Allen 301 and the crackle

[tschnuckel]

I'm playing an Allen 301. Sometimes, but not predictable, the organ makes a crackling sound when powering up and powering down. The crackling can last a few seconds. It sounds like listening to an AM radio in a thunderstorm.
Other times it powers on/off with the normal sound of an amp turning on.


Also, it will make that noise, but not as intense, when playing 8' flutes. The crackle then is accompanied by a thud/boom - sounds like someone stepped on the low pedals with that flute drawn. This two is no predictable. I've tried with cluster chords, fast, slow, high, low, sustained, coupling etc, but can't find a pattern.

Any thoughts?

[jbird604]

Probably a bad amplifier, in need of having all its capacitors changed out. You can test this by disconnecting the speakers from the other amp and by removing all input from the offending amp.

[tschnuckel]

Is changing the capacitors hard? I have soldering skills.

[indianajo]

Uh, no. Sort of like mowing the lawn. The ones that are most likely bad are electrolytic, and have a plus on one end, minuses in balls on a strips pointing at the minus lead, or are tall cans with circles squares and triangles for the various sections. Mark your board with a sharpie for plus, don't get them backwards. Take pictures if you have to pull any wires more than one at a time. See this thread about this very common problem: http://www.organforum.com/forums/sho...146#post280146 Other electrolytic capacitors have the voltage followed by the letters "NP" for non-polar.
Note crackles can also be poorly soldered components, but finding these is so tedious, it is better to start by getting rid of electrolytic capacitors over 20 years old.

[jbird604]

Do you have T-50 amps or S-100 amps or perhaps some newer type that has been substituted into the organ? I'm going to assume that you have T-50's since they were the most common amps used in MOS1 organs until about 1976 or so when the S-100 began to supercede them.

In many early Allen amps, there are several 1000 mfd and 500 mfd electrolytics under the amp chassis, and they are very easy to change out and in my experience they are the ones most likely to be bad. These may even show leaking juice around the ends. There are others of 1 mfd, 5 mfd, 10 mfd, etc. that are also easy to change. I wouldn't worry about the big can-type ones unless you have a large amount of hum in the output. Not that they shouldn't be changed eventually, but they serve only to filter the large voltages and if they fail you will know it for sure.

It's also possible for some of the transistors in the low-level sections of the amp to become noisy, but once you change out the caps, you can go back and re-flow the solder around the transistors and other components, which will probably get rid of any remaining noise. In some amps, there are obscure transistor types that are not easily found for purchase. (Be sure not to spend too much time re-flowing around any one transistor because you don't want to overheat these things.)

I'm assuming, of course, that you first make sure the amp is the source of the noise by disconnecting the two or three RCA plugs from the inputs. If you still hear the crackling in the speaker, the noise can only come from the amp itself. Other possible sources are the DAC boards and anything else in the audio path, such as the expression cells, the mute relays, the "voicing" control on the front panel.... But I have know many of the old T-50 and S-100 amps to become noisy due to bad caps, so that is a good place to start. Needs to be done anyway.

[tschnuckel]

So the sounds disappeared for about a week. But are back. Although they don't happen on power on/off.

I did what you said about 3 hours after it was making the noise (that was a couple weeks ago). I did unhook the other speakers and input from the "bad" amp. There were no sounds at power on/off.

The Amps say type 1000-4.

[jbird604]

I think type 1000 was another designation for the T-50 amps, so my advice applies about replacing all the electrolytic caps that are easily accesible, which includes several 1000 mfd and 500 (470) mfd caps and perhaps some 250 mfd, 100 mfd, etc. Don't worry about the large can-type.

[tschnuckel]

There is a paper cap. that says 006 MFD 200WVDC. I'm thinking it should have the decimal (.006) since there are zeros. Either way I can't find a .006 or 6 uF at a 200V rating. What size can I substitute it with?

Also, when you mention the large can-type you mean the ones on the outside by the transformers, right?

