Looking for old opto isolator
Phil Symonds
I am looking for a replacement for an opto isolator in my Gibson guitar
amplifier made c. 1965-66. It is part of a tremolo circuit. You can find a
schematic at www.schematicheaven.com. The amp model # is GA20-RVT. This
particular item is a cylinder covered with a black sleeve about 3/8" x 2", 2
red wires out one end, 2 green and 1 unshieled wires out the other end. The
part information: Sylvania LDR500 622 PL8224C PPB Made in USA. Or, does
anyone have an old parts book that tells the specs of this thing so I could
build a duplicate? Thanks for your help.
Phil S.
Bob M.
Basically it's a light bulb and a photo-cell in close proximity inside a
(hopefully) light-tight assembly. As the lamp gets brighter, the photo-cell
resistance goes lower. These were often used in audio consoles to replace
the sliding level pots that always got noisy from use.
You'll have to do some experimentation to figure out which light bulb to use
and what values of resistance are needed when the lamp is on and off. You
can make one yourself by taping the same components together and stuffing
them inside something that's absolutely light-tight, like a piece of copper
pipe or even an old 35mm film can. Black electrical tape will NOT work. You
also need to shield it to keep the hum level under control, unless you want
60 Hz hum in the guitar sound.
I thought these things were still available somewhere, but not necessarily
an exact replacement to what you need. I'd say you could adjust some other
circuit components to deal with almost any lamp and photo-cell combination
you can come up with. By the way, you can substitute an LED for the light
bulb, but it will have a much sharper on/off switching point, since the LED
can't really glow with low voltage like the lamp can.
Bob M.
======
"Phil Symonds" <psym...@mica.edu> wrote in message
news:asqvt7$ue17a$1...@ID-170833.news.dfncis.de...
John Fields
<psym...@mica.edu> wrote:
>LDR500 622 PL8224C PPB
---
Try this link:
Seems like what you have is used in a lot of automotive apps!
There doesn't seem to be a data sheet available so you may want to
contact Sylvania for help.
---
John Fields
Professional circuit designer
http://www.austininstruments.com
CJT
Isn't it likely that only the light bulb is burned out, and the
photocell still good?
Bob Weigel
circuit looks simple enough. My recommendation
1) install a cheap neon 115V bulb in place of the old filament drawn in the
schematic. The reason I say this is because you can't hurt the circuit if
it's as drawn. The tube will keep too much current from flowing in the
worst case. And there's a 10K load (the 7W resistor) to the 248V wires.
2) power up the amp and watch the bulb. Can you make it go on and off with
the controls? If so this may be what they used, though they drew a filament
in the diagram. Some of the people who drew old diagrams well..they weren't
perfect or didn't always think or something. :-) Conventions also varied I
guess and maybe the filament was seen as a generic light symbol by many.
Though I'm fairly sure a specific schematic symbol for neon lights existed
then.
3) If the old unit didn't light, and the neon one above did, feel confident
you can tear it apart to examine what's inside the "black sleeve" which is a
heat shrink I assume? If you see a neon bulb inside, then there you have
it. It's likely illuminating a pancake photocell and is the same as the
fender style ones used back in that era. Antique electronics has them I
think.
-Bob
Phil Symonds <psym...@mica.edu> wrote in message
news:asqvt7$ue17a$1...@ID-170833.news.dfncis.de...
Bob Weigel
the problem I've seen is the heat shrink stresses the photocell and it
cracks. I've seen the neon tubes fail also. So if he wants to he can of
course just put a new neon tube in if that's the case I forgot to
ention. -Bob
CJT <chel...@prodigy.net> wrote in message
news:3DF13282...@prodigy.net...
John Fields
<soundd...@montanadsl.net> wrote:
>Wow, that's a cool old amp. I haven't studied that one before. The tremelo
>circuit looks simple enough. My recommendation
>1) install a cheap neon 115V bulb in place of the old filament drawn in the
>schematic.
---
Since the tremolo cicuit is supposed to modulate the volume of the
amp smoothly up and down, a neon lamp won't work properly because
nothing would happen until the gas in the tube became ionized, then
there would be a sharp change in volume.
Lee Blaver
> Just from the part number, I'd say this is a Light Dependent Resistor (LDR).
