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Bogen chb35a and Bogen chb100

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  • #16
    Nope, I am assuming the CT is still connected to ground.. I am assuming "all the wires" from the circuit board were disconnected but that one escaped. It is extremely rare to get a frame short on the heaters.
    Education is what you're left with after you have forgotten what you have learned.

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    • #17
      No g1, it was all disconnected. I just got the OT transformer pulled out too. I'm going to check it with my DMM in a little while. I gotta stop for dinner and a walk but I'll get back to it in a few hours. I'll go back over the PT tonight to be sure I measured everything correctly.

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      • #18
        First, I think the confusion here around the CT could be due to me referring to CT as "ground".

        To be clear, ALL wires from the power transformer are unsoldered and disconnected from the curcuitry, even the CT. When I reported measuring .4ohms to ground from the heaters, what I meant was .4ohm to CT. AND .4ohms between the heaters themselves.

        Seems right to me but I'm obviously a novice. So feel free to confirm or deny.

        Here is the schematic that I'm working from in case you want a reference.

        https://imgur.com/gallery/yBKP2vk

        A few notes on the OT and how it differs from the schematic:

        1.The wire colors on the OT are almost completely different from the schematic. I know it's no biggy but just wanted to state that fact in case of future confusion.
        2. The 70v 80ohm leads are on the primary side of the transformer instead of on the secondary side as the schematic shows. Also schematic shows 80ohms and I'm only measuring 7.6 ohms. Is the discrepancy due to no power being applied to the transformer?

        The two wires referred to as Blue and Brown on the schematic's primary side measure 46.5ohms. Maybe it is worth noting that in real life these 2 wires are both brown so I don't know which is supposed to be Blue and which is supposed to be Brown. Anyways, the 465v Red (actually green) wire to what I am going to call the Blue wire measures 84.9 ohms. 465v Red wire to the brown wires measure 38.9 ohms. This seems off but again novice here.

        The rest of the wires on the secondary all have 0 resistance.

        I also measured all primary wires to all secondary and there is no continuity between the two sides. Likewise, there is no continuity between the 70v 80ohm and the winding for speaker outs.



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        • #19
          OK, the heater wwindings are low number of turns and heavy wire, it is normal for them to measure extremely low resistance. Your meter does not have the resolution to make these measurements.

          1.The wire colors on the OT are almost completely different from the schematic. I know it's no biggy but just wanted to state that fact in case of future confusion.
          A lot of times the wires age and the colors fade on the wires. If we take the end bells off teh transformer we might see that looked like orange or brown was really red. That sort of thing. I don't know that is happening for you, but consider it.

          2. The 70v 80ohm leads are on the primary side of the transformer instead of on the secondary side as the schematic shows. Also schematic shows 80ohms and I'm only measuring 7.6 ohms. Is the discrepancy due to no power being applied to the transformer?
          Schematics show electrical relationships,not mechanical ones. There is only one primary, the red, blue, and brown wires on the left on the drawing. The speaker side windings are all secondaries. The physical transformer has no primary and secondary "sides". it doesn't matter which hole a wire comes out or which side of the thing a wire leaves, that has nothing to do with primary/secondary. The primary is where the energy goes into a transformer, the secondaries are where it comes out. That has nothing to do with where the wires are in the chassis.

          Your ohm meter measures resistance, not impedance. The 70v winding is for distributing sound to multiple speakers, like in the ceiling of a restaurant or the halls of an airport. If you are interested, look up "constant voltage speakers" and the specific example here - "70.7 volt speakers". But for reference, it also has an impedance and that is the 80 ohms. We rarely use that figure. But 80 ohms is the impedance, while your7.6 ohms is merely the resistance of the wire. The low impedance speaker connections will normally read very low resistance. All that looks normal.

          The two wires referred to as Blue and Brown on the schematic's primary side measure 46.5ohms. Maybe it is worth noting that in real life these 2 wires are both brown so I don't know which is supposed to be Blue and which is supposed to be Brown. Anyways, the 465v Red (actually green) wire to what I am going to call the Blue wire measures 84.9 ohms. 465v Red wire to the brown wires measure 38.9 ohms. This seems off but again novice here.
          The center tap of the primary is red on schematic, whatever color you have, it must connect to the 465v node in the power supply. To put it simply, the wire goes over to a filter cap. The other two wires do not. The blue and brown wires - or whatever - each go to pin 9 of a power tube socket. Look at your schematic. No doubt they were wired correctly originally, so don't change them.

          Can we work on terms here? Resistance is always between two points. A wire doesn't have a resistance - unless you mean from one end to the other. It has a resistance to ground, or a resistance to another wire. SO brown#1 to green is XXX ohms and brown #2 to green is ZZZ ohms. And brown #1 to brown #2 is YYY ohns. That measures the resistance - not impedance - of the two halves of the primary and of the whole winding. It is normal for some transformers to measure the two halfs differently.
          Education is what you're left with after you have forgotten what you have learned.

