Mesa Boogie Quad preamp / LDR drama

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Zokk
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Post by Zokk »

Hello
my Quad Preamp has suffered from being stored during near a decade... yes it's a shame :roll:
So of course I will have to change the electrolytics, all the orange drop capacitors which are on the anodes, and of course... the "vactrols"...
Some of them have died at power-on, when I tried to use the preamp after its long sleep in the closet.

Of course, I plan to "roll" my own 19 vactrols, seeing that the most important specs to target seems to be the R "off" which should be greater that 50Mohm to ensure proper insulations in the circuits, and slow speed to ensure popless switching (?).

If I'm right the 7812 regulator feeds the footswitch AND the 7805 regulator.
All the LEDs and TTLs are powered from this +5V rail; but the schematics show an +8V for the front panel LEDs. It seems wrong as on my unit those LEDs a directely connected to the 7805 output.

So most "vactrols" are driven by +5V / 360ohm; they clearly don't reach lower than 1K at R "on"; and the four (LDR21, 22, 23, 24) "LEAD1 and 2" + EQs vactrols seem to be "limited" to have a different range.
I don't understand why... could it cause oscillations if R"on" is lower, say less than 1K ?
or if R"off" is higher than 5Meg ?

The vactrols which switch the EQs and Channels are the slowers, the others have a slighty faster "release".
Time to release to dark resitance max is always a lot longer that Time to reach R"on" in full light.

Let me know if I'm wrong:
I will probably use GL5537-1 LDRs, they seem to reach more than 50Mohm at dark, they are slow enough to mimic the VTL5C4's "release".
But I think also about the GL5528 to mimic the VTL5C1, but this could be a lot too fast to reach R "on" max compared to a real old school opto.
The GL5516 are the slowest of this series, but the don't have a high R"off"; I measured an average 1-2Mohm for the "best" ones.
So really high dark resistance is harder to get than low light resistance!
Anyway modern LEDs will provide enough lumens at 10-12mA to reach a real low R"on" without a doubt.

It is strange to see that the LDRs are exposed to high voltages, way to high according to the datasheets.
The real VTLs are quoted to 100V max.
For sure all the LDRs would fry at power on if the main capacitors are dead...


Please, I have a doubt on a silver, DIP8 component, it takes place of a 555 according to the schematics, but its reference is unknown :hmmm:
IMG_20220707_115742.jpg
I also need some advice about the small axial tantalum capacitors. Are they the same "technology" as the radials ones (those who die in a dramatic shortcut on the power supply rails in old gear)?

I'm open to suggestions, to not forget to do something while the pcb will be reachable... I would prefer avoid to have to extract the pcb twice only because I forgot a pair of resistors, or a capacitor to replace.
Did someone already swap the originals VTL with a "hand rolled" one in a Mesa stuff?

Thanks!

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Post by ppluis0 »

Hi Zokk,

Can you post the datasheet (or the corresponding link) to know the response times of this photocells ?
All the info that can found in the web express the same operational times of 20 and 30 msec for all the GL series of cells.
Also, in the same datasheet, these photoresistors prefers to be illuminated with a 560nm wavelengt to have the best performance, so I think that a 5mm ultra bright green led can be employed to roll your own photocouplers.
BTW, is frequent to replace burn photocells used to channel switching in Mesa amps.

Cheers,
Jose

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Post by Zokk »

Hi Jose
The VTLs are page 41:
https://pdf1.alldatasheet.com/datasheet ... TL5C1.html

the datasheets for the GL55xx series are just "informative":
https://datasheetspdf.com/pdf-file/7568 ... GL5537-1/1

According to the datasheet, R"on" is measured at 10Lux, R"off" is given at 3Mohm for the 5537-1...
Well in the real life, with a nice LED @10mA it gives something else: the average is more like R"on" = 700ohm
R"off" is more like 50Meg after 4-5 seconds.
Which matches the VTL5C1 behaviour at the same current, but is faster to reach a high dark resistance (5C1 is given at 10seconds to 50Meg)

