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A "Sound Technology 1200A Test Panel" that needs attention.
A "Sound Technology 1200A Test Panel" that needs attention.
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Old 20th April 2021, 08:40 PM   #1
zebulon1 is offline zebulon1
diyAudio Member
Join Date: Jan 2016
Default A "Sound Technology 1200A Test Panel" that needs attention.

I restored the power supply, Deoxed the pots and switches. Replaced the RCA jacks on the front and resecured all the BNC connectors on the back.
All the PS voltages are good.
I have the panel connected to the ST1700B and a ST1000A along with the Bench receiver.
At first the test panel was working well. I connected each ST Distortion analyzer, FM generator and the Oscilloscope, making sure the switching and functions worked through the panel.
I have a feeling the op amps are making it difficult to precisely check the functions. Sometimes the signal is there, other times its distorted.
Its the buffered signal that the ST1200 uses for all the test functions where a signal is needed in the test receiver/amp.
It has some of these OPA2605 and the OPA741's there's 14 or so op-amps total in this piece of equipment. I need to find some modern op amps to fit.
Here's an attachment of the snip: test signal board:
Attached Images
File Type: png st1200 test mesurement board 1.PNG (327.2 KB, 83 views)
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Old 21st April 2021, 01:29 AM   #2
rsavas is offline rsavas  Canada
diyAudio Member
Join Date: Aug 2012
Location: Georgina, Ontario
This ST1200A (J36) connects up to the ST1700A ( Source & THD analyzer) as its source and routes the signal to the test receiver.
The test receiver speakers connect to J9-12 to TB1 (1,2 & 1.9) o/p gets routed back to the ST1700A
check Relays K1,K2,K3 Speaker fuses F4,5
I think you need to trace/monitor the signals from input to output.

Last edited by rsavas; 21st April 2021 at 01:48 AM.
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Old 21st April 2021, 11:34 PM   #3
1audio is offline 1audio  United States
diyAudio Member
Join Date: Mar 2004
Location: SF Bay Area
A "Sound Technology 1200A Test Panel" that needs attention.
Working on the ST1200 is a real PITA. Much harder than the analyzers with all the myriad connections on the test panel. I have had some bad connection problems as well. They turned out to be the cable connection to the PCB's. Again deox helps as did NOT plugging in all the way??. Sometimes you just need to move on. Its been fine with the cable slightly loose.

First probable candidate are the switches. At their age they may have some corrosion on the contacts. replacing opamps is only a last resort if everything points at them. You can make an adapter using parts from e-bay. There are adapters for using can opamps in dip sockets but you can reverse that with the pins on the round pattern and sockets on the dip pattern.

I rescanned some of the pages that did not come out well. PM me and I'll resend them.
Demian Martin
Product Design Services
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Old 25th April 2021, 04:50 PM   #4
mgwizard is offline mgwizard
diyAudio Member
Join Date: Mar 2018
Location: Reno, Nv
I have had two ST1200 since new, I have multiples of the op amp go bad and yes they are a PITA to work on. Also don't overlook the relays for bad/intermittent connections.
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