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          • #20
            Originally posted by Enzo View Post
            The center tap of the primary is red on schematic, whatever color you have, it must connect to the 465v node in the power supply. To put it simply, the wire goes over to a filter cap. The other two wires do not. The blue and brown wires - or whatever - each go to pin 9 of a power tube socket. Look at your schematic. No doubt they were wired correctly originally, so don't change them.

            Can we work on terms here? Resistance is always between two points. A wire doesn't have a resistance - unless you mean from one end to the other. It has a resistance to ground, or a resistance to another wire. SO brown#1 to green is XXX ohms and brown #2 to green is ZZZ ohms. And brown #1 to brown #2 is YYY ohns. That measures the resistance - not impedance - of the two halves of the primary and of the whole winding. It is normal for some transformers to measure the two halfs differently.

            Thanks for being patient with me. I can tell that you're under the impression that I'm restoring this amp to stock. At this stage I am just determining what is salvageable.

            In my first post I made rough note of the condition of the amp. I pointed out that the previous owner made modifications, changed some components and also had a small campfire inside the amp at some point....There is black soot under the chassis, the tubes themselves were visually burnt, etc.... The transformer has potentially been changed once already. This could explain the off-color transformer wires or it could be that this schematic is for a different revision of the circuit.

            Thus far I've only been trying to determine if I have viable transformers to work with. The reason I've been measuring resistance of wires from one end to the other and from primary to secondary is to check for shorts; Do I have the wrong idea here? From what I understand I'm checking for any resistance across a winding or any continuity from one winding to the next which would indicate an internal short, correct?




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            • #21
              Oh perfectly valid to check for shorts between windings, and sure, check windings themselves - mainly for opens.

              Even if you won't be leaving teh transformer in teh chassis, if you know which wires went to the tube sockets, you would know whuch equated to the blue and brown ones.

              Red, blue, brown is an AMerican color scheme, other countries don't alwats use that, MArshall comes to mind.
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                Status update:

                So the output transformer is completely removed from the amp.

                I made note of the leads and center tap on the primary side.

                On the secondary I have positively identified the 2 leads for the 70v 80ohm winding. The other secondary winding which provides taps for 2ohm 25v, 4ohm, 8ohm 25cvt, 16ohm and common has been identified however, due to the taps color being inconsistent with the schematic and due to it not being wired as the schemtaic shows I am unsure which of these 5 taps is which.
                .............

                I would just like to reconfirm the resistance on the primary side is what one should expect:

                The Blue and Brown wires of the primary measure 46.5ohms of resistance. Blue to Red CT measures 84.9 of resistance. Brown to Red CT measures 38.9 ohm of resistance. I thought this is the kind of resistance I was checking for?

                Not trying to discount Enzo, who has been extremely helpful, confirming this resistance earlier but there was some miscommunication between us and I'd prefer to err on the side of caution.

                .......

                Aside from the transformers and tube sockets, are there any other components that I should consider salvaging?
                Last edited by DŁTerra; 10-13-2020, 01:03 AM.

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                • #23
                  Could someone be so kind as to upload the pic from post #18 ? I can't see Imgur pics for some reason. Not here or anywhere else.

                  Maybe I'm not following well but this all seems very confusing. Up in post #18 you said the colors are not what you are 'calling' them. Just call them what you see. You are trying to figure out what they are, so there is no room for assumptions. If there are 2 wires that look the same (2 browns), put some tape on them and call one B-1 and the other B-2.
                  Also, I hope you kept track of where they got disconnected from. It would help to tell us that too, and any pictures you may have taken. Are there any numbers on this transformer?
                  From the description you gave of the OT primary, the 84.9 ohms is across the full winding. The brown is sitting at approx. middle of resistance so it would be the C.T.
                  "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                  • #24
                    Click image for larger version

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                    • #25
                      I've got all the wires identified. I was referring to the wires by the color on the schematic for you folks following along.

                      Are these components likely to all be trash or should I attempt to salvage some?

                      I didn't take a lot of photos but this is the amp in the shape in which it was received before I began working on it.


                      Click image for larger version  Name:	20201013_040901.jpg Views:	0 Size:	2.13 MB ID:	915715

                      Poor thang has the black lung


                      Click image for larger version  Name:	20201013_040608.jpg Views:	0 Size:	4.57 MB ID:	915714
                      I guess it got happy when it saw my hot soldering iron.
                      Last edited by DŁTerra; 10-13-2020, 09:45 AM.

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                      • #26
                        Here is the OT Click image for larger version

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Views:	82
Size:	1.34 MB
ID:	915718

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