Heath ta-16

[bkahuna]

I CARUMBA,CISCO....WHAT EES GOING ON HERE?

Yep. I had the same problem with a TA-16 I worked on. The LED/LDR does work in the tremolo circuit, but at the expense of attenuating the signal at all times due to the LED remaining lit when depth is turned down.

I ended up using a super bright LED that did not directly illuminate the LDR, and making a white cardboard box to go around the whole assembly. By persistant fiddling with the angles the LED and LDR were aimed, I was able to get an acceptable compromise using the LED light reflected off the white box. At depth of zero, it was dim enough in the box to keep from affecting the LDR too much, but when activated, the super bright LED lit up the inside of the box enough that the ambient/reflected light would give a good tremolo effect.

Believe me, I lost my ass on this repair, but it sort of became a quest!

[wizard 651]

Thanks. I thought I was going nuts here.
Maybe it's the low current of an LED. I might try a 1.5 v. incandescent.

[Mike Patterson]

Wizard, I believe that the light also stayed on in the original LDR. I built my amp new, and the "normal" channel always had more gain than the "effects" channel. The light being on would explain the difference.

Tucker, I don't know what to tell you. I looked at my amp schematic for any clues as to input and output impedances, but couldn't find any. But then, I never was much good at reading schematics.

[bkahuna]

The original lamp in the LDR assy I had was still good (the LDR itself was bad). I dismantled the assembly to see what was going on. The lamp still functioned but was incredibly dim. I mean REALLY dim. I could barely get enough light to affect any of the LDRs I have here. That's why I persisted with the LED/LDR fix. That must have been a really sensitive LDR they used.

Mike, you're saying your amp was ALWAYS weak on the effects channel?
Doh! I spent hours trying to "fix" that (and was actually successful!)

[wizard 651]

I'm still going to try the Silonex part suggested by a member as a direct replacement for the original optocoupler. That's plan "A"
I just wanted to see if the circuit was malfunctioning, or that my observations were typical.
I also used a super bright White LED in my experiments.
Three choices:
1. Trying a very low voltage incandescent... (a 1.5 volt two pin "flashlight bulb" as used in a single AAA "mini-mag light"). The original was an incandescent... I also opened the dead one up with a dremel

2. Re wire the footswitch jack/circuitry so that trem circuit is disconnected/disabled...no power to light bulb/LED, unless footswitch plugged in, (like a fender needs a footpedal or "shorted jack" to function, or

3. substitute the intensity pot with one having a switch at "zero throw", that lifts the LDR's attenuating ground connection, or shuts off all flow to the bulb for no light when off.

I also built one of these at age 14 back in the day, but do not remember the reverb channel being conspiculously more quiet than the "normal" channel.

[bkahuna]

That's funny, I had a list pretty much identical to your list of choices before I was able to come up with something that worked, more or less, normally. I considered all of those ideas at one time or another.

I can tell you that a lamp from a 2)AA mini maglite won't work. The filament resistance was so low it stalled the oscillation. Good luck!

Let us know how the Silonex part works out.

[J M Fahey]

a lamp from a 2)AA mini maglite won't work. The filament resistance was so low it stalled the oscillation.

Agree, they have only a couple ohms when hot, much less when cold.
I used successfully a miniature incandescent lamp, we call it "grain of wheat/corn" , imagine the size, it's much smaller than a neon lamp (or a Christmas tree lamp), is 12Volts rated, comes with two bare copper wires about one inch/2.5cm long, is commonly used to light the dial or face of radios, panel instruments, etc. It's cheap and common but must be bought in a specialised lamp shop, those that advertise to have "10.000 lamps", not in a regular electrical or electronics shop.
And yes, even on 12V they are dim, they shine somewhat orange, to last more.

