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Previous Page SB-200 Restoration, Page 1
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I decided to tackle the power supply section first. This
needed to be working so that other stages could be tested later. The old
leaking, corroded and mouldy HT reservoir capacitors were removed (see top left)
ready for new one's. The 16 rectifiers were then removed and checked. These I
found were a variety of types and some were faulty. One rectifier had a high
reverse leakage, another a high forward resistance and another was
intermittently open-circuit due to a crack where the leadout wire enters the rectifier. It certainly pays to
just replace these. For help on the PSU rebuild I consulted the excellent article on
SB-200 restoration on "The Mules Ham Radio
Page"
The
leaked electrolyte was cleaned off the PCB (top right) with 'electro-clean'
solvent. Once clean, the eight new 1N5408 (3A@1KV) rectifiers were fitted in place
of the 16 original lower working voltage types. New smoothing capacitors were
ordered from Farnell
Electronic Components. I decided to replace them with six of the Panasonic
220uF 450V 105 deg C capacitors, Panasonic Part Number: EETED2W221DA). The
Farnell order code is 4438164. These are PCB snap in types with a 1.42A ripple
current rating (@120Hz, 105 deg C) and an ESR of 603 milli-ohm @ 120Hz. I
decided to use these as I've used them successfully in motor / servo drive PSU's
in the past. The PCB needed to be re-drilled with a mini-drill to take these new capacitors.
The original 'slotted' hole for the positive tag was bridged over by soldering
copper braid across them. The new capacitors were a little smaller than the original so
they fitted nicely within the boundary of the originals (Height 35mm, Diameter
30mm, Lead pitch 10mm). A slight crack was also noticed in the middle of the
PCB. The track that ran over it was bridged by soldering a short piece of
braid across it as a precaution.
The Heathkit SB-200 was powered up slowly using a variable transformer (variac) to allow the capacitors to reform and for voltage tests to be made at reduced supply voltage (if you havn't got one, try a 100W light bulb in series with the supply to reduce the supply voltage). The voltages were checked with an EHT probe (DON'T USE A NORMAL DVM DIRECTLY ON THE 2.4KV HT RAIL !). These voltage tests revealed unevenly split voltages across each capacitor due to the six splitter resistors which had changed value (measuring between 27K and 31K). These were replaced with six Welwyn 33K, 14W, 5% wire-wound vitreous enamel (glass) bodied resistors from Farnell Electronic Components (order code 3270841). The leadouts of the resistors were sleeved with glass sleeving to reduce the chance of shorting / flashover. This restored the balance of the voltages on each capacitor to +/- 5V @ +B 2300V DC). It always pays to check the splitter resistors on series capacitor chains like these. I've seen many capacitors on servo drives and AC inverters that have exploded because the splitter resistors failed and one capacitor exceeded it's working voltage.
Resistor R3, consisting of three 4.7 Meg ohm resistors from the +B connection to the metering circuit was checked next. These as expected had gone high in value and were replaced with 4.7 Meg Ohm 2W high voltage resistors (in new glass sleeving), together with R4 (15K 1W). The SB-200 was slowly powered up again. At full supply the panel meter read just under 2400 Volts, the actual +B HT was 2380 Volts on the EHT probe.
Two modifications were then added ; A glitch resistor of 10 Ohm, 14W (vitreous enamel wirewound, Farnell Electronic Components order code 3270841) in series with the +B supply to reduce tube flashover damage and three 1N5408 diodes in series across R12 (anodes towards ground) to protect the meter circuit if R12 goes open-circuit during a tube flash-over.
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As a final touch, the leadouts on transformer wires had additional glass sleeving added. The completed PSU section is shown in the photographs above.
The next job, the new vales (tubes) and a replacement anode cap.
Two new Colomor 572B valves (tubes) were purchased from Colomor Electronics Ltd. They are very friendly people to talk to. These were to be used for initial tests until the matched pair of "Golden Dragon" 572B's arrive from Chris M5LRO. Chris will match these valves (tubes) himself for you. I found these valves for sale from M5LRO on Ebay. Chris M5LRO now has an online store www.tubesonix.com. He also has a lot of knowledge about the SB-200, it's usual faults and what it's tube requirements are.
