Mesa 50 cal fried resistors continued

[01redcobra]

Working on this 50 cal with fried resistors and I was wondering if there is a way to test the LDRs out of circuit?
Apparently, some big voltage got thru to the LDR voltage and melted a bunch of stuff. I've attached the schematic of the power circuit for reference. The 680ohm 2w got taken out and the 1.5k ohm 5w between a & b on the high voltage supply also. Also the 150 and 470 in the LDR supply look compromised too.50 Caliber+ Power.pdf

[Mick Bailey]

To test the LDR measure the forward voltage drop of the LED side with a multimeter set to the Diode function - it should read around 1.8v. Reverse voltage should be 0v. Mesa fitted different brands, so some may measure higher or slightly lower. It's usually the LED side that fails, so often this is the only test required.

If the LED does measure ok you can connect it with a 9v battery and 1k series resistor. With the LED disconnected the LDR side will read several tens of meg Ohms resistance. With the battery connected it will come down to around 150R or less. The LED polarity is marked on the body - the LED being the thicker leads. The actual resistance values don't matter too much so long as it goes high/low. For actual values you need to look at the spec sheet - they're usually Vactec or Silonex.

The vactrols can be tested in-circuit with the amp powered off using the same method. I have a set of mini-hook test leads especially for this job.

[Mark Black]

Also, I have found if you have two meters on hand just applying the diode test voltage to the LED side will cause a resistance fluctuation on the other side of a working LDR.

[01redcobra]

Thank you for your replies! I used a 2nd meter and saw the change on LDR 5. I checked the LDRs and it appears that L5 is the only one working at this point. I need to pull the leads on LDR 1,2, and 3 because they all have the leads for the LEDs to common points and I can't test each one separately. My next dilemma is what to use for suitable replacements. The Vactrols that are in there have no numbers on them to cross to anything. And the one unit that has numbers (21L883) doesn't cross to anything I can find. Couldn't find anything on Mouser, but found some possibles on Newark.

[Mick Bailey]

I use Vactec VTL5C9, simply because I keep a plenty in stock to replace them in Tremoverbs (which have something like 25).

The two meter method needs to be used with a caveat; some vactrols fail so that their 'on' resistance is too high. In some places this doesn't matter too much, but in others will cause loss of signal (I haven't looked at the rest of the schematic to see if this would be the case with your particular amp). I've had them at quite a few K when they've been like this. In critical positions you need to test them with the LED fully energised - most meters are limited to 2v - and measure the LDR.

Vactrols can also be tested with the amp in standby, though I usually pull the tubes when doing this. You can then see if the LDR is getting its correct voltage and measure the LDR resistance. This checks the switching as well. Assumes no major faults elsewhere though, but a useful check for an amp that isn't switching or has low output on one channel.

[01redcobra]

Thank you Mick, but is there any way to determine what Mesa was using for these LDRs? I've noticed on data sheets that there are a lot of different parameters to be considered, such as on and off resistance and response times. Right now the board is burned to badly to run any voltage tests. I have to rebuild the power supply to the LDRs before I can get any power to them. I'm trying to get all the parts in advance so I can do the board work at the same time. I have attached a pdf of the preamp with equalizer that is run by the LDRs

50 Caliber+ Preamp.pdf

[Mick Bailey]

There are a mixture of types and brands in Mesa amps. They switched from Silonix to Vactec as the company perceived reliability issues with Silonix. There are a number of considerations with Vactrols in amp switching;

1. We want it to act like a switch, so the on and off resistance is important.
2. The reaction speed of the LDR is important to give a quick transition (amps with vactrols don't switch as quickly as relay circuits).
3. The cell voltage needs to be considered - how much voltage is across the cell when it's in the 'off' state. Too much voltage and it will break down.

I've used the VTL5C9 without any complaints in all positions in Mesa amps as a generic type. Cost comes into manufacturing, so you'll see vactrols with lower cell voltage specs, slope, or whatever where they're warranted in different positions. This doesn't mean they can't be replaced with a higher spec.

If you want the exact parts you can contact the factory service department directly - I've found them to be really helpful.

[01redcobra]

Thanks for the input Mick. We'll give it a shot as soon as I get an ok from the customer. These are not the nicest amps to work on.