Zenith 12H090 still chasing sound quaility

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Zenith 12H090 still chasing sound quaility

Post by MrBill on Sun May 25, 2014 11:00 am

I am still not 100% satisfied with the sound quaility of this radio. I was measuring all the tube voltages and found my 6J5GT inverter tube has low readings, they are as follows; P is 56vdc should be 145vdc and K is -1.3v and it should be 5v. I have double check all wiring, resistors and capacitors all are good and have also been replaced. Do I have a weak tube?

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Re: Zenith 12H090 still chasing sound quaility

Post by Wildcat445 on Sun May 25, 2014 11:59 am

The schematic for this radio is in Beitmans, 1946, pages 190--192.

I would recommend that you measure B+ at the cathode of the recitifer, and at each filter cap as a first step, and verify that these readings are correct.

Verify B+ voltages as being correct on the output tubes, and also check again R-26, 2200 ohms, R-27, 47K ohms, R-29, 330K ohms, and finally R-28, 47K ohms.  R-28 supplies B+ to the plate of the 6J5.  I read that you say these resistors are "good", but you don't say whether you actually cut them loose to check or not.  Were the resistors replaced?   If these resistors are actually known good, then you need to start breaking up the circuit to find out what is loading the B+ to the 6J5.  Notice that on the schematic the specs for resistors is 10%.  That is fairly tight for a radio.  You need to make certain that all the resistors in the audio circuits are within 10%.  Good luck.

WC

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Re: Zenith 12H090 still chasing sound quaility

Post by 35Z5 on Sun May 25, 2014 12:49 pm

MrBill wrote:I am still not 100% satisfied with the sound quaility of this radio. I was measuring all the tube voltages and found my 6J5GT inverter tube has low readings, they are as follows; P is 56vdc should be 145vdc and K is -1.3v and it should be 5v. I have double check all wiring, resistors and capacitors all are good and have also been replaced. Do I have a weak tube?

A weak tube would cause a high plate reading and low cathode voltage, a gassy tube just the reverse...

Proceed with other voltage checks as suggested...
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Re: Zenith 12H090 still chasing sound quaility

Post by MrBill on Sun May 25, 2014 10:32 pm

So I went back through all my work and found that I mistakely installed a 1 ohm resistor in R26 instead of the 2200. I replaced the 1 ohm with the correct 2.2k resistor and voltages are now correct on 6J5 p is now 147 and k is 4.6 (R27,28 and 30 have been replaced and R29 & 31 have been checked when the others were replace.
Sound is a little better but I still have a blown speaker affect especially if I turn the bass control on. I have also noticed that the 2 6V6 tubes are not "matched" one is a Zenith the other is a GE. Could it be the speaker as it is a electromagnet type and possibly a weak hum coil. The field coil is marked 660 ohm and it ohms out at 657. The output transformer also ohms out properly. I am new to all this so its been quite a learning curve. Smile Any and all suggestions welcome. I have inspected the speaker there are no rips or tears and I have gentley cleaned it.

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Re: Zenith 12H090 still chasing sound quaility

Post by Wildcat445 on Sun May 25, 2014 10:53 pm

Have you verified correct voltages on all the output tubes?  Be especially critical of bias voltages.  I would probably just sub both output tubes from the start, and note any effect.  

Is the sound distorted immediately when it comes on, or does it get worse as the set warms up?  If it gets worse as the set warms up, I would suspect grid emission in an output tube.  Check again caps C25 a .02 and C39 a .005.  You have checked or replaced the resistors associated with the output tubes.  This would be a good time to break out your signal tracer, if you have one, and place the probe on the center terminal of the volume control.  It is still possible that the distortion starts ahead of the volume control, but we need to eliminate any possibilities in the audio section first.  It still would not hurt to sub the 6J5 and both 6V6's just to eliminate tubes as the culprit.  The output tubes not being matched can certainly cause distortion, usually associated with hum.  Is there any appreciable hum in the output?  You could have a voice coil rubbing in the speaker.  Leaving the old speaker still hooked up, you can sub a PM speaker temporarily using test leads hooked to the voice coil connections of the current speaker.  Then note any difference in performance.  Did you perform the B+ measurements I recommended?  If so, was everything okay there?  If not, you really need to.  Low B+ to the output section can cause all kinds of "chase your tail" problems.  Good luck.

WC

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Re: Zenith 12H090 still chasing sound quaility

Post by Dr. Radio on Sun May 25, 2014 11:47 pm

Just to clarify, a proper restoration of a radio using push-pull output tubes requires "matched output tubes".

