View Full Version : L4b problem
W1DRF
05-06-2011, 06:11 PM
Keep blowing res. R12 notice one tube poping a blue arc on last resistor do i need tubes or is there somthing in the power supply causing this ?
This happens at no drive with amp hooked to dummy load and amp set to all tuning positions.Amp was stored for 10 years.
Help please.
WB2WIK
05-06-2011, 06:22 PM
Likely a bad tube. I assume if you're blowing R12 that means you have the front panel meter switch in the "HV" position, to read plate voltage, as I think it's bypassed entirely in the other positions.
KC8VWM
05-06-2011, 06:50 PM
Your question was do I need new tubes or is it something in the power supply?
The blue flash of death you experienced is common in certain amplifiers stored for a long period of time. In your case it's probably because the 3-500z tube in particular doesn't hold up very well in long term storage. (Amp was stored for 10 years as you say) The tube seals leak, tubes becomes gassy and are very well known to fail and blow stuff up in your power supply as your situation described.:
Edit: I should add that working on amplifiers is extremely dangerous and there are lethal voltages involved even if the amplifier isn't even turned on and/or plugged in. !!!
CAUTION:
You should be entirely familiar with all the required special handling, disassembly and de energizing procedures before starting work on any amplifier. If you are not specifically familiar with these very specific procedures, DO NOT PROCEED because death is likely.
W1DRF
05-06-2011, 06:58 PM
Likely a bad tube. I assume if you're blowing R12 that means you have the front panel meter switch in the "HV" position, to read plate voltage, as I think it's bypassed entirely in the other positions.
NO it was in 3000 watts fwd position.
W1DRF
05-06-2011, 07:04 PM
When i got the amp the r12 was blown in the power supply the previous onwer had replaced 4 of the caps and it blew the r12 when he power it up he then stuck in in the closet till now this amp,is very clean almost as new and in orginal drake boxes.
Thanks for reply.
KC8VWM
05-06-2011, 07:19 PM
How new or clean the amplifer physically appears has absolutely no bearing on it's electrical status nor does it determine if the seals of the tubes are leaking. It's even possible the previously replaced caps are incorrect values causing other parts to blow out in the power supply. It's even entirely possible these caps have dried out during long term storage and become defective for that matter. Just because they were previously replaced, doesn't mean they should be ruled out of the problem causing equasion.
KC8VWM
05-06-2011, 07:33 PM
Replace R12 and remove all the tubes and power it back up with no tubes inside.
Make notes of your meter readings. Come back and tell us your observations.
W1DRF
05-06-2011, 08:22 PM
OK i removed tubes and it took out r12 as soon as i turned it on
Thanks
WB2WIK
05-06-2011, 09:45 PM
That doesn't make sense to me, unless your amp is different from the schematic here: http://wb4hfn.com/DRAKE/DrakeManuals/L4B/Manual_L4B.htm
In this schematic, R12 is a 100K metering resistor for the HV meter only and isn't in the circuit at all in any other meter switch position (according to the diagram, both ends would be grounded in every switch position except "high voltage").
It's in series with 20 Megs of multiplier resistors (R6, R7, R9, R10) and is the ground return for that metering string.
Are we talking about the same "R12?"
Oh WAIT!
I'll bet you're talking about the other R12, which is inside the power supply (0.82 Ohms in series with the B+).
If that's the case, that's like a fuse and if it blows it indicates a fault, probably a short to ground, inside the RF deck.
W1DRF
05-06-2011, 10:23 PM
That doesn't make sense to me, unless your amp is different from the schematic here: http://wb4hfn.com/DRAKE/DrakeManuals/L4B/Manual_L4B.htm
In this schematic, R12 is a 100K metering resistor for the HV meter only and isn't in the circuit at all in any other meter switch position (according to the diagram, both ends would be grounded in every switch position except "high voltage").
It's in series with 20 Megs of multiplier resistors (R6, R7, R9, R10) and is the ground return for that metering string.
Are we talking about the same "R12?"
Oh WAIT!
I'll bet you're talking about the other R12, which is inside the power supply (0.82 Ohms in series with the B+).
If that's the case, that's like a fuse and if it blows it indicates a fault, probably a short to ground, inside the RF deck.
Sorry it is the one in the power supply.