[mashaffer]

http://www.tubesandmore.com/products/C-T-630V

mike

[jbird604]

No need to change little caps like that. Just the 1000 and 500 mfd caps and 250 mfd if present under the chassis. The big cans on top by the transformer are not likely to be bad either.

[tschnuckel]

I replaced the caps. in the three amps except for the ones under 1uF and the two cans on top. But it still makes the sound when playing Flute(and ped) channel stops. Not so much with the Main (principal) stops.

Not sure if this is related, but I have to set the Main amp to Full to balance the flute at 50%. Both the Main and Flute amp indicate they should be set about 50% - there's red polish on the post.

I checked the voltage off the big power supply. The 5V and -5V check out OK and both -27V are -28.3V.
The Bright/Mellow control works and so does the expression shoe.
The instrument does have its share of dust and sat idle for the last 5 years.

I do appreciate everyone's help!
Tyler

[tschnuckel]

Anyone have a suggestion?

I replaced the caps. in the three amps except for the ones under 1uF and the two cans on top. But it still makes the sound when playing Flute(and ped) channel stops. Not so much with the Main (principal) stops.

Not sure if this is related, but I have to set the Main amp to Full to balance the flute at 50%. Both the Main and Flute amp indicate they should be set about 50% - there's red polish on the post.

I checked the voltage off the big power supply. The 5V and -5V check out OK and both -27V are -28.3V.
The Bright/Mellow control works and so does the expression shoe.
The instrument does have its share of dust and sat idle for the last 5 years.

I do appreciate everyone's help!
Tyler

[jbird604]

Have you swapped the two amps to see if the problem follows the amp? If the problem is still the amp, there could be a bad transistor in there or soldering joint problem.

Or the problem may be in the DAC board. You can determine this by feeding the output of each DAC channel directly into the GOOD amp to see if the signal is clean at that point. If the DAC is the source of the trouble, you may see some socketed chips on it, and if so, pull the chips, clean the legs, carefully re-insert. You might also check the DAC for soldering problems. There are sometimes some distortion null controls on the DAC boards (some have them, some don't) and you might try adjusting that, IF you first determine that the problem is the DAC. Otherwise, leave them alone.

[indianajo]

With an anywhere problem like this occasional pop, I take data at this point. Get an analog VOM (volt ohmmeter) with a 20 VAC and 2 VAC scale, or a VTVM (vacuum tube voltmeter, a 1960 technology), or an oscilloscope with probe. You have to put a .1 uf cap in series with the probe on a VOM to keep the AC scale from showing readings on DC voltages. Don't expect a DVM (digital voltmeter) to read anything accurately on AC scale on music frequencies, I have had several that don't. I use a 200kohm/volt Simpson 260 meter I bought in 1987. (no longer in production). Clip lead the probes to analog ground and a test point somewhere in the middle of the possible areas. Put a roll of coins on a key. Pick a scale where the meter pointer is somewhere near the middle, watch the meter and wait. When it pops, did you see the meter kick? If so, the problem is before your test point. If not, the problem is after your test point. When you've figured out which half of the system has the problem, pick another test point in the middle of the half that has the problem. Watch for the meter kick when there is a pop. No kick, the problem is after. Etc. You can isolate the problem down to one component by chopping the likely area in two several times. While e-caps are usual suspects, after that I don't like disturbing the things that are working already, only the area that has the problem.
You could also configure an amp-speaker combination as a "sound probe" to listen for the problem with clip leads. But you have to protect the input of the amp from DC shifts with a blocking capacitor as above, and a back to back zener diode clamp after the blocking capacitor to avoid huge DC shifts from tearing up the probe speaker or amp.

[tschnuckel]

Well I wiped the DAC and the Stop Board Array contacts with a bit of Vaseline. I also gently swabbed all the stop tap contacts.
I did see the MOS board was replaced sometime after 2006.

For the time, the noises have gone away!!

All the components have the same three numbers stamped in red except the DAC. Does that mean it has been swapped at some point?
Also, how do you clean the key contacts? Does it involve removing all the screws holding the contact strips?