Magnavox Imperial model 1ST233

[electra225]

While I'm waiting for parts to arrive, I found two terms that I was unsure of their meaning. "Fixed bias" and "self bias". And also remember, I have been watching videos and reading information basically dealing with a guitar amp. Apparently, the guys who listen to music with their amps don't ever work on them, or at least they don't make videos of them working on them. Some of the guitar amp guys are as prone to use slang as the TV guys are. I have learned that fixed bias is grid bias. Self bias is cathode bias.

[electra225]

There is a negative feedback loop on this amp on both bass amps. I wasn't sure what its purpose was, or how it was supposed to work. Since this is an ultra- linear amp, a portion of the audio, on the secondary of the output transformer, is fed back to the grid of the amplifier tube. There is a 15K ohm resistor paralleled with a 100pf capacitor in the feedback loop. This signal is fed to the grid of the 12AT7 amplifier tubes out of phase, which means the more treble you feed to the 12AT7 from this loop, the more bass you get in the speaker. The caps in the feedback loop are ceramic, so they are probably still okay. I'm going to change the 15K resistors, since I have to lift one end to get a good reading. This is an area I need to investigate once I get the amp fired back up to make sure I'm not losing bass response. I'm beginning to wonder if Magnavox didn't design this amp to be complicated, just for the sake of complication. There are about a dozen couplets in the tuner, many of which deal in some regard with tone control. Bass compensation, treble compensation, timbre compensation, cathode follower bias compensation, yada, yada.....

[William]

What you just posted is Greek to me, but I can tell you the Magnavox Imperial is complicated and fought me the whole way as I was going through mine. My last two hurdles were having one side of the volume pot bad and finally the switch on the back of the function switch that would allow the changer to turn off everything on the last record. I was able to find a replacement for the volume pot, but I had to do bypass surgery for the switch. Even though there many a time I wanted to chuck it out the window I am so glad I did not as it is the best sounding console I own.

Bill

[electra225]

I'm trying to decide whether what I'm finding in this amp is due to my not fully servicing the amp in 2015, not knowing what to look for, or if components failed in use, or a combination of both. I still haven't found a good tutorial on servicing and repairing stereo audio amps. Everything I have found is concerning guitar amps. The principles are the same, but it would be nice to see something dealing strictly with stereo audio amps, like those found in console stereos. Uncle Doug's videos seem to be the most complete, simplest to follow and are very well put together.

I have learned that idle current, cathode current and plate current are all basically the same. Like in TV repair, there is a lot of slang used in amplifier service. Uncle Doug doesn't use slang. I still have a million questions written down that I would like to find answers for. I am learning a lot, but still have a lot to learn. I am in totally unchartered territory for me. I have no idea what the outcome is going to be yet. I will be intimately familiar with the electronics in this old Imperial stereo, I can tell you that for sure.....

The plan yet for this afternoon is to dig into the tuner and see what I can find wrong. I have an eye tube that doesn't deflect, a trait shared with the CG. I need a new 6E5 which I may have upstairs. I hope the high-value resistor for the eye tube is in the chassis, not in the socket.

[electra225]

Okay, here we go again.....

There are 50+ resistors in the tuner I need to check. I'll run the resistance tests like I did in the amp and note the results. I believe my focus will be to figure out why I have strangled bass and to see why the eye tube doesn't deflect. Right now, I'm not going to give any priority to radio operation. It was satisfactory before I took everything apart, outside of the traditional "no AFC" FM drift. I don't want to have to do a complete alignment on the tuner. I still need to do something with the changer, and I want to be able to use this stereo while I am still young enough to use it. The three 6EU7 tubes in the tuner will be a focus. The output tubes in the amp share bias components, but the 6EU7 audio tube in the tuner has separate bias components for each triode half of the tube. I ran the component values of the bias components thru Uncle Doug's recommended formula. I found the bias to be fairly flat from 30hz on up to about 10,000 hz. This stage, then, shouldn't be where bass response is being attenuated. I believe that honor to be in the 12AT7 in the amp chassis, but I don't know that for sure yet. Drifted resistors are the bane of the TV guys. I've never worked on an audio device, or even a radio, that had as many drifted components as this stereo does. The jury is still out on converting the 6EU7's to 12AX7's. I believe, at this time, I'll leave well enough alone. If I have success in "finding" the bass response this thing should have, maybe I'll revisit the issue.