> Basically it's a light bulb and a photo-cell in close proximity inside a
> (hopefully) light-tight assembly. As the lamp gets brighter, the photo-cell
> resistance goes lower. These were often used in audio consoles to replace
> the sliding level pots that always got noisy from use.
>
>
Also used to be used in some 1970s UK telephones...
Lee
Bob Weigel
EXACT SAME THING. The way the circuit is designed gives an amount of
current control in the tube that allows for enough smoothness to make a nice
tremelo. (I recommend adding a capacitor in those circuits across the lamp
btw; about .015uF seemed ideal on the ones I worked on. This reduces the
popping sound that can sometimes occur to a greater extent than others for
reasons I don't totally understand yet. I've worked on a lot of these amps
and a couple of them had excessive noise and this really helped.) Fender's
circuit simply hooks a pancake photocell to ground from the center lug of a
pot whose outside legs are ground and signal to inverter stage as I recall.
And THEY SOUND SMOOTH ENOUGH apparently. Very popular circuits..sooo...
since you are a professional circuit designer I'm sure you don't need my
help to figure out how it all works. Just look. -Bob
John Fields <jfi...@texas.net> wrote in message
news:3df847b4...@news.texas.net...
Bob Weigel
lot of absorbtion and decay of the light on the components contributing to
the perceived smoothness as well. I just got done working on another old
super reverb. It's certainly not a sine wave LFO! And it doesn't go all
that slow either btw which helps mask the perception of abrupt change. But
it's good enough to sound like something other than cheeze because they're
still one of the most sought after amps for guitar work and people still use
that effect with pride :-) . -Bob
John Fields <jfi...@texas.net> wrote in message
news:3df847b4...@news.texas.net...
John Fields
<soundd...@montanadsl.net> wrote:
>I'm sure you don't need my
>help to figure out how it all works. Just look.
---
I did, Bob, and even though I didn't spend a huge amount of time
going through the circuit's operation I noticed that the notation on
the schematic indicates that there's about 10V across the
filament(?) of Z1, and there's about about 8 mA. going through it if
R49 is at 50% of its range. 10V isn't enough to keep a neon lamp
ionized, while 8mA doesn't seem like enough to keep an incandescent
lit, but if you take a look at I1, you'll see the symbol for a neon
lamp with an integral resistor, so it's not like they didn't know
how to draw. ISTM that if it was a neon lamp without an integral
resistor they would just have drawn it without the resistor, so I'll
vote for the incandescent lamp. Also, a visit to Zenith's website
yields a large number of automotive uses for the part number the OP
gave, so since the 10V given on the schematic is pretty close to
12V, seems like that pulls it even further into the incandsescent
camp. There's also that 10k 7W resistor... seems like a little
overkill for a neon. Also, take a look at the plate family of
curves for a 12AU7 and it seems there's something a little heavier
than a neon lamp being driven, but I could be wrong...
John Fields
wrote:
>Also, a visit to Zenith's website
---
Sylvania
Phil Symonds
in particular, the discussion by John and Bob. Just so you know, the guitar
amp folks who claim to have hacked apart this object (theirs, not mine) tell
me that it is an incandescent bulb, but none of them were able to offer
anything but trial and error on the bulb spec. I think the engineers
understand this particlar challenge a whole lot better than the amp techs.
I was particularly impressed with the hit on the Sylvania web site. I was
there, but couldn't get the hit, so that was a real bonus. I'll be trying
the autoparts route if my other luck find doesn't pan out. I got a private
communication from another engineer who thinks he may actually have a spare
for me. Thanks for taking a few minutes to help me out.
Phil Symonds
Bob Weigel
one of these things. THey use almost ALWAYS (listen to me people...I repair
these things for a living for crying out loud!) NEON LAMPS! This WAS the
standard way of doing it during the years these were made by Fender and I
seem to recall some gibsons used the same. I just haven't seen near as many
of them and can't recall a broken one. They are much less common of an amp.
The only thing that would make me think any different is the symbol
used...but like I say I've seen that done and it be neon before I believe.
So... my suggestion was completely logical based on years of troubleshooting
things. -Bob
Ross Herbert <rher...@bigpond.net.au> wrote in message
news:3DF47DC2...@bigpond.net.au...
