Halden, a good and most interestion shot!!

I guess you have pealed back the top layer of insulation, which we see at the top.

I guess it was taken more from the left side so the right looks a bit smaller?

I believe I can see the 6.3 at the left, then a 5V very close to the 6.3, but going right is seems there is a big gap before the other 5V, but it seems that there are large wire windings ?just below. Is that right? Or what about my “gap”.

Here is a bit of the Quoted text without the picture:

Rick, good to hear your furnace works in the summer, etc etc,

But Re the switch, what is the obvious problem now, and what is causing it? ?I can imagine many reasons, but perhaps it boils down to opening or closing or both, and just how many times it has been switched under load. ?Like tubes and caps, they only last so long working 12 hour shifts.?

So use the wall switch or some other switch, or just Marr/Marrette two wires together [with gloves on]. With the latter you can see up close what is going on.? While you are thinking about the “switch”, use your back-up brain to ask the transformer how it feels about the situation going on right next door. Maybe it cannot take this kind of life for long either. You know it thongs sometimes due to saturation causing magnetostriction, but insulation breakdown does not have a voice, just an eventual aroma when it is too late. ?So if the switch is bad, what are its ratings? Ya, all hidden from day 1 no doubt. ?Can it be changed?, likely a real tough project???. I still don’t know what it physically really is, let alone its electrical properties, nobody really knows I guess. ?

If the thing is bad, what kind of voltage is it sending to the transformer, during make and during break ? … I’m just asking!! Again likely

The “Quoted text” below is a fragment only

Halden, good information. I have not yet tried to understand the finals, bias etc, but I guess you are saying when going to STBY it takes a bit of time to unload the power draw, and it takes longer for the L26 to settle down to STBY current and /or “off” current

?

Re “You ask the very important question:? "does it?"?”?? I’m not sure about what you mean, however if it is about testing using the 6.3, I was meaning to test it going from STBY to OFF ?and/ or from MOX,,, ?with the normal pause. ?Depending on what you see you might shorten the pause, but don’t do it for me.

I can’t say anything about the finals, and since it is the transformer that fails,? and might be related to the “no pause” syndrome, my attention went that way.

It is not clear to me ?that this is a swinging choke or not, and if it is ?just what the rating means[my lack of education], so when the tube resistance is added, the 3 seconds might be shortened, but the choke seems to ?cause the dominant ?time delay

As an aside, in looking into power supplies, there is so much time/space spent on PS for old ?audio amps in radios etc etc that some ideas about PS get cast in stone, but it seems for Ham use and other two way stuff the PS needs to run for long periods mostly unloaded in standby, so some PS “rules” don’t apply here. ?

Hi Halden, I’m blue inserted in ????your original black

I noted your idea in the "Re-membered (new to me) HT-37" thread about turning the transmitter off suddenly, possibly leaving some current in L26 with no place to go.? At first I was hopeful that you had found the explanation for the note in the manual but became disappointed in that respect as I examined the circuit.? The top (from the perspective of the HT-37 manual's schematic) of L26 connects to C69 which would hold its voltage in the 800-1000 V range briefly as it begins to discharge at turn-off.? The bottom connects to the filament and the transformer winding powering it.? If that part of the circuit suddenly opens and current suddenly stops, that voltage would drop in order to pull more current into the inductor.? But that part of the circuit is still intact and can draw current from the secondary through the tube because it is still hot.?? That’s my way of thinking too?? ?Also, the current doesn't stop suddenly upon switching out of MOX.? The bias voltage doesn't change instantaneously, so there's time for the inductor current to change gradually.?? So a “pause” is needed !! ??????? ? Opening the primary by turning the switch off doesn't prevent current from flowing in the secondary. ?Agree! Sort-of,? But ?that winding by itself is an inductor but is coupled to the others by flux changes, and as long as it does not saturate, it couples/transforms voltage/current back to the primary etc ??so somewhat repeating…? ??And the voltage in the HHV winding attempts/does shoot ?negative, and couples/transforms to the primary which has just open-circuited, so the voltage on the windings is no longer controlled by the AC power source, and so the winding voltages might go anywhere. That is the basic jist, but not my whole story. ?

