On Wed, 9 Jul 97 17:39:36 -0700 Rich Measures <measures@vc.net> writes:
>>...snip...
>>Makes me wonder how many SB-220 series have lost the filament xfmr
>due to
>>the infamous C-4??
>>
>>On a slightly different note Rich makes mention of filament xfmr
>>protection from a tube short on his Web page but it is a bit
>confusing.
>>Maybe you can elaborate a bit Rich.
Thanks for the info.
>In a SB-220, during intermittent push-push parasitic oscillations at
>c.110MHz, lateral mechanical stress is applied to the 3-500Z's hot
>tungsten filament (1800 degrees K). This stress is due to the
>apparently
>large burst of grid current associated with such oscillations. (see
>photo on page 15 in the September, 1994 *QST*). After several such
>oscillations, the filament may be bent far enough to touch the grid,
>resulting in a filament to grid short. The filament center-tap is the
>DC
>cathode/filament return. The center-tap is connected to the +120V
>power
>supply during receive in order to cut off the flow of anode current.
>When the filament/cathode is bent far enough laterally to touch the
>grounded-grid, a short to ground is placed on the +120V power supply
>during receive. However, there is no fuse for the filament
>transformer
>from which this supply receives its AC source. Thus, a short on the
>+120V supply causes the unfused filament transformer to summarily melt
>
>down. One solutions is to install a 10k ohm, 2w resistor between the
>+120v supply and the relay's bias switching contacts. In the event
>that
>a 3-500Z develops a grid/filament short, the short-circuit current
>will
>be limited to about 0.12A. Another solution is to convert the SB-220
>from voltage-cutoff bias to resistor-cutoff bias (a.k.a. 'self-bias').
>
>The existing 100k, 0.5w resistor on the relay contacts can be used for
>
>this purpose.
>>Summertime suggestions for SB-220 series owners:
>>
>>1. Check those HV filter caps, replace if showing leakage. If no
>>leakage, measure the voltage across each one. In the SSB position
>they
>>should be roughly 370VDC and a +/- variant of 10-15V is OK with the
>30K
>>resistors. A wide range of readings and it is time to consider
>>replacements. The Harbach assembly is an excellent choice since he
>uses
>>1% matched CGS caps and 100K resistors. They still fit into the
>original
>>coffin with no air circulation but it is better than original.
>>
>Matsushita/Panasonic also makes suitable electrolytics---which are
>available from Digikey. The part # is P6443 for 330uF at 450 V. The
>first price break occurs at 10 units, so if you buy 2 sets (16) and
>share
>one set with a friend you can save some money (the price for 16 is
>$9.95
>each). Digi-Key's telephone number is 1-800-344-4539.
That is still an 85C cap and is identical to the CDE 380LX seies
available for $8.93 from Mouser. A better choice is the 105C 381LX
(Mouser # 5985-450V330) and $10.69 in 10 qty. Without air circulation I
would prefer the CGS Computer Grade and opt for the 381LX on a new PC
board. BTW, if you go with the 220MF 381LX the price is $8.14 in 10 qty.
The Mouser # is 1-800-346-6873.
Heath was not
>known for generously sized filter caps. IMO, a little extra C is
>better.
200MF was larger than the others of the era.
>>2. Replace the 200pf red rectangular grid bypass caps with 500V
>dipped
>>silver micas. Later amps used 115pf supposedly to better compensate
>for
>>grid XL. I use 120-130pf since they are on hand...work fine. Those
>old
>>block style caps are known to crack and go bang.
>
>I would be inclined to replace the grid to ground 1A RFCs (whose
>fusing
>current is around 15a) with frangible resistors or fast acting 0.25A
>fuses. However, when this is done, an additional 1000pF or so is
>needed
>to protect the grid fusing element from 10m RF.
I do not like that method at all and remove any that I find on repair
jobs. The correct choke rating is 100ma not 1A and they will do the same
fuse job as a questionable resistor circuit. I am VERY comfortable with
the original Heath grid circuit.
>- OTOH, The grids may also be directly grounded.
And when the tube goes bang it is toast ala the AL-82 and others of
similar design.
>
>>3. Check the output bandswitch for burnt contacts.
>
>Some photographs of what seem to be parasitic-arced bandswitches are
>available on my Web site.
With a new suppressor the only arcing is due to undersized components and
cockpit error.
>>4. Measure the value of the parasitic suppressor resistor. It should
>be
>>47 Ohms. If over 60 it is time to replace with a new one or
>alternative
>>offerings.
