All signs point to open PT primary (??!!!)

[BrianS]

I've got a Fender Hot Rod Deluxe in the shop right now, and I want to run this issue by you guys before I contact the customer.  First off, here is what the customer told me about the amp, via email:

"The amp died in the usual way, it was being played through at the time and suddenly lost sound then smelled of burnt electronics. On removal of the chassis I quickly found the burnt 5W resistors and thought the repair was within my skill set (oops!). I replaced both resistors (I still have two spares if needed) and reassembled the unit for testing with the old tubes but the unit still doesn't work. I fear I may damaged the circuit tracings.  I did not test the preamp section to see if the signal was getting that far."

So, I checked the work that the customer did, and it is rough looking, but it all ohmed out fine. At that point, I went through and looked for broken traces & solder joints...everything looked good, so I plugged the amp in with no tubes installed.  The "on" lamp did not come on, and I measured no DC volts on the circuit board.  I did measure 120 VAC at the switch and on the circuit board. The fuse is good. I double checked all the connectors & switches several times, and they seem good.

 I also ohmed out the secondary of the PT and didn't get any opens or dead shorts, but when I ohmed the primary, it measured and open circuit. 

All my research points to this being a very unlikely scenario.  My only thought is that maybe the customer yanked on one of the primary wires when trying to disconnect it from the circuit board, and broke the connection between the hook up and magnet wire.  There is no obvious damage to the PT or any components in that part of the circuit.

Thoughts?


Brian

[kagliostro]

In some PT (battery charger transformers) I dismounted, in series with the primary, there was a 105°C Temperature Fuse, so I was able to substitute the Temperature Fuses and repair the battery chargers

I don't know if Temperature Fuse were used by Fender ...........


K

[BrianS]

^There is some kind of component that may be like that in line with the primary...I'm not familiar with what it is.  However, I measure 120VAC on both sides of it, so I assume it is working correctly.

[eleventeen]

Place your ohmmeter leads on the blades of the AC plug and flip the on-off switch. Should alternate between *almost* short and open provided you have a good fuse. Offhand I do not know if on this model the mains AC is led to the transformer using PC board traces or if the tranny wires are directly soldered to fusepost and AC entry/line cord.


It may be that the electrical fault or, as you say, rough handling destroyed the PT. Yes, they burn out on occasion but it's pretty rare. What is less rare is for an inexperienced user to put a 15 amp fuse in the thing when the original blows, then something else REALLY blows when they turn it back on......and then it's time to get the right sized fuse in there! So when you get the amp, it has the right sized fuse yet nothing works and you have the burned smell. It the PT gets its power through some PC-board traces, certainly those can open up. Or there can be a thermistor or other type of surge-suppression thing in there. You have to check some kind of primitive things in this situation, IMO.

[kagliostro]

In the schematic there is an NTC between Fuse and PT, did you checked it ?

http://el34world.com/charts/Schematics/files/fender/Fender_hotrod_deluxe.pdf

Service Manual
http://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDEQFjAA&url=http%3A%2F%2Fwww.elektroda.pl%2Frtvforum%2Fdownload.php%3Fid%3D322434%2526%25238206%253B&ei=6ABkU_D4IdSZ0QXp3oBo&usg=AFQjCNFTAdu7IY8V_WxzcVGBB8XRdskn2A&bvm=bv.65788261,d.bGQ

K

[BrianS]

Eleventeen, the PT is connected via the circuit board.  I did your "switch flip" test and measured an open circuit regardless of switch position.

I did test the thermistor by measuring the AC volts on either side of it.  It is passing 120VAC.  Manual says it should measure 10 ohms.  It measures 11.9 ohms in circuit.

[eleventeen]

Well, then you have an "open circuit" obviously; to isolate exactly where that break is, you have to divide the whole "loop" of the input circuit into segments. It could be a melted trace on the PC board, it could be a cooked tranny. Obviously, your preference is "anything but a cooked tranny". We take the burnt smell as a pretty bad but not necessarily fatal sign.


You say there are PC traces that connect the line to the tranny input wires. Fine. Can you get to both ends of each segment and work your meter probes in there to test for continuity?


What I am saying, perhaps clumsily, is that in the very simple loop that powers up the PT, you could have a 6-8-10 solder connections and five or six wire (or PC-trace) segments. AC input plug > fusepost > fusepost > switch > switch > PC trace > PC trace > PT wire---<<tranny in here>----PT wire > PC trace > PC trace > AC in connector (or line cord)


Ordinarily we give little thought to this; we assume that a wire or a PC trace is whole from one end to the other, but here you had a serious overload condition happen to the amp. Stuff could well have been burnt. Heck, it might have melted the innards of the fusepost. Or the AC switch. (only partially being ridiculous; when you know a serious overload has occurred, often you have to check out stuff you ordinarily give no thought to.)