> Bob Weigel wrote:
> >
> > And yes it is likely. Especially if they used an actual bulb. MORE
often
> > the problem I've seen is the heat shrink stresses the photocell and it
> > cracks. I've seen the neon tubes fail also. So if he wants to he can
of
> > course just put a new neon tube in if that's the case I forgot to
> > ention. -Bob
> >
>
> > >
> > > Isn't it likely that only the light bulb is burned out, and the
> > > photocell still good?
> > >
>
> It is almost certainly an incandescent bulb device... LED's weren't
> exactly commonplace in 1965 and the design for this thing would most
> likely have been many years old by the time it was manufactured. Once
> the muso electronics designers got something right they weren't too keen
> to change it either, so the light bulb is sure to be the answer.
>
> A resistance check will tell you very quickly... the lack of any meter
> indication will mean that the bulb is open circuit.
>
> Ross Herbert
Bob Weigel
and I didn't work the logic there. It must be a light in this one then. My
recommendation would have taken about the time it takes to talk about it
though. It wouldn't have worked, the scope would have shown the oscillation
attempting to happen, and we could have seen that ti's time to take apart
the thing and figure out exactly what value they used. 10 minutes of work.
I can't talk about it for half an hour. But it's good to know some people
did it differently. -Bob
John Fields <jfi...@texas.net> wrote in message
news:3df5a963....@news.texas.net...
Bob Weigel
from the diagram. BEWARE though..there could be a failing component that
DREW MORE CURRENT than the light was capable of handling! Note C30. If
it's leaking it will give a positive voltage on the grid if any "depth" is
dialed up. ALso, note R49 which can adjust the current. Get a reading on
it. Someone could have turned it at some point for all we know and it could
be even shorting the cathode to ground. As you can see, there should be a
bit over 2V on the cathode. Were that with the resistor all the way up,
that would mean a current through the tube of 2/500 or 4 mA. However as
oscillation occurs, this value is going to change. But anyway you get the
ballpark figure that you need a lamp that's maybe 12V or less rated and has
a fairly small current capacity. Anyway, if I were you I'd power it up and
put a vtvm on that grid with the depth turned all the way up and see if
there's any leakage before you even stick a lamp in again. And record R49's
value then turn it all the way up to 500ohms. I haven't taken time to read
the spec on the tube used there. You should have a copy of the essential
characteristics if you work on these much. Antique electronics has it. -Bob
Phil Symonds <psym...@mica.edu> wrote in message
news:at2gm6$v5lp6$1...@ID-170833.news.dfncis.de...
John Fields
<rher...@bigpond.net.au> wrote:
>It is almost certainly an incandescent bulb device... LED's weren't
>exactly commonplace in 1965 and the design for this thing would most
>likely have been many years old by the time it was manufactured. Once
>the muso electronics designers got something right they weren't too keen
>to change it either, so the light bulb is sure to be the answer.
>
>A resistance check will tell you very quickly... the lack of any meter
>indication will mean that the bulb is open circuit.
---
Or neon?^)
Phil Symonds
Thanks again for the additional info. I'll pass it along to the tech who is
working on the amp. I broke it while fooling with the grounding scheme,
trying to lick a hum...probably a bad decision on my part with regard to a
ground wire. When this happened, I threw in the towel. I don't know
enough. However, as I changed the adjustment on the pot (R49) the voltage
in the circuit did change (true before and after I broke it). When I "ohm"
R49, it indicates that it is operational and "in spec." I can't say much
about C30 except that it looks to be a relatively new replacement. Anyhow,
thanks for explaining what these actually do. BTW, this was a lush and
robust tremolo when it was working. I hope to get it restored. I'll post
the results.
Phil
Phil Symonds
I'm told by the guitar amp folks who have hacked this part open to repair it
that this one has an incandescent bulb. See my earlier post and the end of
the thread. I understand that Fender used a neon and using the Fender part
would require a circuit mod. We'd like to avoid this -- it won't sound the
same. Thanks for the discussion.
Phil
"John Fields" <jfi...@texas.net> wrote in message
news:3df704f3...@news.texas.net...
