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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Jul Mon 31, 2017 4:33 pm 
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Could my avc be the problem? Should I replace the last three resistors in the radio even if they still measure in tolerance? Just a side note: I have a small fluorescent light above my workbench. When it is on, there is no interference, but when it is off, the radio gets interference that I would typically hear from a fluorescent light.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Jul Mon 31, 2017 11:01 pm 
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Okay, I did absolutely nothing except re-solder the 75 to volume control connection, and the volume has increased a bit. Still not as loud as it is supposed to be though. When I tune around on the dial (with no antenna) I can hear slight oscillations as well as a crackling/popping sound that goes away and comes back at times. It is a clear day outside, so thunderstorms are not the cause. Also, I know that avc is working, since a very loud local station comes in at the same volume as a distant one.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Wed 02, 2017 2:47 am 
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Turns out my 74 (part no.) section of the candohm was bad, so now I need to replace the whole thing. I should get parts in the next few days, and will update this thread with any new results. Hopefully, this is the problem. The resistance should be 1.2k but instead measures 12k! All the other sections of the candohm unfortunately test fine, so it looks like one bad section was the death of the candohm and hopefully the cause of the low volume in the radio.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Wed 02, 2017 2:50 am 
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For now bridge it with a close value you have on hand and try it...
Phil

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Wed 02, 2017 2:23 pm 
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Unfortunately, I don't have any resistors near the correct value to put in parallel with the circuit, so I have to wait until I get all the replacement resistors. If the candohm isn't the source of the problem and is not broken, then I guess I just replaced something in the radio that is most likely to fail sometime in the future. Quick question: could I detach the tabs on the candohm and glue them in place using non-conductive epoxy? This way, I won't have to remove the original candohm and will be able to use it as a terminal strip.

I was thinking of doing something like this: https://www.youtube.com/watch?v=usouMbihkm8

(Electrolytic replacement 4:37)

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Wed 02, 2017 9:17 pm 
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If the candohm reads 10x the proper resistance, that is certainly a problem. I would replace all parts of the candohm with individual resistors mounted on a proper terminal strip, and not attempt to use the candohm at all for mounting.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Wed 02, 2017 11:50 pm 
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Isn't there a trick that if you clamp it down in the bad section that the proper connection will be made again? I thought I remember hearing something about this.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Thu 03, 2017 3:10 pm 
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5Y4GT wrote:
Isn't there a trick that if you clamp it down in the bad section that the proper connection will be made again? I thought I remember hearing something about this.

There are many situations where you could at least temporarily get something working by doing that.
But---as a general rule---Don't. Fix it correctly.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Thu 03, 2017 6:02 pm 
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Okay, thanks! I just wanted to know what works best.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 05, 2017 12:05 am 
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I can't seem to find a 5-inch terminal strip anywhere. The original candohm is 5 inches long, and I don't want to drill any mounting holes for a new terminal strip, since if I do, I will destroy the original chassis label on the back. Does anyone know where I can find a 5-inch terminal strip with mounting holes at the end and 5 or more lugs?

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 05, 2017 1:25 pm 
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RadioDaze, for example, has many terminal strips. You might have to use two terminal strips mounted end to end to achieve what you're looking for. Link provided only as reference.

http://www.radiodaze.com/terminal-strip ... red&page=2

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 05, 2017 10:41 pm 
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there are many ways to do this. One is to use 1-terminal strips, and solder the ground end of each one to a terminal on the existing candohm.

You can also use cable ties to strap the new parts to the old candohm, and then make the connections "in the air".
slightly better would be to cut a piece of fiberglass circuit board to the right size, mount all the parts on it, and then strap the assy to the candohm

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 05, 2017 11:45 pm 
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I thought I might use chassis mount resistors, and then JB Weld them to the chassis. This way I won't have to worry about mounting holes.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sun 06, 2017 11:17 am 
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Chassis mount resistors like to have their flat side smack up against the chassis, to conduct heat away. Any cement there would interfere with that function. I guess it would be OK to cement just the tabs to the chassis, although I'm not a fan of that method.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sun 06, 2017 2:16 pm 
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5Y4GT wrote:
I thought I might use chassis mount resistors, and then JB Weld them to the chassis. This way I won't have to worry about mounting holes.


Another supporter of bad idea: Shortcuts like that have ramifications. If there is no room in the rats nest, put them on the sides. Bolts & perhaps heatsink compound rather than glue.

Marc


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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sun 06, 2017 5:22 pm 
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I just wanted to thank everyone for all of their advice on my candohm replacement. In the end, I have decided to use Mark's (pixellany) suggestion of putting one-pin terminal strips on top of each existing candohm pin. I know I need terminals with a ground connection that is separate from the lug on the top of the terminals, and have found some on radiodaze that I plan to order. Thanks again for the help!

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Mon 07, 2017 12:42 am 
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What I have done with some of those long tapped resistors / candohm's and it can't be done with all, is to strip all of the wire off it & then utilise the existing clamps.

Marc


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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Fri 11, 2017 3:06 pm 
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Okay....

I replaced the candohm and barely anything changed. The volume slightly increased and the hum from touching the 75 tube increased as well. All coils in this radio check good, so any ideas on what the problem could be? Also, static and crackling sounds intermittently occur when I am using the radio. I also had a station seemingly "disappear" at one point. I have absolutely no idea about what is going on!

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Fri 11, 2017 7:48 pm 
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Also, a resistor (#34) reads 20k ohms on the schematic, but measures 25k ohms. Could a drift this big (over 20%) cause an issue like the one I am describing in the radio?

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 12, 2017 12:23 am 
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This is getting long. It may need a "go back & review?

Factors of significance:
Weak tubes: In my experience its everything else and tube failures are at the lower end of cause.
If the voltages are wrong you cannot expect the set to run properly;
If an electrolytic capacitor leaks more than it should, that can be a critical load & do damage: (They have to leak a little to sustain their polarity).
If a non polarised cap leaks, in the case of a coupling cap it can bleed positive onto the control grid. The control grid is meant to be negative to a set point, that will kill it.

Alignment: This by far (when everything else is correct, including good signal) is the biggest cause of "deaf" radios. Wrong frequency & too much signal, often applied wrongly all contribute to getting it very wrong. When you have changed RF parts, most manufacturers, recommend re-alignment. Once a set is running & I am happy with it (to some degree) Alignment of the IF precedes any adjustments in the mixer oscillator area.

As far as I am concerned we all need to adopt personal "standards" & methodology. These can be formulated from info on the forum. Resistors have tolerances, in most cases my repairs are commercial and the same standard applies if it isn't. If its resistance is more than 10% out on the high side it gets replaced, most of the unstable ones will be more than 20%. The proper time to check & replace them, was when you changed the dodgy caps. That can save a huge amount of time & frustration.

Personally I would be inclined to look at all of the resistors. Some cannot be accurately measured in circuit, or with Ecaps across them, but the general rule is that if it measures high in situ, it has failed. It is never uncommon to find several out of spec.. on a set in the line here, it had six. With Pentagrids I tend to give the oscillator more volts (tolerance is around 15-20%) when changing from old to modern resistance values. Closer AGC resistance value, resistors can be found in low watt resistors: AGC is more about control voltage, not current.

Marc


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