[Amps] L7 problems

Alek Petkovic vk6apk at bigpond.com
Sat Dec 10 19:09:59 EST 2016 

If its simple to disconnect all the wires on T1, then a megger will tell 
you if it is sick. Check between windings and frame, etc.

Solid state ones are cheap as chips on ebay and hand wound ones are 
usually even cheaper at flea markets.

73, Alek.

On 11/12/2016 2:03 AM, Paul Kraemer wrote:
> There are no feedthroughs on an L7, just disc ceramics
> Paul
>
> -----Original Message----- From: Alek Petkovic Sent: Friday, December 
> 09, 2016 8:36 PM To: AMPS Subject: Re: [Amps] L7 problems
> Did you check the filament feedthroughs like the other Jim suggested?
>
> 73, Alek. VK6APK
>
> On 10/12/2016 10:10 AM, Paul Kraemer wrote:
>> Jim
>> Good to hear from you.  I was hoping you would have comments.
>>
>> I had already made the change on the keying going to a 100K 5w and 
>> series string of 1N5408 in ct for bias.  The relay shorts the 
>> resistor.  Just like my ES4L4B kit includes with the soft start and 
>> soft key.  I also don't like putting the 120v on the ct and running 
>> zero bias on keyed.
>>
>> I had also done the diodes HV- to gnd and the one across the grid meter.
>>
>> Of course I made sure along the way the changes didn't make any 
>> difference to the problem
>>
>> Interlock was checked first off and has a stiff bit of pvc cable 
>> jacket in there to keep it defeated while I work on it.
>>
>> Feel like only thing left in the equation is something is wrong with 
>> that transformer.  I have some lv high amp industrial transformers 
>> that with a little adjust from a variac I can run at least one tube 
>> on the bench and I'll disconnect primary and secondary of the Drake 
>> transformer.  Don't need the relay power just to test so that should 
>> say yay or nay and I can send the transformer to Heyboer for rewind 
>> if it shows that IS the culprit.  I'll let everyone know how it works 
>> out.
>>
>> Any notes or history would be appreciated
>>
>> Paul K0UYA
>>
>>
>> -----Original Message----- From: Jim Thomson
>> Sent: Friday, December 09, 2016 12:05 AM
>> To: amps at contesting.com
>> Subject: [Amps] L7 problems
>>
>> Date: Thu, 8 Dec 2016 20:52:20 -0600
>> From: "Paul Kraemer" <elespe at lisco.com>
>> To: <amps at contesting.com>
>> Subject: [Amps] L7 problems
>>
>> Group
>> I have an L7 that was sent to me for repair and I am getting nowhere 
>> with it.
>>
>> The problem is the grid meter has a small negative reading in standby 
>> even with NO tubes installed.
>> If the antenna relay is keyed the reading goes to zero.  The amp does 
>> work if you ignore that pesky off scale negative reading in standby.  
>> The amp does same thing on three different supplies so it is NOT the 
>> power supply. I have an L7 of my own that is fine on the supplies so 
>> it is the L7 at fault.
>>
>> Here is an A/B comparison of the bad amp (customers) and a good one 
>> (mine)
>>
>> The ONLY things I have been able to observe between bad and good is:
>> (1) Bad the center tap of the filament winding is off center by about
>> 0.2vac.  In other words, it should be the center and instead measures 
>> like
>> 2.6vac one side to center and 2.8vac other side to center whereas 
>> good will
>> measure EXACTLY the same either side to center
>> (2) Bad there is a measureable 25mv (dc plus 60hz)to ground on the 
>> negative connection
>> of the HV supply.  Since that voltage is across a 1 ohm resistor it 
>> makes
>> the amount of negative grid current.  If the amp is keyed the voltage 
>> goes
>> nearly to zero as it should be all the time.  Good amp is ZERO all 
>> the time.
>> Same supply, same location, just swap the linear.
>>
>> I hope someone can give some insight to this.
>>
>> Thanks
>>
>> Paul K0YTA
>>
>> ##  I had the exact identical problem on a hb  4-1000 GG amp years 
>> ago.  Neg grid current on RX..... then rises to zero when u hit the 
>> PTT  (no drive applied).
>> With drive applied, it operated normally.  Lemme dig out my old notes 
>> as see if I can find it.  But it only happens when I cranked the B+  
>> way up to 5600 no load..on RX.
>> I also own 4 x drake L4Bs...with mating 4 x L4PS power supplies. L4PS 
>> is identical to the L7PS.   I have not had the neg grid current issue 
>> on the drake amps.
>> I did have one of them with a fil to cathode short..that resulted in 
>> idle current on RX.  One bad tube, thats it.
>>
>> ##  IF the CT is not dead center on the fil xfmr sec, you will hear 
>> 60 hz hum over the air.  A buddy re wound a fil xfmr for a hb 2 x 
>> 3-500Z amp. The loaded voltage was a
>> bit high, so he took turns off of one end only. Voltage was back down 
>> to 5.0 vac..... but the CT  was no longer dead center, but 
>> offset....since he removed turns from ONE end
>> only.   You could hear the slight bit of hum over the air.... but in 
>> his case, it was a lot more off, vs  ur .2 vac offset.
>>
>> ##  I had another buddy with the neg grid current issue on RX...on a 
>> drake amp, dont recall if it was an L4B or L7.  Again I will attempt 
>> to dig up notes.
>> Check the B+ interlock circuit.  On the L4B...with top lid removed, 
>> it just doesnt short the B+ to the chassis.  It also shorts the B- to 
>> the chassis..and does that
>> 1st, a split second b4 the B+ gets shorted to chassis.  You can see 
>> that on the spring loaded plunger rod assy.  Rls it by hand in slow 
>> motion... and you will there is
>> 2 x sets of contacts.... with the lower set grnding the B- to 
>> chassis...then followed by grnd the B+ to chassis....last.
>>
>> ##  On my l4bs.... I tossed the pair of 50 watt bleeder resistors in 
>> the HV supply.  The no load B+ didnt budge...2650 on RX.   2500 under 
>> load of 800 ma.
>> Ok, oem deal was to use a 7k 5 watt  resistor in series with the 
>> former bleeders.  The V drop across the 7k was + 130 vdc..used to cut 
>> the tubes off.  On CW position,
>> its  +90 vdc to cut the tubes off.  Its a dumb fubar method imo, 
>> since the CT goes open when TR 3PDT relay is in mid air.    With big 
>> bleeders removed, you no longer have any
>> 90/130 vdc for cut off bias.   No problem just use a 100k at 3 watt 
>> mof..or a pair in parallel for redundancy... in the ct lead.   Then 
>> re wire the center tr relay contacts so they
>> just short out the 100 k..when on TX.
>>
>> ##  RVS  connected 1N5408 / 6A10s  wired between B-  and chassis.   
>> Then the meters are protected.  B- cant wander more than  +/- .7 vdc.
>>
>> stay tuned.
>>
>> Jim   VE7RF
>>
>>
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>

-- 
 From sunny Binningup
Our new house.
https://17coronation.wordpress.com


Family Businesses:
SP Electrical. www.spelectrical.net.au
The Velvet Fox. www.facebook.com/TheVelvetFox
Smart Design Electrical. 0417 602 032
Perth Interiors. https://www.instagram.com/perthinteriors/


---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

More information about the Amps mailing list