I've made several tests with the "slower" GL5516, and it's really a bit slower but still really snappy (if used in enveloppe followers, or LowPass Gates); it can achieve less than 100ohm @ 5mA, but only 3Meg after 5 seconds in the dark.
Also in a batch of 200 of the same reference, you'll find values in a really wide range, some LDRs are simply not usable, others are in the "midway", and a bunch are a lot "wider".
The time response is totally random for those LDR, you'll have to build a test rig and sort them by tracing curves.
Even the datasheets for the VTL and the GL series are not really clear: the "release time" is measured only until it reaches 100K at dark. So yes 30ms...
But if you look for 50Meg it will be longer of course, I mean sometimes really longer.

If you look for a linear response, you may want to try a simple 3mm IR LED; they used this in the old VTL series.
Maybe IR wavelenght affects the LDR in a nice way, which gives more or less a linear curve. Specially if driven @ 40mA!
It's not important to match the datasheet wavelenght sensitivity: any bright white LED will change the resistance of a LDR.
What matters is to find the good range, and nice curve, with a low consumption.

Another thing often forgotten: LDRs are temperature sensitive.
High temperatures increase resistance; in the Quad it's not really cold when tubes are glowing...
So maybe the VTL5C1 has a R"on" (@10mA and 45°C) around 2K or more, in the Quad. Is it still considered as "switching"?

I've not found any example of vactrol "hack" in Boogies... but you're right, a huge part of their products used vactrols switching systems, and LDR failure seems very common.
Show your Boogies fitted with DIY vactrols please.

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Post by n8b5150 »

Is there a reason you want to replace the film caps? I would leave those alone unless they are leaky.

As far as the vactrols, if the dark resistance is sufficient to prevent signal bleed and the light resistance is within spec, it's working as designed. When I did warranty work for Mesa, there was a service note for various amps that had failed vactrols due to high "on" current through the led. The fix was to increase the current limiting resistor and replace the failed vactrols. This was over a decade ago, so my memory is a little foggy.

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Post by Zokk »

It is so difficult to access to the tracks on the pcb that I would like to change all the critical components at once.
The anode caps are really important and they live in tough conditions... they would probably fail before the other film caps I think.
Also they are not so expensive compared to the optos (if I would like to get the "real" ones).

I am really annoyed by the choice of the 5C4, it's the most difficult to mimmic with a DIY combo.
It has a "release" curve which is really "fast" to reach 400Meg compared to what I get with the GL55xx series LDR.
And I've done other tests which show that the 5528s are way slower than the 5537s.
But the 5528 reaches easily the typical R"on" of the 5C4 without the need to change the original 380ohm limiting resistors.

So you don't think this deep 5C4-style "release" curve is important to switch properly the LEAD and EQ sections?

At the moment I'm thinking about rolling all the "5C1" vactrols with the GL5537-1 and 5mm flat transparent orange LED.
But to get the 5C4 R"on", I would use a GL5528 + an ultrabright 18000mcd white LED... but this will give a very very slow pseudo-5C4.

Anyway I can't go further messing with R"off" curves as I've tested much of the GL55xx series without finding something better than the 5537-1.
All the other ones (5506, 5516, 5528 that I've tested) are way slower to reach high resistance; to give an idea, it takes about 3 seconds to reach 2Meg for a 5528; about 5 seconds for a 5516... While the 5537-1 is already at more than 50Meg after 3 seconds.

Thanks for the information about how Mesa handled those vactrol failures.
However I think the LEDs were not really faulty, they run at 10mA which is nice; the LDRs on the other side can have a really high voltage across them. And I've seen more dead LDR than dead LEDs (mostly in pedals and synths).

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Post by n8b5150 »

The fast release is desirable for sure, but I know the pain of sorting ldr's for both fast release and low on resistance. It sounds like you're on the right track. I can't recommend a work-a-like as I always had access to the original vactrols when I was in the business. I'm just getting back into repair and building and boy have things changed in the past 10 years!

