Fender AA1069 Doghouse resistor failure, wattage question

[NotWesYet]

Hi All,

I recently rebuilt a '71 Fender Pro Reverb. The tech had modified/mangled it so I was uncertain as to what circuit was present. Given the serial number and rectifier choice I went with the AA1069.

Everything except the transformers are new...everything from pots to caps to circuit board to wiring. With the sole exception of the bias balance pot as I could not find one with the corresponding connections specifically the center tap.

I powered it on and all seemed fine for a moment. Then I attempted to adjust the bias balance and blew a fuse. I did not see any loose connections, so I put in a new fuse and tried again.

The 4.7K resistor on the doghouse board went up in smoke and the fuse blew.

Since it has been a while since I worked on this project I double checked all values and found I may have used 1W resistors based upon the values for a '71 Super Reverb.

I have found others information that indicates they should be 3w.

Since I am planning to replace all resistors on the board as I can't recall what I put in at this point, I wondered about your thoughts on values and any other area I should check that may have caused the problem?

Thanks again for everyone's help!

[2deaf]

Sounds like it is drawing excessive current downstream from the 4.7K resistor and the 4.7K resistor is the victim, not the cause.

[NotWesYet]

Sounds like it is drawing excessive current downstream from the 4.7K resistor and the 4.7K resistor is the victim, not the cause.

Thank you.

So the insufficient wattage rating of the resistor is not why it failed, but rather excessive current could you suggest any usual suspects? The caps are new, as are the resistors.

I have been meticulous in following the schematic and layout, but I am lost at this point.

[sluckey]

The 4.7K resistor on the doghouse board went up in smoke and the fuse blew.

Fender used 1 watt resistors in that position. 1W is fine. Many people (myself included) would replace that 4.7K and 1K with 3W metal oxide resistors. I agree that the failed resistor is a casualty, not the cause.

That resistor feeds node D which supplies six triodes in that amp. But they are all very low current loads.

The most likely causes to burn up that 4.7K in an amp that has just been totally rebuilt/rewired would be...

1. Node D filter cap defective or connected backwards.
2. Wiring error downstream of that resistor.

With power off and the filter caps drained, measure resistance between the 4.7K (node D end) and chassis. What have you?

[NotWesYet]

The 4.7K resistor on the doghouse board went up in smoke and the fuse blew.

Fender used 1 watt resistors in that position. 1W is fine. Many people (myself included) would replace that 4.7K and 1K with 3W metal oxide resistors. I agree that the failed resistor is a casualty, not the cause.

That resistor feeds node D which supplies six triodes in that amp. But they are all very low current loads.

The most likely causes to burn up that 4.7K in an amp that has just been totally rebuilt/rewired would be...

1. Node D filter cap defective or connected backwards.
2. Wiring error downstream of that resistor.

With power off and the filter caps drained, measure resistance between the 4.7K (node D end) and chassis. What have you?

Thanks! I will work on this tomorrow and report back.

I did recently replace those filter capacitors as I realized I had not doubled checked the position of polarity. I had two backwards. Assuming I have the correct, could prior inversion lead to a failure of an of the accompanying resistors?

[sluckey]

I did recently replace those filter capacitors as I realized I had not doubled checked the position of polarity. I had two backwards. Assuming I have the correct, could prior inversion lead to a failure of an of the accompanying resistors?

There's only one more resistor . Failure due to a reverse cap would likely be obvious. I would not trust a filter cap that has been installed backwards. They have been compromised, probably permanently. I suggest replacing the two backwards ones with fresh ones.

[NotWesYet]

I did recently replace those filter capacitors as I realized I had not doubled checked the position of polarity. I had two backwards. Assuming I have the correct, could prior inversion lead to a failure of an of the accompanying resistors?

There's only one more resistor . Failure due to a reverse cap would likely be obvious. I would not trust a filter cap that has been installed backwards. They have been compromised, probably permanently. I suggest replacing the two backwards ones with fresh ones.

I did replace them once I realized I had them in backwards and had powered the amp on only to have the fuse blow. Those were the caps on the board, not the ones in the "doghouse."

[pdf64]

Any reverse polarity ecaps would draw a lot of current, so any resistors feeding them would likely over dissipate, and would best be replaced.

[NotWesYet]

It was, as we used to say in tech support "an ID10T error."

I connected the wire form the dog house to wrong point. One over from where it should have been.

Time to check all components and, once again double check all my connections.

Thanks for everyone's help. By the end of all this I might just get my amp up and running and not electrocute myself.

[dude]

From experience most issues like you described are misplaced wiring, exactly what what happened in your case. It’s easy to do, l’ve done the same thing a few times in the beginning. A good way to check the wiring is to start at one end and check every wire connection, making sure it’s in the right place. Then use a marker and mark that connection, once you have every connection marked and as positive as you can get, use a light bulb limiter to start the first time.
Finding a wiring mistake can be extremely frustrating, we’ve all been there a few times.

[NotWesYet]

From experience most issues like you described are misplaced wiring, exactly what what happened in your case. It’s easy to do, l’ve done the same thing a few times in the beginning. A good way to check the wiring is to start at one end and check every wire connection, making sure it’s in the right place. Then use a marker and mark that connection, once you have every connection marked and as positive as you can get, use a light bulb limiter to start the first time.
Finding a wiring mistake can be extremely frustrating, we’ve all been there a few times.

That's a great idea!

I created a spreadsheet to log every connection on the board but where I ran into problems is that I did not have the matching colors of wiring for the doghouse and had to transpose... I stopped working on it for awhile and when I went back I had to get back up to speed.

[NotWesYet]

Update. I double checked everything.

I was following the doghouse wiring from a Super Reverb as provided by a fellow forum member. So I found a bandmaster reverb diagram from Rob Robinette that seemed to match the original.

Please be aware that after the tech worked on the amp on and off for years, I could not determine what circuit it was. So many things were wrong and did not match either.

I replaced the resistors, rewired the doghouse board. Then checked every connection making sure it match the AA1069 circuit.

It powers on without incident and all tube light up. The balance control seems to have no effect and using a VHT bias meter I am getting nothing.

The tubes are new and I tried two different rectifiers. I have som ideas where to look next, but I'd appreciate everyone's opinion and suggestions.

Thanks!

[DummyLoad]

voltage measurements of the power supply nodes - A B C D? perhaps you damaged the choke or the 1K upstream when the 4.7K blew? 


--pete

[NotWesYet]

voltage measurements of the power supply nodes - A B C D? perhaps you damaged the choke or the 1K upstream when the 4.7K blew? 


--pete

Thank you! I did replace both the 4.7k and the 1k. I will check the nodes, as well as the choke. Given it is 49 years old, it might be a good idea to change it anyway!

[NotWesYet]

Progress! I replaced the choke as suggested. I doubled check all wiring, as well. Redid a couple of solder joints.

It now powers on, no smoke, explosions or fuse blowing.;-) Power tubes glow equally, but dimly and I get no voltage reading from the VHT bias meter. Rectifier glows but not strongly. It is new.

I double checked the standby switch wiring as it is almost behaving as if it is in standby.

I am going back to the amp repair book to study and then going to try to check every component.

I am close and hoping to resolve the issue without electrocuting myself.

Any further suggestions on usual suspects?

Once again, thanks for everyone's help. And Happy New Year!