W1DRF
05-06-2011, 10:35 PM
Would this res. smoke if the case interlock switch was faulty,i checked hv input to ground with cover in place no tubes and it showed grounded removed cover on amp and checked switch still grounded moved stem around on switch and it went open if i turn it it goes froim grounded to open.
WB2WIK
05-06-2011, 10:51 PM
Would this res. smoke if the case interlock switch was faulty,i checked hv input to ground with cover in place no tubes and it showed grounded removed cover on amp and checked switch still grounded moved stem around on switch and it went open if i turn it it goes froim grounded to open.
Of course it would.
Actually the interlock, if shorted, should blow the 15A circuit breaker in the power supply; however the .82 Ohm resistor may well be a "weak link" and blow first.
It is the weak link usually, so far as smoke goes it is really full to the top.
WB2WIK
05-06-2011, 11:51 PM
It is the weak link usually, so far as smoke goes it is really full to the top.
Yep.
I don't remember (it's been 20 years or more since I owned an L-4B) what the HV interlock was. Was it a spring loaded plunger that shorts against the chassis if the cover's removed?
W1DRF
05-06-2011, 11:53 PM
Update I moved amp and something rattled found one of the front panel spacers it has been moving around inside amp i wonder if it has been there since assy at drake as far as i can tell front has never been off but one of the spacers was missing didnt notice it till i found one inside, amp powers up now and all meter reading are zero with no tubes.
WB2WIK
05-07-2011, 08:18 PM
Update I moved amp and something rattled found one of the front panel spacers it has been moving around inside amp i wonder if it has been there since assy at drake as far as i can tell front has never been off but one of the spacers was missing didnt notice it till i found one inside, amp powers up now and all meter reading are zero with no tubes.
The meter readings should be zero except for the high voltage...that should read normally upscale to indicate the HV. Doesn't it do that??
KC8VWM
05-07-2011, 09:21 PM
Getting back to the previous cap replacement. Can you confirm if the replacement caps are of the correct specification according to the schematic and polarity installed properly etc.?
Rationale:
The original owner replaced the caps, it blew R12, he gave up working on the amplifier and so it was stored for 10 years. So do I understand correctly that it was never confirmed the replacement caps ever worked inside this amplifier?
Questions to ask: Why did he replace the caps in the first place? Was there other suspected problems occuring at the time? What was happening that lead to replacing the caps?
..Was it just preventative maintenance?
..Was he rebuilding it?
For example, if he was rebuilding it, the rattling parts inside suggest he had other things apart inside? ...If so, did he change or replace anything else inside that could be causing the problem? Did he rewire something the wrong way when putting it back together to cause a short circuit issue? etc.. etc...
We need to confirm Steve's question before proceeding further. Is there no HV reading on the meter? Next we need to determine why.
WB2WIK
05-07-2011, 11:20 PM
The caps precede R12 in the circuit, so even terrible caps wouldn't blow R12. A short in the RF deck would.
Bad caps would blow the 15A breaker in the primary, and possibly even the rectifiers if the breaker wasn't fast enough.
Although I don't know who designed the L-4B, it appears R12 in the power supply is there as a fusable link to prevent PS damage from a short or heavy arc in the RF deck. If you lose B+ and keep driving the tubes with RF, grid current should run very high but thankfully 3-500Zs can handle quite a lot of grid current without damage, so this may have been a smart idea.
In any case, there should be HV "showing" on the panel meter as soon as you turn the amp on. If not, either R12 is still open or something else is wrong.
W1DRF
05-09-2011, 01:05 AM
Now it is tripping one of the 15 amp breakers so i removed both board one had several bad diodes have new board and caps on order from Harbaugh electronics hope this fixes the blasted thing or it goes back in the closet to stay.
Thanks guys for all of the replys i will post if the results from board replacement.
W1DRF
05-10-2011, 09:32 PM
Now it is tripping one of the 15 amp breakers so i removed both board one had several bad diodes have new board and caps on order from Harbaugh electronics hope this fixes the blasted thing or it goes back in the closet to stay.
Thanks guys for all of the replys i will post if the results from board replacement.
New board installed and these boards are nice and easy to assy. Plate volts =2000 holding grid current for now to 200ma and plate current to .3 betwen 400 and 450 watts into dummyload will run it there for now till all the codwebs are gone.
I would like to thank everyone that replied to this thread.
