Ebay Sansui 7900Z Volume not working correctly

[Plomaman27]

I just made an offer on eBay for a nice looking 7900Z and was pleased to see the owner accept my offer pretty quickly. They had the Buy It Now @ $200 so I think I did well... although I cant be sure until I get it home and check it out. All lights work, unit powers up and has good audio output but the volume up and down buttons not working at all. Hoping its just a loose connector to the motor or something similar (and minor. Has anyone owned one of these 7900Z's?

 

[Sansuiman]

7900Z (as well as 8900ZDB and 9900Z) uses a strange contraption for the volume control. Inside the unit is a conventional dual ganged audio taper potentiometer to control the volume. However, it is linked by a length of dial cord to a small DC motor and gear box which drives the pot and a sliding backlit film visible through a narrow window on the front panel to simulate a digital bargraph display for the volume level.

There are up and down buttons on the front panel which control the operation of the motor inside, and in turn the pot and the level display "bargraph". All in all, a very convoluted way to achieve a digital volume control (that really isn't) but hey, they were going for a certain look and feel and I guess this was the easiest way to get there at the time.

Anyway, the dial cord could just be slipping, could be broken or have jumped off somewhere, or the motor or its associated drive circuitry could be faulty. None of this should be too difficult to fix and get the volume control functional.

In more general terms, it is a good receiver and still kind of overlooked. Not exactly to everyone's taste with that early 80's digital light show thing it has going on, but it sounds good (it is a direct coupled output stage). Not quite built to the same standard as it's predecessors in the G series, but still a quality piece albeit with some cost cutting evident.

If you don't mind the near seizure inducing theatrics of the digital displays in a dark room, it will do quite nicely. I have a 9900Z and it was a steal at the time so I gambled on it. Glad I did as it's a sleeper, or at least was one at the time I bought it about 12 years ago. I'd say you did quite well at the price offered if the volume control is the only thing that needs repair.

 

[ETLS]

Questions of value belong in the dollars and sense forum.

 

[kevzep]

Questions of value belong in the dollars and sense forum.

The OP is actually asking a technical question about the volume control, the title is missleading.

 

[Dan ʕ•ᴥ•ʔ]

Corrected the title

 

[Plomaman27]

If you don't mind the near seizure inducing theatrics of the digital displays in a dark room, it will do quite nicely.

Lol, didnt think of that I may have to put some window tint over it to reduce the length of the seizures However there are two other problems with this as-is purchase.... the AM antenna is missing from the socket mount and the balance knob is staticky. Hopefully the AM antenna wont be too hard to locate and repair. As for the balance pot, it should be fine after a shot of deoxit and that goes for all of the pots and toggle switches. I believe this baby puts out 100 wpc which will be a welcome bump in power to run a pair of Infinity Kappa 7 speakers.

 

[Sansuiman]

Yes, 100WPC. 8900ZDB was 125/ch and 9900Z was 160/ch. All the controls should be cleaned at this point, not just the balance. The 7900Z also uses a spectrum analyzer display with two fewer bands than the 8900 or 9900.

I'd not worry too much about AM antenna. An external antenna can be easily attached, unless you are just going for original condition, then yes, you'll need the antenna parts. Those won't be so easy to find as they are somewhat unique to the Z series receivers, and finding one being parted isn't terribly common.

 

[UncleBingo]

See if the vol pot motor makes noise when pressing the volume up/down buttons. If not, replace the tactile switches under the buttons. I've done this on 2 of them so far.It isn't difficult.

 

[Plomaman27]

Aha so there is a spectrum analyzer display!... for a second I thought the seller was confusing this receiver with some test equipment items on eBay when he referred to a spectrum analyzer display: ) That should be interesting. How would an external AM antenna be attached? I know there are FM antenna terminals there but not sure where the AM would be hooked up. Would you suggest doing a re-cap of the power circuitry?

 

[Plomaman27]

Aha so there is a spectrum analyzer display!... for a second I thought the seller was confusing this receiver with some test equipment items on eBay when he referred to a spectrum analyzer display: ) That should be interesting. How would an external AM antenna be attached? I know there are FM antenna terminals there but not sure where the AM would be hooked up. Would you suggest doing a re-cap of the power circuitry?

OK I can now see where an external AM antenna would be an easy hookup to the designated AM terminal lug.

