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 »

JMPGuitars wrote:
Fri 04/01/22 1:56 pm
After rebuilding the circuit, do you have a different OT you can test?
Yes, I have extra of these amps with the same circuit. The PT and OT in those amps are from a different year and manufacturer, but should be the same spec.

Is it the different OT to try first? And why?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by JMPGuitars »

yello wrote:
Fri 04/01/22 2:57 pm
Is it the different OT to try first? And why?
Because, some things may only fail under load. You know that you only have a short with the power tubes connected. You can have everything else loaded, and no short, so less likely the PT. You can have only the power tubes and rectifier loaded, and the amp shorts. If it's not the circuit components, it's logically the OT. If with another OT it still shorts, then you either have 2 defective OTs (highly unlikely) or you missed something else.

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

Post by Bieworm »

Josh is stating things I think too are suspicious.
Some years ago I had a very expensive amp (still have it) doing weird stuff. It had volume loss , made crackling noise and started whining sometimes. It didn't do so immediately, but after warming up.
This turned out to be the choke. The amp tech explained this to me as the choke being under load for some time started warming up and the windings were malfunctioning due to the miniscule swelling. There was a tiny fault in the winding that would appear after behaving in certain circumstances.
I know this is not the same thing , but just to point out these hidden problems are in fact a possibility.. even in other parts, such as resistors and stuff.
My advice is to swap the OT and see if that solves your problem. You're bound to radical things since you need and lack lab equipment to look for it at tech level.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Sat 04/02/22 4:12 am
Josh is stating things I think too are suspicious.
Some years ago I had a very expensive amp (still have it) doing weird stuff. It had volume loss , made crackling noise and started whining sometimes. It didn't do so immediately, but after warming up.
This turned out to be the choke. The amp tech explained this to me as the choke being under load for some time started warming up and the windings were malfunctioning due to the miniscule swelling. There was a tiny fault in the winding that would appear after behaving in certain circumstances.
I know this is not the same thing , but just to point out these hidden problems are in fact a possibility.. even in other parts, such as resistors and stuff.
My advice is to swap the OT and see if that solves your problem. You're bound to radical things since you need and lack lab equipment to look for it at tech level.
After I finish getting the new circuit parts in, I will take a closer look at the OT, and swap in the other one if the short remains with the new circuit parts all updated. I just gotta order some resistors tomorrow, I thought I had 560k but I don't.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

While waiting for parts, I'm exploring removing the final parts from the turret board that are for the now unused film input. You can see it/them on the schematic here:

Image

Looks like I can remove:

R201, C202, R204, R223, and R224. Does that look right?

I already took out the tube socket thing with its resistors, R203 which is a pot and R235 which connects pot to v1.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

yello wrote:
Fri 04/08/22 12:38 pm
While waiting for parts, I'm exploring removing the final parts from the turret board that are for the now unused film input. You can see it/them on the schematic here:

Image

Looks like I can remove:

R201, C202, R204, R223, and R224. Does that look right?

I already took out the tube socket thing with its resistors, R203 which is a pot and R235 which connects pot to v1.
I don't know what you're aiming for, but if you want to remove those .. go ahead.
Looks to me when you remove R235 + R204 + R224 and R223 that part is completely out of the system.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Mon 04/11/22 7:17 am
I don't know what you're aiming for, but if you want to remove those .. go ahead.
Looks to me when you remove R235 + R204 + R224 and R223 that part is completely out of the system.
Thanks for the feedback Bieworm (I'm back in town after a few days away from a computer).

My goal was simply to remove extraneous parts and wires that aren't used. That way there is more room to troubleshoot in a very cramped chassis both now and in the future if mods or repairs are needed.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Sat 04/02/22 4:12 am
My advice is to swap the OT and see if that solves your problem. You're bound to radical things since you need and lack lab equipment to look for it at tech level.
I installed the other OT into the amp and tested it today - the exact problem remains. With no tubes, just the rectifiers, or with adding in preamp tubes, there is no overcorrect and my light bulb limiter stays dim.

Once I pop in the first power tube, turn on and caps charge, then the light bulb glows very brightly.

The only thing left in this amp that hasn't been touched is the PT. I do have an extra from the same amp I pulled the OT from. I imagine that is next, given everything else is new. I have gone over everything else several times to make sure that there is not an errant wire, short, etc. somewhere and I can't see any problem anywhere.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

I mentioned that some time ago. There could be a faulty winding that's on the threshold of failing , it just needs enough current draw to do do. Hence the problem only occurring when the power tubes are installed. It's a theory, but well possible.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by JMPGuitars »

Bieworm wrote:
Sun 04/17/22 3:06 am
I mentioned that some time ago. There could be a faulty winding that's on the threshold of failing , it just needs enough current draw to do do. Hence the problem only occurring when the power tubes are installed. It's a theory, but well possible.
"Enough current to do do" - That's a funny way of saying S*&t the bed. 😂😂
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

JMPGuitars wrote:
Sun 04/17/22 8:06 am
Bieworm wrote:
Sun 04/17/22 3:06 am
I mentioned that some time ago. There could be a faulty winding that's on the threshold of failing , it just needs enough current draw to do do. Hence the problem only occurring when the power tubes are installed. It's a theory, but well possible.
"Enough current to do do" - That's a funny way of saying S*&t the bed. 😂😂
Cr@p!! Lolll!! *to do so...
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Sun 04/17/22 3:06 am
I mentioned that some time ago. There could be a faulty winding that's on the threshold of failing , it just needs enough current draw to do do. Hence the problem only occurring when the power tubes are installed. It's a theory, but well possible.
Indeed, good theory given what I have experienced so far.