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Post by n8b5150 »

Here is the service bulletin I was referring to (it was for the Tremoverbs):

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

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Post by Zokk »

Thank you! This is interesting!
I'm tempted to "read between the lines": is it possible that they advised to change the resistors and optos because they changed their vactrol manufacturer and the "new" Vactec component has different specs?
It seems the Silonix (Silonex?) parts needed to be driven harder to achieve low R"on" at such currents that it would fry the Vactec LEDs.
Did they use the two types of vactrols before this note was released?

Old school optical switching... yes you should see something different in today's amps!
Now everything is µC controlled, smt mounted, lead-free... not "repair-friendly".
Anyway I think things have gone bad since manufacturers begin to use pcbs in tube amps :slap:

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Post by n8b5150 »

I never encountered an amp with Silonex, but did repair at least a few Tremoverbs that required the replacement resistors/vactrols. I can't speak to why they were driving the LEDs so hard, but I speculate it had to do with getting a low "on" resistance.

As far as PCBs, I think it's all down to application of the technology and quality of the boards. Most are made for cheap and easy production. I always thought the Mesa's were good quality and serviceable, just a bit over-engineered and over-complicated. Many techs won't touch them for obvious reasons, but I always enjoyed working on them in a sick way.

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Post by Zokk »

Hi, I've just finished to replace the vactrols and the electros capacitors...
Accessing the pcb was not as difficult as I thought, just a bit of sport to work with the preamp in vertical position laying on its left side (like a book) to access both pcb sides at the same time to remove the solder properly (iron at max on the component side, sucker gun at 500°C on the track side).

First I have to say that my DIY vactrols are not performing nicely.
I've changed my mind several times to choose the "best" LED/LDR combo, I've found the VTL5C4s have 250ohm R"on" resistance in the Quad, and the VTL5C1s are about 650ohm in the circuit. Those are real measures from the real components from my Quad; my first estimations about the 5C4 were totally off...
I've found several dead 5C1s but all the 5C4s were ok. Anyway I've removed all the originals vactrols to avoid further failures.

So the 5C4 are simulated with hi-bright 5mm white LED (vf 1.9V) + GL5528 LDR... those are very slow to recover high R"off" resistance but they were the only ones in the right R"on" range without modifications in the Quad. Exact 250ohm at 5v + 1K5 resistor in the Quad.

The 5C1 are simulated with orange transparent flat 5mm LEDs (vf 1.8V) + GL5537-1, and their R"on" is a bit lower than the originals, the majority in my batch were in the 550ohm range. So it's a bit lower than the originals.

Results are not nice when switching channels and rythm/lead. I can hear loud pops, hoomps, crossfades with level changes and distorsion etc etc... not usable for live or rehearshals. Maybe the 5537s are too fast? I will try to replace some of the "critical" fake 5C1 by my fake 5C4s which are slower.
But all other switchings are ok (pushpull pots and EQ): totally silent.

I've not changed the anodes caps, not enough money to get all the parts now, sadly...

>>> Now I have another problem that drives me mad:
input vol rythm2 pot and lead1 pot are acting like those "two-stroke engine set screw", if you see what I mean.
There is a curve like a bell between zero and "3" where I can get the max signal level in the middle of the "bell", but before and after there's nothing...
Those pots are also scratchy and microphonics now; tap on them and you hear it in the signal output; but they test ok with the dmm... :scratch:

I've tried the simple tricks like swapping tubes, and of course cleaning the pots tracks, with no results.
Grounds are ok too... my new solder joins are also ok, no damage on the pcb, and cleaned the flux.

Do you have an idea to explain why those pots are acting crazy?

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Post by n8b5150 »

Check for DC on the pots. You may have a leaky cap or two.

I'd have to look over the schematic, but I doubt the ldr's are too fast. If anything, they may be too slow going to a high resistance.

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