John Fields
<psym...@mica.edu> wrote:
>Bob,
>Thanks again for the additional info. I'll pass it along to the tech who is
>working on the amp. I broke it while fooling with the grounding scheme,
>trying to lick a hum...probably a bad decision on my part with regard to a
>ground wire. When this happened, I threw in the towel. I don't know
>enough. However, as I changed the adjustment on the pot (R49) the voltage
>in the circuit did change (true before and after I broke it).
---
That's interesting. If the voltage on the wire connected to the 10k
7W resistor changed as you adjusted R49, then the lamp wasn't burned
out and the V5B was OK too!
---
>When I "ohm"
>R49, it indicates that it is operational and "in spec." I can't say much
>about C30 except that it looks to be a relatively new replacement. Anyhow,
>thanks for explaining what these actually do. BTW, this was a lush and
>robust tremolo when it was working. I hope to get it restored. I'll post
>the results.
---
Here's a little more for ya!^) V3b and its surrounding circuitry
comprise a phase shift oscillator, the output amplitude of which is
controlled by the DEPTH pot and the frequency of which is controlled
by the FREQUENCY pot. The oscillator's output is fed to the grid of
V5B where it is used to vary the current flowing through Z1's lamp.
This change in current through the lamp causes its optical output to
vary (the higher the current the brighter the lamp) and as the
brightness of the lamp varies, the resistance of Z1's LDR varies
also; The brighter the lamp the lower the resistance of the LDR. As
the resistance of the LDR varies, its output (notice there is _NO_
DC coupling, pretty slick IMO) goes to the grid of V4A, thence from
the plate of V4A through some shaping into the grid of V5A. At this
point all of the audio from either front end has been combined and
the signal on the grid of V5A is used to drive the primary of T3,
the secondary of which drives the grids of the output stage.
Just as an aside, I had thought originally that the current
available from the 12AT7 required for the lamp to drive the LDR was
a little low, but after looking through some incandescent lamp
catalogs there are a lot of choices available.
BTW, R49 is used to adjust the quiescent brightness of Z1's lamp.
That is, for the tremolo to work right it never needs to bump into a
clamp, so it needs to be adjusted so that no matter where the DEPTH
pot is set there'll always be some headroom and footroom left over.
Phil Symonds
This is a real nice posting (the website). Whodathunk? Thanks.
Philnews:3DF58C8C...@bigpond.net.au...
> Bob Weigel wrote:
> >
> > Ross...who the heck said anything about LED's? I've never seen an LED
in
> > one of these things. THey use almost ALWAYS (listen to me people...I
repair
> > these things for a living for crying out loud!) NEON LAMPS! This WAS the
> > standard way of doing it during the years these were made by Fender and
I
> > seem to recall some gibsons used the same. I just haven't seen near as
many
> > of them and can't recall a broken one. They are much less common of an
amp.
> > The only thing that would make me think any different is the symbol
> > used...but like I say I've seen that done and it be neon before I
believe.
> > So... my suggestion was completely logical based on years of
troubleshooting
> > things. -Bob
>
> devices, which I grant you, must be formidable. Nor was I saying that I
> had any practical experience in the repair of the devices used in the
> tremolo circuitry. I do however have some experience with constructing
> light coupled devices and I have built quite a number over the years
> from the mid 70's onwards using incandescent bulbs & LDR's as well as
> led's and photo-transistors. The fact is that there are more tremolo
> circuits in amplifiers in circulation today using either
> incandescent/ldr (older amps) and led/LDR (or LED/phototransistor), than
> those using NEON/LDR, therefore the likelihood that the tremolo
> schematic in the GA20-RVT in question is less likely to be the latter
> arrangement. As far as I am aware only Fender used the NEON/LDR
> "Trem-Roach" assembly in one or two models.
>
> Since a NEON bulb is more difficult to modulate to produce a variation
> in light intensity compared to using an incandescent bulb it seems
> reasonable that the latter would be used as a preference in the majority
> of tremolo circuits until LED's appeared.
>
> Just as an example of what is required to replace the NEON/LDR device
> with a modern approach I invite readers to take a look at the complexity
> of one suggestion for achieving such a solution.
