Replacing Resistors with different value

Thread Starter

niterida

Joined Dec 27, 2019
29
I am trying to repair a Crown CDI 1000 amplifier soft-start / power on circuit. It has 6 48k7 1% resistors which are hard (read expensive) to get. I can get 47k or 51k resistors everywhere. Will it work with those slightly different value resistors ?
Thanks :)
 

Ya’akov

Joined Jan 27, 2019
6,561
The fact they are 1% tolerance and a very specific value would suggest they are critical to the circuit’s operation./

Without a schematic, I would say no, they are required.
 

Ya’akov

Joined Jan 27, 2019
6,561
To modify that slightly, in theory you could get a few of the other values and check them on a calibrated meter to find one within 1% of 48.7㏀.
 

Thread Starter

niterida

Joined Dec 27, 2019
29
The fact they are 1% tolerance and a very specific value would suggest they are critical to the circuit’s operation./

Without a schematic, I would say no, they are required.
See attached file. It is R370->374 and R378 on Page 6 as per this snip :
Crown CDI snip.jpg

Futurlec is only 1/4w - I need 1w
Meritek is 1/8w and SMD - I need through hole and they don't seem to do online ordering
Mouser is $46 by the time I pay for shipping :( But this was the only one I could find that had them available and at somewhere near reasonable price.

A (47 K + 1.8 K) will give 48.8 k, which I guess should meet the requirement.
I did think about - thanks for the reminder :)
But even then the local supplier only has 2 x 47K and they don't supply 1.7K and the next shop is 420kms away - the joy of living in the middle of nowhere :(
 

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Thread Starter

niterida

Joined Dec 27, 2019
29
hi niter,
Can you say why you need to change those resistors, are they damaged in some way, or off specification?
E
There was a bad batch of these resistors and these amps were recalled to have them replaced but that was 12yrs ago.
4 of them are registering 22k and 2 are at 40k
 

Alec_t

Joined Sep 17, 2013
13,153
Given that the Zener voltage of a MMSZ5246 can be anywhere in the 15.2V-16.8V range (per the datasheet), I can't see that the 1% tolerance of the voltage dropper resistors is justified/critical.
 

Thread Starter

niterida

Joined Dec 27, 2019
29
Given that the Zener voltage of a MMSZ5246 can be anywhere in the 15.2V-16.8V range (per the datasheet), I can't see that the 1% tolerance of the voltage dropper resistors is justified/critical.
In English does that mean that 47K resistors would work ??
 

BobTPH

Joined Jun 5, 2013
5,762
Is the amp functioning now with those incorrect values? If so, replacing them with 47K would surely not be a problem.
 

Alec_t

Joined Sep 17, 2013
13,153
In English does that mean that 47K resistors would work ??
Yes, IMO (providing V_BULK isn't excessive).
Not doin so well so far are they
How do you know that they were the correct values initially, since the batch was faulty?
And we still don't know what V_BULK is, so can't assess what resistor ratings should have been used for a robust design.

Edit:
Even if V_bulk were 320V (e.g. from rectified 240V mains), 47k resistors would be dissipating about 0.45W each, so would get pretty warm, but the 1W rating would be reasonable. I'd be tempted to replace with 47k 2W 5% resistors to err on the cautious side. They're cheap and hopefully readily obtainable.

Edit 2:
Hopefully the Zeners have survived. Resistors dropping to 22k should still have allowed the Zeners to run within their rated power limit (200mW-500mW depending on physical size and format) with V_bulk = 320V, unless the problem was caused by considerable V_bulk over-voltage. A Zener failing short-circuit could account for the soft-start not working.
 
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Thread Starter

niterida

Joined Dec 27, 2019
29
Yes, IMO (providing V_BULK isn't excessive).
Thank you :)

How do you know that they were the correct values initially, since the batch was faulty?
Not sure what you mean ? The resistors are 48k7 on the schematics and 48k7 by the colour bands but measure at 22k and 40k.
And I was actually referring to Liverpool Football Club not having a good start to the new season .......

And we still don't know what V_BULK is, so can't assess what resistor ratings should have been used for a robust design.
Its 240v input if that helps ?
 
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