RM20B - Constantly Blowing External Fuse - PLEASE HELP!!

[schismatic]

Hi guys,

Recently my RM20B combo stopped working completely. I pulled out the external fuse (the one immediately next to the power socket), and it had blown. Not a problem I thought, I'll go to my local electronics shop and pick up some fuses.

As per the ratings on the back of the amp, I picked up 250V/1.6A rated slo blo fuses - I am in the UK on 240V AC mains.

I placed the new fuse in the socket, inserted correctly, switched the power back on and it immediately blew. I tried another fuse in case I had not inserted the external fuse into the correct position, made extra sure it was correct and tried again. Nothing . The only sign of life that I received was the red LED on my Recto preamp board flashing for about half a second. After that, the amp appears to be effectively dead.

I decided to do some more fault finding based on things that I knew I had the knowledge and capability to measure. I took our the Hi Voltage Fuse (250mA Fast-Blo) and measured the resistance across it using a multimeter. It picked up resistance and visually looked in tact, therefore I am confident that the Hi Voltage fuse has not blown.

As I cannot get power on to the amp, I could not check tube biasing (although I'm not sure that I need to).

I've blown quite a few of these 1.6A fuses now, so I'm running out of ideas and this is where I need some forum help.

What else should I do? Am I use the right rating of fuses? Can I provide you with any further information? I can supply photos if necessary.

I'm desperate to fix this as I need to gig with it in the near future!

Help much appreciated and many thanks in advance!

 

[joey_truelove]

Does the fuse blow on standby or when switching the power on?

 

[schismatic]

Thanks for replying so promptly.

I double checked the full process:

1. Ensured Power Button was off and the second switch was set to 'Off' (as opposed to 'Live' or 'Silent').

2. Inserted fresh fuse, ensuring in correct position for UK voltage (240V).

3. Inserted power cable.

4. Turned power on at the mains.

At this point, nothing happened, so I removed the fuse to check it. It had not blown at this point.

5. I re-inserted the fuse.

6. I switched on the Power button on the front of the amp.

At this point, a small quick flash occurred in the external fuse area. I switched everything off, removed the fuse and the 1.6A slo-blo that I had just inserted had indeed blown. The Hi Voltage fuse is still in tact.

 

[schismatic]

Apologies for bumping this but I still have a dead amp and I'm very close to turning it into an expensive 1x12 can unless I can get some further guidance.

Is there anything else I can do to troubleshoot my issue?

 

[Jaded Faith]

When your amp blows a fuse, it's trying to tell you something is wrong. If you don't locate and address why there is too much current flowing through the part of the circuit the fuse is intended to protect you will spend a lot of money on fuses only to watch them burn up.

The fuse you are blowing is the Mains fuse. The first thing to check is to make sure the voltage selector where the power cord plugs in as set properly for your line voltage.

You mentioned that you pulled the HT fuse (the 250mA one) and checked it for resistance. I'll assume you meant continuity? Or that the resistance reading was "0"? That's what you want to check there. You really need to rule this fuse out.

The HT fuse blowing would be indicative of either a shorted tube, power cap or transformer. If you are certain the HT fuse is good and it's only the Mains fuse blowing, that may be indicative of more serious problems that will require a tech.

Some last things to try in this order:

- Check the HT and mains fuse for continuity
- Replace all the tubes with fresh ones or known good ones. You should spray the pins of them with contact cleaner and gently work them in/out of the socket a few times to make sure they are not dirty.
- Set the Bias almost all the way down
- Power the amp on

Does it stay on? If so, bias it properly per the manual and enjoy the amp. It would appear a tube was bad and taking out your fuses.

Does it blow again? This is out of your hands and you need to take it to a pro.

Hope that helps,
Rob

 

[schismatic]

Many thanks for your reply Rob.

When checking the HT fuse (250mA) I checked that there was a resistance present across it. I believe it to be 100% in tact because it presented a resistance to me on my meter, and I have added some weight to this through a visual inspection which shows what appears to the eye as an in tact fuse. So for the purposes of this discussion now, we should assume the HT fuse to be operating correctly.

I have double-checked each time that my voltage selector is properly lined up for the correctly line voltage. For me in the UK, that is 240V. I'm absolutely sure it is correct.

I have tried removing all tubes and re-inserting them, but this did not solve my problem.

The only thing I haven't tried is adjusting the bias prior to powering on, however I was under the impression I couldn't bias without power on (my knowledge of the circuitry is not excellent!).

I however am expecting that the mains fuse will once again blow, which will lead me to your final suggestion - a pro needs to look at it.

 

[webrthomson]

Hi guys,

Recently my RM20B combo stopped working completely. I pulled out the external fuse (the one immediately next to the power socket), and it had blown. Not a problem I thought, I'll go to my local electronics shop and pick up some fuses.

As per the ratings on the back of the amp, I picked up 250V/1.6A rated slo blo fuses - I am in the UK on 240V AC mains.