If my reasoning above is wrong, installing a 1.3 kV PIV diode from the bottom of L26 (cathode) to ground (anode) would prevent a voltage spike here from going much below 0V.? well, seems ok, but is that what is needed?? Does the voltage actually go negative ? and is that the condition that agitates the transformer failure area? It is too complex for me: when in the cycle does the switch begin to open?, and can/does it absorb [much]energy from the choke? That is a very tough one!? However as I think I said way back, perhaps the heaters/filaments? absorb enough energy to prevent HHV overvoltage, at least if the “pause” is there. Some time back I tried to suggest that monitoring the 6.3 voltage ?should give clues as to what is going on in the transformer at switch off. If that voltage just dies nicely to zero, my idea becomes hogwash, but any spikes in it means spikes elsewhere, so keep a watch, and maybe get some HHVprobes.

?

Today's measurements found:

L26 DC resistance is 157 ohms.? Measuring across a 1.0-ohm resistor, 10.4 mA flows in STBY and 42.0 mA flows in MOX mode.? Of this, 3.3 mA should flow through the bleeder resistors and a little bit might leak through the old capacitors.? This leaves a bit less than 20 mA each to bias the 6146s.? That might be a little too low. So STBY is ? of MOX current ?? ???Did you first set that minus 49 volt bias that I posted in the other thread?

With thanks to Russ for the idea, I installed a 5-ohm (at 25 C) inrush limiter (TDK-EPCOS, 8.5 mm diameter) marked "NTC 5.0".? After warm-up, it drops 0.79 V.? This should moderate the 49A pulses I occasionally get when turning the unit on with the same polarity as the remanence at the previous turn-off.? Next Mouser order, I'll probably buy an CL-60 or CL-70 for this.? Or I might install a second NTC 5.0 in series with the one I have.

Cheers,

Halden

Hi Paul, my emails inbox is quiet today.. so..

?

RE “Jim also mentioned the mechanics of the HT-37’s Operation switch. ?I’m glad someone finally mentioned it as a possible root (or contributing) cause of the transformer failure problem. ?

I can’t count the number of times I’ve mistakenly relied only on a schematics and have ignored physical attributes like cable routing and effects caused by wafer switch movement - the kind of things not easily recognized on a schematic diagram.?” ?

?

Some time back on another “topic” I was asking about that switch because I wondered about having the AC ON/OFF run thru a wafer. Finally from the schematic, it appeared that it must be a toggle switch buried within the wafer part, and driven by the same shaft somehow.?? The AC ON/OFF? part? if hard to find? using the Bama manuals? schematics, and the parts list is no help. Somebody must have looked at it? ?

?

Jim, Re your “I would think that one would have to see what the OPERATIONS switch is doing in those first three positions. Based on the manual this seem a most obvious ?place to begin.? Thanks to recent info posted, it seems that the finals are significantly, unloaded ?when in standby ??

?

Dave Mentioned part of this . here is the whole bit.

So perhaps on some units the bias is not minus 49 and leaves the current draw ?too low allowing the B++ volts to reach 1000. As has been reported.

Back to the pause; Their statement confirms that in standby the finals unload ?and we guess the B++ current slowly decreases in the choke, 5V4 and transformer,? which then allows a reasonable turn-off.? Reverse logic says the pause in standby is to allow the current/energy in the choke to subside.? ?As a reminder, when the transformer is powered by 117/60Hz, it controls the transformer voltages on all windings, but once the source is open, the choke has yet to discharge energy and will put out a lot of volts to do so, ?and the 5V4 connects it to the transformer HHV, so I wonder what takes place over the next little bit? ?

?