Zenith 12H090 lives again. Small sound issue need assistance

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Zenith 12H090 lives again. Small sound issue need assistance

Post by MrBill on Mon Apr 28, 2014 9:18 pm

I have owned this Zenith for 4 years and just a few months a go I repaired mt Trav-Ler 5170 so I was feeling brave and attempted to recap the Zeinth which went pretty well, I did replace the old rubber coated wires and made new harnesses for the power/audio and the tuning eye and replaced the 5Y3GT because while inspecting it there was a loose piece of metal floating inside.

I have not worked on the phonograph yet. Powered it up for the first time yesterday, I did have it protected by a 3 amp fuse and also made on of those power isolator with a 150watt light bulb. It did not go boom and after a few minutes it started to play but the sound comes up very slowly and distorted. It does clear up to a point but not perfect. The tone control does work as it has an affect on the sound but it is very bassy and distorts as I turn up the volume. I have since removed the power isolated and operated it full currect. The wiring diagram is available on line mine is the 11C21 chassis

It has very good reception on both the am and fm. I am assuming the 2 6V6GT maybe the problem but I am only guessing and do not have a tube tester.  Here are the readings from the 6V6GT tubes; Tube #1 3=255vdc 4=268 5= -.2 8=15.15
Tube#2 3=259.5 4=264 5=1.9 8=15.01
In the wiring diagram it shows letters instead of numbers and I used r-type.org GE Tube data to reference the pin arrangement.
The diagram also shows voltages for those letters. I believe I converted them correctly. 1=NC 2=H 3=P 4=CS 5=G 7=H K=8
The diagram states the following voltages K=14.5 P=268 CS=280 G= -1.5
What advise do you folks have?

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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by Wildcat445 on Tue Apr 29, 2014 12:28 am

I have not had the chance to pull the schematic up for this set as of yet. At first blush, reading the voltage measurements you provided, I would not like the fact that you apparently have higher voltage on the screens of your output tubes than you do on the plates. I would suggest that perhaps a resistor has drifted in value, affecting the voltage on the plates. If you are using a digital meter, this may affect the readings somewhat. This, of course, is merely a guess, based upon what little information we have. The reason for the "bassy" sound might be twofold. First, those old Zeniths tended to sound rather bassy anyway, and secondly, you may have an incorrect capacitor value in the tone control or audio circuit somewhere. I would probably recommend checking every resistor and capacitor from the volume control to the speaker for correct value and correct lead dress and wiring connections. Good luck.

WC

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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by MrBill on Tue Apr 29, 2014 7:15 am

WC,

Yes I was using a digital meter (Fluke 87 V). I have an old Eico VTVM M#249 that was given to me I have not checked it out yet and it even has the UniProbe. I will check everything you suggested but forgive my ignorance what is "correct lead dress?" Thanks for your help.

Bill

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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by Wildcat445 on Tue Apr 29, 2014 10:35 am

Sorry, MrBill.  I should have explained that better.  

Lead dress is how the wires are run on the chassis.  Certain wires, like those carrying AC (filament wires) can cause hum and other problems if run too close to certain other wires, such as those in high-gain circuits, like those found in the audio section.  If the original wiring is disturbed to an appreciable degree during repair or restoration, problems can be self-induced that did not exist before.  You mentioned that you replaced much of the rotted old rubber wiring.  It is vital that wires to tube filaments be run dressed close to the metal chassis.  Most experienced restorers take detailed digital pictures of a chassis in its original condition, in order to document lead dress issues.  

I hate using digital meters for old radio work.  I like them for measuring resistance.  I would can the digital meter in favor of an old Simpson 260 or a good VTVM.  Once you use a VTVM, you will be spoiled.  But that is just my opinion.  Whatever works best for you is what you should be using.  Hope this helps, and good luck.

WC

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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by MrBill on Tue Apr 29, 2014 1:18 pm

WC,

Thanks very much for all your help. I will check out my Eico 249 if it works properly I will use that. My Fluke is for what I do for a living which is Direct digital controls for HVAC equipment. I will keep you posted on my progress.

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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by MrBill on Tue Apr 29, 2014 8:47 pm

I tested all the resistors tonight, Found 3 bad, all around the 2 6V6GT (PWR AMP) tubes. Funny this is where the 5Y3GT Rec tube is and its called the Power audio chassis. Someone had added a 30mfd cap where the dual cap had 1 side failed plus the 5Y3GT tube has metal pieces floating in it so I replaced it before start up. The resistors are R26,R27,R30. R27 was actually cracked on the bottom and not reading at all. I will post back when these are replaced.

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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by Wildcat445 on Tue Apr 29, 2014 8:56 pm

How were the resistors "bad."? Had they drifted high or low in value? By how much? Were they burned? You mentioned that one was cracked. Had it burned? Have you determined what caused that failure?

WC

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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by MrBill on Tue Apr 29, 2014 9:16 pm

R27 is YVOS 47K 10% looks like it burned there is a piece broken off the bottom and I can move each section away from each other.
R26 is YVO 47K 20% reads 64.2 so it is drifting high? 20% high would be 56.4 and R30 YPY 470K is reading .574M drifting low?

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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by Dr. Radio on Tue Apr 29, 2014 11:35 pm

If I'm reading your numbers right, one of your output tubes has a positive voltage on the grid--never good. Leaky cap, mis-wire, bad tube are all suspects.

I'll have to study the schematic more later, on my smartphone now...to much work with a small screen Wink
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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by Wildcat445 on Wed Apr 30, 2014 12:26 am

I missed that, Doc. Perhaps some grid emission on an output tube?

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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by MrBill on Wed Apr 30, 2014 8:41 am

I looked back over my notes and I typed incorrectly on tube#2 yermial 5 is actually -1.9 Tube #1 has -.2 that reading concerns me as the wiring diagram calls for -1.3.
I appreciate all the help!!!

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Re: Zenith 12H090 lives again. Small sound issue need assistance

Post by MrBill on Sat May 03, 2014 11:15 am

I replaced R27,28 & 30 today and that has solved te sound issue. I appreciate everyones help. Now on to my Zeinth 7H24Z that has a 39ohm resisitor reading .64M it is in the circuit that powers all the tube heaters so no tubes light up.

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Re: Zenith 12H090 lives again. Small sound issue need assistance

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