Zenith SF2500 Reverse Phase Question

[dstroup]

I recently acquired a 1961/1962 Zenith SFF2500 console. It has a 4D20 amplifier (3 channel).

The original turntable has been replaced with a Garrand SL 55B and a Realistic 42-2930 Solid State Stereo Pre-Amp was added as well.

I would like to connect my iPod to this console. In a separate thread here on audiokarma it was suggested I remove the Pre-amp if I intend to play my iPod through the console. That said, I came across some information that indicated I may end up with out of phase audio since I’ll be using the TT inputs.

My questions are:

1. Will playing my iPod through the console result in out of phase audio?

2. If so, can I remedy this issue simply by reversing the polarity of one (or more) of the speakers via reversing the speaker wire connections on the amplifier or speaker(s)?

3. If reversing the polarity of speaker(s) will work, which speaker / speakers do I need to apply this remedy to?

I have added some photos below to help illustrate the issue.

Thanks!

 

[cademan]

If it's factory wired (except for the preamp), there should be no out-of-phase issues. Don't know who gave you that misinformation but being that it is a 3 channel amplifier, has no bearing on any phase issues. Motorola makes 3 channel amps with a lot of their consoles (I have one), and I use mine with my MP3 player connected to the phono inputs with absolutely no phase issues whatsoever.

That added preamp was so someone could use a magnetic cartridge turntable with a ceramic cartridge input. Your original turntable input is like a line level input and can be used for other things like a tape deck, tuner, or CD player.

Leave the speakers alone!

 

[gadget73]

It shouldn't invert the phase. Its got 2 active amplifier sections per channel if I read this right, one triode in what appears to be a 12ax7 and the 6bq5. Broadly speaking, even numbers of tube sections do not invert phase.

 

[dstroup]

Thanks for your replies! I'll update after trying out the ipod with it.

gadget73: yes, you are correct: 12ax7 and 6bq5

Thanks again!

 

[dstroup]

UPDATE: I hooked up the ipod and the console played very nicely when set to MONO mode, but when set to STEREO the left speaker does not produce sound?

I checked all the connections and have used DeOxit and DeOxit Gold on all the pots including the Bass knob which is used to switch between Stereo and Mono (Pull knob = Mono, Push down knob = Stereo).

Any suggestions on what might be going on here and how to fix it? Thanks!

 

[dstroup]

UPDATE: I tried the following remedies and now I notice I do get very faint audio out of the Left speaker if I turn up the Loudness knob to around 4 and I have the volume on the ipod up to around 75%. This may have been there before and I just didn't notice it?

Reapplied DeOxit and DeOxit Gold again and worked the knobs

Swapped the RCA leads (female) that connect to the Phono inputs (male)

Reseated the RCA connections

Double checked all the Tubes and speaker wires were seated properly

Tried a different ipod and different songs/music

Still just faint audio on the Left speaker when set to Stereo mode?!!?

 

[cademan]

Something you may not know about these Zenith amps is that the balance control is not really a balance control but rather a volume control for the other channel. The marked loudness & balance controls are basically volume 1 and volume 2, or volume left & volume right.

Set both the loudness & balance controls fully counter clockwise and then turn them both together clockwise and see if that doesn't help your problem.

When both channels (speakers) work in mono mode, then at least you (we) know that the amplifier is working properly.

 

[dstroup]

Thanks for the info cademan! I'll have to do a more methodical check of the loudness & balance control tomorrow, then i'll post an update.

 

[gadget73]

Hm, its possible that the way it does this 3 channel thing is that it requires an inverted phase input on one side. If thats the case, I don't know that you can easily do that with an ipod since the ground side is common to both channels. Maybe a small 1:1 isolation transformer on one channel with the phase reversed so it works would make it behave.

Quick and dirty test, set it to stereo and unplug one of the leads. If you get sound out of one channel at a time with it set to stereo, then it wants an inverted phase on one channel to work right.

 

[century tek]

Check to see if both channels are working by plugging the turntable back in with the preamp. I know for a fact that that Garrard TT cartridge does not invert one channel. If one side doesn't work in stereo with the Garrard, then it would certainly be plausible that the original TT had a cartridge that was inverted.

Probably the best way to see if both channel inputs share a common ground would be to take an ohm meter to the ground side of both male RCA input jacks!

 

[gadget73]

The phase could invert on the Garrard if someone flipped the wires around. Actually that would be a simple way of making the magic happen if it needed the goofy inverse phase thing. I've done it by accident myself.

 

[century tek]

The phase could invert on the Garrard if someone flipped the wires around. Actually that would be a simple way of making the magic happen if it needed the goofy inverse phase thing. I've done it by accident myself.

This is true but who's gonna know that except people like you and me and people here?

 

[dstroup]

UPDATE: Here’s the results of tests suggested.

