Fender Hot Rod Deluxe, no lights, no sound, no bueno

[Rambo]

Hi, ive recently been having a bit bother with my fender hot rod deluxe.
Im not a amp tech but i am a domestic electrician. I have very little experience with electronics so im a bit out of my realm and im looking for quick answers.

Scenario.
Was recording some music, went down stairs to make a snadwich, came back up stairs to a loud humming sound coming from the amp and a burning smell.
A package that was on top of the amp fell down the back and had stated to smolder on the tubes.

Turned the amp off, turned it back on 10 minutes later and no lights/ sound.
checked the fuses (obvs), they are fine.
checked the first power switch , its fine 230 going through.

Power then seems to be going to the first transformer but theres no power at the standby.

Does this mean the transformers knackered?

[nevetslab]

I gather this package that was on top of the amp had uncapped liquid which tipped over and seeped into the amp by first flooding the control panel? That's dangerous. 230VAC AC Mains. We can't yet see where you're from, as the forum's software just changed a couple days ago. No matter.

With NO Pilot Light and No Sound, checking the fuse was the first obvious action. With the AC mains cord disconnected, and the rear panel off, can you see where the liquid flowed on the main and tube PCB, as well as on the chassis? Granted, you can't see the other side of the chassis, and can only see some of the surface of the chassis behind the PCB's, while you can see all of the chassis by the power xfmr. Normally, if you've lost the pilot light, those are socketed, and sometimes have seating problems if they haven't burnt out/gone open. The Pilot light is on the Heater Winding of the Secondary side of the Xfmr.

I assume you have a multimeter., so in spite not having experience in electronics maintenance, you'll only get so far, but we can guide you. With the AC mains fuse intact (located in the bottom chassis panel by the AC Mains Cord entrance). Make sure the connections to/from the fuse holder are solidly connected, as well as ALL of the Primary wires to/from the fuse, AC Power Switch, PCB Primary terminals.. One side goes to the DPDT AC Power Switch, with the other side of the Switch AC input comes from the Mains cord. Using your multimeter in the lowest ohms setting (zero it if you have that option), check the reading with the probes shorted for starters. Then, with the Power Switch set to ON, see what your reading is between the AC Mains pins of the plug. You're reading the primary side of the power xfmr, and should be a low reading, well below 100 ohms If it's open circuit, it's either open fuse, open In-Rush Current Thermistor (on the main PCB....black large 2-leaded disc near the AC mains wires from the switch/power xfmr), OR....an open Primary on the xfmr. You can check the Primary of the xfmr by following the set of wires (seven of them on Export models) to see if there's an open internal Thermal Switch, which would be connected to CP3/VIO wire & CP5/blk/Red wire). Then the upper primary coil wires are CP5/Blk/Red & CP7/Blk/Grn, while the lower primary coil wires are Blk/Yel, Wht, Wht/Blk & Blk. Where they're plugged in depends on whether it's wired for 230V or 240V. The PCB terminals for those are CP8, CP9, CP10 & CP4. There should be low resistance on both the upper and lower Primary wires, and the Thermal Switch wires VIO & Blk/Red should likewise be nearly zero ohms. If you're getting open circuit readings on the primary wires & in-line Thermal Switch, you have an open Primary.

The Thermal Switch which is ahead of all these is found between Terminals CP1 & CP3. It's soldered into the PCB, and probably starts out around 10 ohms cold, and decreases in resistance as AC Mains current flows thru it, probably down to less than 2 ohms (I forget the actual readings).

Less likely, though also in the primary circuit path is the power switch itself.

I've attached the Schematic for the Hot Rod Deluxe below. We can proceed further after you've gone thru these checks.

Hot_Rod_Deluxe_Schematic.pdf

[Rambo]

Wow, thanks brother, what a great response.
you really know what you’re talking about.
this will give me something to chew on today and I’ll come back with some intel later.

the package didn’t have any liquid in it though so there’s been no short circuit like that.

from Glasgow

[Rambo]

Right,so there is power flowing through the dpdt switch.

i located the open in rush thermistor but im struggling to get my leads on the wee ends that join into the board. Its saying 13.4ohms ? is that right?

moved on to the primary of the transformer cp3/ cp5 and im getting an open circuit between them 2.

Whats your thoughts?

[nevetslab]

Right,so there is power flowing through the dpdt switch.

i located the open in rush thermistor but im struggling to get my leads on the wee ends that join into the board. Its saying 13.4ohms ? is that right?

moved on to the primary of the transformer cp3/ cp5 and im getting an open circuit between them 2.