Sorry if this sounds redundant or rambling. I use this topic as notes and reference as I work. I hope I'm not boring you guys with my babbling. It appears I have run this old stereo far past its "due date". I feel kinda bad about that.

[electra225]

I need to remove a shield under the tuner before I can run my resistance checks, so I have been poking around, testing individual resistors in the audio circuits. I found a 220K ohm plate load resistor in V10, a 6EU7 audio tube, that had measured 562K ohms. A 2.2K ohm cathode bias resistor on the same tube measured 3.465K ohms. These resistors being drifted in value can upset the bias on this tube. The resistors in the various couplets I have checked are well within factory specs, and, in fact, the furthest one out I have found is less than 7% out of spec, so they are good, thank goodness. I measured R7, a 220K ohm resistor to pin 1 of the 6BA6 RF amp that measured 233K. While more than 5% out of spec., it's less than the 15% given on the drawing. The cathode bypass cap on V10 is original. The filter cap can has been restuffed. I believe that and probably testing tubes was all I did in the tuner on the first pass in 2015. I'll respect the 15% tolerance for the front end, then use 5% in the audio sections. There is a 330 ohm resistor on Pin 2 of V9, the phono amp, that I can't find on the drawing (yet). It measures 1.152K ohms.

[electra225]

There is a 480K ohm resistor in the eye tube circuit, between pins 2 and 4 that measures more than 800K ohms. In old radios, the resistor that is in the eye tube socket usually goes pretty much open, to infinity. How far can that resistor drift high before it affects the operation of the eye tube? This eye tube is almost dead, but never had much, if any, deflection. The resistance measurements on the tuner circuits, per the drawing, are all over the place. There are a lot of "problem areas" in the tuner, more than I found in the amp.

[William]

Even though I only understand half of what you are taking about, Greg, I appreciate your posts as I hope to have some of what you are saying sink into my feeble old brain. When I went through my Imperial I found a bunch of drifted resistors in my tuner too. They all got replaced. Keep up the good work, and please, keep posting your progress.

Bill

[electra225]

Component-by-component troubleshooting is really tedious work. I have checked 31 resistors in the tuner. 26 of them are bad. And, not just by my 5% standards. Many of the 26 that are bad are drifted way above the 15% tolerance specified in the drawing. Nearly all the resistors in the three audio tubes are over the 15% limit. I'm not ready to make any predictions yet, but one would think that finding 90% of the resistors and all the audio tubes and rectifiers bad, that replacements should make a difference. I'll have to remove the volume control to change a couple of 1 Meg resistors that have drifted. These are in the tape input circuit. There are 45 resistors in the tuner, plus those in couplets. There are three paper caps (one is a Goodall!) plus two electrolytics in the tuner chassis that will have to be replaced as well.

Up to this point, I have been shotgunning the resistors. When I get the parts, I'll finish the amp then power up. I'll make sure I didn't create another fault, note performance, then power down and do likewise in the tuner. I'll do it stage by stage, starting with the audio section. I'll power up after each stage, since this project could go south fairly quickly if I make a mistake and don't catch it early. I'll work toward the antenna. The eye tube circuit will take some serious troubleshooting. Every component checks bad. I'm in the process of making a diagram to mark the wires and that. I had to take a break.

My research for the evening concerns the hum control. The three 6EU7 audio tubes in the tuner receive filament voltage via a separate tap on the secondary of the power transformer. The hum control is a 2K ohm pot that works in the filament circuit of these three tubes somehow. The control itself is a semi-hidden component that is obviously not intended to be monkeyed with by "happy hands at home". Does this control vary the filament voltage to the three tubes? I surmise that this setup is intended to be an isolation method of some kind, but what is it isolating? I hope I can fill in the blanks when I run the voltage checks on the tuner.

[TC Chris]

Do a search on "hum balance control." It reduces hum in the early stages via the filament AC. This is from a guitar site:

The hum balance control (a grounded pot connected between the two heater filiments),
found on many later silverface amps gives you the ability to find the “sweet spot”
between the heater filaments for the least amount of hum.
Although using an existing heater winding center tap or the twin 100 ohm resistor
method theoretically gives you a precise center tap ground, this method doesn't always
have the ability to balance ancillary parts of the amp that may be causing slight phase
differerences between the heater filiments, which in turn cause increased hum.

Chris Campbell

[electra225]

Thanks, Chris. I don't have hum. What I wonder is does that hum control lower the filament voltage?