>>
>If the original suppressor resistor burned up, IMO, it wasn't because
>the
>resistor had a manufacturing defect.
That is not what I said and please dont try and continue your silly
Tom-Rich war on my nickel. After 25 years of RF heating that single 47
Ohm 2W carbon composition resistor increases in value. There are better
ways to build the suppressor, check out the new 5W metal oxide resistors
in the latest Mouser catalog. I cant even smoke one in a 6M FL-2100 key
down for 2 minutes.
>>5. Check the 500pf 80M loading mica, C-56; they like to split open.
>>
>The 2000v rating is difficult to find. Try Spectrum Electronics,
>first.
2000V is not the gating spec; current is the key. I have maybe 20 or so
here including the later dipped variety. Other options are a pair of
250-300pf 1KV in parallel. I have even used 500V CD-19 case size dipped
micas for years at the 1200-1500W level as 80 or 160M additional loading
caps with no failures. I suspect that there is no internal difference in
the 500 and 1000V versions since they are identical in all dimensions.
The CD-19 and even the CDV-19 (1KV) sizes are at Mouser. Spectrum has no
stock.
>6. Check the plate blocking cap, C-29 for any signs of cracking. I
>>sometimes wonder if this is really a RF rated cap since I have found
>many
>>bad ones.
>
>It is not an RF-rated cap.
>
>>7. Install a pair of back to back diodes across the meter leads. Any
>>1N400x series is fine. Original meters are virtually extinct and the
>>diodes will protect from a tube flash over or short.
>>
>Original meters are still available: SPECTRUM ELECTRONICS (616)
>742-0613 They reportedly carry the SB-220's current-limiting filament
>
>transformer and well as other parts.
>>8. Burnish the relay contacts. I have yet to replace one due to hot
>>switching but a little PM goes a long way. Make sure it engages with
>a
>>good clunk. Remove and compress the spring a bit if necessary for
>good
>>action.
>
>Hotswitching is assured when a stock SB-220 is used with a
>faster-switching modern radio. The fix is to install high speed
>switching. (see Fig 7 on my Web site)
Sell your stuff on ur own nickel. Besides, who wants a 300W spike from
their new Rice Box to get into the amp any faster anyway. The old 220
relay is just fine for most of us, thank you.
ttp://www.vcnet.com/measures
>Also see 1/94 *QST*, 'The Nearly Perfect Amplifier'. To test for hot
>switching, press the push-to-talk button on a mobile microphone. If
>the
>SWR indication on the radio temporarily surges at the instant the
>button
>is pressed, hot switching is indicated.
>>
>>9. Most SB-220's and early 221's will have the small Johnson 154-9
>Tune
>>cap or the OEP 26-131. This cap is prone to arcing since it is only
>>.075"/3KV spacing. Later amps and Heath replacements were the OEP
>26-164
>>which is about .090/3500V spacing. It will still arc on occasion when
>the
>>amp is mistuned but not melt down as did the original. There is no
>>Johnson/Cardwell equivalent. If there is enough interest I will have
>the
>>26-164 reproduced.
>>
>- IMO, the voltage withstanding ability of the original tune-C is
>lower
>than the voltage withstanding ability of the output bandswitch.
>- IMO, the voltage withstanding ability of the wider-spaced tune-C is
>higher than the voltage withstanding ability of the output bandswitch.
>
>When something goes wrong, either the bandswitch OR the tune-C is
>likely
>to arc. The choice is yours. Those who would rather file down arc
>marks
>on the tune-C plates should probably opt for the closer spaced tune-C.
>
>However, those who would rather replace crispy-crittered bandswitches
>should probably go with the wider spaced tune-C.
Learning how to tune the amp works well also. The 220 is in bad need of a
pair of 6:1 reduction drives and a good logging scale. Tuning up every
day on the same band is nuts but yet some hams havent a clue what they
are doing. Others want to play God. All I get are the basket cases.
>>10. Check the fan. After many years and no lubrication they slow
>down a
>>lot. Replace with the Harbach unit...spend the extra $1 for the high
>>speed version, particularly if you live in a 50Hz world.
>>
>...and if you live in the 60Hz world, the high speed fan will
>reportedly
>suck chrome plating off a trailer hitch.
>Rich---
And keep the tubes a bit happier when some bozo needs 2 minutes to tune
up.
73...Carl KM1H
>R. L. Measures, 805-386-3734, AG6K
>
>
--------- End forwarded message ----------
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