 

[Plomaman27]

See if the vol pot motor makes noise when pressing the volume up/down buttons. If not, replace the tactile switches under the buttons. I've done this on 2 of them so far.It isn't difficult.

Good idea. Are those tactile switches obtainable at Mouser or Newark or are they a specialty item specifically for Sansui stuff.

 

[Sansuiman]

The switches are of a type I've not seen elsewhere. They are larger than the commonly seen micro-tactile switches seen now in a lot of equipment. These are larger, with a "button" about 1/4" in diameter. I've not gone looking them, but can't imagine a suitable replacement should be difficult to find from one of the usual suppliers.

If you plan on keeping it functional long term, a recap will be in order. These are 1980/81 production so nearly 40 years old now. The clock is ticking loudly on all those electrolytic caps inside.

The spectrum analyzer is more of a toy than a serious effort, it is a blue VFD with 8 bands marked on the window beneath the display. There is no "y" axis however, marked in dB or otherwise, to indicate an absolute level for each band. Further, there is a dial to adjust the sensitivity of the analyzer display and can be turned all the way down so it isn't visible at all.

Essentially, it is just there to give a light show based upon the real time spectrum analysis of the currently selected source. There is no peak hold function present on it either. So, nice to look at but not all that useful. If you really need a RTA in your system, better to invest in one that is properly calibrated of 12 or 14 bands with selectable peak hold capability.

 

[UncleBingo]

Good idea. Are those tactile switches obtainable at Mouser or Newark or are they a specialty item specifically for Sansui stuff.

I got them from Mouser iirc.

 

[Plomaman27]

If you plan on keeping it functional long term, a recap will be in order. These are 1980/81 production so nearly 40 years old now. The clock is ticking loudly on all those electrolytic caps inside.

Yes I totally concur. Especially since the speakers they'll be driving are known to load receivers something fierce.. Infinity Kappa 7's can go down to 2 ohms depending. Now comes the "fun" part.... matching parts lists with whats currently available in a useable size filter cap for the application. Do you have a favorite brand/source for electrolytic caps for vintage receivers?

 

[Plomaman27]

I got them from Mouser iirc.

Thanks, I'm familiar with Mouser but not sure what iirc refers to...

 

[KingBubba]

If I remember correctly.

 

[Sansuiman]

Might be careful with attaching difficult/low impedance speaker loads on this receiver. Min rated impedance is 8 ohms, unlike many earlier Sansui models that were ok at 4 ohms. Will probably be ok if you aren't listening at levels that could trigger earthquakes in Hong Kong, but something to keep in mind.

Make sure you have only one set of speakers in use at any given time though, or you'll likely do some damage. Or, get an impedance matching external speaker switch box to adjust the impedance presented to the receiver.

 

[superdog]

No help on the technical aspect but I had a couple of these units and I really liked they way they sound.It was the motorized volume that I didn't like.Thats what sent them packing.

 

[Plomaman27]

See next post...

 

[Plomaman27]

Will probably be ok if you aren't listening at levels that could trigger earthquakes in Hong Kong, but something to keep in mind.

For the record, I would never crank a vintage unit up high enough to quake Hong Kong... Los Angeles maybe but never Hong Kong.... lol
I just brought my vintage Sansui 7790Z "eBay" receiver home. I removed the top cover (all wood and immediately went to get a dust mask because of the large amount of dust inside. All 40 years worth it appears. Every square inch was colored grey....the buss fuses were completely covered and none of the lettering on the boards was visible. I used a long bristle medium brush to loosen the top layers of dust and then a stiff dry acid brush to brush most of it loose between components while sucking it all up with a crevice attachment of a vacuum cleaner. The receiver powered up without incident followed by the comforting click of the speaker relay. Yes! I wanted to check out the volume control first so I pushed on the UP button and the motor started turning easily.... it kept turning as long as I applied pressure to the button.. Pushing the DOWN button also turned the motor in the reverse direction and the associated tuning cord. Even the bargraph shows increasing and decreasing volume levels. Tuning to a local radio station caused the tuning bar increments to increase to 5 bars so the tuner is working good as well. BUT....with volume maxxed out on the bargraph there was only faint music coming from both L/R speakers. Looks like the volume problem wasnt in the volume button gizmo but rather somewhere downline from there... tomorrow I will post some pics and start tracing with the scope.