In the end, you will probably have to pull the board out, one way or another....if only for inspection purposes. You'd have to do so to connect the new PT wires (if yours is dead) unless you're planning to splice to the cut-off stubs of the old one. Unless something obvious (or obvious after you measure it and find out it is bad) can be fixed without board removal. Sometimes it is better to proceed directly to board removal, sometimes you can cheat and avoid the exercise. I have rarely been able to figure out which way until I actually do it. It is indeed unfortunate that in most cases, taking an amp board out of the chassis is usually quite tedious because of having to remove all the knobs and unscrew all the pot-bushings.


Sometimes you can detect a burnt trace or at least some sort of trace damage by shining a very bright light on the PC board; depends upon the board material. That would likely not be helpful on single-sided cheapo phenolic boards that Fender uses. On fiberglas-epoxy (usually green) boards it can be very useful.

[BrianS]

I had to pull the CB right away in order to double check the customer's solder job on the resistors he replaced. And I've checked all the connections, traces & solder joints in the PT area. They all seem fine.  And I have 120VAC right up to the primary...on the board. I will double check, though. 

FWIW, the resistors that initially burned up really cooked the traces...it's very obvious and I'm guessing that is the burned smell the customer noticed initially.  I read up on this and apparently it's a fairly common problem in the HRD.  I did not find any mention that this problem would take out the PT, however.  I've got an email to the customer asking if he may have yanked the wires when he initially pulled the board.

[smackoj]

I have a friend who wanted me to mod his HRD. I did quite a bit of web research on the several different models produced. Three things stood out that could be involved here with this one. They have flimsy pcbs that are thin and cheaply built so they flex more than they should (especially during removal from the chassis). They also are notorious for bad solder joints or cracked solder joints that are hard to see. And, if the pcb is taken out of the chassis without following the proper techniques, then all kinds of bad things can and do happen.

p.s. I never did do any mods to my friend's HR Deville. He decided to buy a nice Rivera Era Princeton and he sold the Deville. Smart fella.

good luck amigo.   

[sluckey]

but when I ohmed the primary, it measured and open circuit.

If your meter probes were connected directly to the transformer primary leads, then the primary is indeed open, regardless of how it got that way. If you pull the PT and remove the end bells you may get lucky and spot a broken wire that can be repaired.

But if your meter probes were NOT connected directly to the transformer, you cannot say with certainty that the PT is bad.

[BrianS]

Well, I got a new PT from Classic Tone and installed this morning.  Got my voltages back...but there are still problems.  Something is drawing too much current or something.  Help?

-Power On, Standby Off:  All the voltages supplied by the brown and green secondary wires seem fine.  Amp sits ready for the standby to be flipped.

-Power On, Standby On:  As soon as the standby switch is flipped, there is a buzzing sound, my leak light lights up, and the power tubes have a blue line of electricity develop inside of them (noticed this when I shut my bench lights off).  Fuse does not blow.

If I take the power tubes out (brand new tubes, fwiw), or all the tubes for that matter, the buzzing sound is there when the standby switch is thrown, but the leak light does not light up.

After I first wired in the new PT, I had the buzzing sound, but I was able to bias the tubes without the leak light coming on.  I did not leave the amp on in this condition for more than a minute.  I went to plug a guitar in and found that some solder joints were bad on the input jack, so I went through and touched up all the solder joints on all the jacks...and any other suspicious ones as well.

After that, I turned the amp on and the leak light lit up when the standby switch was thrown, and the buzzing sound was still there.  And that is where I am at right now. 

I've cleaned all the pots/jacks/switches, including the bias pot.  I left the leads on the transformer full length.  I've double checked solder joints.  I've tapped on components.  I've measured voltages.  I'm starting to go nuts...

[tubeswell]

So the voltages are all okay now?


And there's no 'blue-line' in the output tubes any more?


Could be a bad pre-amp triode - did you try subbing them out one at a time with a known good tube?

[PRR]

There's a two-part problem.

* Burnt resistors

* Open transformer

I smell a third part.

Power tubes lost bias. Tubes sucked heavy current. Resistors burned first. Then PT primary.

You fixed the resistors and the primary. However you may still have bad bias, causing heavy sucking.

