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FDP Forum / Amp Mods, Repairs, and Projects / Measure resistors in place?

RDR
Contributing Member
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I tried to think

but nothing happened!
Jun 18th, 2017 01:21 PM   Edit   Profile   Print Topic   Search Topic

Hi. My 66 Bandmaster tremelo is getting weak. I'm gonna dive in soon. Have a new bug. While I'm in there, can you check value of resistors while in place?
Thanks!

Peegoo
Contributing Member
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I walk

between the raindrops
Jun 18th, 2017 02:12 PM   Edit   Profile   Print Topic   Search Topic

Negative!

You need to desolder one end before metering 'em.

guitarcapo

U.S.A.

Jun 19th, 2017 10:18 AM   Edit   Profile   Print Topic   Search Topic

The problem you run into is that the resistor might be in parallel with something else in the circuit and your meter reading could be less than the actual value. It will never be MORE, though. If you get a reading HIGHER than it should be by a significant amount, it is probably burned "open"

ejm

usa

Jun 19th, 2017 02:43 PM   Edit   Profile   Print Topic   Search Topic

The to be safe 100% foolproof answer is: No, you cannot measure resistance accurately/correctly in circuit.

Also, one other thing that was not mentioned: You cannot measure resistance with power applied to the circuit.

However, both of these answers are not 100% correct.

It IS possible to measure resistors correctly in circuit, BUT it depends upon the circuit. You need to have the schematic (probably) and know what you are doing.

With power applied and measuring resistance, once again, it depends upon the circuit.

So the answer is no you can't, but yes you can.

Hope that clears it up.

(This message was last edited by ejm at 04:45 PM, Jun 19th, 2017)

RDR
Contributing Member
********

I tried to think

but nothing happened!
Jun 19th, 2017 03:16 PM   Edit   Profile   Print Topic   Search Topic

I thought the answers would be as above. Thought I'd ask anyway, thanks for all the answers!

pdf64

UK

Jun 19th, 2017 06:15 PM   Edit   Profile   Print Topic   Search Topic

Assuming that the amp is de-energised, the vast majority of the resistors in a bandmaster can be measured in circuit.
Caps may need to charge up but after a few seconds the meter should read the resistor value fine.
There are only a few that have parallel resistive paths and will measure low, these include vol and treble pot tracks, the 820 and 100 ohm feedback resistors, and the 68k and 1M input resistors.
For the feedback resistors, the parallel path can be removed by de-soldering the wire at the speaker jack socket, thereby avoiding stress to the resistors.
For the input circuit resistors, a jack plug can help to overcome the shorting jack socket switch action with the latter.

Any measurement will have measurement error.
I suspect that the heat / physical stress of removal may well affect such old resistors adversely.

I don't think parts should be removed for no good reason.
As part of a campaign replacement, or if they've been identified as faulty, then fine, but removing resistors just to measure them, especially with no plan in place, seems pointless.

How far off nominal are you prepared to accept? Within their tolerance band? But to fail any just outside that seems unfair, as they may only have drifted 1% from when they were new 6 decades ago.

But if they pass your measurement criteria, what are you going to do?
Re-fitting old used resistors seems a bad idea surely?
ie the 100 ohm often fails due to physical stress from the board flexing; after the stress of removal, are you really going to refit it?

schematic

(This message was last edited by pdf64 at 08:48 AM, Jun 20th, 2017)

RDR
Contributing Member
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I tried to think

but nothing happened!
Jun 21st, 2017 07:42 AM   Edit   Profile   Print Topic   Search Topic

Thanks again pdf64!

pdf64

UK

Jun 21st, 2017 09:56 AM   Edit   Profile   Print Topic   Search Topic

You're welcome - repost if you get any inexplicable wacky readings.

FDP Forum / Amp Mods, Repairs, and Projects / Measure resistors in place?




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