How to repair LDR Mesa Boogie amp

[mikeydee77]

LDRs seem to be super expensive these days (c£9) and so with the prospect of about 10 to replace on a 30y old Boogie MKIV B that I am refurbishing I thought I would try fixing them.... You can remove the LED by slicing off the resin cap holding in the LED and if you are fortunate the old LED will drop out easily. A couple of times it got stuck in there sideways which was fun to remove.

The thing I would like help with is sealing up the replacement LED. The original resin is very dark to prevent light entry and very hard. Ideally I would like to use whatever Vactrol uses (if that is that the manufacturer?) so they can be fixed again in the future if needed.

I also read about changes on boogies to reduce the switching voltage to prevent failure... if anyone has details would like to hear about that too...

Thanks

[Mick Bailey]

Vactrol was a Vactec brand originally that's now become a generic term for a LED/LDR combo and other manufacturers make them too (Silonix, for example). The slightest light leakage affects these devices but I did discover some really cheap automotive satin black paint from a Pound shop that completely blocked out the light when the vactrol was resealed with 5-minute epoxy. I've also used adhesive lined heatshrink over the repaired unit and pinched it together to seal over the leads.

The only specifics I know about the switching voltage relates to Tremoverbs. I've had a few of these and they always suffer from vactrol problems. There's a service bulletin floating around on the web which details the problem;

TECH BULLETIN: "Tremoverb" Update
*NOTE: This update does not apply to very early units
having a 120ohm, 10W sandblock resistor near pwr transformer.
The "Tremoverb" amplifier has two 47-ohm, 1/4-watt resistors
(one is located mext to the pair of relays and is connected
to LDR #3 - Location 26-E on diagram below. The other is
next to and connected to LDR #13 - location 14-B below).
Please replace both of these 47-ohm resistors with 220-ohm, 1/2 watt resistors.
Also, the 68-ohm resistor that is connected to LDR's 11 & 12
(location 14-C below) should be increased to 360-ohms.
Finally, please REPLACE LDR's #3 (type 5C4/2), #11 (5C9), #12 (5C9) and #13 (5C4).

*NOTE: LDR's manufactured by "VACTEC"
(has 3 lines of print on top) are more reliable
than LDR's made by "SILONIX" (has single 4-
digit number). If you encounter an amp having
several "SILONIX" LDR's, please call
Mesa/Boogie to arrange a factory repair

[mikeydee77]

Thanks for the reply and the tip about the epoxy Mick... Yep two coats of black paint make a x10 meg ohm difference I measured.

I had been trying some construction adhesive (sticks like sh!t) but that is prone to cracking and tries slightly soft. I also was considering the araldite metal glue which is black two part paste but I suspect that would be impossible to remove. The other one I was thinking of trying is the "gorilla glue".

Fun stuff...

[mikeydee77]

[drewl]

We have tons of the adhesive lined heat shrink at work I use for things like this.

Nice tip for fixing these, thanks.

[Harrysong]

LDRs seem to be super expensive these days (c£9) and so with the prospect of about 10 to replace on a 30y old Boogie MKIV B that I am refurbishing I thought I would try fixing them.... You can remove the LED by slicing off the resin cap holding in the LED and if you are fortunate the old LED will drop out easily. A couple of times it got stuck in there sideways which was fun to remove.

The thing I would like help with is sealing up the replacement LED. The original resin is very dark to prevent light entry and very hard. Ideally I would like to use whatever Vactrol uses (if that is that the manufacturer?) so they can be fixed again in the future if needed.

I also read about changes on boogies to reduce the switching voltage to prevent failure... if anyone has details would like to hear about that too...

Thanks

Hi. How did you remove the resin? I was trying to do so with An xacto knife but it’s hard as a rock!!!I couldn’t do it.
thanks!

[mikeydee77]

It is...

With your ldr on its side cut into the resin near where it joins the plastic body. Once you get the seal broken it drops out.

I use what we call a Stanley knife I think known as a box cutter in the US. You can apply quite a bit of downward pressure.

Hope it works

[Harrysong]

Ok. Will try. Thank you.

[Zouto]

You could also replace the LDRs with Omron G3VM-401s.