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Author Topic: Drake l-4b low plate volts  (Read 4691 times)
HS0ZJU
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Posts: 163




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« on: February 20, 2012, 07:41:01 AM »

My l4b is showing 2400 volts no load. I believe it should show 2600 volts. it has the harbach ps upgrade. what would be the proper trouble shooting steps.

73 marc hs0zju
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AD4U
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Posts: 2150




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« Reply #1 on: February 20, 2012, 08:20:34 AM »

Before you assume something is wrong........

What is the input (line) voltage?  Does it match the specified power transformer input voltage?

Check the HV metering resistors.  They may be out-of-spec (too high in resistance) due to age (40+ years old) and / or heat from operation.

Clean the HV metering selector switch contacts.

Do you have access to a Triplett 630 VOM or a similar meter that can read HV directly?  The Triplett can read up to 6000 volts.  If so compare the reading on the VOM to see if your HV is really low.

If all this fails you may have a bad PS diode or perhaps some PS caps need to be replaced.

Dick  AD4U
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HS0ZJU
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Posts: 163




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« Reply #2 on: February 20, 2012, 05:35:57 PM »

Hi Dick


The line voltage is 230volts and is stable.   I dont have a meter capable of testing the hv but may be able to borrow one. Another  issue/sympton i discovered today is the cabinet is hot (has voltage).  I will check the metering later today an post the results.....

73 marc  hs0zju
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W9PMZ
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« Reply #3 on: February 21, 2012, 02:32:19 AM »

What's the point?  According to the RAST website HF is limited to 200W...

Wish it wasn't so complicated to get a license to operate there.

73,

Carl - W9PMZ
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HS0ZJU
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Posts: 163




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« Reply #4 on: February 21, 2012, 06:53:31 AM »

What's the point?  According to the RAST website HF is limited to 200W...

Wish it wasn't so complicated to get a license to operate there.

73,

Carl - W9PMZ
hi carl

Luckily I hold a general class fcc license.....Being  you have a fcc license and are a US citizen then getting your thai license is easy......

Back to the drake.  The hot cabinet was caused by improper gnd. Now fixed. Still have to verify the plate voltage.....
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W9PMZ
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« Reply #5 on: February 21, 2012, 07:04:20 AM »

I go there a lot for business, to Laem Chabang.  Two weeks at a time.  Seems too complicated.

I might go before Sonkran for 3 months this time and I looked into the requirements.  Not having transportation is the issue...

Hope you get your amp fixed!

Regards,

Carl - W9PMZ
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VR2AX
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Posts: 575




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« Reply #6 on: February 21, 2012, 04:58:16 PM »

Marc,

The L-4/L-4B and companion pwer supply are designed for the now old fashioned "240V" supply voltage. Only with 240V would you get 2600+V off load.

In Hong Kong, line voltage is nominally 220V, but measured on my Fluke DVM, is usually slightly less, around 215-217V.

My HV shows 2400V approx off load. The power supply has been rebuilt. It is caused by the low supply voltage.

Low supply voltage also means the tube filaments have less voltage, so emmission is rduced.

73s,

Wyn
VR2AX

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W1QJ
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« Reply #7 on: February 21, 2012, 05:20:45 PM »

Wyn, did you ever get the 6M L4B going?  Lou
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VR2AX
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« Reply #8 on: February 21, 2012, 10:08:30 PM »

Lou, I tested it last summer briefly and it worked fine. I have been working in XX9 since July and shipped all my gear to the UK from HK. If I get a license here I guess I will have to ship it back out ;-) Wyn
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W1QJ
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« Reply #9 on: February 22, 2012, 05:01:56 AM »

OK Great, good luck to you!  Nice to see you in here on discussions. Lou
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W8JI
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« Reply #10 on: February 22, 2012, 05:10:28 AM »

My l4b is showing 2400 volts no load. I believe it should show 2600 volts. it has the harbach ps upgrade. what would be the proper trouble shooting steps.

