66 Pro Reverb blowing fuses after I replaced some components

[sthomas]

I have started to renew an old amp: Replaced filter caps and some electrolytics on the board. I put in a 3 prong cord. The transformer wires are white and black. I hooked up the white as hot and changed some resistors on the preamp section, and the resitor and rectifiier on the bias. I also put new resistors on the input on the effects channel.

It is noisy on start up and blows the fuse after a few chords.

It does not pop the fuse when no power tubes are in. I changed the power tubes and then it popped the fuse again.

Any suggestions. I am new and not too technical.

Thank you

[Enzo]

White is neutral and black is hot, but reversing them shouldn't matter here.

You replaced the bias rectifier?? I am not sure I have ever had to replace the bias rectifier in a Fender amp. But you do realize that bias is a negative voltage, so you may have installed it backwards. The stripe end is towards the transformer wire, and the other end goes to the bias filter cap.

Also, the bias filter caps must be "upside down" their + end is grounded. Make sure that is right.

It doesn;t blow fuses with the power tubes out? FIne. Probe the socket pins with a volt meter. With teh amop running, make sure there is B+ voltage on both pins 3 and 4 of the sockets, and importantly, that ther is NEGATIVE voltage on the pin 5s.

[sthomas]

Enzo Thank you for the info

White is neutral and black is hot, but reversing them shouldn't matter here.

You replaced the bias rectifier?? I am not sure I have ever had to replace the bias rectifier in a Fender amp. But you do realize that bias is a negative voltage, so you may have installed it backwards. The stripe end is towards the transformer wire, and the other end goes to the bias filter cap.

Also, the bias filter caps must be "upside down" their + end is grounded. Make sure that is right.

It doesn;t blow fuses with the power tubes out? FIne. Probe the socket pins with a volt meter. With teh amop running, make sure there is B+ voltage on both pins 3 and 4 of the sockets, and importantly, that ther is NEGATIVE voltage on the pin 5s.

Hi Enzo, I put the recifier with the stripe going to toward the resistor that links the rectifier to the blue and red transformer wire. Also the positive end of my bias cap is hooked to the ground and the negative side is linked to the non-striped side of the rectifier.

Any advice on how to probe the tube pins would be appreciated. Do I probe the red to ground and the black to the pins to take the reading? Also what setting should I put my meter to?: I have 600, 200, 20, 2000M and 200M settings available.

Also, when probing I assume the standby switch should be deactivated. Correct?

Thank you

[Enzo]

When taking voltage readings, your black probe is the reference. The black probe should go to ground unless otherwise directed. Think of it like measuring how tall something is. Your zero end of the ruler is always on the floor, and the other end tells the height. And the unless otherwise directed part would be like the thing is not on the floor, but on a table. Then you'd start from teh table not the floor.

The reb probe then is used to take the readings.

We are looking to see if the operating voltages are present and correct at the tube socket. The amp must be completely on all the switches - power and standby - must be in the on position - the operate position. Otherwise the voltages would be turned off, and voltages are notoriously difficult to measure when they are turned off.

SO ground the black probe and visit the socket pins with the red. You are measuring DC voltages. Your B+ won't be over 500v, but it will be over 300v, so that 600v scale would be where to start. The bias voltage will be negative and probably in the 30-50v area. You can read that on the 600v scale, or step down to the 200v scale.

When you don;t know what a voltage will be, always start on the highest voltage scale, you can always change if if the reading is too small.


It is entirely possible the whole problem is a failed power tube.


And from your own admission and the nature of your questions, you are not very experienced. The voltages inside these amps can KILL YOU. Be very careful inside the amp.

[sthomas]

Thanks again

When taking voltage readings, your black probe is the reference. The black probe should go to ground unless otherwise directed. Think of it like measuring how tall something is. Your zero end of the ruler is always on the floor, and the other end tells the height. And the unless otherwise directed part would be like the thing is not on the floor, but on a table. Then you'd start from teh table not the floor.

The reb probe then is used to take the readings.

We are looking to see if the operating voltages are present and correct at the tube socket. The amp must be completely on all the switches - power and standby - must be in the on position - the operate position. Otherwise the voltages would be turned off, and voltages are notoriously difficult to measure when they are turned off.

SO ground the black probe and visit the socket pins with the red. You are measuring DC voltages. Your B+ won't be over 500v, but it will be over 300v, so that 600v scale would be where to start. The bias voltage will be negative and probably in the 30-50v area. You can read that on the 600v scale, or step down to the 200v scale.

When you don;t know what a voltage will be, always start on the highest voltage scale, you can always change if if the reading is too small.


It is entirely possible the whole problem is a failed power tube.


And from your own admission and the nature of your questions, you are not very experienced. The voltages inside these amps can KILL YOU. Be very careful inside the amp.

Enzo,
I really appreciate all of this information. I am aware of the dangers (draining caps when working on it when power is not needed), but being new @ this I must keep reminding myself of the importance of safety. Being out of town now, I will need to check the voltage correctly tomorrow and let you (or any other interested user) know what I come up with .

