Harmon Kardon HK690i, tuner acting weird

[Mrx3750]

I have an HK 690i that doesn't have that many hours on it. The unit spent much of its life in storage before it was given to me. I've gotten years of reliable service out of this receiver, but just yesterday it decided to do something weird.

I found myself getting nothing but static from the tuner, in spite of it being tuned to a strong local station. Then when I tried to change frequencies, it started scanning for frequencies in the 75-90 Mhz range instead of 88 to 108. I couldn't pick up a single station on either AM or FM.

After fiddling around with it, I set it to auto scan, and then it picked up a station and displayed the right frequency. However it refused to exit from auto scan, and when I turned off the unit and turned it back on, the static returned.

I very much like this receiver, and from what I saw online, this has happened to other HK690i's.

[Mrx3750]

Re: Harmon Kardon HK690i, tuner acting weird

Still no clue, but wondering if it's the tuner IC...

[jabbba]

Re: Harmon Kardon HK690i, tuner acting weird

Have yuo tried downloading the service manual, after a quick search i found:- :-http://servicemanuals.pro/servicemanuals/harman_kardon/service_manual/audio/av_receiver/hk690i/download.html.
If thats no good eserviceinfo.com will have something as well

[Mrx3750]

Re: Harmon Kardon HK690i, tuner acting weird

Yeah I have the service manual. That's why I think it's the IC.

However the service manual doesn't specifically mention this problem. It just gives a brief description of how the FM circuit works.

[Longbow]

Re: Harmon Kardon HK690i, tuner acting weird

Seems like others have discovered the problem, but nobody has fixed it yet. The first bit of valuable information is that both AM and FM are affected. Since both sections are varactor tuned, I would first start looking around in the prescaler/pll synth section. Of course start with the correct B+, free of ripple, etc. Look in the PLL, AM and FM sections for correct B+. Second look at the crystal frequency X701. The various functions of IC702 are documented and might lead to other checks.

Naturally you would get static if the tuners are tuned out-of-band. The frequency display isn't really looking at the front end, it just indicates what the PLL thinks it should be. Luckily, the problem exists in AM, so your simple audio or function generator will serve as a known signal source. Compare what the display reads on AM when your generator is set to 1 MHz, for example. Again, with the problem showing up on both bands, I would start with the PLL section. Look at the varactor tuning line voltage for clues like hunting, lack of lock-in behavior, etc. Keep us up to date.

[Mrx3750]

Re: Harmon Kardon HK690i, tuner acting weird

Hey guys. It's been awhile. I haven't been able to do much electronics work these past few months. However I got back to this and I realized that a grounding strap had a broken solder connection. I resoldered it, and it fixed the tuner, somewhat.

The tuner now works in auto mode. It locks onto stations and doesn't cut out or act weird. However manual tuning still doesn't work, and I discovered that if I switch to another mode and go back to tuner, the display reads 89.6 Mhz. However it's still tuned to 101.1 and plays music from that station. Powering the unit off and back on corrects the display.

Edit: manual tuning started working after I pressed the Memory button. Display still says 89.6 if I switch modes though.

[Longbow]

Re: Harmon Kardon HK690i, tuner acting weird

Soldering a grounding strap? If you get a huge difference from soldering a grounding strap, it suggests to me that you still need to look closely at the regulated supply voltages in the tuner area. I'm almost certain you will find noise or other variations there. Any rf cables with corroded connectors? Will the tuner switch to AM? If so, don't waste your time in FM mode just yet. IC702 is where your troubleshooting should start. The data sheet suggests a 7.2 MHz crystal frequency. Look at the front panel button board - presets and up/down switches may be partially shorted or disconnected. Post voltages. A few photos of the tuner area would be interesting.

[kc8adu]

Re: Harmon Kardon HK690i, tuner acting weird

something is messing with the configuration info.eeprom corruption likely.
it thinks its set up for the japanese market.they use the 75-90mhz range.
or pullup/pulldown resistor cracked joints around the micro.

[Longbow]

Re: Harmon Kardon HK690i, tuner acting weird

I'm not ready to write this off to a bad EPROM just yet. It doesn't have a dedicated EEPROM chip, and the software requirements for these functions are almost zero. As suggested, you should check the external board labeled "Frequency Step Selector". Clean the switch and check the wiring. Pins 29, 30 tell the micro what system to use. Pin 24 will be H for Euro mode. My other post suggested 7.2 MHz crystal frequency. The manual page 6 says it is 4.5 MHz.

There is a good block diagram and circuit explanation in the manual. There are even chip diagrams on the side of the schematics. One way to get some clues is to perform the AM/FM section alignment procedures. At some point you will find something you can't adjust. It works, try it.

[Mrx3750]

Re: Harmon Kardon HK690i, tuner acting weird

I'm not ready to write this off to a bad EPROM just yet. It doesn't have a dedicated EEPROM chip, and the software requirements for these functions are almost zero. As suggested, you should check the external board labeled "Frequency Step Selector". Clean the switch and check the wiring. .

Where would this be exactly?

Far as I can tell AM works. In that it seems to be in the right band and the frequency display works. I didn't hook up an antenna because I can't recall the last time I ever listened to AM radio. If I need to test it I supposed I could. Only AM station I know of is WWJ.

