Noisy Heathkit TA-16

[bware]

I have a Heathkit TA-16 solid state guitar amp that has low level white noise. The volume controls do not completely silence the incoming signal. I've replaced all the electrolytic capacitors.

[awdman]

Does it change when you turn the volume up or down, if you do not have your instrument jacked up does it make the noise and what about when your touching some metal part of the frame?

[bware]

The volume does go up, but it never goes completely off. The noise is always there, regardless of grounding, etc. It's white noise being generated by something. Do transistors go bad after 40 years?

[Enzo]

Transistors go bad whenever they want to, there is no timetable.

[bware]

Would a bad transistor generate white noise?

[J M Fahey]

Please post or link the schematic to be able to give you a more precise response.
Is it "unbearable level" white noise or just some low level hiss, only audible in a relatively silent room.
The last one might even be normal.

[bware]

http://www.epanorama.rackhost.net/schematicsforfree/Audio/Pro_Audio/Products/Misc/Heathkit%20TA16%20Guitar%20Amplifier%20Sheet%20A.gif
http://www.epanorama.rackhost.net/schematicsforfree/Audio/Pro_Audio/Products/Misc/Heathkit%20TA16%20Guitar%20Amplifier%20Sheet%20B.gif
The noise is fairly high. It may be related to not being able to zero the volume.

[J M Fahey]

Hi bware.
It's an *ooooold* design, and has a couple of minor design flaws.
To be certain where the noise comes from, you should ground  Q3's and Q4's collectors and see if the noise stops. Problem is, they both carry some positive DC voltage so you should audio ground them with a 10uF x 25V electrolytic. It will pop loudly when you do so.
You might turn the amp off, tack solder the capacitor from collector to ground, and turn the amp on.
Do it on one channel, on the other, and on both, and later also on Q5's collector, the reverb re-amplifier.
Now that I see it after such a long time, that hiss *might* even be "normal", you have three "noisemakers" hooked all the time to the power amp, with no controls whatsoever.
The treble control has a flaw, does the hiss exist still with it on "5" ?(on a 0/10 scale), or only on "10" ?

[Big Kahuna]

Hi Noisy Heathkit
   Hope I'm not to late to help but here goes. I'm going to retire soon, I played alittle guitar in junior high school and always wanted to play so now I'll have the time to relearn and have some fun, stay out of the wife's hair. She bought me a Fender guitar for fathers day and I needed an amp. I looked in Craigs list and found this 1966 heathkit amp. I have built many kits from Heathkit from a bug zapper to color TVs. I went and looked at the amp and it should have been in the trash. I gave the guy a few bucks for it and took it. Got it home took the amp out of the cabinet looked around a bit and didn't see anything wrong. I plugged it in and got nothing out of it. I looked the amp over real good and found a burnt resistor, I already had a schematic for it. I hunted up the resistor soldered it in fired that thing up and I got a nice loud hum when I touched the input. Oh the reverb tank was also missing had I seen that I would have left the amp where I found it. I hookup the ax now I haven't played in 47 years and it sounded like it, but made enough noise to see how this amp worked. It sounded lousey, I know that capacitors go bad over time, I pulled one end of several caps and they all measured right on the money. Then I pulled the end of some resistors, I struck gold, every one was off value not by a "few" ohms but by thousands of ohms. Depending on the original value of the resistor If it was supposed to be 500 ohms it measured 550, 600 ohms. Some were off 10,000 ohms. Just for an example a 220k resistor measured 236k a 6.8k measured 7.2k. Someone put in a 90 ohm 10 watt resistor in place of a 270 ohm one watt. The person who built it had to be blind and used a propane torch. The tremolo didn't work because of a part called an LDR I scratched my head and said WTF is an LDR. I Googled it and found out its a photo cell, the same thing that turns on the lights outside of some houses when you walk by. I had to figure out how that worked cause I wasn't about to pick up a phone and tell the person I need an LDR can you bring one over? Where do you find one. So I figured out how it worked and made one with a bright LED and a photo cell. So I finished rebuilding the amp hunted down a reverb tank out in my shed and tried it out. This thing sounds great only I can't get the reverb to work, this little piss ant transistor don't have the chutzpah to shake the springs in the reverb tank hard enough to be picked up at the other end. If I connect a small amp to the output of the Heath amp going to the tank I can get reverb. So if your having a problem like noise coming from your amp, like many others said check the capacitors but also check the resistors too, check all the solder joints, I found a few bad ones. Transistors normally don't do what your saying they just throw up thier hands and say f*** it I quit. Another thing If you have the schematic and got it on line from www.vintage-radio.info the values to about 6 resistors are wrong. The true value is printed on the circuitboard under the resistors. Heath must have made a revision and we don't have the new schematic. Some one else had a post on this site about losing volume or the amp's not as loud as it used to be that could be that LDR R-56, if the little bulb in there stays on that will activate the photo resistor lowering its resistance which will make the amp lose volume. Thats how that works the bulb blinks according to how you adjust the knobs, then the bulb shines on the photo resistor who's resistance will drop when it sees light. Look at the schematic one end of the bulb is grounded. Well have fun hope you get your amp working again, hope this was alittle help to you.

