an hour ago, I was replacing the power cord on a nice old MacIntosh stereo amp. I carefully removed the old power cord - didn't want to scratch anything. Got the new cord, stripped and tinned it, and soldered it to the terminals on the inside. Of course I forgot to run it through the hole in the chassis first. So I had to do it over. And believe me I am experienced enough to know better.

I only pointed out your error to say I don't want you to worry about bending the wires, they had nothing to do with the problem.

[Suburbanite]

Oh I didn't take offence, im always wiring up xlr's without puting the sleaves on first, If I make up 10 leads i'm garenteed to do it 3 times

Any way went and got the resistor and replaced it tonight, IT LIVES

though I still have an issue with the clean channel losing volume itermitently, I't doesn't seem to happen so bad when you turn it past 2 but 2 + has a tendancy to drown out the entire band, damn but this amp is loud. I'm starting to suspect the main volume pot, can't work out a way to test it and prodding the insides with a stick (probe) hasn't pointed any fingers, Ive changed the valves, reflowed the main inputs, also could be the effects loop. I'm gonna make up a "y" lead and try the effects loop out

[Don Moose]

I wonder if one of the volume pots on the clean channel just has a dead spot or a place where the wiper doesn't make good contact when it's warm.

Does the shaft wiggle at all?

Does that make the symptom worse or happen at cooler temps?

Hope this helps!

[tim]

Suburbanite,
If it will make you feel any better, back several months ago I was working on a troublesome amp, several different problems with it and I thought I had finally had fixed the last problem...it was just one of those amps that you fix one problem and another one is exposed. About the time that I was wrapping it up and getting ready to test it, my wife calls from the kitchen and says that supper is ready. I head out to the kitchen to eat, knowing that the test would take longer than she wanted to wait for me....about 45 minutes later I get back into the shop and turn on the amp........nothing. I checked to make sure the pilot light worked, knowing full well that it was working when I shut off the amp. grabbed my meter...no 6.3 volts....no HV, nada...checked the fuse...OK, checked the trafo primaries...no 115. checked the main switch...seemd Ok....what the heck was wrong with this amp??!!!! It was just working an hour ago!!!! I was pulling my hair out...what more could be wrong with this amp???
At this point I walked out of the shop shaking my head, wanting to toss this amp into the river behind the house...I sat on the river bank for a while and smoked a cigar, calmed down a bit and then walked back into my shop....as soon as I walked into the shop, I saw the problem and I was so embarrassed...when I left to go have supper, I'd reached under the bench and unplugged the amp out of the current limit setup I have so little hands couldn't get in there while I was out of the shop...I normally don't do that but my young grandkids were over at the house and I didn't want them hurting themselves....cripes!
How much time was wasted from the time I went back to the shop after supper until I figured out what was wrong??? 45 minutes! and I've been doing this stuff for over 20 years!
We all have our forehead smacking moments...don't let it get to ya...

Tim
Electronic Sound Technologies

[Suburbanite]

The wiper feels fine, in fact everything seems fine other than this ghost fault, I had it open last night while it was having one of it's fits, and nothing I prodded or poked or wiggled seemed to make a difference, all though sometimes if you whack the volume up it goes away, like it needs to clear it's throat, also I could do with a bit more time than an hour here an hour there.

Ah well, wouldn't mind so much if it wasn't my main amp, luckily have a transistor amp for a backup but its not the same

[Suburbanite]

Found it,

After all this time, the fault is in the channel changing button, I am so damn happy right now. I only ever used the clean channel so I didn't bother with a foot switch, I plugged a foot switch in to find out whether the foot switch was latching or non latching and suddenly my sound cleaned up, I unplug it and it starts the trouble I've been having, plug it back in and.....gone.....no trouble it bypasses the dodgy switch. I think I might just link out the foot switch jack for the mo' or just put an empty jack in it until I work out how to put on a nice simple toggle switch.