This was working great not long ago. I had it serviced and several caps replaced but recently, I turned it on and there was a loud hum for about a second before the fuse blew. Replaced the fuse twice with the same results. Any thoughts on what might be the culprit? Have multimeter. Thanks- Chews.
Try with power tubes removed.
No more fuses! Limiting lamp (http://www.ssguitar.com/index.php?topic=2093.0) naow.
Perhaps not all the suspect caps were replaced.
This is fairly typical of a valve amp that has been in storage for a long time, and the electrolytic caps in the power supply can de-form or lose their electrolytic layer and go low resistance.
Depending on how bad they are they can sometimes be re-formed by energising via a low wattage (20-40W) limiting lamp for a while. If they don't show a drop in current within a few minutes then replace.
Should also check the main rectifier diodes.
Ok- I built a limiter lamp and plugged the Peaver into it. I have a 15 watt bulb in it right now. Don't have a 20 to 40 handy- figured a 15 would be ok for now.
When I turn on the amp the bulb comes on probably its full brightness. The fuse does not blow. The amp doesn't look like it is coming to life though- no glowing tubes- no power light.
I've been letting it set for several minutes like this- not quite sure how to test the capacitors or where the rectifier diodes are. Doesn't look like any of the capacitors in the power section have been replaced. Several on the preamp board have been replaced however.
10-15 minutes passed and light still at full brightness and not life in amp. Turned off, turned on a few times... nothing.
Did you take out the power tubes yet?
Yes- turned it on without the power tubes in and still shorts out somewhere.
I would want to increase the light bulb to 40 watts. Check to see if the bulb goes dim with tubes removed and a higher wattage bulb.
Also Roly had suggested to test the rectifier diodes. If your multimeter has a diode check setting this can be easily done. Set meter to diode setting, check black meter on Cathode and red meter on Anode. You will sometimes see a combined reading of two diodes in series if the diodes are still in circuit. To get a more accurate test on the diodes in series you will have to lift one leg on the component.
Also, to point out that there is High voltage on this amp and it will be best to learn to drain the caps. Do you know how? Just want to make sure none of those 350v caps have a charge while you are touching anything in the amp. Use voltmeter, connect black probe to ground, to test either side of those 350v filter caps to confirm if there is a stored charge. If there is a charge you need to be able to know how to drain the caps.
Here is the schematic for all to share here and a picture of testing a diode. Good luck!
EDIT: Also could be a shorted filter cap too. At least test the diodes to clear them from the equation.
Ok- I discharged caps per instructional video that said hook one end of wire to chassis and the other to pin 1 of a tube socket. I guess it worked- I didn't touch the caps to find out.
Then I tested the diodes with my multimeter- it indeed has a diode testing feature. The ones at the front (top in the pic) all tested roughly between 7k and 10k. I tested the ones at the back and they all tested very close to each other at around 5 M. I guess this means they are OK?
I'll need to stop at the store and pick up a 40 watt bulb- I have plenty of 60 watters laying around if one of those might work.
Found a 40 watt bulb-
I turned on the amp and I guess it was probably half the brightness as it should be.
Turned off and removed tubes and turned back on- still seemed to be about the same brightness.
Turned off and replaced tubes and turned back on- light bulb got WAY dim-- almost barely on and for a brief moment the power indicator lamp on the front of the amp came on -- but then went out. Bulb still very dim.
??
From what I am reading it seems that the amp is not pulling excessive current. The power indicator will also be dim as the voltage will be lower with the light bulb limiter. It would seem that you measured the diodes with ohms setting on the meter and not diode check, still a valid way to test. It can be confusing measuring diodes in circuit, but you can also check to see if any are shorted. If we see resistance across them then they are not shorted.
As far as not touching the caps to find out if they were discharged... Use your meter still to confirm that the voltage has been drained. There are ways you can even use your meter to monitor, in real time, the voltage being drained and all you need are clip on leads or something.
Still something is causing the amp to blow fuses... The 60 watt bulb would have been fine too. What I notice is that a 40 watt bulb will glow brighter than a 60 or 100 watt bulb on a working amp.
Read this on diode checking too...
http://www.allaboutcircuits.com/vol_3/chpt_3/2.html
I am a bit stumped as to what the problem is at the very moment. I would double check those diodes and run the amp at 60 watt bulb too. Let's also see what anyone else here says in addition to my comments. BTW do you have a speaker or load plugged in on the output?
Edit: Just to point out that when I wrote "If we see resistance across them then they are not shorted" that I mean high resistance. A shorted diode will have very low resistance both ways like a piece of wire. Just thought I better make my statement hold true there that's all.
I did not have a load hooked up to it. I plugged it into an 8 ohm cab and switched to a 60 watt bulb. Turned it on and the bulb got very dim and the power indicator lamp on the amp came on and stayed on. Very faint glow in the power tubes too.
I just rechecked the front diodes (this time using the correct diode testing settings) and they all read right at .495 +/- .001 or .002. The rear diodes are all measuring around .52.
The dim bulb indicates the short is not present right now.
It's possible one of your power tubes has an intermittent short/failure.
ARITE- I think I found the problem.
