HK690i - no phono - everything else OK

[Bill York]

I've had this 690i for 30 years - bought it new in 1986. Yesterday I pulled it from the dusty basement, tried it out, and found that it powers up but did not work on all inputs. So I took off the covers and ran it through the dishwasher - no soap...

Now all the buttons work and the tone and balance and speaker switches work fine.

The bad news - the phone input does not work. No pops, no crackles, no sound whatsoever. The same turntable - an HK T35C - plays fine through another receiver and it even produces audio when pumped through Tape1 or Tape2 input. Of course, the audio level is VERY LOW when played through the Tape inputs.

I've downloaded the Technical Manual and started looking it over.

I gently poked at all circuits around the phono section with a plastic pen while audio was playing through to check for any loose connections on the circuit board.

I pushed the phono selector button - even the MC on/off switch - several dozen times - still no sound whatsoever.

Now you might think "hey, why not get an external pre-amp and use that?" - call it pride. This HK unit has awesome phono sound. How nice it would be to have it working again.

A bit more info - one or two caps on the FM board have leaked out but I could not see any physical damage on the phono board.

If anyone has suggestions, I'd be much appreciative!

Happy New Year!

Bill

 

[bradfro]

If you haven't already done so, I would get some DeOxit and clean all the controls including the selector. I don't know how the selector switch on this works, it maybe electronic and not a mechanical switch. But it's still a good idea to clean all the controls and switches before you do anything else

The other likely problem is a cold solder joint. I know that some of these HK units have issues with cold solder joints. You would need to pull the board that has the phono preamp assuming it's the vertically mounted board and check the solder joints. If the phono preamp is on the main board, you'll have to remove the bottom cover.

If these are not issues, then it gets down to troubleshooting the phono\preamp board.

The leaky capacitors are probably just glue that is used to glue down the caps during manufacturing. A picture would be helpful in determining this

 

[Bill York]

Excellent and thanks for the fast reply. I ordered some DeOxit yesterday and will apply it when it gets here. I suspect one of three failure points:

1. The selector switch -- note that the same switch bank controls the PHONO/AM/FM/AUX selection and all but phono produce signal
2. The MC / MM switch -- I think this one controls a relay but I could not find the relay on the phono PCB
3. One of the connector cables -- there are several connector cables attaching to the phono PCB, lots of possible bad connections

Of course the phono PCB could have a bad chip or component or solder joint too

Here's a picture of the FM board with the leaky cap, I don't have a picture of the phono board yet.

http://audiokarma.org/forums/index.php?media/hk_690i-cropped.8861

 

[bradfro]

The larger cap in the lower center of the picture, that's glue. It should be removed when recapped, because that glue can become conductive and corrosive.

 

[Bill York]

I dug in for a few more hours today. Using the schematic, I was able to observe a few things.

1) signal connected directly to the post-phono-pre-amp stage propagates through cleanly
2) injecting signal to various points in the circuit results in clean or spotty propagation
3) shorting (by accident) a pair of resistors caused a spark and then strong pre-amp for a few seconds

My conclusions based on this:

a) some of the electrolytic caps are bad
b) shorting them gave them a "jump start" which let them work for a small amount of time
c) the rest of the circuits and switches are functional

So, if this is correct, it sounds like I need to pull this circuit board and replace all the caps. It likely would not make sense to pull the whole board and cherry pick out a few caps.

Thoughts?

 

[GoRNR]

Hopefully the accidental short didn't result in further damage. It's not often when you cause a spark inside solid state gear that nothing bad happened. Be careful if probing around with the unit powered up. If you have or can get a schematic along with a parts layout, it may help with your troubleshooting and tracing the signal path components in the phono stage. Are these in your "Technical Manual"? Due to age if there are ordinary electrolytic or tantalum capacitors that couple the audio through the phono amp circuit, they could be degraded and need replaced. I don't know if you have FETs or BJT transistors in the circuit, but they could be culprits. If you can ID components in the signal path from a schematic, you can narrow down your troubleshooting. Also, don't assume you don't have connection problems somewhere as was mentioned previously. Intermittent contacts on switches and solder joints cause many problems.

As for replacing ALL of the capacitors- you may want to troubleshoot your current problem first, then move on to replacing capacitors if it's necessary. Hopefully if changing out all the capacitors, you aren't planning to do so in one evolution- this method of replacement could result in new problems and then you won't know which replacement(s) caused the problem. Much wisdom here on AK regarding doing multiple component swaps without checking work after each replacement. If you decide you really want to replace all those capacitors, maybe do one or two at a time, recheck your work and continue. In this manner you minimize the risk of new problems while still replacing all capacitors eventually. Some others on this board my suggest you are wasting time and money and putting your treasured HK receiver at risk by replacing ALL of the capacitors. In the end it's your time, money and receiver-- so you have to be the one that's happy with the results. Just go into your project with your eyes open.

 

[Bill York]

You are correct. Replacing all the caps is rather risky. The first challenge - aside from pulling the phono pre-amp section from the system is determining which cap to replace.

I've done a bit of electronics work as a hobbyist so I'm comfortable with soldering and circuit testing etc. I'm not clear on how to test for a bad component though. Could you suggest an on-line resource to get started?

Also, my electronics cleaner arrived and I will use that liberally on the knobs and switches before digging into the caps work.

Thanks, and I'll keep you posted.

Bill

 

[Bill York]

The HK690i is now completely dead.

After several cleaning attempts - one with DeOxit - I was unable to restore the phono circuit. At one point I was able to use the AUX circuit and play records with an external Phono Amp. It sounded great then started to crackle, pop, fade, and resume normal volume.

Another trip through with the cleaning fluid, the amp POP'd when first turned on and then no sound. A quick circuit check revealed no blown fuses.

Alas, the cleaning fluid must have shorted something out. I clearly did not wait long enough after cleaning.

Thanks everyone for their help.