I will admit that in haste I took some readings @ 600v but I put the red to ground and the black to the pins. Whether it is valuable information or not, Both pin #3's read -491 and both pin # 4's read -497 and both pin 5's fluctuated between -001 and -004. Also when I probed pin #3 on both tubes, a slight noise came out of the speakers. I hope I did not screw things up.

Thanks again,

Scott

[Enzo]

Getting your meter leads backwards merely reverses the polarity of the reading. So you got -491v instead of 491 volts. No harm. Touching those pin 3s and getting a little noise is normal.

Those pin 5 readings confirm my original suspicion, you have lost your bias to the tubes. I am looing for -40v, not -4v. And that -4 you did get was with the meter reversed, so it was really +4.

Go back to the bias suply we discussed. Let's learn a term or two. The line end or banded end of a diode is the cathode. The non-line end is the anode. You have that filter cap with the + end to ground and the - end to the anode of the diode. What voltage is at the point they meet? In other words how much voltage is across (end to end) the filter cap? Better be about -50vDC.

If there is voltage there, move over to that little control mounted nearby off the corner of the parts board. It is the control that adjusts with a screwdriver. Got -50 or so on its pins?

[sthomas]

Great information

Hi Enzo,

Later today when I get back home I will follow your instructions.

Thanks again, this is great stuff!

Scott

[sthomas]

very low reading

Getting your meter leads backwards merely reverses the polarity of the reading. So you got -491v instead of 491 volts. No harm. Touching those pin 3s and getting a little noise is normal.

Those pin 5 readings confirm my original suspicion, you have lost your bias to the tubes. I am looing for -40v, not -4v. And that -4 you did get was with the meter reversed, so it was really +4.

Go back to the bias suply we discussed. Let's learn a term or two. The line end or banded end of a diode is the cathode. The non-line end is the anode. You have that filter cap with the + end to ground and the - end to the anode of the diode. What voltage is at the point they meet? In other words how much voltage is across (end to end) the filter cap? Better be about -50vDC.

If there is voltage there, move over to that little control mounted nearby off the corner of the parts board. It is the control that adjusts with a screwdriver. Got -50 or so on its pins?

Hi Enzo, I tested and even after adjusting the bias screw both ways, the best reading I got was -004. What now?

Thansk

[Enzo]

twiddling the bias control will do nothing if the bias power supply is not functioning. That's like turning up the furnace when the gas is shut off.

I asked what voltage you had across the bias filter cap? Well? And then at the terminals of the bias pot? Just center the bias control until you get some actual bias.

You don;t have any bias at the tube, so we need to find out where it went. SO we start at the bias filter cap.

[sthomas]

twiddling the bias control will do nothing if the bias power supply is not functioning. That's like turning up the furnace when the gas is shut off.

I asked what voltage you had across the bias filter cap? Well? And then at the terminals of the bias pot? Just center the bias control until you get some actual bias.

You don;t have any bias at the tube, so we need to find out where it went. SO we start at the bias filter cap.

I did test the voltage of the bias cap a # of times but forgot to report it. Last night and a couple times today the reading was minimal and fluctuating from .7 to -.3. @ the filter cap negative side and almost zero @ pin 5's. However after reading your reply here I probed one more time for good measure and something very strange has happened though.

With the black probe grounded and the red probe @ the reading points I just got:

-69.6 @ the bias cap
-69.6 @ the leg of the pot leading to the bias cap with pot screw centered
-59.8 @ the leg of the pot leading to the resistors with the pot screw centered
-58.4 @ both pin 5's.

I just did a reading one more time and got 0 @ all points, then scraped along the wire @ the bias cap and it came back up to-69+. Tried again and it went to zero (zero @ the #5 pins too), fiddled again and it went back up @ all points. Just to be sure it was not my meter, throughout the these exercises I tested pins 3 & 4 and the always read as before. What could it be?

[Enzo]

Attach clip wires to the end of your meter probes,and then clip those to the points you are testing. Then with hands now free, wiggle each wire that is involved, and push each involved component side to side a bit. Use a wooden chopstick or some other insulated thing for this. Watch the meter. ANy chance that resistor before the diode might be cracked?

You replaced the cap and diode already, right? Did you replace the resistor? I think the resistor and cathode of the diode share an eyelet hole, so they were resoldered, but did you resolder the other end of the resistor? That shares the eyelet with teh transformer wire, right?

You have an intermittant problem. Looks to me like if you get voltage at the bias filter cap, it will be present all the way to the tubes. SO if we just concentrate on keeping voltage on that cap, we ought to solve this. So clip the meter to that cap and start poking.

And just so you know, and especially on older stuff, I often have to scrape along the wire leads of parts to get electrical contact with my meter probes. I usually rub up and down a couple times as I start. Like making a circle or two with a ball point pen before writing to get the ink moving.

