Classic Hifi Care

After we got the floor done, I decided to use the Imperial for awhile and let the Symphony rest. I hadn't plugged this in in probably over a year. I still believe this is the prettiest cabinet style Magnavox ever built.

That is beautiful, Greg, and I like the new floor too.

Bill

That sliding door is the nice touch.

As is the restrained and thoughtful design.

Chris Campbell

I posted in Chat about a tape I found with my great-grandmother singing on her 100th birthday. If you look at the left corner of the Imperial in the picture above, that is her picture with her husband who passed in 1948. That picture was taken in color, which was a big deal post WW2.

Good looking set and good looking floor.

I fought replacing the flooring tooth and nail. Finally, the wife and I compromised as to design. I wanted something that would go with the mahogany on the Imperial and with the pink whorehouse lamp that belongs thereon. She was NOT excited about that prospect. The design we agreed on looks nice with my old "junk" and with the wall paint and other "set top appointments". (Her "junk".) Oh, and Charlie the Chicken. God help us if the flooring clashed with ol' Charlie. Don't ask......

Every time I play my beloved old Imperial, it seems like it sounds different each time. I see the eye tube has died since I used it last. Frank Sinatra sounds like he is singing down the hall, not on the stage. "Poor soundstage" would be the gripe on AudioKarma. Something is failing in this stereo. Some component or components are not as young as they used to be. I have a line ahead of it, but I need to get on this thing. I know you guys are tired of me blabbing on about this, but it bothers me that I went thru this thing and missed something. Maybe it will get Dave's 12AX7 conversion. I need to take a hard look at the tape input circuit. FM radio, particularly, doesn't sound too bad, certainly better than the tape input. I put JJ tubes in the amp, and am beginning to wonder if I don't have a weak one in Channel 2. Whatever is causing this, is common to both channels. I used to have no bass when first started, then it would come in about ten minutes later. That sometimes doesn't happen now. I post these comments so I don't forget them and so maybe someone will read them and have a suggestion as to where to look. I don't intend to be critical or anything like that. I missed something. I need to figure out WHAT......

Edit: The Imperial played about two hours this morning before the bass started working. Must like Frank Sinatra after all....Now it sounds like a Magnavox. What is going on?.......

Sounds like my gripe ... fix it and something else blows or fails down the line ... it is getting to the point where you need to rebuild the entire set to get proper operation

I thought I DID rebuild the whole thing back in 2014. It may be such a thing that something wore out since I "fixed" it. This stuff was made to break and have to be fixed. We have to remember that. It's playing a Jim Reeves CD now and sounds fine. It's been running all day, since about 8am this morning. I still believe a tube SOMEWHERE has low emissions and needs to get really warm before it starts conducting.

Since i posted about the Imperial, several issues have developed. First, the eye tube has gone almost totally dark. Second, this stereo has almost zero fidelity. It sounds monotone. Zero bass, but poorly defined treble as well. The timbre control and the tone controls appear to do nothing. It sounds like the stereo is setting in a culvert, or a pipe. It sounds like the speakers are in a different room from the stereo. It's hard to describe, really. One channel is obviously weaker than the other one. There is a component that is failing or has failed. I will remove the tuner, MPX adapter and amp, connect them on my bench. I have a set of speakers out of a solid state Stereo Theater I will use for test. I will make some speaker wires so I get good contact versus using test leads on the speakers. I will compare schematics for mine and for Bill's newer Imperial. I will seal the speaker boxes. I will go thru the electronics like I have never laid eyes on it before. Go component by component. It has couplets, so we'll take those as troubleshooting dictates. I will at least consider a 6EU7 to 12AX7 conversion. This stereo has reclaimed its spot in the repair queu since it has taken a dump and we use it on the CD changer. I may start a new topic on the Imperial once I get a battle plan established.

I've constructed replacement circuits on small portions of breadboard, using components capable of taking higher power than the originals, to use in base radio equipment when the manufacturer had obsoleted them. If the circuit layout and part lists are available in the service literature, it could be an easy workaround.

I've been watching Uncle Doug videos on cathode biasing. There were unanswered questions from the first time I went thru this stereo, in 2014. I never did, for sure, get to know where the cathode bias was. It had a burned cathode bias resistor that was causing a huge hum. I got that sorted, then stopped. I should have figured out how "hot" the output tubes were running in the amp. We have a little blurb in Getting Started on how to use an old tube base to make a doodad to measure cathode current in output tubes. Uncle Doug offers mathematical calculations on how to bias tubes, which I am watching and taking notes. Math ain't my strong suite, so this will be something new for me. I'd like to get to the bottom of why this stereo has such poor performance.

I'm a long way from reconstructing couplets at this point, but I appreciate your suggestion. I'm beginning to wonder if the issue I am having with this old stereo may be at least partially self-inflicted......

Here's the type of probes I use for measurements with wiring around tube bases. With care, they can get in through the wiring and grab a single lead. There may be other sources on Ebay, but these were the fastest ones to come up. Pomona Test equipment made a lot of them.

https://www.ebay.com/itm/165242523988?i ... R_LQ5LLDYw

I have a set of those. My first avenue of investigation will be to figure out what the plate current and cathode current is in the bass amplifier output stages. I have long wondered whether the bass amp was working hard enough. Plate current on P-P 6V6's, according to the RCA tube manual, should be 90 ma per tube, max. If the plate current is now about, say, 40 ma per tube, that would mean the tubes aren't "hot" enough, not working hard enough. What maladies, if any, that would cause would be unknown at this point. I reckon the procedure would be to make the plate current right, then see how it sounds.