>
> http://www.geofex.com/Article_Folders/sstremolo/sstremolo.htm
>
> I don't know what arrangement Gibson used in the GA20-RVT but it would
> almost certainly have been incandescent/ldr.
>
> A NEON would probably have a longer life than a incandescent bulb but
> usually the bulb type selected for these applications had a quite rugged
> filament support assembly and were thus able to withstand considerable
> rough handling, plus the production was simpler and cheaper.
>
> Ross Herbert
Phil Symonds
This is really amazing to someone like me who knows very little. There is
actually at least one person (I'm sure there are many) who can understand
how these work by reading the schematic! Thanks for the explanation and the
glimmer of light! Maybe this is an easy fix.
When I speak with the tech next week I'll see if he can entertain this
discussion. I think he will. He is actually an boutique amp designer of
some modest fame, having reworked the original design of the Vox WahWah
pedal into something better. In fact, your diagnosis (that the lamp still
works) may explain his reaction when I explained to him about the voltage
thing. He didn't want to give away his cards at the time (good business
sense), but he probably suspected that the opto-isolator is still working
and that something else in the circuit needs attention. Given his
experience and expertise, I won't be surprised if this part of the job turns
out to be a quick fix. It's the hum on the reverb channel that will
probably take some work. I was moving around the grounds when I caused the
tremolo problem -- and it became obvious how little I know and how this amp
is not an appropriate first project.
BTW, V5 is a 12AU7 on my particular amp even though the available schematic
shows a 6EU7. Gibson was not consistent in its production. The amp
actually came with a schematic glued in the cabinet and with this tube, a
few minor differences, and different parts numbers (cap and res #'s are in
the same places but different sequence numbers), it's very much the same.
My schematic (can't get it published because I'd have to remove the speaker
to get a good picture -- it ain't broke and I'm not gonna fix it) states,
"voltages vary widely with R38 assemblies", R38 being the opto-isolator and
the in and out voltages being 124 and 136, a 12V difference! This probably
doesn't matter much...these are all dual triodes.
Again, thanks very much.
Phil
Bob Weigel
GA-20rtv. It wasn't used in only a few models. It was in ALL that had
tremelo from Fender as far as I'm aware. Like I say haven't seen Gibsons
with a problem and so haven't had to dig into them. But seems we've gotten
confirmation that it was an incandescent lamp from a testimonial and our
evaluation of the circuit. My "what the ...." remark was to express my
confusion...beause it sounded like you were saying someone suggested they
might have used LED's in '68 or whatever. :-) So I was
kind of doing a double take and...still am. -Bob
Ross Herbert <rher...@bigpond.net.au> wrote in message
news:3DF58C8C...@bigpond.net.au...
> Bob Weigel wrote:
> >
> > Ross...who the heck said anything about LED's? I've never seen an LED
in
> > one of these things. THey use almost ALWAYS (listen to me people...I
repair
> > these things for a living for crying out loud!) NEON LAMPS! This WAS the
> > standard way of doing it during the years these were made by Fender and
I
> > seem to recall some gibsons used the same. I just haven't seen near as
many
> > of them and can't recall a broken one. They are much less common of an
amp.
> > The only thing that would make me think any different is the symbol
> > used...but like I say I've seen that done and it be neon before I
believe.
> > So... my suggestion was completely logical based on years of
troubleshooting
> > things. -Bob
>
Bob Weigel
terms of sane productivity. I mean...c'mon. It's very very very seldom
I've seen a tremelo tube fail. They don't run that hot and who cares if
they are noisy or u-phonic? Cheap crappy tubes work just fine. :-) I often
get the 12AX7WA for less than $4.00 The sovtek tube actually tends to last
better than some of the cheap tubes and..keep a spare ya know? So my
recommendation is...leave it stock. Unless you have nightmares about it
failing in the middle of that special moment of waviness by some freak of
nature. Then..yeah build something even a little more reliable. :-) It
hasn't made my top 100 projects list yet. Hehhe. -Bob
Phil Symonds <psym...@mica.edu> wrote in message
news:at58sr$10d1kr$1...@ID-170833.news.dfncis.de...