I plugged the amp into a regular outlet and sure enough it fired up like normal. I gave the tubes a bit of a wiggle just to make sure they were seated good and then "POP" goes the fuse.
I put another fuse in and plugged it back into the limiter and when I wiggled the tube closest to the power transformer, it would crackle. On closer inspection and probing with a wooden skewer, a little black diode bridging two of the pins was sparking when I poked it and shorting out. I unplugged everything and took a close look at it and there is a crack running almost the length of this little guy.
Guess I'm off to the electronics store! :dbtu:
Edit to add: This diode is marked "2873." Are they readily available? Should I use something else/better? I'm thinking I should replace the one in the other socket as well just for good measure?
Those are SR-2873 diodes, rated for 2000V. Probably no longer available except through peavey.
Some people use 3 1N4007's in series instead.
PV would have them. There is nothing absolute about their value, so if I run out of the ones PV uses, I just grab a couple R3000 from my Fender bins. All those types do the same thing.
Given their position on the OP valve sockets these will be diodes intended to protect the OPT if the amp if fired up without a load, e.g.;
(http://www.ozvalveamps.org/eti/eti456scan31an-p.gif)
I would therefore give the whole OPT secondary circuit a really good scrute for any possible intermittent opens, lazy OP socket contacts, broken braids on the speaker(s), &c.
Just ordered two of these from Tube Depot: R3000 .25A / 3000V High Voltage Protection Diode
Checked with Peavey and they did have the original diodes but they had a minimum order $$ plus shipping would have made them too much to justify when I was able to get the R3000's for less than $3 shipped.
Should be here in a few days. :dbtu:
I just got the diodes. I want to be sure I only need one per tube socket. They are really small compared to the originals.
Also, which end goes to ground? There's a silver stripe going around one end of the diode.
Thanks!
Think about in terms of just replacing the part in the same orientation that the other part was connected. You have the old diodes and the new diodes with a stripe on one end of both type of parts. The striped end is the cathode and the other side is the anode. Looking at your pictures it is easy to see how to just replace the part in the correct orientation. Also, look back at your schematic and find those diodes(CR16 & CR17) on the power amp, the anode is clearly going to chassis ground. Still we can assume the amp was working great before and the old diodes were original, thereby correctly installed.
Edit: Also the peavey 2873 diode is 2kv rating with 250ma(0.25A) rating. So the R3000 .25A / 3000V rating means that just one will be sufficient. Actually the R3000 is a higher rated part as you can see. I have read some other data sheets on the R3000 that had it at 200ma(0.2A), but that might have been a different manufacturer. If they say that is the rating then it must be accurate, I hope.
Double Edit: All the info is actually stated on the tube depot site where you bought the part. Telling you the cathode goes the plate of the tube and anode to ground. Also, tells you what the part is designed for in the fender amp example.
https://tubedepot.com/products/r3000-25a-3000v-high-voltage-protection-diode
Don't forget to read Roly's post again(reply#15) and check over the items he listed that might have caused this diode to go bad.
Replaced the diode and checked for frayed wires and such- couldn't find anything obvious that might cause the diode to fail.
Fired it up and it seems to be working fine again. Played it for about an hour and sounds right to me. Of course I'll keep my eye (or ear) on it. If it fails again anytime soon I'll know to look elsewhere for the cause.
Thanks everyone for the help on this! I really appreciate it. :dbtu:
Chews.
Frayed wires are not what those diodes care about. They die when large transient voltages occur and they must deal with them. The kind that occur when the speaker goes intermittent or the connections to it fail.
Hi guys; my first post. I have here a Classic VT series 100, 2 tube 50 watt. Was a combo, someone cut it to a head. I was asked to repair it. I have read many posts about blowing fuses, and checked many components including all diodes.. Can't find the problem. I have less of a legit troubleshooter, more of a 'check for breaks' education.
The preamp board has a couple ugly repairs, a few off-value E caps. C10 350V is leaking and reads 15 microF.
One of the large power board 100 microF Ecaps reads .39 nanofarad. That seems a bit too far out of range.
With tubes out, the input power fuse blows immediately.
I dont want to throw parts at this thing since it isnt mine.
Can someone help?
Hi, always better to start a new thread for a different amp, instead of hopping on the discussion of someone else's amp.
Pull the tubes.
Isolate your problem. It is in the circuits or in the power supply. You have all the fuses in the power supply? Note where they go and remove them. Does the amp still blow the main fuse? If so, then your transformer is suspect. Rare, but it happens. If the amp now will hold a fuse, put the secondary fuses back in one at a time to see which one makes the mains fuse go off.
By the way, no point in wasting fuses, look up "light bulb limiter" then make one and use it.
SOMETHING is blowing your fuses. The transformer itself, or something associated with the secondary supplies. We need to determine WHAT. Usually the high voltage circuits are responsible for fuses blowing. Very often one of the flyback diodes is shorted. Those are the ones from pin 3 of each power tune to ground. Rarely but happens, the output transformer itself. Is there a low resistance to ground from either pin 3?
Filter caps don't usually "short out" but it happens. be aware that your ohm meter works on just a volt or two, and a cap tested at a volt or two may "check" fine, but leak like a screen door at 200v or more, and since these caps do work at hundreds of volts we ned to keep that in mind.