1. Check to see if adjusting the “Loudness” and “Balance” knobs will produce the correct volume from all speakers.

RESULT: Adjusting the “Loudness” knob only results in very faint audio coming from the LEFT speaker (no sound from other speakers). Adjusting the “Balance” knob only results in normal audio coming through the RIGHT and CENTER speakers and nothing from the LEFT speaker (Although it is a bit hard to tell if anything is coming from the LEFT speaker when the other speakers are producing audio).

2. When set to STEREO unplug one of the leads - if you get sound out of one channel at a time then it indicates that it wants an inverted phase on one channel to work correctly.

RESULT: Pulling the BLACK RCA plug results in very faint audio coming out of only the RIGHT Speaker. Pulling the RED RCA plug appears to make no difference in the audio coming out of the speakers (i.e. no sound from the left speaker, only audio through the right and center speakers).

3. Plug back in the TT with the pre-amplifier and see if one side doesn’t work in stereo - that would indicate that it was an inverted signal since the Garrand TT does not invert.

RESULT: The LEFT speaker does not produce audio when set to Stereo, even when I alternately adjust the “Loudness” and “Balance” knobs.

In addition:
When set to Mono and I only turn up the “Loudness” knob I get no sound from any speakers.

When set to Stereo and I only turn up the “Balance” knob I get audio only from the Right and Center speakers.

When set to Mono and I only turn up the “Balance” knob I get audio coming from all speakers.

Tomorrow I’ll have my Ohm Meter and I'll run that test as well.

I'm getting the feeling that I have the inverted signal phase problem I was initially concerned about.

Is there any risk / issue with swapping the + and - wires on the Left speaker just to see if that fixes the problem? If not, I'd like to give it a try.

 

[century tek]

Swapping the + and - left speaker wires isn't going to do anything if you are not getting any sound out of the left channel in stereo mode. There appears to be a problem somewhere else. Sounds like an input problem so some more in-depth troubleshooting is needed. Possible problems that I can think of off the top of my head:

1. Left RCA input cable is bad.
2. Left (loudness) potentiometer may be bad.
3. A resistor inside the amp for the left channel input might be open.
4. Cold solder connection.
5. Bad connection somewhere underneath inside amplifier associated with the left channel.
6. With the light off or in a darkened room, look at the 12AX7 tube carefully and see if both heaters (the part that glows inside the tube) to both halves are glowing.
7. Anything that anyone else can think of.

As mentioned by Cademan, leave the speakers alone. It appears that you are having a problem with the left input somewhere. Both output tubes (6BQ5's) are working if you are getting sound out of both left & right speakers in mono mode. The 6BQ5 tubes are the speaker outputs. Since it uses one tube for each speaker, this is called SE or Single Ended where one tube drives the left speaker, and one tube drives the right speaker.

For example, if one of the 6BQ5 tubes wasn't working, you would only get sound out of the left speaker only or the right speaker only in either stereo or mono mode, but since both speakers are working in mono mode only with the "balance" (left) control, then both the 6BQ5 tubes are working and you have a left input problem somewhere.

Tell us what state or city you are in and maybe an AK member close by could stop in and possibly help you out!

 

[dstroup]

Thanks for the suggestions century tek! I have a few addition questions / comments and have attached some photos to help illustrate:

1. Regarding the RCA cable being bad:
How would I replace this cable? Do I need to connect a new cable by soldering or can I connect it to the amplifier using some kind of plug (i.e. banana plugs)? I don’t understand why the connection to the amplifier has 3 holes, 1 of which appears to have the wire braid around the 2 main cables going into it? Is there a way to cut and strip a regular RCA cable and attach it to the amplifier ( again, banana plugs?). Please advise. Thanks!

2. You suggested previously that check to see if both channel inputs share a common ground use of an ohm meter to the ground side of both male RCA input jacks. Have my previous tests eliminated the need to do this? If not, what/where would I be connecting the probes of the Ohm meter to in order to check this?

3. I’ll look at the 12AX7 tube also to see if both heaters / halves are glowing as well and update afterwards.

Thanks again!

PS: I’m in Seattle, WA

 

[century tek]

Regarding the braided cable, the braid itself is the ground connection which is also a common ground for both channels, and being braided, it cuts down on any outside interference. The other two wires on the wafer end going into the amp, are the right & left inputs.

With a meter set on the ohms setting, you can easily see if the cable is bad by checking for continuity between each end of the cable. The other end with the 3 wires going into the amp, just simply unplugs from the amp so you can easily check it without actually pulling the amp out. Carefully pry up the wafer plug with a small flat head screwdriver evenly all around the edges so as not to break it and it will come out. I have a Zenith amp with this same kind of plug so I will show you a pic of what it looks like out of the socket.

Personally, I don't think the cable is your problem but it's nice to at least eliminate it from the equation.

EDIT: Here's pics.

 

[dstroup]

century tek: Thanks for the information! The photos really help!

Unfortunately, I wasn't able to check on anything today. I will be unable to do further checks until Monday at the earliest. I will use your suggestions and update this thread on Monday.

Thanks again!