[Roly]

The person who built it had to be blind and used a propane torch.

This is something to keep in mind with anything that has "Heathkit" written on the front - it wasn't built in a factory but by an amateur who may, or may not, have been suitably skilled.  The upside is that it often doesn't take too much to make a great improvement, you know, like getting some parts in the right place at long last.   

It's not so much the absolute value of a component as the percentage deviation from the marked value.  Generally with amplifiers anything within 10% will be okay, but wider variations, particularly if the component shows signs of distress, are suspect.

A "LDR" is a Light Dependent Resistor which does what its name suggests.  It is also sometimes called a Cadmium-Sulfide or CdS cell.  It is actually quite rare for these to fail by themselves without being abused in some way - the associated lamp being the much more common problem.  They also come in a wide range of characteristics so finding a suitable replacement is often a bit of a hit and miss affair.

Most parts retailers stock a couple of types, one is a "low resistance" type, the other a "high resistance" type.  These circuits almost always use the higher resistance type(s) that generally have a longer zig-zag track.

the same thing that turns on the lights outside of some houses when you walk by

LDR's are used to control some devices such as night lights, but the sensor in motion detectors is actually a silicon photocell that is vertically split down the middle, and in conjunction with the fly-eye lens detects a flutter from anything moving across its field of view.

hunted down a reverb tank out in my shed and tried it out. This thing sounds great only I can't get the reverb to work, this little piss ant transistor don't have the chutzpah to shake the springs in the reverb tank hard enough

This will very likely be because the tank you dug out has a low impedance drive coil (maybe ex-valve/tube amp?) and this amp needs one with a high impedance drive coil.  Sadly there seems to be no indication at all on the circuit what type of tank it used, but companies like Accutronic still make a range of tanks including high impedance ones.  Alternatively a small chip amp like an LM380 following Q11 should drive a low impedance tank crazy.

Transistors certainly do often throw in the towel, but they can do all sorts of other things as well, such as go noisy, leaky, intermittent; just when I think I've seen it all something new jumps out at me.  So most of the time they are go/no-go, but particularly in a 60's build I'm not at all surprised by a residual noise problem; hiss, rumble, or both.

Good to hear you are getting back into playing again in retirement; me too - you're never too old to be in a band.  :tu:

[Enzo]

yeah welcome back to the game, Kahuna.

I have to point out that a 220k resistor that meansures 236k is off by 16k, or 7%, well within a 10% tolerance.  A 500 ohm that measures 600 is off by 20%.  Don;t recall what they used in the Heath, but all the old Fenders we know and love were made with 20% parts.   7.2k instead of 6.8k is 6% off.   One can make a case for having the exact value parts, but the amp will still work with a wide variation in part values.

I think Roly hit your reverb issue on the nose.  Wrong pan type.  But even with the right pan, if the drive is not sufficient, it isnlt because it is a transistor circuit.  The original circuit was perfectly capable of driving the stock reverb pan.   If the circuit has FAILED, well that is the issue then, not the transistor-ness of it.

[Big Kahuna]

I have a Heathkit TA-16 solid state guitar amp that has low level white noise. The volume controls do not completely silence the incoming signal. I've replaced all the electrolytic capacitors.