[sthomas]

moving forward

Attach clip wires to the end of your meter probes,and then clip those to the points you are testing. Then with hands now free, wiggle each wire that is involved, and push each involved component side to side a bit. Use a wooden chopstick or some other insulated thing for this. Watch the meter. ANy chance that resistor before the diode might be cracked?

You replaced the cap and diode already, right? Did you replace the resistor? I think the resistor and cathode of the diode share an eyelet hole, so they were resoldered, but did you resolder the other end of the resistor? That shares the eyelet with teh transformer wire, right?

You have an intermittant problem. Looks to me like if you get voltage at the bias filter cap, it will be present all the way to the tubes. SO if we just concentrate on keeping voltage on that cap, we ought to solve this. So clip the meter to that cap and start poking.

And just so you know, and especially on older stuff, I often have to scrape along the wire leads of parts to get electrical contact with my meter probes. I usually rub up and down a couple times as I start. Like making a circle or two with a ball point pen before writing to get the ink moving.

Enzo, yes I replaced the resistor between the diode and the transformer wire. As far as I know I soldered it well. I will do as you suggested. So I will clip he bias cap and start to check. So far it seems like it may be the junction where the negative end of the bias cap, the diode, and the white wire that leads to the 22m resistor to the trem speed caps and vibrato pedal.

Are there any specific locations where I should concentrate?

Also were the negative readings -69.6 @ the bias cap, etc. meaninful? Too high? Too low?

[sthomas]

Checked it, plugged guitar in, it works

Enzo, yes I replaced the resistor between the diode and the transformer wire. As far as I know I soldered it well. I will do as you suggested. So I will clip he bias cap and start to check. So far it seems like it may be the junction where the negative end of the bias cap, the diode, and the white wire that leads to the 22m resistor to the trem speed caps and vibrato pedal.

Are there any specific locations where I should concentrate?

Also were the negative readings -69.6 @ the bias cap, etc. meaninful? Too high? Too low?

Hi Enzo, I did some probing and after a bit lost the bias @ one point when I was probing @ the on off switch. I moved the on off switch wires around a bit then rechecked and the bias cap was still @ zero. I tugged on the white wire that runs from - bias cap side to the 220k resistor by the tremelo caps and it biased again with similar readings as before. I probed some more all over the place and the bias held. I then decided to put in the power tubes, x my fingers and try it out. Plugged into input 1 of effects channel and away I went. It sounds great (reverb & vib. are not plugged in though) The only thing I noticed is with amp volume @ zero there is a louder hum that comes out of the speakers. Once I turn the amp up a bit the hum is reduced to what seems like the normal cycle hum.

Back before the fuses started popping, there were lots of hissing and slight crackling noises, even after I had replaced the electro caps etc. Now it is way more quiet, only the hum. I sure would like to find out for sure what this intermittent bias loss is coming from. Any suggestions? Thank you very much for all your help.

Scott

[Enzo]

One BIG suggestion: Leave the power tubes out of the amp until you are SURE the bias is stable. With bias coming and going, all you can do is damage something with the power tubes in.

The voltage is fine, we just need it to stay there. We can worry later if we need to change it.

OK< again let's isolate the problem. The bias goes away at the filter cap. There is a resistor between that cap on the small bias board and the one terminal on the bias control, right? And also there is the white wire over to the part board. Now when we lose it, either the voltage is being shorted to ground somehow, or it is just not getting to the cap. I tend to doubt the control is bad, since that resistor would prevent a shorted control from completely grounding out the bias. And at the othet end of the white wire, anything on the other end of the 2.2M resistor will have darn little effect, but if a stray bit of wire at that end touches ground, pfft, no bias.

If the bias were being shorted to ground, then the 1k resistor and diode would probably get hot.

So. Next time the voltage disappears from teh cap, switch teh meter to AC volts and see if AC is present at the joint of transformer wire asn resistor, and again at joint of resistor and diode. If no voltage is coming from the transformer wire, then no bias can result. And that transformer wire may carry the AC< but of the solder in that eyelet is not making good contact with both the wire and the resistor lead, then that voltage doen;t pass througgh the resistor, right? So we follow the AC fron transformer wire up to the diode.

And on the other side, if we suspect the bias is being shorted out, unsolder and pull the white wire out.

What is that resistor from the little board over to the control? My schematic says 33k I think - orange-orange-orange. In any case, tug on it a little to see if it comes free.

And unplug the amp, and that small bias board is held down by two screws I think. Take them out and look underneath just for clues. An untrimmed wire, or a wire poking through and touching the chassis maybe

[sthomas]

Excellent

Enzo, Later today I will follow your instructions. Several days ago I removed the little bias board to check the wires. I have resoldered that whole board. I may give all the give all the contacts a good cleaning and resolder. The resistor on the little board is 470-1W. There is no resistor going from the little board to the control, some layouts show one but the pro-reverb AA165 does not have one.

I wonder if the bias control pot is just dirty? I have not cleaned it, just turned it back and forth several times, and it seems like the intermittent bias loss is happening less and less. Also I measured the resistor on the bias control pot @ 6k and it calls for a 27k resistor.