[TenTec] Titan II Bias issue

[Dukes HiFi]

The schematic is not wrong.

The circuit is not a full wave rectifier, it is a voltage doubler.

That is why you see 140 DC from a 70m VAC transformer.

Gary


> On Mar 25, 2017, at 7:28 PM, kr7o at vhfdx.com wrote:
> 
> Hi Jim,
> 
> Thank you for the reply.  The amp appears to be working now.  Replaced two failed transistors on the screen board which were biasing the tube on all the time.  Also changed the bias voltage jumper to get the idle current within spec.
> 
> The amp tripped once while doing test tune-ups on each of the bands, so hopefully the tube is not gassy.  I will run it gently for a while until I am confident in the tube and repairs.
> 
> The transformer secondary for the bias circuit is 70vac, so there appears to be a typo in the manual regarding -130v on RX.
> 
> 
> At 04:12 AM 3/23/2017 +0000, you wrote:
>> Hello OM,
>> 
>>  Not familiar with the Titan II, but when my Titan 1 425 failed with
>> the exact same symptoms, the B- bias diode
>> 
>> in the power supply "opened up" and was not supplying bias to the
>> tubes.. Chances are that diode is "open".
>> 
>> 73, Jim W3KA
>> 
>> On 2017-03-21 05:27, kr7o at vhfdx.com wrote:
>> 
>> > I am repairing an amp that was from a friend's estate and have fixed several issues, but now when I switch the amp to Operate, considerable heat is coming out of the exhaust, so it appears the tube is partially biased on all the time.  I have loaded the amp many times up to about 1kw output, so I am reasonably confident that the tube is OK.
>> >
>> > According to the manual, the cutoff voltage is supposed to be -130v, but with 70vac on the primary into a half wave bridge, I cannot understand where they are getting -130v from.  My bias voltage is only -80v in RX.  I get up to -95v with the bridge unloaded, which is close to what would be expected from a half wave bridge.  The screen voltage supply appears to be functioning normally.
>> >
>> > I will admit, I am not much of a tech, but I DO know how to work around high voltage and avoid exposed plate voltage unless absolutely necessary.  I have gotten this far, and would love to get this amp running if anyone has experience with this amp and does not mind helping me sort this out.  I have more history and measurements, but I will spare the list.
>> >
>> > Thanks,
>> >
>> > 73, Robert KR7O/YB2ARO, DM07ba/OI52ee  (ex.  N7STU)
>> > DXCC  6M #1122
>> > WAZ  6M #112
>> > VUCC  6M, 2M
>> >
>> > www.vhfdx.com [1] (KR7O/YB2ARO pages)
>> > Looking for old VHF QSL cards and logbooks.
>> >
>> > _______________________________________________
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>> > TenTec at contesting.com
>> > http://lists.contesting.com/mailman/listinfo/tentec
>> 
>> 
>> Links:
>> ------
>> [1] http://www.vhfdx.com
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> 
> 73, Robert KR7O/YB2ARO, DM07ba/OI52ee  (ex.  N7STU)
> DXCC  6M #1122
> WAZ  6M #112
> VUCC  6M, 2M
> 
> www.vhfdx.com (KR7O/YB2ARO pages)
> Looking for old VHF QSL cards and logbooks.
> 
> 
> _______________________________________________
> TenTec mailing list
> TenTec at contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec