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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 12, 2017 1:24 pm 
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Location: Cleona, PA
There is something that I think was never resolved which could cause low volume. Originally you had measured across the high and low terminals of the volume control and found 5K. Then you disconnected the wires, measured again, and got 500K, which is the correct resistance of the control. Was the 5K discrepancy ever resolved? What does it measure across the control now, in-circuit?

One of the problems is that the draftsman got himself into a bind when drawing the area around the control, but instead of fixing the jam up he just kept on going. I have redrawn that area of the schematic to clarify. If you trace the wiring you'll see that there is nothing connected to the volume control that could drag down its 500K resistance (assuming resistors and caps are all good and not shorted.) Something may be miswired. Take it line by line and trace your wiring, double check that components are correct size and good.


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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 12, 2017 3:03 pm 
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I think that since I had the 75 tube in place, the control was still in-circuit, causing the resistance to be off. Should the resistance be correct when the control is in-circuit? I am begining to believe that something is up with the 75 tube. Plate measures 190v instead of 95v and the section of the volume control I disconnected went to the cathode of the tube. I have traced all of the wires leading to pin 2 of the 75 tube, and everything connected in some way to that pin has been replaced and is in-spec. I also want to note that all of the coils in the set measure OK.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 12, 2017 4:09 pm 
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I have four wires connected to the volume control. Two wires are on the high side and one of them goes to resistor 54. I believe the other one goes to one side of the secondary of the IF transformer.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 12, 2017 8:34 pm 
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Turns out the wire that I thought was connected to the secondary of the IF transformer was actually a ground connection, shorting the high side of the volume control to ground. When I removed the wire from the pot, the resistance across it read the appropriate 500k ohms. My only problem now is finding where that wire goes to because it probably provides the ground for something. It is connected to the same ground terminal as R57.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 12, 2017 8:53 pm 
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Location: Cleona, PA
With the set turned off and measuring resistance, whether the tube is in or out won't make any difference in this case. The grid and cathode are just sitting there isolated in a vacuum doing nothing. Let's forget the "extra wire" for a moment. Have you traced all the wires that I show and are they correct to my diagram? How does the set play now?

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sat 12, 2017 11:33 pm 
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Tracing all of the wires back, they are in the right places (except for the mystery wire). I have not tested the set because I believe that the mystery wire provides the ground for something, although I don't see any components that don't already have a ground. I think the wire was my problem and was shorting the high side of the volume control to ground and messing up my resistance. I don't know if completely removing the wire would harm the set. I think the wire is original and probably needed to make the radio operational.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sun 13, 2017 12:01 am 
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It was working with low volume before. You removed the short to ground. It wasn't a "power short" involving sparks. It was just a signal short of insignificant voltage. Now it should work fine. Who knows who may have been in there before and added things, or whatever that wire went to is now grounded by some other method. Go ahead and try it out.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sun 13, 2017 12:32 am 
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I also noticed that the police band coil may be wired incorrectly. Above is the schematic version, below is the way it was wired in the radio. I think the way it is now is original, since an original capacitor was installed in the same place as the new one is now. Maybe the ground wire was meant to be used by the coil, but it never got connected and was just loose.


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AB 450 Police Band Coil.pdf [22.88 KiB]
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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sun 13, 2017 1:12 am 
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I get quite a few radios that have been "got at", often several times over the devices existence, before it ended up here. Parts that fall out & wires without an owner & wrong parts, are never unusual. We all have Drongo's that get in over their head, mess it up & flip it, to save face.

Tampering is the first thing that you look for: not power. With a lost wire end, note if it is original and of the same type that was used originally. That obviously tells you if it has been messed with. The wire has a finite length and if it is not recycled, can only have been on a limited number of nodes. Sometimes, (if you are lucky,) a close examination of the solder (if any) will expose a node with the other half of the pattern, providing it has not been re-flowed.

At the end of the day with sets like that, you have take frustration out of the equation, be prepared to walk away & come back with a fresh approach, & check every bit of it bit by bit, against the schematic. On several occasions I have ended up, like has been done for you, redrawing it, sometimes entirely from scratch. Often I will colour code the wires on the drawing to help trace them. And put different coloured dots on the pins or nodes (circuit board cleaner usually removes). Putting a dot on the nodes checked, as they did in manufacture can also help.