Whats your thoughts?

The DCR of the Thermistor seems right. Now, with regards to the power xfmr's internal thermal switch, which appears between CP3 & CP5, THAT is a problem, and is no doubt why you're NOT getting power to the amp. Now, A simple fix would be to move the Primary Lead wire BLK/RED from CP5 to CP3. This bypasses that thermal switch. BUT....we DON'T know WHY the thermal switch opened. Did you get DCR readings on the remainder of the two Primary windings taps?

Do you have any means of measuring AC Mains current that is flowing into the amp? If so, we need to disconnect the secondary windings from the main PCB. This is to allow measuring just the excitation current of the power xfmr, unloaded. You would need to unplug (and mark each with a suitable Sharpie pen) CP11, CP12, (both RED), CP 13 & CP14 (both GRN) which are plugged into the Tube PCB below and tedious to reach, and CP15 & CP16 (BRN).

If you don't have any means to measure AC mains current flowing into the amp, what most of us have in our testing tools is a Light Bulb Limiter. It's built so it places an incandescent light bulb, typically 100W size, in series with the circuit load. Basically a go/no-go indicator advising abnormal amount of current is flowing. I normally have whatever I'm working on plugged into my AC Power Analyzer which gives me AC Amps/AC Watts/AC Volts simultaneously, and is fed from a 30Amp Variac.

If after building one of these, and with the secondary of the xfmr disconnected (open circuit), and the light bulb glows brightly, that indicates there's a short in the primary or secondary of the power xfmr.

I think you can find plans to build one of these using the Search function to find it on our forum. I'll post this then see what I can find, and edit this with the details.

https://www.thegearpage.net/board/in...hread.1607972/

[Rambo]

im getting;

cp5 - cp7 = 7.7ohm
blk/yellow - white = open circuit
white/black - black = 0.09ohm

my multi meter will measure ac current also

[Rambo]

do you think its a good idea to by pass the thermal switch (cp3 to cp5) without knowing the full scipt yet?

[nevetslab]

im getting;

cp5 - cp7 = 7.7ohm
blk/yellow - white = open circuit
white/black - black = 0.09ohm

my multi meter will measure ac current also

Regarding the bypassing the Primary internal thermal switch, I DID preface it with wanting to know WHY it opened. That's the reason for building the Light Bulb Limiter. If you have access to a variac, allowing starting the AC Mains voltage from 0V being applied to the unloaded power xfmr, if you see loads of current flowing immediately, they you know why the transformer heated up and caused the thermal switch to open. These used in the Fender Amps is a one-time shot..it opens, and now no current will ever flow thru it.

BLK/YEL to WHT is not a good sign. What do you have between BLK/YEL and BLK, or BLK/YEL and WHT/BLK? If we have an open primary coil on the pair of primaries which MAY be what your BLK/YEL to WHT is indicating, you'll need to replace the power xfm

If you DO use the AC Current mode on your multimeter to measure Primary current into the amp, PLEASE make the connections safe, as you're now in deadly territory.

[Rambo]

black/yellow - black = 8.6ohms
black/yellow - white/black = 7.8ohms

[Rambo]

do you want me to take the primary current reading? or am i in the market for a new transformer?
they are so bloody expensive!!

[Enzo]

do you think its a good idea to by pass the thermal switch (cp3 to cp5) without knowing the full scipt yet?

Bypassing the thermal switch is not proposed as a solution or cure, it is a test procedure. We only need to power up that way briefly to answer the immediate questions.

[Rambo]

thanks, do you think based on the readings that it is the power transformer that i the problem?

[g1]

Your measurement of open circuit between blk/yel & white indicates a bad transformer. Double check it.

[Rambo]

thanks a lot boys, really helpful chat! any recommendations on a replacement or hould i just buy the fender one?

[nevetslab]

black/yellow - black = 8.6ohms
black/yellow - white/black = 7.8ohms

While we don't know why there is no reading between the BLK/YEL and WHT wires, the upper coil between the BLK/RED and BLK/GRN wires and the lower coil between the BLK/YEL and BLK (240V) or WHT/BLK (230V) are very similar, that DOEs indicate you have the windings intact for both 230V and 240V wiring. And, only NOT having the thermal switch in place, which is measuring open circuit. I would now check the xfmr wired either for 230V or 240V and bypass the thermal switch to see if the excitation current of the xfmr is nominal (under 50mA flowing typically). Meaning move the BLK/RED wire to CP3, leaving the VIO wire disconnected.