Author Topic: Missed it by "that much" - a recapping tale  (Read 427 times)

Motorola Minion

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Missed it by "that much" - a recapping tale
« on: April 20, 2018, 01:56:50 PM »
After performing a full recapping on a mid-level 1964 Zenith stereo amplifier that uses SE6BQ5 and 1-12AX7 , I was greeted with a loud, irritating oscillation upon power-up.

Rather than hearing silence with the volume set at minimum, a loud 700 hertz (or thereabouts) blast filled the shop and beyond one early morning in February.
Turning up volume, audio from the restored tuner came through but still, I had just enough time to kill power to stop it.

Not fast enough, this prompted the upstairs tenant to complain to my normally patient landlord/owner (who is Amish) via a "building manager" (a demanding RE agent that he hired to manage tenants), prompting me to be banned from my shop before noon. Sheisse!!! I am preparing a new space in a barn on the same property. More on that in another post, probably in chat.

Coming back later that day when the coast was clear, the volume could not be reduced, the tone controls had little effect and neither did the bass-boost switch.

No usable clues except pulling the 12AX7 made it quiet. Not knowing where to start, both the Zenith factory and SAMs Schematics were double-checked to see if my nearly 100% re-cap missed any values.

Even the normally OK Zenith-made ceramic disc caps of .01 and .0033mf, 470 and 220 pf had cracks in them, prompting a replacement along with bumblebee, dipped paper caps and electrolytics including some resistors. I want this console to sound as good as possible, being the last of the all-tube Zeniths.

 Today, I thought to bring in the chassis and schemos and scrutinize it over my lunch break under very bright lights. The cause found was a capacitor in each channel being off by one order of magnitude, I used a .0033 instead of a .033 mf. This was in the feedback loop that runs through the bass-boost switch.

Lesson learned: Even though I wrap leads of replacement caps around the existing cap leads before soldering and clipping old caps out, I missed it somehow ???
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Dave

Harbourmaster

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Re: Missed it by "that much" - a recapping tale
« Reply #1 on: April 20, 2018, 07:46:28 PM »
Could happen to anyone.  :o


Sorry to hear about the dustup over your work space.
-- Aloha, Ken

No Console Left Behind!

Motorola Minion

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Re: Missed it by "that much" - a recapping tale
« Reply #2 on: April 23, 2018, 12:28:38 PM »
Ken- I do get in the shop a few times a week, right after work. And if a rare time when the tenant is away, I hope to close this thread by confirming the Zenith sounds like new. HiFifun sold me a set of MR-105 radial speakers which I cant wait to hear on this console.

The shop is more serious now because its limited to 30 minutes of a test record and I'm not blasting a 3-Dog Night record to test the limits, well at least not when someone is home ;)
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Dave

Motorola Minion

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Re: Missed it by "that much" - a recapping tale
« Reply #3 on: April 25, 2018, 02:45:50 PM »
I changed the two caps of wrong value. The constant feedback is gone but if the treble is turned up, it returns.

Bass boost works and sound is better with the original Zenith-branded Amperex tubes installed*, not great but that could be the speakers.

One important thing I noticed is that weak output tubes (tired RCA 6BQ5's used for testing) allow plate and screen voltages to rise considerably.

*With the weak tubes, normal 12 volts cathode voltage dropped to 5 volts. This under-bias causes other issues as well. The common cathode resistor is correct as is the 40 mf bypass cap across it.


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Dave

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Re: Missed it by "that much" - a recapping tale
« Reply #4 on: April 27, 2018, 01:38:08 PM »
I'm going to play this for a few hours to see what happens. Sometimes amps need to wake up after being dormant then recapped. I have another Zenith console with this same amp, all original. I may need to do a power supply re-cap and see how it sounds in comparison.
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Dave

Motorola Minion

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Re: Missed it by "that much" - a recapping tale
« Reply #5 on: May 01, 2018, 10:46:34 AM »
There is a 8+lug terminal strip between the two output tubes. This is where I replaced two .022 grid signal coupling caps. the two lugs toward the back of the chassis had both the caps on feeding the opposite tube's grid:stupid:

Well now I know how bad the original speakers are. I think all six could be replaced with two new 10" woofers and two horns (Foster OEM, from a Zenith SS console).

The good news is the cabinet is low and long with the speakers in separate chambers, allowing for fully enclosing the rear and adding a tuned port or insulation.

Both 10" woofers sound pretty bad when the bass boost is on, which is to say "boomy". Turning up the treble brought very little in fidelity, so either the two 4" mids and two 3.5" cone tweets are shot. It cant be as simple as a crossover cap:sigh:.

This console is worth the effort mainly due to its solid-built walnut cabinet, 2g record changer and quality electronics, now restored.[/
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Dave

TC Chris

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Re: Missed it by "that much" - a recapping tale
« Reply #6 on: May 01, 2018, 09:35:33 PM »
You can test those speakers with a simple 1.5 volt cell.  If connecting it up kinda intermittently (drag  a lead across a speaker terminal) makes for scratchy noise the speaker is functioning.  It might avoid lots of work without any effect.  Of course, you could have some deformed voice coil forms, but that's unlikely--the most common failure mode is burn-out (open circuit) from the amp clipping. 

Chris Campbell

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Re: Missed it by "that much" - a recapping tale
« Reply #7 on: May 03, 2018, 09:33:44 AM »
Zenith extension speakers I got a while back, sound good though they were made for a solid state amp. I disconnected them and tried the console's again yesterday and it actually sounded better, as if the speakers are shaking off or stretching ??? The amp itself seems to be recovering too.

Lets try removing and regluing them and checking for rips as needed. The battery trick I never heard of to verify good free-air movement.
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Dave