This may look tedious, but often, that is the only path to success. Always take photos before you attack: Even if it is blatantly wrong.

Marc


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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sun 13, 2017 12:25 pm 
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"drongo"....?
Sounds like what some call a "knuckle dragger"

Electronics that have benefitted from the efforts of others can indeed be a nightmare. In same cases, we're motivated to ask "What were they thinking?" when the reality is often that they weren't.......

I'm working on a set now that was "restored" 20 years ago by someone that allegedly knew what they were doing. I have found at least one mod where the person's notes indicate the the circuit could not work as shown on the schematic. A few other discrepancies appears to be simple errors.

The worst situations arise from people who have an incomplete or erroneous view of what is supposed to be happening, and then do random experiments to try and realize that view.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Sun 13, 2017 12:50 pm 
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I believe we are on the same page here. When you have fixed a lot of radios, you see enough to easily fill a foolscap page. I have noted two swines of Autodyne's that were that, as they been had been modified and butchered by the usual incompetents & the only wiring diagrams, I was ever going to get, were the ones I got by tediously reverse engineering what I had. Then looking at others of its period & type, and in one case manufacturer. To get it back to as close as possible, to the original concept.

Drongo: A more acceptable way of putting grossly incompetent, etcetera : When unprintable adjectives are more applicable. :evil:

Marc


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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Mon 14, 2017 1:30 am 
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I have good news...IT WORKS :D . I just want to thank everyone who helped me in figuring out the problem; I probably wouldn't have been able to figure it out without you guys. Now it's time to tackle the cabinet restoration (tackle probably isn't the correct word, since I am almost done patching missing veneer and should be able to lacquer the cabinet by the end of the month.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Aug Mon 14, 2017 4:59 am 
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Excellent: Persistence often has its rewards. Asking for help is can achieve. Sets that fail to play ball are educational. You probably subliminally learned a lot about how it works & diagnosis with this one.

Now you see the correct repair order; You fix the chassis & burn test it, listening to it, whist you sort out the cabinet.

Marc


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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Sep Sat 23, 2017 8:55 pm 
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Okay, just a quick question: The radio is fine, but I think my candohm repair looks very sloppy. Can I just put a resistor in parallel with the bad section of the candohm to get it to the correct resistance, or is this a bad idea? All of the other sections in the candohm are fine.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Oct Mon 30, 2017 1:27 pm 
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Another question: the radio is working fine, but I was wondering if I should add an aux imput, because I was planning on selling the radio.

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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Oct Mon 30, 2017 2:30 pm 
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5Y4GT wrote:
Another question: the radio is working fine, but I was wondering if I should add an aux imput, because I was planning on selling the radio.


Aux input? For what purpose? It is what is it is! It's working: That's how it was built and that's how someone looking for an original working (albeit repaired) radio will want it. Be happy leave it alone: You may well devalue it.

Marc


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 Post subject: Re: American Bosch Model 450 Low Volume Issue
PostPosted: Dec Mon 04, 2017 10:36 pm 
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One final problem: When I replaced the missing volume control on this radio, I found that the shaft for the control was about 1/4 inch shorter than the other shafts, and thus the shaft would not stick out far enough when the radio sat in the cabinet. What should I do to fix this? I think that a shaft extender would be too big and would extend the shaft too far. What should I add to the shaft to make it the 1/4 inch longer that it needs to be? I don't want to buy a can of JB Weld just to use it for one problem.

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 Post subject: Re: American Bosch Model 450 Incorrect Shaft Length
PostPosted: Dec Tue 05, 2017 12:14 pm 
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I have deleted that error: Lots went scramble when, for about the sixth time this year the ISP's system crashed and was still not working 12hrs later, by then there was no data at all? There were several windows open at the time. Some of the mail in was also affected & I did do a couple of bulk send to recipients of the same email, & several of these did not get through? Who knows what went wrong?

I am now playing catch up.

With the shaft, I normally make my own shaft extenders (joy of having a lathe etc.). That depending on the situation involves cutting the shaft shorter, but leaving enough to get a socket onto. Most times I make a one piece one, but I save the off cut in case I choose to make a socket. I do this as I find some of the commercial ones frightfully expensive and not always what you want. "Jobbing" for one item, or very few, is a big time killer due in many cases the setup time.

Attached was cropped out of a bigger pic but should give an idea of an option. Its a one piece job

Marc


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