Heath ta-16

[roobliz]

There was a thread a few months back about repairing a Heathkit TA-16. I just completed repairing one, here's what I found. TREMOLO/REVERB both were not working. Using the schematic, I found some missing voltages. There was a 13V zener diode installed backwards, reversing it corrected the problem. The LDR was bad in the TREMOLO circuit. I replaced it with a LED and photoresistor(PHOTO) under the chassis where it's nice and dark. The REVERB was out due to some broken wires inside the pan, easy fix! The TREBLE control on the reverb channel was NW, I just followed the wires from the control pot and found a broken solder joint. Look over the whole board for broken solder joints, in total I found 3 more that I'm sure would affect the amps performance. I replaced all the electrolytic caps and now the amp is 100% working!!! Hopefully this helps someone out there...

[Enzo]

Way to go. You never know what you'll find in these, since they were all put together by amateurs from kits

[Mike Patterson]

Do you have mfr. part nos. for the items you used? I have a TA-16 that I built back in 1968, and have recently refurbished. I would love to have the tremolo working again. Any help and/or advice would be greatly appreciated.

Thanks.

[roobliz]

Mike- The original LDR contains a small bulb and a photoresistor in the sealed package you see on your board. The bulb eventually burns out which causes the failure, the voltage for the bulb part of the LDR is approx 2 volts which is the operating V for most LEDs. I believe any photoresistor will work to turn the sound on/off in relation to the light on/off... Check your local electronics store, both items should be readily available. Good Luck!

[Mike Patterson]

Roobliz,

Thank you for your answer. I do have another question: Did you try to replace the bulb in the original LDR? Would this be possible? I read in a post on Harmony Central where someone said he had replaced the bulb, but the gentleman left no means of contact. I would like to keep the amp as original as possible. TYIA,

Mike

[roobliz]

Hmm... I guess, it would be a tedious procedure though. You'd have to heat up the package to get the bulb out. Finding a working light dependant resistor/bulb package with similar specs could be an option. I think the alternative repair would be the best way to go, the tremolo works very well this way.

These amps have a distinct tone about them, they just sound late 1960's to me.

[Mike Patterson]

Thanks for the info. I was hoping that the LDR case was snapped together, and could be pried apart. I still might try to disassemble it. It will make a good winter project.

My amp has a tendency to fry the power supply diodes every 8-10 years. That, and blowing speakers have been my main problems. I never used the tremolo, so that wasn't an issue when it was my main (read: only) amp. Do you use yours much?

[plott2670]

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

NSL 32SR3

About $2.50US + shipping.

When I was kid, I made good money putting the TA-16, TA-17 and a few TA-27s together for $50, half what Heathkit charged. Helped pay for my first car!

Blessings!
-charley

[Mike Patterson]

Charley,

Thank you! I didn't know Allied was still in business. That looks like it should work. I'll let you know. Just don't hold your breath, OK?

[Tucker]

Do you happen to know the reverb tank model that Heath sent out with these amp kits, I just found one of these on the side of the road in pretty good working order just without the tank..?

[Mike Patterson]

Originally Posted by Tucker

Do you happen to know the reverb tank model that Heath sent out with these amp kits, I just found one of these on the side of the road in pretty good working order just without the tank..?

Does the amp still have the tank mounting plate screwed in the bottom of the cabinet? I'm not sure of the part #, but it's a long (16-3/4") 2 spring tank.

[wizard 651]

Originally Posted by plott2670

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

NSL 32SR3

About $2.50US + shipping.

When I was kid, I made good money putting the TA-16, TA-17 and a few TA-27s together for $50, half what Heathkit charged. Helped pay for my first car!

Blessings!
-charley

The allied site shows several silonex optos with very similar ratings. How did you settle on this model as the direct replacement? Not doubting...looking for guidance. really appreciate the info already! saves me a lot of trial and error going thru my bin of photoresistors. Thnx

[wizard 651]

Roobliz's fabbed LED/LDR not a complete fix for my TA-16 trem!

I tried the LED/photoresistor setup. Ingenius and neat. LED flickered and pulsed the audio, sounding like tremolo. Depth and frequency controls work as designed, (LED freq. and intensity respond), but with "intensity" control at zero, the LED stays on, even when trem circuit shorted by footswitch!
Dim, (but lighted) LED, (in what should be the "off" state), attenuates the signal, by reducing LDR resistance, and creating path to ground. Various spec LED's did not change situation.

Following experiments tried without success:

1. Various resistor values in series with LED. Using a Decade resistance box, I increased resistance between LED and trem circuit. Increasing resistance reduced the intesity of the unwanted "off" state residual glow. However, oscillator ceased fluctuating in response to increasing resistance before LED would go completely out.

2. Replaced C29, 30 and 31 in belief they could be shorted or leaking. No change.

3. Tried substituting 8 and 6 volt "grain of wheat" bulbs for LED, (thinking circuit designed around small incandescent bulb, might not be shutting off a low current device like LED). Results: incandescent bulb would not light at all, regardless of control setting.
Bulb did not "stay on" (with the intensity control at zero, like the LED did), but it would not light or flicker with intensity full on, (like the LED would). Final experiment with the bulb: when connected between 13 v. supply for trem circuit, and other bulb lead to a ground, bulb would light up. Also, supply voltage within specs at 13.2 v.

Obvious alternative is to use Silonex direct equivalent as suggested by PLOT2670, (thanks Charlie), but questions remain as to explaining these bizzare results.

Do I have something else defective going on here? Since I have observed the LED flicker and respond to intensity control, showing the circuit works, the xstrs are neither shorted nor open. I can understand that a circuit designed before the LED might exhibit some peculiarities teamed with such a low current device. However, I was surprised that a 6v. 35 ma. incandescent bulb did not funtion at all, (it was the smallest I had on hand).

At this point, my ability to diagnose this is "like a monkey poking at it with a stick"

Any ideas out there? Are the transistors leaking sufficiently to give me unwanted lighting of the LED, yet weak enough that they will not pass enough current to light a conventional bulb?

One last piece of the mystery! Although the LED flickered in response to intensity and rate control, this fluctuation was either non existant or not observable as voltage change on a typical high resistance digital VOM. I would have expected to see some voltage swing or fluctuation.

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

[Tucker]

Thank you Mike,

the plate is there but its blank.

Am I able to use just about any tank with the circuit that's built into the amp?

I am not that interested in the having the thing sound just like every other
TA-16 so can I substitute tanks and to that end anybody know what my limitations are (if any) in terms of choosing one?

[J M Fahey]

Hah !!
I LOVED the little yellow guy in a Mexican hat dancing calypso under the invisible string.
Well, at least that's what it looks like to my none too sharp eyes.
Besides that, congratulations on your restoration project.

[bkahuna]

Originally Posted by wizard 651

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?