harmon/kardon sub-ts14 makes high-pitched squeal sound

[axlmastr]

I have a harmon/kardon sub-ts14. When I turn the amp power on, it starts making a squealing noise though the woofer. It sounds like a single tone higher pitched frequency similar to the sound you get from a shortwave radio when the signal is weak or oscillating. This sub does not oscillate this tone- it is steady. The sub amp is unplugged from any inputs or outputs and does this when power is switched on. The system is about 3 years old and used very little. Any ideas what this may be? What puked on the board that I can fix?

[Per Hansson]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

Well we would need some pictures to see if there is anything wrong with the boards visually...

[stj]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

probably a bad cap in the psu allowing the frequency of the source/psu to pass into the amp stage.

[axlmastr]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

I'll try to grab some pics. This isn't my first rodeo here. I already visually inspected the board under magnification as well as generally. No sign of the usual "visible" failures. I've done over a 800 motherboards & 130 OEM psu's in the last several years. DVD players, routers, switches/hubs, car amps, lcd monitors/televisions- they all suck these days. And it doesn't matter what the price paid is either. Thanks to the many veterans here for your help back in the early days of my re-capping this e-junk. Now I'm e-scrapping a lot of things because no one wants to pay for repairs.

[Toasty]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

Any volume control, or does not respond to that?
Any changes with switch positions? filter, phase...?

Verify voltages from PSU and scope them for noise. If you can hear it, it's under 20kHz. SMPS typically operate well above that from 30kHz-100kHz or more. Could be a harmonic, tho...

Once they're confirmed, I'd be looking for a bad ground or compensation cap in the pre-amp/amp. Maybe a resistor in their has gone high or cracked?

Toast

[Toasty]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

Another question - did this happen suddenly? one day to the next? same day?
Or was it unused for a while and then it started?

[Toasty]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

*BUMP* Any more on this axlmastr?

[axlmastr]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

Sorry about the delay guys. I have been knee deep in laptops to fix and this was a sideline job. I took some pics of the amplifier's input power (PSU) where some of you suggested I look. I request your comments on what you see in these pics as I note a particular area of the PCB that could be the contributor or a sign of what before it is failing. I see a large resistor with both ends rather charred but still intact. A heat marks on the PCB are notable with on solder joint looking a bit "over-cooked". The sound is present the minute the unit is powered without an input signal. It emanates from the speaker with a set tone and gains amplitude when the volume potentiometer is dialed up. It just happened one day. Hit the power off and the sound goes away with a dying whistle. Any suggestions on what to remove and replace on that board would be appreciated.

[axlmastr]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

*BUMP* Any further suggestions? Others who own this sub have posted the same symptom on the web as well. I note again that one resistor in one of the pictures had gotten hot and melted (charred) a solder joint on one end. You can tell which one because of the brown heat mark on the board. This is on the pre-amp board where the input signal is converted to the output signal going to the speaker driver. I re-soldered that one resistor and confirmed continuity with no change in symptom. I did not change the resistor because its value had not changed when I removed it from the board for testing. The board where the AC coming from the wall is converted looks pristine. According to the person who owned this the subwoofer it stopped "working" after a pretty good rain storm but where it was located they didn't hear the "whistle" coming from the woofer that I heard. I took it home and plugged it up to my A/V system lowered the gain to 1/4 and turned the amps power switch on. I immediately heard a "whistle" sound like the one you would hear from a AM or HAM radio in between frequencies. The sound is steady and will change in amplitude as the volume/gain pot is increased and does not oscillate. I unhooked the inputs from my A/V system ans listened. The sound is still there with the unit powered and no input signal. Filter/phase switches have no affect. It seems like the same instance when you have a ground loop issue in a car audio system and can hear the alternator whine. Anyone have an answer or some test procedures I can try to fix this?

Toasty has a good point about a compensation cap but I am not sure where that would be located. Pictures of both boards have been provided with a closeups of certain areas. He also mentioned a resistor "going high" from damage. I explained my findings of one particular resistor in the above text. I am not sure at what stage that component is located in the circuit to ascertain its relevance.

stj had mentioned "a bad cap in the PSU allowing the frequency of the source/psu to pass into the amp stage". Any thoughts on this after reading what led to the symptom?

Sorry for the lengthy and winded post but I would prefer anyone here to help me since this is such a great resource over the years. There are a lot of intelligent and skillful people supporting this great forum and I appreciate any info you can offer.

