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National panasonic R 806 8 track

havols

havols

Well-Known Member
Hi friends from HK
I purchased a beautifull 8 track national panasonic model R 806.
It is a 8 track cartridgr play and recorder.
In very good condition, and still 100% original.
But it has a problem.
When I play, using a good one cartridge, it starts playing the sound very well, but within a few minutes, there's a hiss that becomes constant, and drowns out all the sound.
Could someone help me find the problem?
The schematic is on HIFI engine.
Thanks for your help.
 
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Greetings from RojoLand!

I have the same model. (RS-806US in my case)

Your hiss issue could be due to bad electrolytic capacitors (now around 50 years old), bad transistors, or dirty switch contacts. (Does the hiss affect both channels or just one?) You'll need an oscilloscope to signal-trace the amplifier circuit. If that hiss is quite loud, chances are it's in the early stages of the amp.

Before hunting bad components, though, first clean the record-play switch S8 with a good contact cleaner such as DeoxIT®, then work the Record button back and forth several times. That might do the trick. If not, then you need the scope to find bad components. The set has old germanium PNP transistors that not only are obsolete, their replacements are now obsolete too.

Take care,
—
J. E. Knox "The Victor Freak"
 
Thank you
I will check the capacitors
I think I'll do a full recap of the unit later.
I will post pictures here soon
 
I don t have a osciloscope, so i can t trace the audio signal. Can i measure some voltages? The problem could be in power supply...or only in amp section?
 
Greetings from RojoLand!

Voltage checks won't be of much help. A hissy amp can have normal node voltages. You really need some means of monitoring the signal at various points in the amplifier in order to find where the hiss originates. I would not suspect the power supply; if the PS were at fault you would more likely have NO audio rather than noisy audio.

Take care,
—
J. E. Knox "The Victor Freak"
 
Hi friends.
Thank you for help.
I started the recap on my 8 track yesterday.
Since I don't have an oscilloscope, I decided to do a total recap.
I know this is not recommended as a problem-fix technique, due to the risk of creating new bugs without fixing the initial one, but I'll go this route anyway.
I'm doing it in stages, splitting each channel into 3 parts, and testing after each replacement 4-5 capacitors. And how there are electrolytics in these unit. There is a lot of then.

Well, so far I've swapped 80% of the capacitors on one channel, missing only one plastic-body capacitor.
I also lack the bigger capacitors (1000uf 16v & 25v) that I don't have at hand.
One thing I noticed yesterday while testing:

When I play the 8 track, the sound comes fine on 2 channels at first. Soon after, a hiss starts in one channel, and then the hiss takes over the 2 channels.
If I press the FF key, the tape restarts, and the hiss goes away again, playng fine in both of channels, and a few seconds later the hiss restarts on one channel and then the hiss comes from both channels.

Can this could help me to find the problem?
I haven't done any head cleaning yet.

Greetings
 
Greetings from RojoLand!

As I recall, there is (or seems to be) some sort of auto-level control on that machine. Adjusting the record levels on my rig is a tad finicky. Whether that is affecting your hiss issue or not, I can't say for sure. (If there is in fact an ALC, I will be disabling it!)

Those larger capacitors are most likely related to the power supply. Get those changed ASAP.

Take care,
—
J. E. Knox "The Victor Freak"
 
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