1957 Dynavox Table Top

[Hi-Fi-Mogul]

I'm in the process of reviewing the hot chassis design of
this 50C5 PP amplifier, so I can draw a schematic.
A 12AX7 and selenium rectifier are the other components.

There are no Dynavox schematics published for this era
of these players.

I am dispirited today, after I saw where some yokel has
been messing with the amplifier wiring.

I see where there is a wire connected off the AC supply that connects
to a speaker terminal !! WTH ?

I have scant experience with electronics, but this is
really dangerous.
I'm glad I didn't power it up previously, but this is not
what I ever do anyway.

I'm using the RCA RS-158 chassis as a rough guide.

Some of the resistors are out of spec, but some color
bands (such as second value band) are faded.
I'll have to just try some middle value resistors later
after I get this dang wiring sorted out.

Oh boy, another opportunity for learning. [ : )


.

[electra225]

I wonder if Dynavox may not have made their equipment. That cabinet looks Webcor/VM-ish to me. The RCA schematic you posted might be pretty close to what you actually have. I wonder if the wire from the line to the speaker might be back to common negative for some reason, hum prevention, perhaps? Since the Dynavox has a VM changer, you might look in VM schematics to see if you can come up with something closer?

[William]

I agree with Greg, the grille and cabinet look like my VM 1280A. The 1280A is single ended, has a PT and a 6X9 speaker but the cabinets look very similar. I see it has a 1/4 inch jack in the front, is that for a aux speaker or audio input? Also, in case you are missing one, there is an "E" clip stuck to the magnet of the woofer.

https://www.thevoiceofmusic.com/popup_p ... TITLE=1280

Bill

[TC Chris]

The posted schematic (was it RCA?) had a jack for using the speaker alone for stereo from another amp.

Chris Campbell

[electra225]

I posted a comment, then deleted it. I answered my own question.......

[Hi-Fi-Mogul]

I made a diagram of the power supply.

The 3 wires are all attached to one tab of the terminal strip.

That tab mounts the strip to the chassis frame.

The amplifier's components' B- connections are to the chassis.

It seems to me that this wiring set-up is the "return" route for
AC and B- current ?

And, yes I snipped the 2 resistors to make room for viewing.

.

[electra225]

Are you planning to replace the selenium rectifier? Those things are notoriously unreliable in their old age.

Your schematic looks good to the terminal strip. Unless my eyes are not seeing right, the way you have the terminal strip drawn would make a dead short. The terminal strip, as drawn, is more of a note than a schematic symbol. It might be less confusing if you would figure out where the wires on that terminal strip start and where each one terminates. You can still keep the drawing (or a photo) of the terminal strip so you don't get confused.

[Hi-Fi-Mogul]

Look at the photo closely.
The 3 wires and the capacitor are all soldered to that one tab of the terminal strip.

That tab's lug is riveted to the chassis.

The black wire from the filter cap can is on the
other side of that terminal strip tab, so not
seen in the close-up.

The seleniums always are replaced, yes.

[Hi-Fi-Mogul]

Making slow progress on hand scribing a schematic.

Question : There is a 0.05 mF cap connecting both plates
of the two 50C5 output tubes ?

Is that a correct application ?

Thank you.

[electra225]

That one .05 cap may be used as plate bypass for both output tubes.

[Hi-Fi-Mogul]

Ah ha.
So, being at the final stages of the amplifier,
this cap would be filtering some type of frequency "noise" ?

[TC Chris]

I have seen caps to ground labelled at "tone capacitor" or some such,and those are basically shunting high frequencies to ground (because people then expected audio to lack HF, and also because distorted HF are annoying). Some HF cutoff is helpful to avoid ultrasonic oscillation, but put it too low and it affects the audio adversely.

Chris Campbell

[electra225]

If you look at the schematic for the RCA (?) amp Mr. Mogul posted earlier, it has a rather unique tone control circuit. Typically, the bass and treble controls are both either ahead of or immediately behind the first audio amp. In this circuit, the bass is ahead of the amp, the treble is behind the amp. There is no plate bypass or tone compensation cap used. The volume control is tapped. A rather exotic circuit for a record player amp.

[Hi-Fi-Mogul]

In the DynaVox amp, the phono cartridge signal first enters
the Treble control, then the Volume, and finally the Bass control.

Trying to hand draw a schematic continues to be difficult for me,
as I wish to draw the diagram as organized as a Sams.

Thanks for all the inputs !

[electra225]

If you look at the RCA schematic you posted, the cartridge enters both the bass control and the volume control, since one end of both controls are connected together. The Dynavox chassis apparently is fairly close to how the RCA schematic is drawn. As to your drawing a schematic as neatly as Sams did, remember they were in business for several decades, so they have a head start on you with experience.....

[Hi-Fi-Mogul]

After accidentally blowing the new e-lytic caps
last month, I have the Dynavox all back together.
I put in a new tonearm that would accept a Pfanstiehl P-228
cartridge.
The old arm had a turn-around crystal cart that was shot.

There is more hum than I'd like, but not extreme.
I tried swapping another 12AX7, with no real change.

I will lastly swap out one at a time of the two 50C5
power tubes to see if it helps.

When I replaced the blown capacitors, I used 250 Volt caps.
With the previous 160 Volt caps, the rectified voltage ran
170 Volts on initial power up.

She's quite a good looking player.
Soundwise, Dynavox amps don't have as good
a sound compared to other PP 50C5 amps I've serviced.
I'll take her to the antique shop next week.

[electra225]

You may have already done this to get rid of hum. Try running a redundant ground from changer to chassis, being mindful of the hot chassis.

[William]

Looks great, Mr. Mogul.

[Hi-Fi-Mogul]

Greg, I had connected a ground wire as suggested.

I tried new 50C5 tubes, and the hum persists.

The sound is much better at higher volume, but
so is the hum (vol dependent).

It LOOKS good, though ! [ : )

Anyway, it'll be outta the house, and that's good.

[TC Chris]

The RCA schematic shows a cap from chassis to ground. Does yours have one, and did you replace it with a safety cap? I had an amp that started humming. I lifted that chassis to ground cap and hum stopped. No, I didn't replace it with a safety cap; didn't know what the were back in 1966 when I did that.

Chris Campbell