The grounding strap I mentioned. There's a metal can near the back of the board close to the antenna connections, and the strap goes from it to the chassis. It was soldered on there but the joint broke. Resoldering that solved the frequency locking issue. I ran it for over an hour and it was stable.

[Longbow]

Re: Harmon Kardon HK690i, tuner acting weird

Where would this be exactly?

You can see it on the wiring diagram near the rear of the service manual as PCB16. It is probably mounted on the back panel.

Far as I can tell AM works. In that it seems to be in the right band and the frequency display works.

To me, that fact would indicate that 90% of the tuner circuitry is working.

I didn't hook up an antenna because I can't recall the last time I ever listened to AM radio. If I need to test it I supposed I could.

It is a brilliant idea is because both AM and FM use many of the same circuit functions and testing at 1 MHz is much easier than at 100 MHz.

There's a metal can near the back of the board close to the antenna connections, and the strap goes from it to the chassis. It was soldered on there but the joint broke. Resoldering that solved the frequency locking issue. I ran it for over an hour and it was stable.

The metal can contains the FM front end. Grounding straps around the metal can are for shielding purposes. I wonder if you might be near a very strong FM station, because you just increased your shielding by soldering the braid. In cases of strong FM interference, strange things can happen to digital tuning and readouts. Disconnect the FM antenna and see if anything changes. For that matter, disconnect your speaker leads too, and use headphones. These are usual tests to help diagnose RFI problems.

[Mrx3750]

Re: Harmon Kardon HK690i, tuner acting weird

I don't think that's the issue. I'm not really close to any FM station, and I need an antenna to pick up anything. Plus the tuner is in a basement which doesn't help with signal.

I did hook up a piece of wire for an AM antenna, and AM seems to be fine. The display even remembers what station it's tuned to on AM.

Got some more weirdness on FM though. I was listening to it and pressed the memory preset button for that station, to turn it off. The station cut out a few seconds later, and the tuner wouldn't budge. It was still locked on 101.1, but no music played. I pressed the memory preset button again, and then it started working right as rain again.

[Longbow]

Re: Harmon Kardon HK690i, tuner acting weird

Great, so RFI doesn't seem to be an issue. AM is working normally. The main tuning micro and display IC are working, in that case. So the tuner is erratic in FM only. (BTW memorizing a station often causes momentary muting. Or else it goes into memorize mode and waits for you press a preset button until returning to normal.)

Can the tuner can be tuned to a known FM station (regardless of the display)? Can you get undistorted output in stereo mode on just one station? If so, then the actual RF tuner and demodulators work, but the tuning loop is confused. The prescaler would be the next area to investigate. Since the tuning circuit is a PLL, everything in the loop will look bad if only one thing is wrong. In any case, observing the activity on the varactor tuning lines (TP1) whilst changing stations can sometimes give clues. Check this area, the phase detector loop filter (marked LPF) and power supply lines for suspicious electrolytics. Don't forget your 30v regulator Q75.

[Mrx3750]

Re: Harmon Kardon HK690i, tuner acting weird

When I change stations, the display reverts back to normal. Yes, tuning to a known FM station produces clear undistorted sound.

I noticed something else. I switched between AM and FM a few times, and the display went from 89.6 to 89.4, but it was still playing music from 101.1

Okay so i need to find TP 1. I assume I test that with a multimeter. What am I looking for?

[Mrx3750]

Re: Harmon Kardon HK690i, tuner acting weird

Long time no post. Last 6 months or so have been hectic, but I finally got back to this, and I've finally fixed it! I'm posting this mainly to give this thread some much needed closure, and because someone in the future might have the same issue and stumble across this thread.

The problem was the tuner IC, specifically the TC9147AP IC. I eventually discovered that this IC is used in other receivers from the 1980's, and its failure can cause similar issues. Also explained why it sometimes worked, or half worked.

Well I ran into a problem. TC9147AP IC's aren't exactly easy to find. The only sources I could find either claimed to have it and didn't, or were really sketchy Chinese outfits. However I discovered that the TC9147BP is a perfect drop in substitute, and was much easier to get ahold of.

I also replaced a 0.047f 5.5v super capacitor, mainly because I couldn't test it properly, and I figured that since it was related to IC function, might as well replace it to be on the safe side.

Tuner's been working like new for a week now. Let's hope this is the last issue I have with that.

[Longbow]

Re: Harmon Kardon HK690i, tuner acting weird

Glad you got your tuner working again. I am personally reluctant to suspect bad IC's unless there are obvious signs of incorrect operation from that IC. In your case, AM was working normally. Many if not most of the functions inside the IC were therefore working OK. Anything is possible, and the IC was apparently faulty in this case.

The 1/2 farad backup cap comes up quite often in troubleshooting. Most people say they can't adequately test it. Well it is nearly impossible to read this value on a cap tester and who knows what esr these things have. We really don't care about the esr, because the cap's function is to hold a d.c. charge if the power is out. The test is simply to hook up your meter and monitor the voltage on the cap whilst unplugging the unit. The voltage should stay up for 10 to 30 minutes. The exact time is usually listed in the O.M. A quick discharge means the cap is dead. Incidentally if the backup cap is faulty, it can often cause erratic problems with the micro. Many designs will go into mystery mode when the backup voltage changes even when the power is good.