Sharp HK-9000 with a weird electric behavior.

[ruilaureano]

Hello everyone,

When I had problems with my Sharp VZ-2500, this forum saved me. Now I need help from someone much more knowledgeable than me again:

I found a Sharp HK-9000, early model (with no Dolby B) with the tapes not working. I installed new belts and idler wheels today. When I was putting everything back together and adjusting tape speed I noticed the clock going haywire (random flashing) and resetting the battery seemed to solve it.

But eventually I noticed this:

A- the HK9000 is always on as long as the power cable is connected to the back.
B- setting the power switch to off does not disconnect the HK9000.
C- the clock is blank.
D- everything else seems normal, no weird sounds even.

I then proceeded to:
Try to run the radio from batteries: after installing them, moving the radio to another room and connecting the speakers, the clock came back and the power button resumed function (OFF meant OFF). But I couldn't turn the radion on from the batteries (although this might be because they are too weak?)
But right after this short moment of normalcy, when I connected the power cable, the issue resumed (see A, B, C and D).

Does anyone have any suggestions at to what I could do?

My kindest regards,

Alex

 

[ruilaureano]

Since my post I've seen the radio do the following:

Connected to the mains but with the power switch turned off, the radio switched itself on, while the clock turned off. Started to leave the radio disconnected from the mains all the time.

During a moment where it was behaving normal I noticed that when I pressed play on one of the decks, the clock turned off. Pressing with my thumb on the front plate near the decks repeated this behavior. My hypothesis is that when I mounted the two decks, something was left touching something it shouldn't...

So, to test this, I've removed the decks and have been using the tuner and checking if the issue turns up with the decks removed. So far, it hasn't.

Then I will connect first one deck then the other to the board (without installing then in the chassis) and test for the issue.

 

[ruilaureano]

The issue also comes up using one or both of the decks while unmounted in the chassis: boombox is on with power switch off.

Could this problem be caused by a static electricity build up in the power switch/clock board?

The power switch sits in a separate board, the same board where the digital clock is, and it has a ceramic disc capacitor that must touch one of the decks (which is physically touching the other deck through a metal bar).

I've removed that capacitor and tested it and it seems to be working: it has 104M written on it. I understand this means 10 times 10000 picofarads, or 100 nanofarads. My multimeter reads 85 nanofarads. The M letter seems to indicate a 20% variance is acceptable. So 85 is 15% variance, which must mean it is working? If nothing else works I will try to replace this capacitor.

I am sure this issue is not an issue and it has an obvious solution for someone knowledgeable in electronics... Going to study the service manual carefully for clues about what I must be doing wrong when assembling the radio.

 

[hopey]

Check the proximity switches on the tape deck as these can keep the power on. These switches can also have faulty insulation so can ground the supply voltage which could affect the Clock. I found this problem on a Sharp 7750 with a slow tape motor it was the Reed switch lucky I could swap it with the other one as they are wired different.

 

[ruilaureano]

Thank you so much for your suggestions, as I have been stuck with this one. I will report on any progress. Kind regards

 

[ruilaureano]

A report on progress. After checking all the reed switches and noticing nothing unusual I decided to try to isolate the problem - which deck - if any - was causing the issue. So I connect one alone, then the other alone, then both, and each time... no issue. So I carefully assemble the radio and... so far, so good. The static (?) problem seems to have gone away and I still don't know what caused it.

The remaining problem is I couldn't find replacement belts for both decks so the recording deck is slowing down as tape progresses, probably because the belts I had are a bit too large. And there are no HK9000 belt kits for sale on ebay. I will have to get close enough belts and go by trial and error.

 

[ruilaureano]

New problem. Both decks, when playing after a few minutes start making a low vibrating noise. If I touch the panel I can feel the vibration, which stops if I press harder. It doesn't sound when the deck is in pause so it seems it needs the spindle wheel turning for the sound to occur. I guess I'll have to tear it appart and troubleshoot.