Ampeg Reverbrocket blowing fuse

[mpl006]

I have an early 90's Ampeg Reverbrocket R12R reissue that has developed a slight problem. When turning on the amp, still in standby, everything works great, tubes start glowing, fuses stay together, but when I turn it off of standby, everything goes wrong. First the amp starts humming with increasing volume, same freq., until the fuse starts glowing and then blows.

Here are some of the observations I have made:

1. Problem happens if there are tubes in it or not, solid-state rectifier.
2. OT shows about 130 DC ohms on either side of primary.
3. Problem does not happen if I remove the center tap of the OT primary.

The two things I have tried so far have been to change the tubes with known good tubes with no success and also changed all electrolytic caps. The other problem that I found is the resistor that goes between the center tap and the tube grids was spilt in have so I change it out with the same value when I changed the caps. I am now at a lose and would love to get some advice. I am an Electrical Engineer that loves to fix my own stuff, so I am fighting having to bring it to the shop.

Any advice would be greatly appreciated.

[52 Bill]

Originally Posted by mpl006

1. Problem happens if there are tubes in it or not, solid-state rectifier.
2. OT shows about 130 DC ohms on either side of primary.
3. Problem does not happen if I remove the center tap of the OT primary.

Are you saying that the amp hums and then blows the fuse even with no tubes installed?

Pulling the CT of the output transformer would lead me to a first guess that the output transformer is shorted. Try reading resisitance from the primary to the case/ground.

When you pull off the OT CT, do you have normal power supply voltage readings?

[mpl006]

Thanks for the reply,

Yes the amp hums and blows the fuse no matter if they are in or not.

I checked the resistance from the case to the primary and I don't remember what I got but I don't think it was shorted to it, I will check again when I get home.

What part of the power supply should I test? I haven't gotten into to much voltage testing becuase I wasn't confident with myself when it happened, over a year and a half now, but since then I have built a JTM 45 so I feel a little better about myself.

[52 Bill]

Check the voltages directly after the rectifier, say on the filter caps. Just want to know that everything is ok there and limit your search to the problem area.

The transformer will be a problem if there is any resistance between the case and the primary winding, it should read infinity. When you take this reading, don't hold the meter leads in both of your hands, as it will read the resistance of your skin.

Thinking about it, be sure to check the power tube sockets and the pc traces around them for shorts, burn marks, carbon traces, etc.

[Amp Kat]

Also check your standby switch and see if it is working properly but be sure your caps are discharged before doing these test. If the standby is open it will not allow it to discharge as fast with such a high impedance trickle effect and could be some HV there. Check the switch from ground to each side. Also agree if you get 130 ohms across windings you should get almost exactly 65 ohms from each leg to the CT and if it's open or way less than 65 I would suspect a bad OT. That is a cool amp and have one in my shop right now. It has some nice features like the locking key and the attentuator and they really sound nice so we need get you back up.

[Twist]

I don't remember if this model has them but look for two diodes in series from pin 3 to ground on the output tube sockets. They may be shorted.

[mpl006]

O.K. was able to do a few more tests today.

Checked resistance between primary legs to case of transformer: completely opened. Meter did not act as though I had hooked it up to anything.

Checked resistance between primary CT and case: same result. Infinite resistance.

Checked resistance between secondary legs and primary legs: Infinite resistance once again.

Checked resistance between secondary legs and case: 0.1 ohms. Continuity check showed short. Wiggled wires and the resistance went to infinity. Wiggled them again and back to 0.1 ohms.

Checked the stand by switch and it checked out ok.

Checked voltages and were within 10% of the voltages I found on the schematic at schematic heaven.

Got on here to list my findings and saw the reply from Twist so I went to check the diodes and my meter was showing them as opened. I tested the meter against some known good diodes to make sure I was getting an accurate reading and my meter is showing a short even with no leads plugged in. Guess I get to get a good meter now.

