Military projector amp - last edited 9/26/22 - layout help needed

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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Is there a certain model Siglent you recommend?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

I have the 1102 CML
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by JMPGuitars »

yello wrote:
Tue 02/22/22 1:03 pm
Is there a certain model Siglent you recommend?
That depends on your budget. Bieworm has the SDS1102CML+ which is $299. The SDS1104X-E is $499, and a great scope. I use the SDS2104X+ (upgraded to SDS2504X+).
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

While I consider the scope purchase, and potentially wait for it to arrive, are there particular components to consider replacing first like disc capacitors (I already replaced one that was microphonic), or particular parts of the circuit to start exploring in?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

Don't start replacing stuff randomly. You might create other problems along the way.
Try the non destructive way first

You could try eliminating what's good or bad when you take another same amp.
Take the power tubes out of this one.. make a long alligator clip wire and clip it between the same points in both amps. Always the grid input of a preamp triode.. starting from V1
This is what I did at some point when I couldn't find the problem in an amp. Watch out.. it's noisy too with the long antenna wire😉
But you should get the scope! You're not planning on stopping the amp prodding business anytime soon me guesses 😃😃

Still recommended as plan A, the scoping. That will definitely give you more insights you will benefit from in the future
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by JMPGuitars »

As always, step 1 is knowing and understanding your VOLTAGES.

You need to take voltages in the amp in question, and I assume you have a few more nearly identical amps you can take voltages from to make a comparison.

That kind of static could have a few causes, and knowing what's going on with your voltages may help you determine that.

Bieworm is correct, randomly replacing items could cause entirely new problems. Start with voltages, voltages, and more voltages. And be safe in the process.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Anyone have some good online resources for learning how to safely take voltage readings?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by JMPGuitars »

yello wrote:
Tue 02/22/22 3:45 pm
Anyone have some good online resources for learning how to safely take voltage readings?
I'm sure Bieworm or somebody else could supply you with some useful links, but in the meantime, here's some basics.

1. Safety first. Read every warning about working on tube amps on the entire internet.
2. Make sure your amp is properly grounded. If it's not grounded, replace the power cable/jack with a properly grounded connection.
3. Seriously, don't touch the amp if it isn't grounded properly.
4. Using a good quality DMM (Digital MutliMeter), connect the ground probe to ground with whatever clamping attachment (alligator etc.) came with it.
5. Keep one arm in your pocket, or behind your back. Make sure you handle the probe safely, only touch ONE spot at a time, don't slip and short anything out.
6. Set the DMM to the correct voltage type (AC or DC).

Being careful, and taking your time, test your wall voltage at the amp's inlet, then work through the rest of your AC voltages at the PT including the heaters.

Set the DMM to DC and start measuring your DC voltages starting from the B+ line, and get all the relevant tube voltages (cathode, screen, anode). For reference, look at one of my voltage test charts in the downloads section to get an idea of what we look at.

Also, BE CAREFUL.

Thanks,
Josh
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Great advice, the chart will be handy to guide me.

I turned the amp on now to check out the sound again and it wouldn’t turn on. Fuse was blown. Opened up the amp and noticed that R210, a 1meg feedback resistor for v201 was cracked.

Possible that part was failing and causing the static, then it finally gave up and blew the fuse?
Last edited by yello on Mon 03/14/22 3:14 pm, edited 1 time in total.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

It's possible.. you know what to do next now 😉
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Wed 02/23/22 5:57 am
It's possible.. you know what to do next now 😉
Yep! I assume I need to replace the broken resistor, then check voltages while I wait for my new oscilloscope to show up.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Parts arrived - after I replace the broken resistor, I'm ready to check voltages.

Do I take voltage readings with all tubes in? Power tubes out? With speaker load?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

All tubes in. Speaker attached
The fuse doesn't blow I presume?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Just replaced the cracked resistor and...fuse blew.

Not currently using my light bulb limiter to start it up as I can't find a non-LED light bulb in Seattle.

I only have one 8/10amp 125v fuse left...

What is a good next step to figure out why I am blowing fuses? I do have two more sets of rectifier tubes I can try out (amp used dual 6x4). Just not enough fuses, gotta find some to that spec to order today.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

Fuses blow mostly due to shorts. Shorts can be measured .. look for it
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

I can't visually see any components or wires shorting yet. How do I measure to find them?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

It's likely the resistor was cracked due to a fault in that part of the system. Start there...

Or..

Process of elimination. Pull all tubes. Take your lightbulb current limiter and look if it's not in the B+ line. If the lamp is lighting up strongly without tubes there's a big chance your power transformer is shorted. It's also likely the filter caps are kaput. If it's tube rectified then that also is one of the suspects.

OTOH, if the lightbulb limiter doesn't light up, then there could be a short in the preamp or power amp. Put tubes in, one by one until the limiter lights up. That will narrow your problem area down
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

It's likely the resistor was cracked due to a fault in that part of the system. Start there...

Or..

Process of elimination. Pull all tubes. Take your lightbulb current limiter and look if it's not in the B+ line. If the lamp is lighting up strongly without tubes there's a big chance your power transformer is shorted. It's also likely the filter caps are kaput. If it's tube rectified then that also is one of the suspects.

OTOH, if the lightbulb limiter doesn't light up, then there could be a short in the preamp or power amp. Put tubes in, one by one until the limiter lights up. That will narrow your problem area down
Cracked resistor was a preamp tube negative feedback resistor, I'll being taking a look there.

Thank you for all the suggestions. Thankfully I was able to source a 100w light bulb for my limiter.

Quick questions in regards to your suggestions:

1) Does a new fuse need to be installed to try amp without tubes while using the light bulb limiter?
2) The schematic calls for 8/10 amp 125v fuses. Looks like the modern equivalent is 1 amp 125v fuse?

Anecdotally, the amp was working 10 days ago, albeit with an annoying crackling on all notes (unsure what the problem was, maybe the cracked or failing resistor). I changed out one very microphonic resistor, and cleaned the tube sockets with Deoxit D5, after that is when I started blowing fuses. And each time the amp was turned on, the pilot light would light up right before fuse blew. The amp is running new electrolytic filter caps. I do have a correct spec and tested set of the plug in filter caps that I could try if needed to isolate my new caps as a potential source of the problem if needed, though I tested the new ones before install and they were correct.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

Yes you need a fuse to get continuity. If you would bridge the fuse when using a lightbulb limiter it would be ok, but there's always the chance you forget to fuse it afterwards.
A 1A fuse would be ok too.
I would also closely examine the rectifier and your transformer's high voltages. Maybe disconnect the B+ after the 1st filter cap and measure the DC from the rectifier.
Also check if there is no short in the heater circuit. Those 2 checks come to mind when you mention crackling noises.
You did check the tubes I presume?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Yes, prior to blowing fuses I had tried different tubes in every socket in an attempt to see if my noise issue was tubes. To no avail, noise had remained.
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