[TC Chris]

On hi-fi amps, my recall is that it balances the AC on preamp tube filaments. I'm looking at the schematic for Heath A-9 amplifier. Here's a link.

https://www.google.com/url?sa=i&url=htt ... AdAAAAABAE

The filaments and hum balance are at the bottom of the diagram.

Chris Campbell

[electra225]

I guess I don't understand what it means to "balance the AC". That control has got to draw voltage to ground. I am chasing a weak output/weak bass condition. If this control lowers filament voltage, wouldn't that weaken the stages with the controlled tubes? Something about this isn't getting thru to me yet......

[electra225]

In addition to the hum control, another avenue for consideration is this. In the tuner, on the 6EU7 amp, they use a 25uf at 25 volt cathode bypass capacitor. Ditto in the bass output tubes and on the treble amp output tubes. But on the first audio tubes in the amp, a 12AT7 in the bass amps and a 12AX7 in the treble amps, they use a .1 uf at 630 volt orange drop capacitor. The tiny value of the cathode bypass cap will attenuate bass and will accentuate treble. Why do the feed the same frequencies to the bass amp as they do the treble amp? The component values on the first audio tubes are the same between the bass and the treble amps. Why would Magnavox want flat response in the 6EU7 amp in the tuner as well as from the output tubes in the amp, but attenuate bass in the first audio tubes in the amp? The possibilities are that the Sams drawing is wrong, that I put in the wrong value caps by a factor of ten, or that Magnavox designed it with that value.

[electra225]

I've been digging into the eye tube circuit this afternoon. There is a 470K ohm resistor in the eye tube socket that has drifted to 652K ohms, an increase of 38.29%. There is a 1.5 Megohm resistor in the AGC line that has reduced in value to 562K ohms. The AGC line in the tuner is rather more complicated than I typically find, but there are two other high-value resistors in the AGC line that have drifted in value as well, one higher, one lower. Out of the 45 resistors in the tuner, not counting those in couplets, which all test good, only 9 are still within spec. Most are out over 15%. The eye tube socket's construction has renewed my faith in the quality construction I have always associated with Magnavox tube stereos. It is made to come apart easily and be put back together easily for service. You don't have to drill out a rivet, then create some method of holding the socket together again. Changing the dreaded high-value resistor in the eye tube socket will be a lot easier. I've done some cleaning and polishing and am now getting ready to make charts for the final resistance readings on the amp and tuner, and a chart for biasing. I have quite the bundle of documentation on this project. I have some pictures as well. Now I need to figure out how to save screen shots so I can share those.

[walyfd]

No way a pro or even novice here but I'd suspect a wacky resistor.

I have 3... count them THREE sony dk555 r2r tape recorders for that fisher executive with dead right channels... everything I look at is its gotta be a resistor. I changed every cap, scrubbed the slide switch to reflection and still nothing.

My fisher 500B would suddenly scream on right channel and it was an out of spec resistor buried in the multiplex that heat opened.

[electra225]

At least 80%, maybe more of the resistors in the amp and tuner are out of tolerance. The 1.5 Megohm resistor in the eye tube circuit is on the "printed wiring" board on top of the tuner chassis. I'll replace the resistors, then check voltage, after I replace the 6E5 eye tube. I'll wager the CG has the same issue with the eye tube. It moves a little, but not near like it should.

This is part of the documentation I'm recording for this project. Hopefully this will bail me out if I still have an issue after I go to all the work I've done....

I have shotgunned this project, so that is why I make all these charts. If you look at the picture of the bottom of the amp chassis, you'll see the two orange drop caps, located at the upper right hand corner, that are the cathode bypass caps for the 12AT7 amplifier on the left side above the output tubes. These are the caps that may hold the secret to bass response in this amplifier....

Just some general pictures of the project. The baked, long-suffering Multiplex adapter that is housed in the warmest part of the cabinet.

[Conelrad]

Greg, A hum balance pot is used to create a balance between the two supply wires and chassis ground. Often time a 'one side grounded' scenario will inject enough hum in high-gain stages to make a difference, whereas the pot allows one to adjust for minimum hum as tubes age and/or replaced.

Since the pot resistance is high compared to that of the filament string, there is no appreciable loss in voltage to those filaments.

D

[William]

What are the brownish looking things in the upper right-hand corner of the amp? Are they caps?

Bill

[walyfd]

That's spotless...