Studio preamp clean ch too gainy

[Cron]

Hi, When i got my studio preamp the clean ch was as clean as a bell up to 7 or 8 on the gain control. It seems like its been getting dirtier and dirtier. Now its dirty-ish on 3 gain, even with single coils. Ive swapped tubes, no difference. The lead ch isnt bleeding into it (adjusting lead gain or volume has no effect on the clean ch)

Any ideas?

 

[elvis]

LDR

 

[Cron]

If you had to guess, which one would be the culprit?

 

[elvis]

All of them. If one went, the others are soon to follow.

 

[Cron]

Hoping for some clarification, i measured the LDRs. If anyone can help me find the problem id be very thankful. I named them left to right looking at the front.

The 4 ldrs all measure 5.58v from + to ground (back of pots) on clean (rhythm). I guess they would be the same as these 4 have the same supply. Theres one other ldr with a different model no on the right behind the graphic eq board that measures 7.7v on clean (rhythm) between + and ground (back of pots) and 0.58 between it's + and - leads.
On the lead ch the 4 ldrs all measure 1.5v and the one on the far right measures 0.6 to ground.

On the clean channel the 4 on the left give a different reading over their + and - leads (2.6v) compared to + and ground (back of pots) which is weird isnt it? There is continuity between the - leads and the back of the pots where i took the ground measurement from. On the lead channel they measure the same on their + and - leads as they do between + and back of pots.


Resistance on clean (rhythm) from left to right: 8.5m , 1.68m, 550k, 40k, 22k

Resistances on Lead from left to right: 4.5k, 26k, 14k. 1.3k. 0.3k

Most of these resistances seemed to fluctuate very slightly.

Anything look suspicious there?

I have one spare vactec vtl5c1 ldr which i believe is the right replacement part for the 4 on the left so id really like to try to fix this myself without randomly pulling out ldrs in hope of finding the right one. If someone can confirm that all their measurements look iffy then ill order the parts and replace them all.

Please help!

 

[elvis]

It's not very helpful to measure resistances or DC voltages on a semiconductor switch. If the switch is completely broken, maybe. But yours work - they just have some bleed-through. You might be able to catch something with a test signal and an oscilloscope, but only if you really knew what you were looking for.

A bench tech would use the symptom to narrow down which one seems to not switch correctly, then change it, try it, etc. Much like finding a bad tube.

You can try measuring the resistance across the switches in BOTH direction, with the switches controls set in BOTH positions. That will give you 4 measurements per switch. If one switch is significantly different that the others, it's probably bad. Measuring supply voltage is not so helpful, because that comes from the circuit, not the chip.

 

[daveyboogie]

Hey...
I've pulled LDRs out of my Quad that I KNEW to be faulty but then couldn't get them to go wrong on the bench under the meter! They're strange things. Just replace them all ... it's just easier - Re elvis.

Also check plate voltage into V1A. If the resistor's breaking down a low voltage could cause your problem too.

 

[yeti]

how hard is it to replace the LDRs in these to DIY.

what do they cost? relatively cheap and simple im hoping?

 

[Whoopysnorp]

I replaced LDRs on a Studio .22 before. It was pretty easy. The components themselves cost a few bucks each. I just tried to find them on mouser.com but that site is so gargantuan that I couldn't do it. They definitely have them, though. Don't bother attempting it if you're not comfortable soldering on PCBs.

I've actually always found that I need to keep my Studio Pre's volume down just below 3 in order to get a really clean sound when using guitars with humbuckers. Maybe my LDRs need replacing too.

 

[skoora]

Thread resurrection

Just got a Studio Pre and the clean channel with my PAF loaded V is clean all the way to 10. The lead channel is a little fuzzier than I remember these being. Usually you can get a nice sharp attack and it can't get it like I remember. My Stiletto actually sounds better gain wise. I just got it, so I guess I'll try tubes first.