With tubes out, measure grid bias voltages. Several dozen Volts -negative-.

With 1-Ohm resistors or Bias-Checker, measure tube currents. 40mA is ballpark. 100mA is way high. 200mA will domino-burn everything all over again.

[BrianS]

This amp is driving me nuts.  Here's where I'm at:

-I replaced the OT as well...the old one was doing weird things.  Let's just leave it at that and know that the OT and PT are now new and good.

-With power tubes out, the amp fires up and I get decent voltage readings...somewhat high, but not outrageous.  There is a buzzing sound coming from the power supply area, but I can't locate precisely where it originates.  I also do not know if this is normal or not.  Leak light does not light up.

-With power tubes in, amp fires up and the B+ voltage drops significantly (if memory serves, somewhere around 382 volts...should be around 430v).  Leak light lights up and there is still the same, unchanged buzzing sound.  I measured the bias voltage and with the bias pot turned all the way down, I get 54mV (Fender recommends 60mV).  As I turn it up, the leak light gets brighter.  As such, I did not try to get it up to 60mV for fear of burning something up. 

-The tubes are supposed to be brand new.  I do not have a spare set of 6L6GC to test.  I do have a pair of old RCA 6L6GBs (I know, I know...), and I installed them just as a little test:  with bias pot all the way down, and around 12 or so mV at the bias point, the leak light has a very dull, almost unnoticeable glow.  Turn the bias pot up just a little, and it starts to glow hotter.  Not sure if I learned anything from this test...well, pretty sure I didn't.

I've measured and inspected most the key resistors in the output section and they seem fine.  As mentioned in an earlier post, I've looked all over for bad solder joints and have fixed all I found.

What am I overlooking?

[PRR]

What is a "leak light"??

[Ritchie200]

Just thinking out loud...  If you lost a bridge diode, that could be the cause of the buzzing you hear - and you only hear it when you go off standby possibly indicating something downstream in that circuit.  I agree with PRR that there is another fly in this soup.  And like PRR I am wondering what your leak light is?  Is it a light bulb limiter?  If so, the shorted side of the bridge will cause it to glow.  The problem with that is you would lose about 50% voltage as a result.  I don't know what a healthy amp would give you at B+ with the power tubes out, so I am still guessing here.  So long story short, maybe check your bridge diodes.

Good luck!
Jim

[Ritchie200]

Just another thought.  Remembering I have an amp notorious for arcing in the tube sockets, double check your power tube sockets for evidence of arcing and carbon tracks.  If so, clean, clean, and did I mention - clean.

Jim

[BrianS]

Yes, leak light is the light bulb limiter.  We called them leak lights back in school.

I will report back with some voltages I took.

[BrianS]

Well, I found what is buzzing...there is a capacitor that goes between the two red wires on on the secondary of the PT.  I guess you could say it is right before the bridge rectifier circuit...I can't see how it is really part of the rectifier, and I'm not sure what it is for, but it is buzzing like a bee.  When I put my chopstick on it, that quiets it down.  Not sure why I didn't check it before.

Anyway, I don't think this is my problem, or even caused any of the problems...but...   

Is this "little" capacitor some kind of protection device, in case one of the diodes in the bridge shorts?  Page 11 of the link below is where you will find the schematic I am working from:

http://badraven.com/Fender_Manuals/Hot%20Rod%20Deluxe.pdf

With no power tubes in I get 450V off the rectifier.  With tubes in, all my high voltages drop 40 to 50 volts, and the bias current just runs up until I shut it down.  I tried new power tubes, but I get the same condition.  I only left the new tubes in for a few seconds...

Gonna keep picking away...

[HotBluePlates]

Well, I found what is buzzing...there is a capacitor that goes between the two red wires on on the secondary of the PT. 

Looks like C53 on the schematic in the Hoffman schematic section (link at the bottom of this page). Cool. Cut the cap out. Still buzzing?

With no power tubes in I get 450V off the rectifier.  With tubes in, all my high voltages drop 40 to 50 volts, and the bias current just runs up until I shut it down.  I tried new power tubes, but I get the same condition.  I only left the new tubes in for a few seconds...

What is the voltage on pin 5 of the output tubes, with nothing in the sockets?

[BrianS]

Clipped out the buzzing cap.  No buzzing...of course.

Voltages on pin 5 with no power tubes are both -50.3V.

So is that cap some kind of protection device, or what?

EDIT:  problem seems to be resolved.  See here:  http://el34world.com/Forum/index.php?topic=17168.0

Thanks all!