I will check the diodes out tomorrow just to make sure they are good but from what I have read online, the fact that the secondaries are shorted to the case, the OT is gone. Is this correct? If so, does anyone know where I could find a replacement? The closest service center is about 2 hours from where I live and I don't know if they would just order the part for me or insist that they do the repair themselves.

Thanks for all your advice.

[Amp Kat]

Make sure when you get the new meter to check from CT of the OT (which is NOT grounded to case) to the actual B+ supply of each leg of the primary side. If it's 65 ohms I'm thinking your tranny is good and you have another problem.

[52 Bill]

Checked resistance between secondary legs and case: 0.1 ohms. Continuity check showed short. Wiggled wires and the resistance went to infinity. Wiggled them again and back to 0.1 ohms.
Double check the secondary resistance readings to the case that you got. There should be no continuity or reading from the winding to the case. Though this would not cause the fuse to blow at power up.

Checked voltages and were within 10% of the voltages I found on the schematic at schematic heaven.
If all of the voltages are ok without the OT hooked up, then the problem is either the transformer itself or something that the transformer supplies voltage to.

Try this, with no output tubes, hook up the primary center tap of the transformer to the B+ connection as normal. Unhook both primary plate wires that go to the output tubes. Power up and see if the fuse blows. If it does, then the problem is in the transformer, if it doesn't, the problem is in the circuit.

[mpl006]

Ok so I finally got my meter back up and running. I checked the primary sides of the transformer and they are balanced on either side of the CT and none of the leads are ground to the chassis. I recheck the secondary leads and they are both still showing a short to the chassis. I hooked up the CT of the transformer with no power tubes in and the amp let me turn it on and off of standby. I then tried hooking up one of the sides and then tried it again and the amp started humming and the fuse blew. I checked the diodes to ground and they all tested good. I'm am at a lose and am stracthing my head once again.

Thanks again for all of the help so far. Hopefully we can get this thing up and running again.

[Dave Curtis, dB AudioTech]

Checked resistance between secondary legs and case: 0.1 ohms. Continuity check showed short. Wiggled wires and the resistance went to infinity. Wiggled them again and back to 0.1 ohms.

Hmmm...

[52 Bill]

I recheck the secondary leads and they are both still showing a short to the chassis.
Is this with the transformer secondary leads hooked up to the circuit or completely unhooked from circuit?

I hooked up the CT of the transformer with no power tubes in and the amp let me turn it on and off of standby. I then tried hooking up one of the sides and then tried it again and the amp started humming and the fuse blew.
Then there is something shorting the B+ to ground on the tube sockets or in some other part of the power tube circuitry. The only thing that the output transformer supplies voltage to is the plates of the output tubes. Check the power tube sockets for signs of arcing or shorts, etc. If there is a carbon trace on the socket it will act like a resistor and short out the power supply. Use a magnifier and a bright light to visually inspect between the pins. Pay particular attention to areas around the plate and screen pins.

If your amp has flyback diodes connected from the tube plates to ground, make sure these are not shorted.

Also, you said that you replaced a resistor and filter caps early on. Recheck your work, maybe a bridged solder joint, just check everything that you did to be sure that there are no problems there.

[Twist]

If this is a SLM speaker tranny then the black speaker wire is grounded to the frame of the transformer via a small bare wire. Does it have DF-226 stamped on the side? This may not be the case with all of them but I have a couple of new ones in stock and they are grounded this way. You can see the bare wire right under the black ground wire. So if your checking for shorts on the secondary side it will appeared shorted to the frame. As for the other issue with the fuse blowing don't give up. It's something simple I'm sure.

[52 Bill]

Twist- That's why I asked if the secondary was connected in circuit. I didn't know if he was reading the secondary winding or the ground connection to the case.

[Twist]

Remove the flyback diodes completely. It wont hurt to check it without them.

[mpl006]

Well I was finally able to check to diodes with a good meter and two of them on one of the power tubes were shorted. I was then discouraged because I could not get an 4007's local but remembered I had some in a different project that is not as important. I just got through changing them out and the amp sounds great again. Thanks to all of you for your insight and help in figureing this out. I really appreciate it.