DC508A Option 7 installation guide

Nenad Filipovic

Hi Group,

With big thanks to Dennis Tillman I became owner of a DC508A counter intended for my 7L13/TR502 combo. Installation of the required Option 7 upgrade for the counter is not difficult, but some steps are poorly documented and error prone, or scattered in different documents. To resolve this I compiled a set of resources to help with this upgrade:

The above documents and the following instructions should contain all info that you need to complete the Option 7 upgrade.

1. The provided Tekscope article "Tekscope_Vol07_Num5-1_TR_502_Tracking_Generator.pdf" describes the principles of operation of the whole spectrum analyzer - tracking generator - counter combo.

2. TM503 mainframe is recommended to house the TR502 and the counter, however you can also use the larger mainframes. Designated slots must be used for the two, and wired (at the back side of the mainframe motherboard) according to the "TM503_TR502_DC508A_Wiring_Information.pdf" document. Pins denoted on page 19-5 should be interconnected between the counter and the tracking generator (same pins on both).

3. Tracking generator and spectrum analyzer should be connected by a proprietary LEMO connector cable (wiring description in "012-0648-00_TR-502_LEMO_cable .pdf"). As mentioned in this document the original cable is way oversized (stiff for use and difficult to solder). My recommendation would be LiYCY 6x0.14 mm² (26AWG) or equivalent (cable diameter of 4.5-5mm). The LEMO connector (2pc required) part number is FFA.1S.306.CLAC52 (for cables up to 5.2mm thickness, check LEMO product catalog).

4. Tracking generator and counter should be connected by a 50Ohm SMA to BNC short pigtail cable (any decent RF type would do). Example can be seen in any of the operation demonstration photos included.

5. DC508A main PCB has a smaller daughterboard attached to its back side, where components need to be added:
- 74LS76
- 7404
- 7401 (2pcs)
- resistors (3x1kOhm, 2x5.1kOhm)
- 100nF ceramic capacitor
- 18pin standard (100mils pitch) right angled header
Schematic and layout are in the DC508A service manual (the provided manual "DC508A Instruction Manual 1982.pdf" is not OCRed, but contains uncut high-res schematics), and the assembled board can be seen on the "DC508A_Option_7_-_3_-_Daughter_PCB_Connected.jpg" photo.

6. Two wired connectors (10pin and 8pin) are required for the above mentioned PCB extension. The recommended placement and wire routing can be seen on the "DC508A_Option_7_-_2_-_Connector_Wiring.jpg" photo. Header pins are marked by single letters on the PCB, please see the next point regarding wire connections.

7. Before soldering the connector wires, some jumpers need to be removed and added to the DC508A main PCB. All this info is scattered throughout the service manual schematics, and hunting it all down is incredibly annoying and error prone. Please refer to "DC508A_Option_7_-_1_-_Installation_Map.jpg" for jumper modifications and soldering points for connector wires. The map points slightly differ from the letter markings on the back of the DC508A main PCB, but are electrically equivalent (chosen for soldering convenience).

8. Simple operation demonstration can be seen on the three photos included (FM broadcast band and RF signal from TV pattern generator). Performance specs can be found in the Options chapter of the service manual on page 6-1 (40). For spans of 100kHz/div and wider the count resolution is always 100kHz. Spectrum analyzer needs to be well settled thermally for center frequency stability, and even then it may struggle to maintain this 0.1MHz precision. In phase locked modes (spans of 50kHz and narrower) the count resolution is increased, and can be additionally selected using counter's own resolution control. This resolution will affect the time it takes to complete the count, effectively limiting the sweep speed of the spectrum analyzer. Center frequency stability is more-less decent for an instrument of this age.

9. Factory standard Option 7 includes Option 1 - OCXO. This option is not mandatory for primary functions of the Option 7 upgrade.

Hope these instructions help,
Best regards,
Nenad Filipovic