[mxslick]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

Ok, the last post was helpful in that it stopped "working" after a rain storm. That, and the burned solder joint on the resistor leads me to believe that the 4700uf main filter caps have failed. Change them first.

And since there are so few other caps on that main board I'd recap all of them to be sure.

But I feel that replacing the two main filter caps will solve the problem.

[hardwareguy]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

The preamp stage is probably oscillating. Fix the PSU and retest. If that fails, check the bypass cap on the op amps. (its a small cap, usually a ceramic across the supply pins and the ground pins) The ceramic is usually paired with an electrolytic in parallel, check this too. It probably won't be low ESR, it's a general purpose in most cases.

Caution! The squeal you hear may have harmonics in the ultrasonic range, perhaps even into the low RF range. This isn't good for the power stage! If this is occurring, the output devices will become hot in short order.

[axlmastr]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

Thanks to both of you! I'll check and post my findings.

[Ermin]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

@ axlmastr

Have you solved the problem yet bro ? I have the same problem if you could help ?

THX

[axlmastr]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

This post has been up a while and I wanted to refresh it with the addition of the factory schematics. The file I'm uploading is a PDF of the operation/service manual. The service portion with the schematic, board drawings, and parts list begin on pg 15. I have read the responses that were posted. I have a used Devry School of Technology/Bell & Howell Schools Model 34 scope but don't know how to use it or any probes/leads for it. Only a VOM so some testing is not possible. I have not replaced any components yet as some suggested. Maybe the addition of this manual will help some of you pin point your solutions better and I can get his thing out of the dusty corner it is in. I'm not as familiar with the references to the "preamp stage" or "op amps". I do realize I am somewhat in over my head with this one.

[budm]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

Did you check the C16, 17 10UF non-polar 100v? they are part of the ClassD output filter network.

[axlmastr]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

Thanks for the reply budm. I see them on a riser board up from the power board. The board is marked "104DS15000 Amplifier". A Google search shows this is also on a JBL model. The caps are covered in a hard brown glue with a horizontal ferrite cored choke in between them. I cut the glue and peeled them back to check the location number you gave and the values. They are also marked SOLI & 20%. How should I check these as they are visually okay?

What do you think about the other poster's ideas of replacing the two huge Su'scon 4700uf 80V caps? They look fine. I haven't measured across them.

Or replacing the resistor R502 that as I mentioned in my original post had one solder joint (closest to the monster 4700uF cap) melted and charred. There are some brown heat marks in the PCB beneath R501 & R502 as well as Q501 but I have seen these brown marks before on other PCB. Are the brown marks a sign of component stress because of impending failure?

Realize I got this sub to look at after a lightning storm had knocked it unconscious. I took it apart and found the R502 lead had disconnected from the PCB at the solder joint and the lead and solder joint were a bit charred. I cleaned the PCB area trimmed the lead so it was usable and re-soldered it. At this point the unit powered and the high frequency noise was present.

[budm]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

Yes, you should also check the DC filter caps, JBL had the same problem and suggesting to check the main DC filter caps.

[axlmastr]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

I got your message and decided to give this unit some AC and see what measurements I could get. When I flipped the rocker switch for power the relay clicked as usual and within 10 seconds I saw thin smoke rising from beneath the Toroidal coil. It appears the resistor (R502) that was originally charred and detached (the one I resoldered) began to get hot and smoke. I did get a chance to measure the terminals across the two Su'scon 4700uF 80V caps and they measured 142 volts! Could those caps cause a dead short? Where's the 142V coming from? The caps are rated for 80V.

[lti]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

R502 connects to the collector of the TIP32C transistor. Checking the other components connected to the transistors might be a good start.

If the caps really had 142V across them, they would have blown up (unless they were open or dry).

[budm]

Re: harmon/kardon sub-ts14 makes high-pitched squeal sound

You should verify that your meter's battery is OK, the would have explode with that much voltage on it and also blow the main fuse.
By the way, how did you measure the voltage on the main two caps? Did you measure directly on each pair of the cap's legs?
This is BI-POLAR power supply that puts out +/- with ref to ground. The R502 is for supplying the -15V linear regulator (regulated by Q502 and the ref Zener diode D502 which is also the ref for the +15V power supply.
These +/-15 are for the Op-amp. You need to check and see if you have heavy load or short at the Emitter of Q502, may be feeding shorted circuit. You also need to verify of the Q502 is not damaged.
Just set the meter to 200 Ohm scale and check the resistance between the Emitter of Q502 and Ground.