Burning varnish smell!!!!! Oh No :-( PT suddenly got hot

[rogb]

Before I start, I hope all you guys on the East Coast are surviving
OK, please stay safe!

Just switched on my 86 Special for a quick practice, noticed straight
away the volume was low, then the burning varnish smell and the 1A mains fuse blew!
The PT got very hot too. Put in a 500mA fuse and that blew straight away.

It has been working perfectly for months now, and hasn't been touched or tweaked for some time.

This is just what happened when I used the wrong rectifier design and
trashed my first PT way back at the end of the build.

This leads me to suspect one or both of the diodes have gone down and
taken the PT with it.

I will remove them and test them tomorrow, but I think this PT may now be toast I need to get some shares in Hammond and as long as I keep building, you should too...

Any further advice or checks would be really appreciated!
Damn shame as I was going to take the amp to a jam tomorrow night,
oh well...

[Structo]

Man, sorry about that.

Once you smell a fried transformer you will never forget that smell.

Be sure to check all filter capacitors as well.

[kd]

With a 1 amp fuse in the amp, maybe the transformer is not gone. Unhook the secondaries and see if you still have voltage.

Ken

[rogb]

Thanks guys, I will check those points out!

Not holding much hope out for this PT, though I did switch it off quickly, but the .5 amp fuse went instantly, but maybe if a diode has gone bad, maybe luck will be on my side this time

I bought the UF 4007s from eBay, maybe they were poor quality, I will put my regular IN4007s in instead if they test bad.

The only other thing I can add is I have been running the amp pretty well wide open for some time now, using the Dlator to control volume. Do you think this could find a weak component more easily?

[bepone]

Any further advice or checks would be really appreciated!
Damn shame as I was going to take the amp to a jam tomorrow night,
oh well...

hello rogb, step by step, you will find and fix the fault.

1) disconnect PT from the circuit, (all leads, heaters too) and change the fuse. power on and measure voltages on all leads (AC)
if the fuse burns again, you have damaged power transformer and must be replaced

2) if you have all voltages, you can connect heaters in the amp circuit
and try to power again

3) if all ok, pull out all tubes, and connect secondary taps from PT to the circuit and power again the amp without tubes nad measure again DC voltages
------------------------------
we have several possible scenarios in your case, so now i will stop speculating, you must check your PT first, and give some feedback for 1), 2) and 3) to continue

[bepone]

also, schematic will be helpful.. which amp, which pt, etc etc
how i follow this board, this is some 5 Watter with EF86?

[rogb]

Hi bepone, thanks for chiming in, much appreciated

I removed the uf4007 diodes, one tested open. I disconnected all wires from the PT (heaters, HV, CT) and the fuse blew straight away when powered up.
So it looks like the PT is now a hunk of burnin' crap
I will have to order another PT, and will replace the diodes with Vishay IN4007, which I have at hand, and hope to find some good quality UF4007 presently.

Files can be seen here
https://tubeamparchive.com/viewtopic.ph ... highlight=

[VacuumVoodoo]

What you need to do now is some forensic examination of the circuit to find out what caused such massive overload on the PT. The diode failing open circuit wouldn't alone do it. Check the ripple filter caps.
Check also loop jacks for intermittent opens. This could have caused EL84 grid to lose dc path to ground and plate current rushing sky high.

As an observation: whoever recommended to use 1A mains fuse on this amp apparently doesn't know what fuses are for and how to dimension them.
In this particular case mains fuse should have been 225mA High Surge. It would have saved your PT.

[rogb]

Thanks VV, the fuse is standard AX84 as specified on the schematic for the High-Octane at 1A, so I followed that, thinking this is a proven design.
I will go lower next time, for sure, and see how it goes

I have checked the loop jacks and the wiring/switching seems fine.

I will have to check the PS caps, but until I get a new PT, I can do nothing.
There are holidays here in England till Tuesday.

Thanks again, any help is so welcome I can tell you.
Rog

[bepone]

What you need to do now is some forensic examination of the circuit to find out what caused such massive overload on the PT. The diode failing open circuit wouldn't alone do it. Check the ripple filter caps.
Check also loop jacks for intermittent opens. This could have caused EL84 grid to lose dc path to ground and plate current rushing sky high.
.

yes this second thought is dangerous, if 1.grid EL84 loose connection to ground, there is instant max flow of current to the circuit..limited with internal resistance of the power transformer- suspect No1.

[bepone]

checked schematic and there is no grid resistor EL84 to the ground in design??
[IMG:160:112]http://i284.photobucket.com/albums/ll34/bepone/th_86SpecialVoltages_cr.png[/img]
i don't think that this can damage EL84 tube soo much, but there was moments in action, like switching channels maybe or putting some eff in loop, when grid EL84 see no resistance to ground at all? those moments are very short and question is if rushing current can destroy power transformer?

i would solder one cheap resistor 470K to prevent this problem and don't think about it at all in the future..

[VacuumVoodoo]

1A mains fuse blowing in UK after a little while means the current was above 1.5A which equates to ca 350VA power consumption. Something is very wrong with the circuit OR PT itself could have had an isolation breakdown that progressed to a massive short circuit.
Now the £10.000 question to the OP: is there a CE mark on the PT?
Have fun at the Notting Hill Festival!

[rogb]

Hi

The grid reference resistor sounds a good idea, I will fit one, I checked the EL84 data sheet and 470K at Rg1 should be OK.

As to the Hammond 369EX, I can not see a CE mark on the transformer housing. It was supplied by Hammond UK, so I guess they must know what they're doing?

One thing: Mentioning loop jacks etc. The other day, I moved my Dlator/fx from its usual position on top of the HRM where it connects with 6ft leads to the 5w to the top of the 86 Special with short leads, to see if I was losing any top end. I was plugging the leads in and out and switching back and forth quite a lot between clean and OD. It is really too big to sit there so I moved it back and replaced the long cords.
Do you think this may have caused a problem?

[rogb]

Well I spoke to Phil at Bluebell Audio and a new 369EX PT is on its way to me.

This is starting to become a very expensive business so I would like to
take all possible steps to protect my new PT and the v3 EL84 tube.

I am sending the Phillips EL84 to Phil and he will test it if it has gone dead short,
I will use a known good tube for now.

OK, so protection strategies:

Reduce mains fuse to 500mA and see if it holds up

Use 150mA HT CT fuse as I had 500mA in there

Use a 470K grid reference resistor as per bepone's advice.

Any other ideas, please chime in! I can't afford too many more transformers, folks

[alvarezh]

Hi rogb, before you refire it, insert a light bulb current limiter with the tubes out of the circuit. If you don't have one, look here:

http://www.recycledsound.net/10_Build_a ... imiter.pdf

or

http://www.antiqueradio.org/dimbulb.htm

It is a must have item for us amp builders, should cost you about US$10.00 (I'll let you do the converting to BP).

Instructions on its use is on the second link.

Best wishes,