So I replaced the PT. Tested resistance before putting it in, and once installed tested voltage at primaries, heaters, HV, and get proper readings on all. Though this is weird, when I turn the amp on, my light bulb limiter doesn't turn on at all, no glow whatsoever. I tried the bulb in a lamp and it works so its not that the bulb is bad. Even with bulb off I get proper AC for this amp circuit at all points of PT (120v primaries, 290v HV, 7v at heaters). This is with tubes out. Same limiter, same bulb (75w) as used previously, the only conditions that changed was the new PT. That is a new development from the other PT, which glowed dimply with no tubes, rectifiers and preamps, but glowed bright with power tubes - what does it mean that it won't light up at all?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

My bulb never lights up when nothing is wrong...
Does your amp work now?
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Sun 04/17/22 2:51 pm
My bulb never lights up when nothing is wrong...
Does your amp work now?
I used a lower wattage bulb and now I can see the light.

However, I am getting the exact same results: everything normal until I insert a power tube. Tried several to make sure its not the tube, tubes were tested for shorts.

I'll grab one of the 1 amp fuses (amp takes .8 amp fuses but that is the closest new production I could find) but I assume it will just blow if the limiter is glowing brightly.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Sun 04/17/22 2:51 pm
Does your amp work now?
Update - Does the amp work now? Yes, and no.

Good news: For the first time in six weeks it isn't blowing the fuse. I can't explain the difference in how my light bulb limiter is acting, but no fuse blowing is good. I tried a 1 amp 250v new fuse, and also the original spec 8/10 amp 125v fuse, and neither blows. Amp stays on, and it passes signal with a guitar plugged in. I suppose that means that the OT or PT has an internal short appearing under load as Bieworm and JMP predicted. I didn't try the amp with just the new OT without limiter, before new PT, to know if the fuse would blow still with just the OT replaced. Regardless, I'm happy it finally isn't blowing fuses.

Bad news/next step: With no instrument plugged in, amp on, and volume up, there is a squealy type sound. With guitar plugged in, amp on, squeal is gone but there is a good amount of hum and hiss. Amp does not have a shorting input jack with 1m resistor, its the original 1/4" jack isolated from the chassis with fiber washers and no resistor.

I've never rebuilt a full amp before, so don't know what it could be. I replaced every component, and carefully inspected every component including measuring before installing them and comparing to schematic to ensure they were correct, though I recognize there is the possibility for error there. All components have good mechanical connections and solder joints. Do components need time to break-in? Next thought is maybe it is lead dress related, so I need to chopstick, as wires were certainly moved around in this process. Additional thought is that I implemented the suggested mods to improve gain and bass response and I've never heard this circuit with those mods. I'm wondering how those changes naturally interact with the 5-position tone knob and its resistors/caps and functions?

I can take a video this afternoon of what I am hearing to figure out next step.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by JMPGuitars »

Yay! Progress!

A muting jack would be a good idea. But you're right to go after your lead dress. Lead dress is typically the cause of noise issues. Chopstick time!
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by Bieworm »

Try swapping OT primary leads from position. That's a common oscillation source
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

JMPGuitars wrote:
Mon 04/18/22 3:46 pm
Yay! Progress!

A muting jack would be a good idea. But you're right to go after your lead dress. Lead dress is typically the cause of noise issues. Chopstick time!
Funny thing is, the lead dress in these amps is always a mess from the factory 80 years ago. And they are typically without noise issues. The current issues I have are definitely new/wierd in my experience with these amps, but I will for sure chopstick to see if I can improve things.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

Bieworm wrote:
Mon 04/18/22 4:45 pm
Try swapping OT primary leads from position. That's a common oscillation source
I just checked, the OT primary leads are going to the right spots, both per schematic, and per another same amp and the amp I pulled this OT from. That said, maybe that is needed in this instance, its one of the easier things to do so I'll utilize that along with my chop sticking as needed.

Gonna go take a video now.
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Re: Military projector amp problem (Bell and Howell 614/AQ2A) - speaker distorting/overworked

Post by yello »

The noise is somewhat better now that I arranged the wires and bolted the turret board back in its proper place.

Here is the video:

https://youtube.com/shorts/ycipZeNJfw0?feature=share


First is amp totally turned off, nothing plugged into input. You can hear the noise, then I turn the amp volume up a little, not even to 1. Noise is quite loud. Then I plug in guitar and that noise goes away. What is left I think is roughly typical noise for having an Esquire (single coil pickup) plugged in.

Is that particular noise symptomatic of something?

I've never heard that noise before, even without an instrument plugged in, in these types of amps with the same original jack. I imagine it would benefit as noted from a proper shorting jack with 1m to ground. Possibly from a shielded input wire as well?
Last edited by yello on Mon 04/18/22 5:47 pm, edited 1 time in total.
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