It is currently Mon Mar 16, 2020 10:52 am

All times are UTC - 7 hours



Post new topic Reply to topic  [ 9 posts ] 
Author Message
Post subject: Strange standby switch behavior. No power applied
Posted: Sat Apr 23, 2016 6:03 pm
Offline
Hobbyist
Hobbyist

Joined: Fri Feb 19, 2016 9:56 am
Posts: 28
Hi guys.
On the end of a Silverface Showman rebuild. Pretty much all complete and put back together. Anyway, I decided to ohm out the standby switch making sure it fully opens and fully closes when toggled.
DVM leads on each side of the switch:
When the switch is in the up position it reads low resistance as would expect because the standby is off (switch closed). Then in the down position it reads open (o.L on the DVM), also as I would expect. But after a few seconds it starts to conduct. It jumps up to 30meg ohms and drops rapidly.
After several minutes it does begin to finally stabilize at 3meg ohms . If I flip the switch on and off again it reads open again but then starts to conduct after a few seconds, essentially repeating the process.
I'm pretty sure I'm just reading some circuit resistance on the other side of the switch but I just want to double check if this is normal or not.
I don't want to smoke my OPT if the switch is faulty.

Thanks!


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Strange standby switch behavior. No power applied
Posted: Sat Apr 23, 2016 6:57 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14047
Location: Province de Québec, Canada
You can't smoke or damage your amp ( or OPT) with a standby switch defect.

Your stanby switch reading is normal.


Top
Profile
Post subject: Re: Strange standby switch behavior. No power applied
Posted: Sat Apr 23, 2016 9:00 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
You are reading the static DC resistance of the circuit. To read it properly, you need to pull the switch out of the circuit. Then, read the resistance across the switch contacts.

:)


Top
Profile
Post subject: Re: Strange standby switch behavior. No power applied
Posted: Sat Apr 23, 2016 11:33 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26416
Location: Tombstone Territory
BMW2002Ti wrote:
You are reading the static DC resistance of the circuit. To read it properly, you need to pull the switch out of the circuit. Then, read the resistance across the switch contacts.


+1

The switch must be isolated before any meaningful measurements can be made.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Strange standby switch behavior. No power applied
Posted: Mon Apr 25, 2016 8:54 am
Offline
Hobbyist
Hobbyist

Joined: Fri Feb 19, 2016 9:56 am
Posts: 28
That's what I suspected. I just really wanted to be sure.

Thanks


Top
Profile
Post subject: Re: Strange standby switch behavior. No power applied
Posted: Mon Apr 25, 2016 11:32 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26416
Location: Tombstone Territory
So did the standby switch check out or not?

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Strange standby switch behavior. No power applied
Posted: Mon Apr 25, 2016 5:41 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14047
Location: Province de Québec, Canada
oKors wrote:
That's what I suspected. I just really wanted to be sure.

Thanks

Why do you want to check the Stanby switch. ?

If you put the right wires on the switch the amp should work .
If you don't put the right wires at the right place, the problem won't be the switch but the wiring.


Top
Profile
Post subject: Re: Strange standby switch behavior. No power applied
Posted: Mon Apr 25, 2016 8:31 pm
Offline
Professional Musician
Professional Musician

Joined: Thu Aug 07, 2014 3:25 pm
Posts: 1023
It's not unusual to read ascending or descending resistances when measuring in-circuit. The small current that the ohm meter uses to measure resistance is slowly charging or discharging a capacitor somewhere in the circuit.
In your case, it would be the power supply capacitors.
You can interpret that as an open circuit. If the ohm meter is charging the caps, that means the resistance is so high that it can't drain the Ohm meters tiny current.

To properly measure a switch, you should check it a the edge of its action, just before it snaps.
This is the point where the spring tension is at its lowest and the contacts more likely to shhow their real condition.
You should read less than 2 ohms closed and have it stable right up to the edge of action


Top
Profile
Post subject: Re: Strange standby switch behavior. No power applied
Posted: Tue Apr 26, 2016 12:24 pm
Offline
Hobbyist
Hobbyist

Joined: Fri Feb 19, 2016 9:56 am
Posts: 28
stratele52 wrote:
Why do you want to check the Stanby switch. ?

If you put the right wires on the switch the amp should work .
If you don't put the right wires at the right place, the problem won't be the switch but the wiring.


This amp had a catastrophic melt down 20 years ago. The PT melted down and left a tar-like residue from the vapor across the whole board. I'm rebuilding it now but the pots were contaminated, wires burnt up, switches sticky. I've cleaned everything out very thoroughly but I'm at the point where I'm about to flip the switch on this amp with it hooked up to a current limiter.
The standby switch was the only switch I could not physically disassemble to clean. Me checking it was an afterthought. I wanted to make sure for some reason the residue didn't coat the inside and create a short while the switch was supposed to be open, or in standby. During said afterthought, when I checked it in-circuit I got the funny readings and wanted to double check.

This is my first amp rebuild... ever.
I'm just being overly cautious at this point. I've got quite a few dollars invested in this to get it where its at today.

I have not had a chance to unsolder one of the leads going to the switch but at this point I'm pretty confident its fine.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 9 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: