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

[nevetslab]

How do i measure the excitation current of the transfomrer?

Ive moved blk/red to cp3 and left the violet wire disconnected.

At this point when i turn the dpdt the indicator lamp comes on and the tubes start to heat up

Measuring the excitation current is done with the secondary wires disconnected as I had described in an earlier post. The transformer needs to be unloaded. However, as you're now finding the pilot lamp lit up again and seeing the tubes heating up, we're still not knowing WHY the thermal switch between CP3 and CP5 opened up. It could be from the transformer, or could also be in the amplifier circuits that's causing too much current draw, if that was the cause of the thermal switch opening..

This measurement is at dangerous potentials, so if you have any doubts, you might want to pass on this AC Mains current measurement.

As it seems all you have is your multimeter, which can measure AC current, you would have to place your meter leads IN LINE with the input to the power transformer, which is on 230VAC potential. If you have insulated clips for the end of your probe leads, or a separate set of leads that have those, so there's NO CHANCE of them coming off and falling onto any other part of the circuit, you could place it between CP3 (where BLK/RED was connected) and the BLK/RED lead. CP5 and CP6 are joined together, not being used now and fully isolated, so you could plug the BLK/RED lead there and the other test lead to CP6. Then reconnect AC mains cord and power up. If you're not disconnecting the secondary wires, you're now measuring normal AC Mains current that the amp is drawing. I forget off hand what the nominal mains current draw is, but seem to recall the nominal wattage at 120VAC is around 60-70W, which would be in the 0.5A range. At 240 V, this would be under 300mA.

As the power xfmr is behind the chassis, inaccessible from the open rear panel (without removing the chassis from the cabinet), there's no easy way to see if this is heating up. But, seeing mains current way in excess of 300mA at idle would be an indicator.

You can get at the cathode resistors test point of the output tubes with our meter on DCV, lowest range (200mV) and see if the voltage across the cathode resistors are around 60mV (30mA plate current per tube, nominal). That's TP 30 on the component placement drawing. It's measured between Ground and TP30. The tube PCB parts are on the bottom of the tube foil side (which is face-up), so you have to reach for it, though the component lead is right at the left corner, very accessible. Ground can be the chassis....where the AC mains cord ground wire is attached.

[Rambo]

How do i measure the excitation current of the transfomrer?

Ive moved blk/red to cp3 and left the violet wire disconnected.

At this point when i turn the dpdt the indicator lamp comes on and the tubes start to heat up

[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.

[Rambo]

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

[g1]

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

[Rambo]

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

[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]

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!!

[Rambo]

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

[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]

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

[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

[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]

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?

[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