This does not mean brand names!

Matching in the electronic aspect as both tubes need to match each other in operating characteristics under proper bias conditions. You can "match" tubes using a good transconductance tube tester, not a simple emissions tester.
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Re: Zenith 12H090 still chasing sound quaility

Post by MrBill on Wed May 28, 2014 8:46 pm

I'm still new to tube radios and the term B+ voltage so i looked it up. My understanding is the high voltage DC current to the tubes. I measured all the tubes and compared them to the wiring diagram.  Here are the tubes and the first voltage is from the diagram the second is my reading.  
6AG5 GS 150v /136V  G -1.5 /-1.2   p 235v/235v
6SB7Y GS105v/105v  P 260V/255V G 0V/0V  GO -4/-1.2
6SG7/GT   P 265V/254V  H 110V/106V   G -1.3/-1.5  
7W7 P 265V/255V  GS 75V/65V
6SH7  P  70V/110V GS  45V/59V  G -0.5/ -0.15
6S8GT  K -1.3V/-1.13   P  110V/125V

There is a very slight hum when the set comes on and the volume is all the way down and the distortion is worse when it first starts up and sound quaility improves as the set warms up.
I did measure these with my Fluke meter. I have a Eico 249 but I need to follow the maintenance instructions to be sure it is in good working order. I do have a signal tracer but it is a Lafayette KT 208 that was never assembled and I am not going to use the original capacitors (paper)and some of the resistors because they are out of range. I will get these meters up and running. In the mean time i will replace the tubes and then try the speaker. What ohms should the test speaker be?

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Re: Zenith 12H090 still chasing sound quaility

Post by 35Z5 on Wed May 28, 2014 9:51 pm

If sound quality starts to improve within two to five minutes problem is likely a tube, the 6J5 could have been damaged when the one ohm cathode resistor was in place...
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Re: Zenith 12H090 still chasing sound quaility

Post by Wildcat445 on Thu May 29, 2014 9:17 am

You can use whatever PM speaker you have on hand. An 8 ohm will work fine. I would seriously start subbing tubes at this point. The 6J5 and the 6V6's for starters. What were the voltages on the 6V6's?

WC

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Re: Zenith 12H090 still chasing sound quaility

Post by MrBill on Thu May 29, 2014 11:27 am

6V6GT tubes; Tube #1 3=268v/255vdc 4=280v/268 5= -1.3v/ -1.2 8=14.5v/15.15
Tube#2 3=268v/259.5 4=280v/264v 5= -1.3v/ -1.9 8=14.5v/15.01

The first number is from the diagram and the second is my reading..Thanks guys for all your help. I have new tubes coming in and I will try the pm speaker as suggested. Smile

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Re: Zenith 12H090 still chasing sound quaility

Post by Wildcat445 on Thu May 29, 2014 12:24 pm

By the readings you are giving, voltages looks okay at this point.  A signal tracer would sure be nice in this case.  Or a scope.  Sub the tubes and speaker and see what happens.  This problem could still be ahead of the volume control.  We need to absolutely clear the audio section of suspicion, first.  Have you cleaned all the controls and the band switch?  If not, you should do that.   Have you tried wiggling the tubes in their sockets, especially the Loktal ones?   You're getting there.  Good luck.

Edit.....Sorry about not explaining nomenclature for you, Mr Bill. I assumed you know what B+ was. If you are unsure what I am talking about in the future, please just ask. There are no silly questions. We are here to help you with your project and to help you learn. We all started like you are.

WC

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Re: Zenith 12H090 still chasing sound quaility

Post by MrBill on Thu May 29, 2014 12:35 pm

WC,

I cleaned all the controls and band switches with De-Ox-Id and even the tube sockets. I will do it again and wiggle them to see what happens. I will get my Signal Tracer built as I am really enjoying this hobby. Funny, for the past 7 yrs. i have worked in direct digital controls programming in the HVAC industry for a living (HVAC for 35yrs.) and work on this old technology as a hobby. What a contrast!!! Smile

What type of scope do you suggest?

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Re: Zenith 12H090 still chasing sound quaility

Post by 35Z5 on Thu May 29, 2014 5:19 pm

Well a signal tracer can be a good tool but for AM radios a scope isn't really necessary, though once in awhile can be helpful... I do use mine regurarly for homebrew transmitter building and monitoring... Unlike for TV troubleshooting, there aren't any representative waveforms in the service literature for any radio I've seen... Plan on self teaching yourself if you want to use a scope on a regular basis...

