Receiver has a hum - Harmon Kardon 230A

Thank you all for the responses. I apologize for leaving the post hanging. I have been out of town and unable to post a reply.

Here are some items that I have done along with some additional information:

1. The unit is not hooked up to a cable TV, outside antennae or computer. It is only hooked up to indoor rabbit ears antennae, tuner, and speakers. I do not suspect a ground loop.

1. I disconnected everything, antennae, speakers, and tuner. Listening through headphones the hum is still present.

2. I shorted each RCA input to ground. Hum still present.

1. The hum is inaudible with the volume knob turned all the way down. The hum is present on both channels. The hum showed up suddenly.

2. I removed the case and bottom and inspected all grey capacitors. There is no hint of corrosion or a bulge in the end seals.

1. I failed to mention in my original post that for years the volume (pot?) emitted audible static when the volume was adjusted. Somebody mentioned that this should probably be replaced. Probably long overdue for replacement and perhaps the source of the new pesky hum?

John

Someone may have already suggested this, but do look at the output of any voltage regulators. Could be a dried up capacitor near one of them. I just looked over at the Harman dealer site - no service data there. I'll see if I have a manual at work and post back.

Mark Z.

The volume control was recently mentioned as a possible trouble site...

When I was collecting KLH Model Eight radios, I had one that suffered from hum and volume control scratchiness. Much to my surprise, simply flushing the control cleared up the hum.

I sprayed cleaner in the volume control, hum still present. Something else.. the hum is audible no matter what input is selected from the input switch, FM, AM, Phono, tape/aux. But there is a Monitor switch and when switched from Source to Monitor, the hum ceases.

John C.

This suggests that there's nothing wrong with the power supply, as the tape monitor is part of the overall input-selection circuitry.

That is odd... maybe a wiring or corrosion problem at the rear of the unit.

I looked at work - no service manual on a 230A, just a 230E which is COMPLETELY different.

Mark Z.

***Major SNIP***

John,

Have you tried feeding a source into the monitor input and see if the unit sounds OK? I'm guessing it will. While I don't know the HK-230A, I have seen receivers that the tape monitor switch feeds the volume control directly (or nearly so) eliminating the the tone control amps.

I strongly suspect that you have a low level (regulated) power supply failure that is causing the hum. Looking at the power supplies that feed the preamp circuits with an oscilloscope is going to be the hot ticket here.

Tim Schwartz Bristol Electronics

Wouldn't checking ESR on the power supply caps be the first thing for a unit of that age? I have found that to generally be the issue on the old stuff I have worked on.

WT

Tim, Yes, when I plugged a source into the monitor input there is no hum. All controls, bass, treble, balance, volume work for the monitor source. Another item of interest- one of the channels begins to cut out after about half a minute while using the monitor input. I can unplug the RCA jacks and reinsert and both channels come back, only to have one of the channels drop out again after about half a minute.

John

I'd try jumping the 3 470uf and the 1000uf capacitors with new good caps. If this doesn't work, I'd sub the electrolytic caps on that stand up board. If you have an esr meter, it would speed up the process. Chuck

This is starting to sound like the possibility of either bad solder joints, corroded internal connectors....

Also, it's been reported here that some older stuff develops loose circuit board to chassis connections which can be solved by merely backing off and reseating any screws which hold them down.

Clean EVERYTHING--controls, switches, jacks etc. Pull and reseat all connectors. Then, with the amp

This is starting to sound like the possibility of either bad solder joints, corroded internal connectors....

Also, it's been reported here that some older stuff develops loose circuit board to chassis connections which can be solved by merely backing off and reseating any screws which hold them down.

Clean EVERYTHING--controls, switches, jacks etc. Pull and reseat all connectors. If the problem still exists, poke around the inside with an insulated probe and see if you can cause any change in the amplitude of the hum...or in the case of the tape input, can get the bad channel to either cut out or in.

Check back....

jak

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what you should do is if you have a tester meter check the voltage regulater and capasitors

Gads. I worked on one of these once. Had these really unusual Sanyo output transistors.

Does it have some sentimental value or something?

Mark Z.

Sorry (again)...should have read the OP more carefully and looked at the picture. My advice about probing was not very timely, having already been attempted by the OP.

That said, to be clear, it should have been done with the unit powered up. It's not obvious whether that was the case, although I'd assume so. I didn't make that clear in my post, either.

Having looked at the pic (finally), it's striking just how old this relic is. It obviously wasn't all that well-built to begin with, IMO. I'm surprised by the diminutive size of the power transformer and lack of heat sinking (Just where are those outputs, anyway?)...not to mention the amount of point-to-point wiring employed.

Nonetheless, my advice is reinforced by the view. There's a lot of potential for problems caused by strictly mechanical aging. While I wouldn't expect this unit to develop a lot of heat; heat-related electrical issues could still develop. Those would manifest as dried out capacitors, or fatigued solder joints...possibly defective transistors.

I think if this one crossed my bench, I'd look at it as much a 'restoration' as a repair. It's from an time where both mechanical and electrical design were still in flux from the tube era. Looking at it I would guess it to be older than stated. It looks more like something from the mid-60's, as opposed to early 70's.

jak

The outputs are those little black bridge rectifier-looking things attached to the rear panel. I once subbed a set of 2SD313's for them, but the basing is the reverse of a TO-220 package, if memory serves. This model was one of my earliest "wiz-bang" repairs where I had to improvise to get the thing running. Even in 1985 or thereabouts there was no support from Harman on this model.

Which goes back to the question, is there some compelling reason to want to fix this thing? It's probably not even worth the effort unless you just need it to fill out your HK collection - they were pretty rare.

Mark Z.

I don't know how well-made this model is -- at least one poster has suggested it's not of the highest quality. But I know how _I_ feel... If something is well-made, I expect it to last "forever". And I don't like it when I breaks down.

I have a LUX Laboratory Reference System, and it all works, despite being over 30 years old. It's my bedroom system, and I doubt I'd ever sell it. If anything broke, I'd try to fix it.

The OP stated that he's had it since new, so I'd guess a fairly strong sentimental attachment.

jak

Thank you all for your assistance and suggestions to help me with this hum problem. I opened it up again today and removed the front too to better inspect connections on the front panel. With the unit on, I tapped all over the unit with a non-conductive tool. I was unable to have any effect on the hum. I loosened and tightened many screws. Especially those that appear to provide some grounding. No change. I found no connections that appeared suspect.

I suppose the next step is to test the caps with an ESR meter. This is probably beyond my ability and will involve the purchase of such a unit. So I think I will button the unit up, and for now keep it on 'Monitor' and use an external tuner on the monitor inputs. There is no hum when using Monitor.

Yes this unit is a relic and in fact purchased by my wife new in the early

70's. Perhaps not worth the service fee for repair, but it provides a connection to the past so I'll keep in for her a bit longer.

Thanks again.

John