MIMF

I have a Fender Hot Rod Deluxe 40W tube amp. I've had it for about 3-4 years and never had any problems. Now, when I flick the power switch, nothing happens. My first thought was the fuse. I took the fuse out and it looks fine. I don't have a spare on hand, so I'll have to get a new one to try this weekend and hopefully that's it. I used a multi-meter to test the continuity and it checked out okay, so I am starting to worry it's more than the fuse.

If one of the tubes were blown, would the amp still turn on? The tubes don't appear to have blown, they appear normal, but of course that's hardly a sufficient test. A friend suggested that the amp would turn on regardless of the tubes, just that the amp wouldn't work properly.

Any other thoughts on how to troubleshoot? I'm assuming if it's not the fuse or tubes, then this is out of my comfort zone. Can anyone recommend a place to have it looked at? I'm in northern NJ just outside of NYC. I'd really prefer not to have to haul it into the city.

Just about any TV repairman would be able to troubleshoot and fix it. I know this is a silly question, but have you tried plugging it into an outlet on a different circuit?

Hi Bob. Yep, I tried it in several other outlets. No luck.

I'm assuming that when you turn it on, the tubes are not lighting up. That would be no power to the heaters, so the fuse, the switches( power and standby) and the power transformer could be at fault.
I feel obligated to say the voltages in a tube amp are lethal, BE CAREFUL!
I'll try to find a schematic so I can be more helpful.
edit: I found a schematic, and it appears the standby switch is after the power transformer (duh ), so if it doesn't light up, it's the power switch, the fuse, or the power transformer. The pilot light is parallel with the heaters, so it should light up when the heaters have power.

Hey Rodger,

When it was working normally, there was a nice little satisfying "thunk" when I turned it on. The jewel light lit up and the tubes would glow. Now, when I flick the switch, all I hear is the sad little click of the switch. Nothing lights up, nothing happens. Essentially, there's no difference when it's plugged into the outlet or not.

It seems like my best course of action would be to replace the fuse regardless of the continuity test. If that fails, then I'll test out the switch. If that fails to fix things, then I'll look at the transformer. Is there a way to test it? Or the only test is to replace it with a new one? If it's not the transformer, then it's time to bring it to a pro. Does that seem like a reasonable plan?

If the continuity is fine on the fuse, then that's not the problem. If the tube heaters are not glowing, then power is not getting through the transformer. This could be due to a damaged power cord, damaged fuse socket (the style used on this series is pretty crappy), damaged power switch, or blown power transformer.
If you open the amp up, be careful not to go anywhere near the filter capacitors! These can retain lethal voltages even if the amp has been powered off and unplugged for extended periods. If you know how to recognize them, and know how to safely test them for retained voltage, do so before proceeding, and also keep a bleeder resistor attached while you are inside the amp. If you don't understand any part of what I've described, you should not be inside the amp.

You'll probably want to take it to a pro for the transformer, it looks like there are 8 electrical connections. On the other hand, it looks like there may be some type of wire connectors so it may not be solder connections. I don't know how to test the transformer, other than hooking it up and measuring the output voltages. It looks like they should be red 220 VAC to the solid state rectifer, green 3.3 VAC to the heaters & jewel light, and I'm not sure about the brown output.
+1 to what Greg said, there's 430VDC out of the rectifer to the filter caps.

Thanks for the advice. I'll go so far as to check the switch and after that I'll take it in for servicing. I've read much about discharging the caps and safety precautions, but I don't have any practical experience. No need to learn the hard way.

I've had a run of things with bad switches, lately, so just out of personal prejudice, that's where I'm going. If the fuse is reading okay with a meter, then it's okay. I would vote for either the switch (first) or the power cable socket (second). It really sounds like no power is getting to the tranny for it to do anything.

I pulled the panel and checked a few things out. Stupid question. It looks like the bulb in the jewel light is blown. Could a blown bulb be the source of my problem? It seems like such a small thing to kill an amp.

I checked the fuse socket with the continuity test and I think it may be fine. With the fuse in, I have continuity between the 2 red arrows. When I remove the fuse, I do not. Seems like that component passed.

I then checked the power cord and switch. I pulled the wires off the switch, marked with the orange arrows and continuity test passed between the blades on the plug and each end, ground checked out too. After putting them back on, I tested the on/off switch. With the switch off, I had no continuity between any combination of the points marked by the red arrows. With the switch in the on position, any combination of those points cause my meter to chirp. I assume that's what I would expect?

This all leads me to believe the next step would be to have the transformer checked out? Am I making the right assumptions?

Eric, good work so far. You've proceeded correctly, and eliminated the power cord, switch, fuse socket and fuse. You're right that the next step would be to check the power transformer.
Don't worry about the pilot light, they often get smokey and look suspect before actually failing, and there's no way it could cause the whole amp to fail. If you want to check if it's still good, test resistance across its two contacts. If you get any reading it's fine, if your meter reads open it needs to be replaced.

Another possibility that occurred to me is that the heater chain might have an inturruption. If this were the case the B+ (high voltage) would still work, but the amp wouldn't do anything and there'd be no indication it was turned on.
Remove all the tubes and measure resistance between the heater pins on one of the preamp sockets. When looking at the tube side of the socket (as opposed to the component side) the pins are numbered anti clockwise starting from the gap. You want to measure resistance between pin 9 and pin 4 or 5 (pin 4 and 5 are connected together).
If you read around 1 ohm on your meter, that's normal, but if you read around 200 ohms, then the heater chain is broken somewhere, and that would cause your problems.

Actually, thinking further on what you've written Eric, you said there's continuity between all 4 contacts on the switch when turned on? So long as you hooked the power cord back up correctly, that would indicate that the primary of the transformer is not failed open, and the fact that the fuse hasn't blown when you turn it on indicates that it hasn't failed short. That means either there's an interruption in the heater chain, or somehow the heater secondary has failed open.

Another thought- that "thunk" you normally hear on power on usually comes from the inrush current through the transformer to charge the filter capacitor(s). Can you get an actual resistance measurement between the blades of the AC plug (unplugged) with the switch in the on position? Also, did you check the resistance of the jewel bulb?

Hi Greg. I checked the resistance of the bulb and I get 0, so it looks a trip to radio shack for me for a new bulb. There was definitely continuity between all 4 contacts of the switch and I am certain I hooked the power cord back the way it was. I'll test the preamp tube sockets next as you recommend. There are 3 of them, does it matter which one?

Charlie- I tested the reistance with the switch in the on position and it measures 16.2 ohm.

Eric, yep, the pilot light is toast, replace it. It doesn't matter which socket, you could even test from the pilot light socket, so long as you remove all the tubes and the pilot light when you do it. In fact it would probably be easier to test from one of the power tube sockets. You would need to test between pins 2 and 7. Again, looking from the tube side, they are counted anti-clockwise starting from the key, which is the notch in the alignment pin.
Measure the resistance between these pins, and report what you find back here.

Also, you haven't mentioned whether the heaters glow when it is turned on, this is important.

The resistance reading you got between the power cord blades with the power switch turned on again indicates that the power transformer primary hasn't failed open, which is good.

It's still likely that the power transformer has failed, but hopefully we can find out how, and maybe even why.