I recently had a distortion issue that I used a tuner from a '80 rack system for signal injection(alternate audio source) and found the amp was fine, I had problems in the IF/detector circuit... In fact audio was so good on FM, I hated to disconnect it, my treble control actually functioned for a change... Making a about face here, I then connected scope and found detected audio waveform was somewhat compressed indicating distortion...
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Re: Zenith 12H090 still chasing sound quaility

Post by Wildcat445 on Thu May 29, 2014 5:36 pm

While I appreciate that a scope may be overkill for this problem, I was trying to make the point that some type of signal tracing method, be it visual or by sound, was needed to run this distortion down. Try subbing the tubes and a different speaker and see what happens. This should eliminate or indict the audio section as a suspect. Good luck.

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Re: Zenith 12H090 still chasing sound quaility

Post by MrBill on Thu May 29, 2014 6:51 pm

Ok, I connected a temporary PM 4 ohm speaker my brother lent me and it sounded much better I would say an 85% improvement. I ran a long wire so I could locate the PM speaker outside the room where the console is and sound quaility was very good. Open the door and walk inside and the EM speaker sound was poor.
So, thanks to you guys we have made a big step forward. I will look for 12-14" PM speaker and from what I am reading I will have to leave output transformer and connect to the voice coil wires to the new PM speaker, unless you guys recommend a different way to wire it. What should I look for in a new PM speaker? ohms?

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Re: Zenith 12H090 still chasing sound quaility

Post by Bill Cahill on Thu May 29, 2014 7:19 pm

Sounds like cone on original speaker is rubbing. Very common problem. It can be professionally rebuilt, and, If you go to a pm speaker it will adversely affect the value of your set. It won't on all radios, but, especially these types of sets it does. I'd recommend a good speaker re coner. Probably the voice coil is either warped, or, physically falling apart. A very common problem....

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Re: Zenith 12H090 still chasing sound quaility

Post by jerryhawthorne on Fri May 30, 2014 6:16 pm

Mr. Bill, the suggestion to have your speaker rebuilt or find an original speaker is probably the best way to go. Replacing your EM speaker with a PM speaker can be done but the field coil on the EM speaker is part of the powersupply and provides filtering for the B+ as well as providing a magnetic field for the original speaker to work. You would need to add a resistor of the proper value to remove and replace the field coil on the original speaker. This would be a substantial wattage resistor. The resistor will not be as efficient at filtering the B+ and as such you will probably need to increase the size of the e-caps in the supply to aid in the filtering and reduce the hum. It is not just a matter of taking out the original and putting in a PM for replacement.
Hope this makes sense.
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Re: Zenith 12H090 still chasing sound quaility

Post by MrBill on Fri May 30, 2014 6:47 pm

Jerry,

Yes it does make sense and I have been in contact with John @ Vintage Radio (NJ) today with the measurements from my speaker. So it will go out next week.
Has anyone had John do work for them?

Thanks to everyones help.

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Re: Zenith 12H090 still chasing sound quaility

Post by Wildcat445 on Fri May 30, 2014 9:55 pm

You have determined that it is the speaker causing your problem?

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Re: Zenith 12H090 still chasing sound quaility

Post by Dr. Radio on Sat May 31, 2014 12:13 am

Wildcat445 wrote:You have determined that it is the speaker causing your problem?

WC


Exactly.

What I would do is disconnect (carefully) one or both of the leads going to the actual voice coil of the factory speaker (the little leads that actually go into the paper cone). Now connect a suitable permanent magnet test speaker to where the voice coil leads originated from. Keep all other connections as-is including the field coil and primary (from the radio chassis) wires to the output transformer.

Your radio will now be "speaking from new paper".

This will tell you if it is speaker distortion or chassis electronic issues.

If you are unclear on this description, please ask or you risk damaging something.
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Re: Zenith 12H090 still chasing sound quaility

Post by Bill Cahill on Sat May 31, 2014 2:07 am

I believe he said he already tried a pm speaker, and, it sounded very good.

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Re: Zenith 12H090 still chasing sound quaility

Post by Dr. Radio on Sat May 31, 2014 3:11 am

Your right!

How did I miss that? Oh yeah, why am I posting this late? Sleep
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Re: Zenith 12H090 still chasing sound quaility

Post by Wildcat445 on Sat May 31, 2014 9:55 am

Yeah, I missed that, too.  Sorry guys.   Sleep  Sleep 

Glad you found your problem, Mr Bill.

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