Before the new valves (tubes) were plugged in, the condition of the 33 ohm grid bias resistors (R21, R22) and the 33 ohm bias voltage resistor R23 were checked. R21 and R23 were starting to go high but were just within tolerance (will be replaced by Ohmite ceramic composition types very soon). R23 however was quite high at 43 ohms. This was replaced immediately with another non inductive carbon resistor. The bias voltages were also checked in both RX and TX condition, the RX cut-off bias was -130V, the TX bias was -2V. It is important to check this to avoid damaging the new valves.
The cooling fan was also checked. There are many web articles that mention that cooling can be a problem on the SB-200. All the dirt and dust was cleaned from the blades and perforated metal chassis using a vacuum cleaner and brush. The motor was powered up (note, mine was a 110V AC motor). The motor didn't sound good and an intermittent vibration could also be felt through the chassis. The motor was hand spun and it felt tight. Inspecting it revealed the cause, dried out felt oiling pads. These were oiled using and oiler pen through the oil holes drilled in the side of the the drive end and non drive end bearing housings. After a short while the motor felt OK again. It was powered up again and now ran OK with no excessive noise or vibration.
When the SB-200 arrived one of the tube anode 'top cap' clips was missing. A search of the workshop 'scrap box' yielded a suitably sized metal spring clip from 'something' ! The clip was cleaned bright using a small sanding disc attachment on a mini drill. A short, stiff piece copper wire was soldered to one end of the clip, the other end was soldered to the end of the VHF damping choke (PC2). The tubes were plugged in and the anode 'top cap' clips fitted. The new one fitted better than the old one which needed a little adjustment to tighten it. See the photo above.
Now for the RX/TX change-over relay...
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This was a mess ! It was originally a very
nice Potter and Brumfield relay with silver contacts. These had gone black due
to oxidisation. There was evidence of flashover around the relay, and the
leadout wire to the SWR bridge was burnt due to a bad joint on the relay contact
arm. The contact resistance was bad as a result of the condition of the relay,
over 6 ohms on transmit. It isn't easy to replace this relay as it's coil
resistance
forms part of the grid bias circuit (with R1, D17, R2, R16, R18), so I decided
to repair it.
The relay was removed and taken apart. The relay is riveted, but the clamp
holding the contact assembly to the coil yoke can be removed. I removed the
return spring then carefully bent the tabs out to remove the clamp. Then I
removed the contact assembly for cleaning / repair. The moving and fixed
contacts were then cleaned with a glass fibre pen and 'electro-clean' solvent.
New wires were attached to the moving contacts made from very flexible fine
stranded braid, then sleeved to insulate them. The relay was then re-assembled and
the contacts and spring pressures set-up. The completed relay (above) was then
re-fitted to the SB-200 and tested. The relay operated OK and the contact
resistance was now OK too.
Finally the remaining resistors were checked and replaced as necessary, and all the axial electrolytic capacitors (C3, C19) were replaced. The RCA phone jacks at the back were found to be cracked and were replaced with new gold plated ones from Maplin Electronics. The band selector switch was found to be very dirty and was gently cleaned up with a glass fibre cleaning pen.
There was a previous modification found on this SB-200 where a low voltage TX switching circuit was built on a piece of veroboard. This was checked (new electrolytic capacitors were fitted) and left in circuit so I could use it to connect to the linear amplifier control relay contacts via the 7 way DIN "Remote" connector of the Kenwood TS-570D. Note, to use this relay, menu option 39, 'Linear Amplifier Control Relay' must be set to 'ON'.
A Rhopoint 'SurgeGuard' SG170 inrush NTC thermistor was also added to the transformer primary input, connected across the terminals joining the two 110V windings in series (see above). This thermistor is rated for 8A with a 4 Ohm cold resistance and 0.07 Ohm hot resistance at 8 amps (working voltage is 265V AC). I've used these before to limit the inrush current on small AC drives or switch-mode PSU's. I added it to the SB-200 to protect the valve heaters and transformer from excessive inrush currents when cold. The red warning label was moved later so that it wasn't directly above the thermistor (after the photo was taken).
Now for final transmission testing................. Next Page