Phil Symonds
Sometimes it's hard to know what's really on someone's mind in a forum like
this. You can't see or hear the person, so there's no body language to go
with it...only the written word. Now, if we were all that literery, maybe
we could get by without the body language, but we are not. This is a bit of
a long winded way of saying thanks again to everyone who wrote in on this.
The discourse has been enormously helpful and I have been able to get quite
a bit of technical info on a very narrow topic, and very quickly at that. I
think all the participants should all be pleased at the result. I, for one,
am very grateful for the help. It's hard for me to tell if the writers know
each other, if they tangle regularly in this forum just for sport, or what.
I'd like to say that no one should go away from the forum with any hard
feelings about what's been said, because the results of the discussion are
much more far-reaching that I ever expected. Did I mention that I also got
a private email from someone who thinks he might have a replacement for me
(parts from that era, but not clearly marked)? Another great benefit of the
forum.
Bob, I'd like to say that your point is on the mark: probably no one is
going to go build that ss tremolo thing just to fix the problem in my amp.
It is unlikely that parts and bench time make this a good economic choice,
not to mention the sonic roulette...never can tell what does what in an old
hand wired all tube amp (well it has ss rectifier, which surprised me).
I've been told that in those years Gibson was known for building the amp du
jour from whatever parts were on the shelf, demonstrated by the accepted
fact that published schematics are not always reliable. I took that article
reference as a sidebar, and a useful one at that, providing a last resort
fix. Also, if we were designing the thing from the ground up today, it
would probably make sense to consider it today.
All that having been said, I think we can say this thread has reached its
conclusion. It's done. Let's put a fork in it.
All my best,
Phil Symonds
Also, having not examined this particular opto-isolator personally, I am not
in a position to say for sure what's inside. Though I feel certain the info
I have that it is incandescent, the whole discussion of the various
possibilities (neon, LED, etc.) was "illuminating" as well (excuse the pun).
There is only good stuff to be said about a broad discussion...it being much
easier to consider several possibilities and then narrowing down to the most
likely.
John Fields
<psym...@mica.edu> wrote:
>All that having been said, I think we can say this thread has reached its
>conclusion. It's done. Let's put a fork in it.
---
Often, a thread assumes a life of its own and will continue with or
without the OP's consent. :-)
Mac
It is advertised as an optocoupler "vibrato LDR for Fender Amps".
Don't know if it would work for you or not. (by the way, the company is
Antique Electronic Supply).
Mac
Phil Symonds <psym...@mica.edu> wrote in message
news:at5mme$10jsft$1...@ID-170833.news.dfncis.de...
Phil Symonds
Bob Weigel
perspective on the LED thing in a humorous way. I have a bent towards that
kind of humor. One of my favorite artists is named Larry Norman and he
often does this act where he pretends the sound is messed up and then ends
it with "is anybody up there?[in the sound booth] I've seen reactions vary
from people laughing hysterically to getting up and leaving because they
felt he was offending the sound person. No personal attack was meant here.
Just missed where LED's got introduced into discussion of an old amp though
I'm of course well aware that ldr's in new one's are more sophistocated
optocoupler devices using LED's in a molded module. -Bob
Phil Symonds <psym...@mica.edu> wrote in message
news:at7ng4$1100ji$1...@ID-170833.news.dfncis.de...
Michael A. Terrell
Reminds of a sound job one time. I rented a portable system for a
company picnic, and they had some unknown local comic for
entertainment. I had set it up and tested it just before they arrived.
The "Comic" walked up and grabbed the foam windscreen on the microphone,
looked around and asked, "Why the hell do they put the microphone. I
was already in a bad mood so I called out, "To keep idiots from spitting
into my microphones!" Kind of upset his timing for a few seconds as he
stood there with a blank look. Then recognition spread across his
face. The light over his head had a dim glow, then he laughed and said,
"I never could figure out what they were." then went back to his act.
After he finished his act he walked over and laughed about it and
said, "Thanks, Man! I really didn't know." and walked off. He was a
strange guy.
--
Michael A. Terrell
Central Florida
John Fields
<psym...@mica.edu> wrote:
>So I've noticed.
---
_And_ helped to continue its non-demise!^)
BTW, I was intrigued by the coupler and emailed Sylvania's tech
support folks for some spec's on the unit. They emailed me back
this morning saying they couldn't find the part number, but would I
send them a description, so I did... We'll see what that yields.
