Studio Preamp channel switching issue...

[Luijo]

Hi all, I'm new to this forum!
I have a Studio Preamp that I got pretty cheap a couple of years ago. I'm amazed how good it sounds when set up right...definetively lives up to the hype!
Only problem I have with it is that the clean channel seems to leak a little of the dirty channel whenever the lead gain is above zero. This makes it impossible to get a really clean sound, there's always this little buzz in the background.
Also when I switch channels the transition is not smooth at all. Sounds like the lead and rythm channels coexist for a brief moment and then everything goes back to normal.
As you might realize both of this issues don't allow me to use this pre live, unless I use a loop box and switch the device in and out becoming in effect a very expensive OD/Dist. box!
I had a friend call Mesa and the guy he talked to told him the problem is the LEDs are tired. I'm an electrical engineer and never heard of tired LEDs in my life!
I thought about asking here before I go and disassemble this thing to find out what the problem is. Is this a regular issues with this devices? If so, how can I get it to work as it is supposed to?
Thanks in advance for your help and taking the time to read this post.

Luijo

 

[MarkMcQ]

Hi all, I'm new to this forum!
Also when I switch channels the transition is not smooth at all. Sounds like the lead and rythm channels coexist for a brief moment and then everything goes back to normal.

Mine does the exact same thing too. Makes it unusable as a proper 2 channel setup as it is. I'd also like to know what the cause is.

 

[Ed Pitman]

Hi Guys,

This is almost definitely an LDR problem - one of them is leaking and bleeding signal from the lead into the clean channel. I think that LDR stands for Light Dependent Resistor - I'm not a teccie but I understand that these can fail over time. In teh UK they are expensive to buy - around 18 UK pounds each - not sure how many of them there are in a Studio. One for your local tech to check out.

Best of luck !

Ed

 

[disassembled]

I agree with Ed that it sounds like a LDR problem. You can buy a new one from Mesa or go through a supplier like alliedelec.com You just have to find one with the same specs. The triaxis uses the part VTL5C9 for it's switching duties made by perkin elmer. They run $3-$6 if you buy them direct from a supplier. Mesa is probably going to charge a little more since they need to make a little profit. Some parts distributors sell these for $9-$10 a piece. There is not any reason to pay this much if you go through a parts supplier.

Call Mesa and ask them for some help. If you can't solder get a tech to do the work.

 

[Ed Pitman]

Good point disassembled. I'm shocked at component prices from the UK distributor.

Let us know if this cures the problem !

 

[Luijo]

Thanks for your opinions guys!
I was already suspecting the LDRs but just thought I'd check out with long time Mesa owners.
There's a tech note from Mesa for the Trem-O-Verb regarding faulty LDRs. I checked my SP for comparison's sake and found that it has the "reliable" units, but you can see that they are not the original ones. So maybe this fix is not meant for the SP. The fix requieres not only changing the LDRs but a few resistors as well, for correct biasing of the LEDs I guess.
Any of you would happen to know the brand and model of the LDRs in your units? That info would be quite helpful because I would be able to compare the tech specs of the LDRs I have vs the ones that come stock and proceed from there.
I even thought of ditching LDRs altogether and replacing them with mini relays, but while that would get rid of any channel-switching problems I would have to build a small board to house the relays and maybe even add a litlle power supply for the relays themselves.......too much trouble considering there must be a sensible solution to this problem.
Any other ideas or help are greatlly appreciated, thank you all again guys.

 

[disassembled]

Also, if you add relays you will probably add an audible pop when the relay is engaged. You would have to add two diodes to the relay's coil as well. It sounds like it would be a pain because of all the added things that need to be done. The LDRs are very reliable and are very cheap (about $3.90 USD) so if a couple went bad after 16 years I wouldn't sweat it.

There is only one company that I know of who manufactures the LDR and the LED in a casing. Look up the part number: VTL5C9 on google. This is what comes inside the Triaxis and has very good dynamic range (the best out of all that the company offers) so I would assume this is the only choice Boogie has when ordering LDRs. I go through Alliedelec.com since they are the cheapest and you don't have a minimum to order. They are currently out of stock so I hope they get some soon (I need to order about 5 to repair my Triaxis FX loop).

I have two different brands of LDRs in my Triaxis but it looks like the newer ones have 100% of the VTL5C9's to do the switching. I guess they were just going through the old stock of LDRs when my unit was built. BTW I only had 2 of the LDR's fail on me (these were not the ones that I am purchasing they are a different brand) so I hope replacing them fixes my problems with the FX loop.

 

[Luijo]

Thanks all for your time and interest in this subject.
There is a difference between the job the LDRs do in the SP and the job they do in the Triaxis.
In the SP they only work as an SPST switch, that's why you need two of them to do the channel switching, so they work as a DPDT(actually 4PDT since you also switch volume pots and lead bright settings). But in the Triaxis they work as programmable resistors, needing two of them to work as a single pot.
So the specs for LDRs working as pots and the ones working as switches are different. That's why I asked before what kind of LDRs you guys have in your SPs so I can compare with mine and see if it is stock or not (schems don't show this info). Mine has the VTL5C1 unit, but in the above mentioned tech note for the TOV Mesa warns against the Solinex units and recomend instead the VACTEC ones (with a small mod to the biasing circuit for the LED part of the LDR).
The difference between units is mainly response time( the faster the better for switches, but not so impotant for pots) and on/off resistance( again, the lower/higher the better for switches, but not so important for pots).
I hope this clears up the importance of knowing the brand and model used in a stock SP, so I can compare specs and see if the problem is a wrong LDR or a misbiased one.
Again, thank you all for your time and interest in this thread.

 

[Luijo]

:roll:

 

[disassembled]

No, I am fully aware what the LDRs do in your amp. The LDRs I was referring switch the Effects loop in and out so yeah, it would be the same as channel switching. I don't know what else to tell you. Call Mesa? This isn't a tech forum so don't expect too much.

 

[Luijo]

Yeah, I get your point, this is not a deeply technical forum.
But this is a very nice forum indeed, so I'll keep coming back to it....lots of interesting opinions and info.
Regarding your suggestion to call Mesa, as stated in my first post I had a friend call them. He has a good friendship with one of the tech guys and all his friend told him was "his LEDs are tired", which is something in my almost 20 years as an electrical engineer I've never heard of....tired LEDs??!!
Maybe my friend missunderstood him or the guy just didn't have a clear idea of how to get rid of the problem and that would be really strange.
Anyway, I guess I'll have to order a few LDRs and check again. I was hoping to find out if anyone here has had the same problem and how they solved it....well, no luck.
Thanks you all guys for your time and patience, have a nice day.