I placed the new fuse in the socket, inserted correctly, switched the power back on and it immediately blew. I tried another fuse in case I had not inserted the external fuse into the correct position, made extra sure it was correct and tried again. Nothing . The only sign of life that I received was the red LED on my Recto preamp board flashing for about half a second. After that, the amp appears to be effectively dead.

I decided to do some more fault finding based on things that I knew I had the knowledge and capability to measure. I took our the Hi Voltage Fuse (250mA Fast-Blo) and measured the resistance across it using a multimeter. It picked up resistance and visually looked in tact, therefore I am confident that the Hi Voltage fuse has not blown.

As I cannot get power on to the amp, I could not check tube biasing (although I'm not sure that I need to).

I've blown quite a few of these 1.6A fuses now, so I'm running out of ideas and this is where I need some forum help.

What else should I do? Am I use the right rating of fuses? Can I provide you with any further information? I can supply photos if necessary.

I'm desperate to fix this as I need to gig with it in the near future!

Help much appreciated and many thanks in advance!

Hey - I'm also in the UK and I'm not sure if this is the case on your amp or not but on my RM4/RM22 I have to use the setting for 230V NOT the 240V one (I've assuming this has the four sided 110/117/230/240 setup).

On the 240V I get instant blows of the fuse much as you are suggesting is happening with your amp - should be easy to check as the fuse amps ratings are the same for 230 and 240 volts - just stick it in with 230 selected and presto!

Hope this helps!

 

[schismatic]

Hey - I'm also in the UK and I'm not sure if this is the case on your amp or not but on my RM4/RM22 I have to use the setting for 230V NOT the 240V one (I've assuming this has the four sided 110/117/230/240 setup).

On the 240V I get instant blows of the fuse much as you are suggesting is happening with your amp - should be easy to check as the fuse amps ratings are the same for 230 and 240 volts - just stick it in with 230 selected and presto!

Hope this helps!

very interesting sir! Hopefully you have led me to a solution. I'll be checking this when I get home after work.

Out of interest, what fuses have you been using for your mains fuse? I've been using (and blowing!) 1.6A 250V slow-blow one.

 

[webrthomson]

Hey - I'm also in the UK and I'm not sure if this is the case on your amp or not but on my RM4/RM22 I have to use the setting for 230V NOT the 240V one (I've assuming this has the four sided 110/117/230/240 setup).

On the 240V I get instant blows of the fuse much as you are suggesting is happening with your amp - should be easy to check as the fuse amps ratings are the same for 230 and 240 volts - just stick it in with 230 selected and presto!

Hope this helps!

very interesting sir! Hopefully you have led me to a solution. I'll be checking this when I get home after work.

Out of interest, what fuses have you been using for your mains fuse? I've been using (and blowing!) 1.6A 250V slow-blow one.

Hey - 1.6A is what is listed in the manual (http://www.randallamplifiers.com/images/stories/manuals/RM20_22MANUAL_9_21_10.pdf) for 220-240V, from memory that sounds right for the RM22 - I'll confirm when I get home!

 

[webrthomson]

Right checked - My RM22 is set to the 220V option (at the bottom where the arrow is) with a 1.6A fuse - works fine

Hope this helps!

 

[schismatic]

Well bugger me, I have a working amp!

220V option - works perfectly. Webrthomson, you are a lifesaver! (and a substantial money saver as well!)

 

[webrthomson]

Well bugger me, I have a working amp!

220V option - works perfectly. Webrthomson, you are a lifesaver! (and a substantial money saver as well!)

No bother - actually the praise should go to my amp tech Dennis Marshall - he happened to mention this to me one day about my Boogies that they run at 220V NOT 240V - somehow it stuck, don't know how little else does

Glad it's working!

 

[Jaded Faith]

Glad to hear you are up and running. Like I mentioned before, fuses do not blow for no reason at all. Something is causing too much current to flow.

I won't question your original statement about being on 240 V AC Mains voltage there (you do live there after all!), I know the UK's nominal voltage has been officially changed to 230 V. Although 240 V is within tolerances and commonly found, I'd stick with the 220 V setting on my amps if I were you. All of the UK Voltage Converters they sell to us here in the US for UK travel (really for European travel as a whole) are for 220 V, which is as good an indicator as any.

 

[schismatic]

Glad to hear you are up and running. Like I mentioned before, fuses do not blow for no reason at all. Something is causing too much current to flow.

I won't question your original statement about being on 240 V AC Mains voltage there (you do live there after all!), I know the UK's nominal voltage has been officially changed to 230 V. Although 240 V is within tolerances and commonly found, I'd stick with the 220 V setting on my amps if I were you. All of the UK Voltage Converters they sell to us here in the US for UK travel (really for European travel as a whole) are for 220 V, which is as good an indicator as any.

Well to be honest I had always thought it was 240V, but you are right that it was officially changed to 230V. 220V is the way forward anyway - will stick with this in future. Thanks for the help along the way.