I have for quite some time been trying to get my FG Squawk & Talk board (AS-2518-61A) to talk properly and finally, it works! I thought I’d put my findings here. Maybe someone will find this useful.
The S&T board produced occasional garbled phrases or words. It could happen at the beginning, middle or end of the phrase or a complete phrase was just rubbish. It happened maybe 10% of the time. The rest of the time the voice was loud and clear.
I searched the web extensively in the hope of finding a solution, but did not find anything. The closest I got was this topic on pinside: “squawk talk occasional garbled voice problem”.
It describes the same problem that I had. Unfortunately there is no final conclusion in that thread.
Well, to cut it short, after having gone through the entire board, new caps, test with all new ICs (PROM, PIA, CPU, TMS5200NL) etc. the problem was still there. As a last try, I checked the resistor R9. According to the schematic and the color codes on the resistor, it should have been 130 kOhm, but when measured it was 150 kOhm!
After replacing the (faulty) resistor with a good one (measured to be 133 kOhm), my FG now talks very clear!
Some technical background info:
Before finding the faulty resistor, I tried replacing the U8 (TMS5200NL) with a new device borrowed from a friend. After this replacement the board worked very well, so I thought the problem was solved. I therefore ordered a new U8.
After getting the new U8, I installed it and expected everything to be singing and dancing, well at least talking ... Great disappointment! There were more garbled phrases than ever before!
So, the conclusion of this must be that there is a spread in some internal parameters between each U8 device causing a slightly different behavior. In fact, the speech synthesizer U8 (TMS5200NL) uses an external resistor (R9) to set/trim the internal clock frequency of the device. This allows to compensate for slight variations in the parameters between different TMS5200NL devices.
I should probably from the very beginning have measured the frequency at TP11 (TMS5200 clock). This clock should have a frequency of 160 kHz (for a speech sample rate of 8 kHz). If this frequency is not correct, there are only two possibilities, either the TMS5200NL is bad or the (real) value of R9 is not matched to the actual TMS5200NL device used.
Considering the huge price difference between a resistor and a new TMS5200NL, checking and trimming the value of R9 to get a good clock frequency out of U8 should probably be the first thing to do whenever the S&T board produces any kind of strange speech.
Even though 130 kOhm might have been a good value of R9 in the early 80’s, the parameters of the still existing TMS5200NL might have drifted over the past 30-35 years, so also another value can possibly give better performance.