Also, EGG makes a family of couplers they call VACTROL's which are
essentially LDR's driven by neon lamps, LED's, or incandescent
lamps. I've got a few of them around here and I'll check tomorrow
to see if they're even remotely like what's in your amp. If one is,
I'll let you know and if you want it tell me where to send it and
it's yours.
No, I'm not queer, I just like to solve problems.
Bob Weigel
post did I get a bunch of undeliverable messages in my box...listing groups
like sci.elec.components and stuff...truncated the middle of that one it
appears! basics and some others were represented fully, but they still got
kicked back. Are these just groups my server doesn't carry? -Bob
Michael A. Terrell <ter...@mfi.net> wrote in message
news:3DF7DB2E...@mfi.net...
Michael A. Terrell
Very few news providers carry all newsgroups. Some rarely update
their list, others are just low budget operations who don't have the
storage capacity for more groups. Look at the list of available groups
under SUBSCRIBE in your news reader to see what is available.
Andre
> So I've noticed.
> Phil
> "John Fields" <jfi...@texas.net> wrote in message
> news:3df86f56....@news.texas.net...
> > On Wed, 11 Dec 2002 11:01:54 -0500, "Phil Symonds"
> > <psym...@mica.edu> wrote:
> >
> >
> > >All that having been said, I think we can say this thread has reached its
> > >conclusion. It's done. Let's put a fork in it.
> > ---
> > Often, a thread assumes a life of its own and will continue with or
> > without the OP's consent. :-)
LOL!
Dave M
Phil,
Having owned one of these amps for many years, and repaired it several
times, I can, with a bit of authority say that this component is, indeed, a
photo resistor, illuminated by a small incandescent bulb. The bulb runs
normally with about 10-12 volts across it, and, as others have posted, about
8-10 ma through it. I had to replace this component three times during the
time that I owned the amp, and I can attest that the incandescent bulb was
burned out every time.
If you can't find an exact replacement part, you might measure the dark
resistance of the LDR and try to find one with a close value. Finding
10-12V incandescent bulb with that low a current rating might be kinda
tough. Types 344, 1869D, 2149D or 7344 would be good ones to try first,
since they're rated at 10V /14ma. A type 7375 bulb in series with a 470 ohm
1/2W resistor might work too.
I've heard that some techs replace these with the Vactrol VTL3A27. It might
work, but it's bulb is rated at 12V/40ma, so it may be a bit rough on the
triode.
Dave M dgma...@atxxt.net
Remove the xx for my correct email address
Roger Johansson
>photo resistor, illuminated by a small incandescent bulb. The bulb runs
>normally with about 10-12 volts across it, and, as others have posted, about
>8-10 ma through it. I had to replace this component three times during the
>time that I owned the amp, and I can attest that the incandescent bulb was
>burned out every time.
It sounds like this lamp could be replaced by an LED and a resistor.
Roger J.
Bob Weigel
to it would be..interesting. No current would flow until a volt more or
less depending on LED of course. Then it would rapidly rise in intensity
after that initial dark time.... probably to current saturation for the
circuit LONG before the end of the cycle. The bulb designed to go in there
would turn on smoothly and continue to increase in intensity until there is
the listed 10V across it. So it would sound totally different with the LED
I'm fairly sure. I don't think the LED would blow out because I don't think
the circuit will allow enough current to blow it. I can't recall what tube
it even used now. But I doubt it will flow that much current. -Bob
Roger Johansson <no-e...@home.se> wrote in message
news:8slivu4asd7qhgtkk...@4ax.com...
Mark Zenier
Bob Weigel <soundd...@montanadsl.net> wrote:
>Yes us strange guys always make the best comedians. :-) Hey, why on my last
>post did I get a bunch of undeliverable messages in my box...listing groups
>like sci.elec.components and stuff...truncated the middle of that one it
>appears! basics and some others were represented fully, but they still got
>kicked back. Are these just groups my server doesn't carry? -Bob
No, somebody in Maine is running a news to email forwarding system
that's FUBAR. It's their own private problem that doesn't affect
the real net news systems.
Mark Zenier mze...@eskimo.com Washington State resident