Very Poor Bass From Panasonic RE-787 AM-FM Stereo Radio
 

I have a very nice RE-787 (circa 1967-69). This radio looks and operates flawlessly except the bass is extremely shallow. My roommate in the Air Force (long ago!) had this exact radio and the bass was excellent. I've heard that some caps probably need to be replaced, and I have the Sams Photofact for this unit, but I have no idea which caps I would need to replace. The volume, bass and treble pots are clean and have absolutely NO scratchiness and even though the amplifier has low output (3) wattage, the sound output is very clean. Just weak bass.

I really like this radio, especially the cool signal seeking motorized tuning, so can anyone guide me on this? I could scan my Sams if you would need that. Thanks for any assistance here.

http://videokarma.org/attachment.php...3&d=1414712172

I'd start with replacing the power supply caps. Bass requires energy and weak caps are a common culprit.

Thanks Celt. Here's a pic of my power supply board. I assume you suggest replacing all 4 caps? (C1, C2, C5 with resistor R141 and C6).

Check the resistors...but they will probably still be okay. But yes, replace those caps as a starting place and see how it does.

Actually, I would check the ripple (A.C.) on the D.C. voltage sources with the volume up pretty high; this ripple voltage should be say less than 1% of D.C. voltage. If the caps were the problem and going bad in the power supply, I'd think you'd be hearing A.C. hum from the speakers.

More likely, your poor low bass issue is in the signal paths and most likely way out of spec electrolytics (check around the volume and tone controls, and pre-amp to pwr-amp coupling caps). A high ESR on a cap in the audio path will usually result in poor/decreased low end frequency response along with reduced audio power.


Tom (PK)

Decoupling caps on the output stage of the amp... Is this stereo...?
If no then most definately look to the output stage.....
Coupling cap from first stage to output stage, and associated parts...

Be sure you check the speakers....

If a strong bass thump comes along and it seems like the thing momentarally
shuts off, then look to the power supply, if not its in the amp....

A lot of those sets had a good size cap coupling the push pull amp to the speakers....
check the cap, resistors, transistors, caps to ground...
Check everything carrying the lion's share of current in the output stage....
Be sure the power switch is working properly.....
You can easily have one of the output transistors with an open emitter or collector
and be lacking in proper drive current to drive the Whooffer to it's propper Whooff- !

.

I have this radio..someone threw it out down my block..absolutely mint condition. And yes it's stereo. It also has a deep bass sound for sure and nice highs.

Thats a damn nice radio. And you are lucky to have good controls
still, the jap stuff was plagued by them back then.
Even though stereo I would suspect coupling caps. In the Panny TV's
of that era the yellow electrolytics were trouble. Reason its both
channels is one goes but you still got bass in the other til that
one goes then its noticed.
Same with double cassette decks. Most came in with 2 bad motors.
The A deck would go then they would use the B deck til that
one failed also.
I will pull the Sams later & give you a list of suspects.

73 Zeno:smoke:

Partial quote:

"Be sure the power switch is working properly....."

I fail to see how a power switch problem might have anything to do with a low frequency response problem. Unless there are very obvious pop-corn and crackling sounds coming from the speakers along with D.C. supply voltages jumping all over the place.

Tom (PK)

I use to work on Panasonic stuff of this age, and they did have a buncha strange
things happen with them.... Including the slide controls, and power switches which
had gone bad and would not pass signal along..... Just running over some of the
strange stuff I remember seeing.... Take it anyway you want.... I'm saying don't
overlook anything with these radios....

Panasonic makes some really great stuff all around, but sometimes tough to
track down some problems cause they ain't always what you first suspect.

.

Have you tried different speakers?

Do all the controls seem to work properly..... Balance too...?

.

Unseen & unheard....
C18 & 28 they are .2 mfd so you may need to use .22
C20, 30, 23, 33,24, 34, 25, & 35
C22 & 32 they are .1 mfd

Output uses + & - voltages. Quick check of output stage is be sure
there is no DC on speakers.
You can use your finger for a signal injector by touching the
base of transistors up to the drivers. When you get a nice loud
bassy hum you know its OK from then on.

73 Zeno:smoke:

Thanks to all who have pitched in here! In answer to some of the posts, all of the controls, bass, treble, left and right balance, volume, and the input switching between phono, AM, FM, FM Stereo work properly with absolutely no scratchiness, popping etc. Volume is strong and undistorted up to a fairly high room (10'x12') filling level. When I turn the unit ON, there is no bass thump even with the bass pot at max. Everything seems to work very cleanly, it's just that at max setting, the bass is very weak....sounds more like the unit has only a tone control rather than dedicated separate bass and treble. Just for fun, I connected a Sony CD player that has adjustable output to the phono input and got no increase in bass performance.

Those small coupling electrolytic caps in radios that age tend to dry out, and their capacitance goes way down. That would make for a lack of bass. Be careful of the circuit boards, it doesn't take much soldering iron work to make a copper trace peel off.

So this radio has a separate bass knob.... Does it appear to have any effect on the
sound...? How about the treble knob.... Comparing both, does it seem they have
an equal range..? You may find the problem is in this part of the circuit.....

Yes, it does have separate bass and treble. The treble works perfectly and attenuates properly. The bass control does not. At normal or slightly high volume if I put my ear very close up against the speaker I can hear a definite bass (with the bass control at max) but 6" away it's not really audible as "bass"......it's as if the bass signal is not really being "pushed/amplified". Kind of like a radio with only a tone control where the bass is not really attenuated but rather the highs would be washed over with more of a midrange sound.

I'm going to get a cap kit and replace all of the caps previously mentioned in this thread and report on the outcome. Probably won't be until later this week. I appreciate your input on this!

Well.... If you hear no hum.... then there is no need to do power supply caps...
If the bass knob has no response, then look to that circuit first.... For example:
the bass knob can be of a common ground, and it not be attached, knock out
bass on both channels.... Break in circuit board.... bad caps near the bass circuit,
and that part of the circuit will be small.... Could be a wire broken off somewhere
inside....

.