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Replacing standby switch in RP65

I need to replace the Hi-Off-Lo standby switch on my RP65. The switch lever has a soldered cap with a spring inside, the cap popped off and the spring is loose. I have tried putting the spring back in and holding on the cap but the switch doesn't work.

I've bought a new simple on-off-on switch but not sure this will do the job.

The old switch was soldered in with a brown wire to the bottom lug, white wire to the middle and red wire (+resistor link to lamp) to the top lug. I've tried wiring up the new switch with the wires to the same lugs but it's not working. In the Up position the lamp lights up but just get a faint distorted sound, nothing in other two positions.

Does anyone know the significance of the spring and cap on the switch lever? Is it somehow involved in reducing the power perhaps?

I can research the wiring and make a better diagnosis of which lugs to use on the new switch, but I'm thinking I need something better than a simple on-off-on switch.

Photos attached if you can help please.

Update Sep 2. I have checked the schematic and the wires are as expected, brown to high power and red to low power. So either the spring in the old switch was doing something clever that I can't fathom, or whatever caused the switch to break has taken out another component. I would appreciate any help you can offer; I re-capped the amp a couple of years ago and it sounded really sweet up until the switch broke. Thanks


Ohm Out the Switch

Was the new switch "Ohm'd Out" before installation? -mgriffin


I didn't do a continuity test

I didn't do a continuity test on it. Is that what you mean by ohmd out.


Function Test

Yes that's what I meant but ohm'd out was a poor choice on my part. I'd call it a function test using a DMM continuity test to map contact behavior relative to toggle position. In other words, before installing the switch, verify it behaves as expected. -mgriffin


Thanks. I will unsolder and

Thanks. I will unsolder and test it. Also use crock clips to test direct connections missing out the switch.


Good Plan

Good plan. I like "crock" clips aka Alligator clips. That's a good one. Some folks call them roach clips in the states. Is your amp an American model(120V) or export model(240V)? American models have a similar ON-OFF-ON switch on the rear panel called GROUND. -mgriffin


It's a UK model 240v. I

It's a UK model 240v. I bought it new in 1979. GP1 board. Both my sons have also used it, one using it as a bass amp at moment. I recapped couple of years ago, guite proud of that. Thanks for your help so far.


Checked, still not solved

Hi, I removed the standby switch and connected the white and brown wires with clips. Still same symptom, very faint distorted sound. Also tried red (low power) wire, no sound. Also tried plugging guitar into low volt effects loop to eliminate front end of pre-amp, this also gave the same faint distorted sound. There's a row of 3 fuses to the left of the circuit board. The 1AT fuse was black and had no continuity which seemed promising. That fuse supplies two yellow wires, one goes to the neon and has continuity. Other goes into the (dog kennel?) and out again, not via a component, just two wires connected, and goes to the valves. I busted a 200mAT fuse getting it out, but it was OK I think.
I connected the fuse holder ends with clips to simulate a working fuse but the symptom was the same. It seems that the problem is related to that 1AT fuse but I'm stuck as to where to go next.
Do you have any guidance please.


Low Side of Secondary

The 1AT fuse is connected to the low side of the power xfmr secondary. It needs to be there for the HV power supply to function. You should also determine why the fuse blew. Take a look at the components under the dog house and see if any have failed or look burnt. Check the diodes and verify they're not shorted. Also, be very careful as there is 350VDC and 700VDC in there when things work right. Anyway, something has gone wrong on the 350V line and it blew the 1AT fuse. This explains why the amp has extremely weak output. You mentioned a recent re-cap. Were the dog house caps replaced? If so, are the polarities correct? Let us know what you find and we'll go from there. -mgriffin


The dog house all looks fine.

The dog house all looks fine. Yes I replaced those 450v 100uF caps and polarity is correct. I also replaced the two 150k 2A resistors which someone recommended. There is a wire I can't see, between the two strips of insulation, connecting C44- with C45+ but there is continuity so I'm assuming it's OK. Diodes are not shorting, I don't know how to measure them, they show no continuity with meter set on low resistance. Tried to check the two resistors too but the resistance changes as the caps are soaking it up.
I checked continuity from the end of the 1A fuse through to a big black resistor labelled 1.5k. This reads as expected on meter. That connects to the valve lug.
The white wire from the standby switch, carrying low or high power, goes to the common lug of the diodes and has continuity.

I'm waiting on some fuses now. I'm cautious about powering up with Croc clips bridging the fuse holders in case I do any more damage. But if you have any more suggestions that don't involve powering up I would appreciate it.

Thank you for help so far


Pull the Tubes

Remove the output tubes before the amp is powered up again. This will minimize current drain on the HV power supply while it is tested for correct output voltages. Before proceeding, ensure your DMM and test leads rated for more than 700VDC. And be careful to not get shocked because it will hurt. Place the +DMM probe at the top of R66 before powering up. After power up, the DMM should read about 350V (In Hi Mode). If that reads good and no smoke is pouring out, power the amp off and wait a few minutes for HV caps to discharge for the next test. Move the +DMM probe to the top of R65 and power the amp On. The DMM should read around 700V give or take. If that reads good and still no smoke pouring out, the HV power supply is in good working order. If not, you must figure out what is wrong and repair it before installing the tubes. Once you're confident the HV supply is working properly, install the tubes and repeat the tests for 350V and 700V. Let us know what you find and good luck. -mgriffin


Thanks, perfect. Fuses are a

Thanks, perfect. Fuses are a couple of days away and then I'm on vacation. I will for sure be back in touch in a couple of weeks.



Hi, amp is working again :)
I resoldered the standby switch and replaced the fuses. Tested voltages per instructions -
No valves
Top of R66 hi power 345v lo power 231v
Top of R65 hi power 692v lo power 462v
With valves
R66 hi 339 lo 227
R65 hi 681 lo 456
Connected speaker and its working, sweet as before.
I think when I tested bypassing the switch I didn't connect in the resistor that goes to the neon. Doh
Original fault I think was failure of standby switch which took out the 1A fuse.
Anyway I now have a 1kv DMM which is handy.
Is there a place a can donate to help the website?
Thanks again


Good Work

Good work. Glad the amp is happy and healthy. Keep the DMM. It's a good tool to have around. Rock on! -mgriffin

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