73 marc hs0zju

Usually the most common problem is the meter multipliers have gone off value. High resistance carbon film resistors are notoriously unstable with age.

It would be a shame to spend hundreds of dollars to repair no fault.

I would never, not in my wildest moments, suggest someone measure HV over 1-2 kV with the cover removed with a hand held or desk VOM. First, you have no idea how good the meter is unless it is new or calibated. Second, it isn't safe.

A better idea is to compare the meter system resistors to new precision resistors, and see if they are off. The meter can be checked for current against a known good current meter with a small safe supply. If the multiplier resistors are right, and the meter correct, you know the high voltage reading is correct.

Nearly all of the time a discrepancey between HV and line voltage is caused by meter multiplier resistors. Filter caps rarely show as idle voltage errors. They show as hum and poor regulation, if low in capacitance. They show as popping or flicks in the meter if breaking down from voltage.

I can't think of many good reasons to poke around inside a live amp with a meter.

73 Tom   
« Last Edit: February 22, 2012, 05:19:18 AM by W8JI » Logged
AD4U
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« Reply #11 on: February 22, 2012, 06:05:35 AM »

One very good reason I can think of to go poking around inside a tube type amp with a suitable VOM is to measure the HV.  I have done this countless times with my Triplett 630.  I did it a couple of days ago when trying to diagnose a problem with an AL80B amp.  

Of course I do not recommend that anybody who does not know what he is doing go poking around inside an amp using a $9.99 VOM with leads that have thin plastic insulation that may not safely handle 120V.

I, like many who post on this site, have built and repaired amps.  A person with the necessary knowledge and experience who is PROPERLY using a Triplett 630 (or similar VOM) that is designed to measure up to 6KV and that has leads with insulation that will safely handle 6KV is not unsafe.

With the amp OFF and the HV discharged connect the (+) lead to the tube anode with an alligator clip.  Connect the (-) lead to the amp chassis with another alligator clip.  Put the VOM selector switch in the 6KV position.  Set the VOM on the bench.  Do not hold it in your hand when making the measurement.  Turn the amp ON and read the HV on the VOM.  Turn the amp OFF and watch the HV bleed off to zero on the VOM.  As a precaution use a "chicken stick" or similar device and ground the tube anode after the VOM reads zero.  Disconnect the VOM leads from the amp.

If one lacks the knowledge and experience, I would not recommend that he try to measure the HV inside an amp nor would I recommend that he try to fly an airplane.  After years of experience, I have the knowledge and experience to do both.

Dick  AD4U

« Last Edit: February 22, 2012, 02:39:01 PM by AD4U » Logged
W8JI
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« Reply #12 on: February 22, 2012, 05:30:08 PM »

I work on amps almost every day, or at least weekly. I can't imagine poking around inside with a normal meter as routine troubleshooting.

The very few times I need to measure high voltages, which amounts to once a year at the most, I use a special probe. It keeps my hand at least a foot from the amplifier, and I try to use special covers.

Nearly all the time, an issue can be fully resolved without poking around with HV on. In the case of a few hundred volts low meter reading, when the primary voltage is good and nothing is humming or smoking, and voltage sag is reasonable, the problem is HV multiplier resistors.

I think it is terrible advice to tell people to measure things inside a PA with HV up, except as a very last resort.

73 Tom

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G3RZP
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« Reply #13 on: February 23, 2012, 12:42:07 AM »

Measuring the HV can be done safely. BUT it requires the correct equipment and safety precautions AND a very good knowledge of what you are doing AND experience.

Otherwise it can be a very good way for the XYL to collect the life insurance.

I learnt how to do it in the high power transmitter development lab (txs up to 750 kW of carrier, high level modulated) at Marconi's some 45 years ago.
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K2LGO
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« Reply #14 on: February 23, 2012, 04:19:16 AM »

Thank you W8JI for your missive about needless checking of HV inside an amp...Point well taken...I have done it in the past, but after reading your advice, I will post a CEASE and DESIST order when it comes to that practice....BOB
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