Heathkit SB-614 Station Monitor questions

Discussion in '"Boat Anchor" & Classic Equipment' started by KM4HNF, Sep 2, 2015.

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  1. KM4HNF

    KM4HNF Ham Member QRZ Page

    Hello guys,

    I have a pretty nice Heathkit SB-614 Station Monitor. Cabinet and everything very clean with no scratches. Inside looks like it was assembled by somebody who was pretty meticulous. It seems to work as it is supposed to do. Scope seems okay.

    My question concerns Page 27 in the assembly manual. There are two 70 mfd electrolytic capacitors metal covered axial type. In pictorial 2-8, they show the capacitors being secured with plastic cable ties. I never saw that in other Heathkit equipment that I have, and wondered were they really necessary to hold those caps in place? The caps are down pretty tight on the PCB, so it seems they would be very unlikely to move at all. Kind of a nit-picky type of question, I know. Just wondered if they served some other purpose that I am unaware of.

    Also, since they appear to be the original electrolytics, should I leave them alone (if it ain't broke, don't fix it), or assume that given their age and being electrolytic, they will eventually fail and possibly cause further damage, and hence replace them. I assume the can caps are much "tougher" than paper caps, but what life expectancy do they have? Just going through all my equipment and trying to avoid issues down the road.

    Thanks in advance.
  2. W5RKL

    W5RKL Ham Member QRZ Page

    Reviewing the SB-614 schematic I noticed there are 2 different grounds as stated and shown in notes 5 and 6 on the main schematic.

    The PC ground is an upside down triangle while the chassis ground is a set of diminishing horizontal lines. The grounds are different and should not come in direct contact with each other.

    Assuming Heath used the zip ties to prevent the caps from moving and coming in contact with metal that connects to the chassis ground, if you replace the caps with modern day capacitors you will not need the zip ties as modern day caps have a plastic covering over the capacitor's metal body.


    If the caps are okay, meaning they don't show signs of bulging at the positive end and there's no evidence the capacitors are failing then you shouldn't replace them. However, if you would feel more comfortable replacing them with a new modern day set of caps, then that's entirely up to you. The caps are in the +180VDC supply.

    Mike W5RKL
    Last edited: Sep 3, 2015
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Based on several decades of repairing, restoring, etc., equipment from this era, I definitely would replace the electrolytic capacitors. The SB-614 was available from 1974 through 1982. As such, the capacitors could be as old as 41-years and, at best, 33-years old. There is a very good chance that they are pretty well "dried out". Replace them with 82 mfd 350-volt capacitors. Such capacitors are available from Mouser at $2.39 each.


    Investing under $5.00 right now, is cheap insurance to prevent failure of the original capacitors which could definitely case considerably more expensive damage "down the road"!

    Glen, K9STH
    KB0TT likes this.
  4. KM4HNF

    KM4HNF Ham Member QRZ Page

    I agree totally,
    Thanks for the link, Glen. and thanks for your input as well Mike.
  5. VE7GPG

    VE7GPG XML Subscriber QRZ Page

    Don't want to hijack the thread, but I am also trying to fix one of these old beasts.
    Took a voltage measurement today to see what the cause of a faint trace and no horizontal position control might be and found
    I have 1400 + on the collector of Q121 which attaches to pin 7 of the crt. Voltage chart has this at 80 V - so definitely not within range.
    I am suspecting an internal short in the tube as I cannot see any way else that could happen.

    Any thoughts?
  6. W9GB

    W9GB Ham Member QRZ Page

    Heathkit SB-614 Schematic Diagram

    CRT is 3RP1A
    WHAT is the Voltage on Q118 Collector ??
    Pins 6 and 7 on the CRT are Deflector 3 (D3) and Deflector 4 (D4)
    Q121 : Motorola MPS-U10

    Central Semiconductor CEN-U10
    NPN silicon power transistor designed for high voltage amplifier applications.
    This device is an electrical equivalent to Motorola’s MPSU10
    Last edited: Dec 6, 2018
  7. W9GB

    W9GB Ham Member QRZ Page

    MAIN BOARD : HIGH VOLTAGE SECTION (Adjacent to Q121 and Q118)
    CHECK for SHORTS; Resistor Values; and HV Capacitor Replacement.
    USE Heathkit Assembly Manual.
    Sphere Research Corporation
    3394 Sunnyside Road
    West Kelowna, BC, Canada V1Z 2V4
    Phone: +1 (250) 769-1834

    CRT Inventory

    RCA, Hitachi, NEC and Others (USED) : $89
    3" Electrostatic CRT. USED, fully tested, very popular tube, semi-flat face. 12 pin base, we have B12A sockets for these. Used in McIntosh MPI-4, Cushman service monitors, and many others. Same as 3RP1A, but face is not perfectly flat, otherwise they are fully interchangeable. (2 left)

    Chinese Manufactured (NEW) : $129
    3" wide flat face electrostatic CRT. This is an excellent tube, with high brightness and crisp focus. Dynamic testing shows excellent performance, and previous customers of these tubes have been very happy with the quality. New, factory boxed, every tube is individually, dynamically tested for intensity and focus. 12 pin base (B12A), we have sockets for these (New Ediswan $15 ea.),
    GREAT for CRT clocks, as shown. Used in McIntosh MPI-4, Heathkit monitors, Cushman service monitors, and many others.
    We found one LAST BATCH of these, no more are available when these few are gone. (25 left).
    Last edited: Dec 6, 2018
  8. VE7GPG

    VE7GPG XML Subscriber QRZ Page


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