[wizard 651]

Thanks everyone. I thought a lower voltage bulb was the way to go, (being more similar to the operating voltage of an LED), but I will try the 12 volt. I have an excellent collection of grain of wheat/rice/corn, bulbs, (LOL) but never thought of increasing the voltage.
Heck, I haven't actually used tremolo for 20 years, (like who does?) but as you all can understand...it bothers us tech geeks when the thing isn't "100 percent".

[erenaud]

I am late to this party, but had a thought. I just swapped the LDR with the Silonex part, and get a very nice trem behavior. But yes, the gain is off due to the drain through the photocell (LDR). Looking at the schematic, the +13VDC is always on , even when the depth is zero or the footswitch is off. With the footswitch off the LED is in series with the two junctions on the transistor and R63 to ground, so it is always going to be on unless the oscillator is running. My suggested fix is to replace the depth pot with a combination pot/spst switch, using the swicth to shut off the voltage when the pot is set to zero.

Any thoughts?

[rrr]

"Allied has a SILONEX part that is listed as a direct replacment for the original Heath opto-coupler:

NSL 32SR3

About $2.50US + shipping."

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The NSL 32r3 is a "moderm " replacement for the original Heathkit part. This part has an LED in place of the incadescent lamp, thus you will have the same reduced power concerns as the home built LED solutions,as the LED will never go dark in the existing circuit. At this point I think the switch on the intensity pot is the easiest answer.

[J M Fahey]

Please post the schematic.
The difference is (as far as I remember, it was 40 years ago) that the lamp was not *voltage* driven but *current* driven, it was in series with a transistor or something like that.
It is designed to *always* have some idle current, which is no big deal on a filament because it might be on the threshold of producing *any* light or even below it, but a LED *always* produces light if some current goes through it.
Anyway, maybe I can suggest you changing some resistor value to adapt it.

[wizard 651]

Heath kit trem replacements.

Thanks for your kind reply. Your diagnosis makes sense.
Yes, we played with a resistor value using a decade box to jumper the in circuit resistor.
The OEMl propriatary Canadian grain of wheat bulb/LDR unit, is a goner. I used an LED and LDR of appropriate value as a replacement.
The unresolved problem, is that with any resistor value that allows the circuit to oscillate the LED, there is severe attenuation of the overall volume of that channel, even without the tremolo engaged, (intensity set to zero). Some improvement of tremolo "off state" volume was obtained, at some resistance values, but the tremolo volume was always noticeably lowder, than the "regular volume of the channel.

I am going to put in a pot with switch to neutralize the loading/attenuation, of this poorly designed trem circuit. That way, I hope to get full volume from the reverb channel, and be able to engage the tremolo when wanted.

A bit of nostalia here. This is a close copy of the one i built in about 1964.

[Boogyman]

Ok, new guy here reviving this old thread I just scored an old TA-16 for $50. Only the normal channel worked, but I found the inputs on the reverb/tremelo channel were wired wrong so now both channels are up and running, and the reverb works fine. The amp sounds pretty good for solid state and has decent power, nice overdrive too.
Unfortunately I have the same problem with the tremelo not working. I'm not a trained electronics guy, just a tinkerer with a multimeter and a soldering iron. Reading up on threads like this really helps, but not knowing much is embarassing. So what does the LDR look like? Is it the little black box on the circuit board in the tremelo circuit? I think I understand how it works, so if I could cut it open with my trusty Dremel I might be able to replace the filament with an LED?

[52 Bill]

Maybe you should start a new thread for your problem, as it may not be the same as the problem in the older posts.

Before you start tearing into the optoisolator, do you know that it is the problem? Is the trem oscillator running?

[Boogyman]

Nope, I don't. How would I test the circuit? Is it the little black box? I'm thinking having it opened up and all plugged in and turned on, then using a multimeter to test for voltage on the board/unit? Or just test for continuity/ohms resistance with it unplugged?
Like I said I'm no electronics guy but I'm quick on the uptake, and short on the funds to pay someone else to do something I may be able to pull off myself.