[Central-Electronics] Access to 100V 6550 Final Tubes Under Socket Wiring

[Whiteheadg]

A snap and burning smell is usually the symptoms of a capacitor going out. Failing electrolytics behave like that and the rig may have been upgraded with some tantalum caps. Have you actually measured the finals 600 V B+, the lower 340 VB+ voltage for other tubes, and the -120 V bias voltages. If it starts to smoke again whenever you turn it on you've got significant current flowing in the failed component which could drag down one of these power supply voltages, whichever it is, and overheat the transformer or a filter choke causing it to fail. Don't let that happen! Be very careful trying to measure the either B+ voltage.  The 600 V B+ for the 6550, and the 340 V B+ for other tubes can be fatal. Note that filter caps for the 600 V supply are two, 250 mf caps in series at the output of the filter choke. The top one in this series pair has its can covered with a black cardboard insulator because the outer can of that electrolytic is hot at 600 V being connected to the + terminal of the lower one of the pair whose can is grounded. Each cap in the series pair is rated at 450V,  so the total capacitance 125 mf with the pair tolerating the higher applied 600 V. Each is bypassed with a 100k, 2W resistor as bleeders and to balance the voltage across each.
 It would be helpful if you could describe in more detail what trouble shooting fault isolation procedures and measurements you have done so far. 73,George, W1BOF
In a message dated 1/6/2025 4:12:20 PM Eastern Standard Time, aj1g at sbcglobal.net writes: 
Thanks for the input!  Don’t believe it to be a HV problem, IIRC the HV power is not on with the rig in standby, but I might be mistaken.  Failure occurred out of nowhere while in standby and I was my self was standing by the rig when it happened and I heard the snap which was immediately followed by smoke and burnt resistor type smell.  Shut the set off and then briefly turned it back on, smell and smoke stared up as soon as the LV B plus rectifier heated up, turned radio off immediately.  Next step - resistance checks.  Manual has a comprehensive table of reference values thankfully.
Sent from my iPhone

> On Jan 6, 2025, at 15:56, JAMES D COMSTOCK <comnoz2 at comcast.net> wrote:
> 
> Chris,
> After 50 years or so I have not seen problems under the 6550 sockets. The toughest to get to was the choke near the bottom of the tubes in the PA section that burned when a tube shorted.
> A common problem that would produce the symptoms you have would be a failed connector between the RF and audio/ps sections. The high voltage supply pin will eventually arc through to the metal around the connector. Both 100V's that I own have had this connector bypassed with a solid wire through the hole left after removing the pin. Jim k0jjf
> 
> 
>> On 01/05/2025 3:58 PM MST Christopher Bowne <aj1g at sbcglobal.net> wrote:
>> 
>> 
>> I’m currently chasing an apparent low voltage B plus driven short somewhere in the vicinity of the 6550s shielded amplifier cage that suddenly  developed with a loud snap with the set in standby and caused some comment to pull a lot of current and emit smoke and nasty burnt component odor.  Have not seen any thing yet and will be doing resistance checks shortly (no pun  intended).  Noticed during visual checks that the 6550s appear to be mounted an elevated sub chassis plate above the main chassis inside the shield cage.  How does one gain access to the underside of this subchassis plate? From what I can see from below the main chassis, any wiring to the 6550s sockets passes up through small holes in the main chassis.  I’m assuming that there may be screen bypass caps hiding up there, pretty sure they would not be hit with any voltage in standby though.
>> 
>> Would like to get the 100V back on the air soon.  I bought it and another one from the estate of Andy WA2CDL two years ago.  He in turn had previously acquired them from the “estate” of another Long Island SK ham.  I say”estate” because Andy saved them and a lot of other equipment from being dumpsterized by the previous owner’s family.  Anyone here on the list know who that owner might have been?
>> 
>> Also, how much traffic is there on this list Nick? I receive it in digest format and only receive postings a few times a year.
>> 
>> 73 de Chris AJ1G Stonington CT
>> Sent from my iPhone
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