saba: SABA Continental 410
Hello SABA experts,
I am restoring a SABA Continental 410US for a friend and have encountered a technical problem with the ratio detector circuit. I have not been able to resolve it and seek the advice of those more expert than I.
The radio (after repairs and alignment) functions well in all respects save one: the ratio detector DC voltage (schematic point X) limits at 17.5V. I believe this is slightly low, as my Freiburg 14 (same circuit) limits at 19.5V.
The FM sensitivity is good; slightly better than my Freiburg 14.
Direct injection of a 10.7 MHZ signal on the control grid of the EBF89 also results in a 17.5V limit.
Removing the Stereo Decoder does not resolve the problem.
The G2 and Anode voltages on the EBF 89 are correct per the schematic. The DC supply voltages are correct. The Anode and G2 currents are correct. Correct means within 2% of spec.
The 5uF Elko has been exchanged (precautionary) with no effect. Also, the EBF89 has been exchanged with no effect. The EM84 was exchanged, as it was dim; it had no effect on the performance.
The following resistors measure correct: R381, R306, R303,R311,R314, R315, R703-R706, R301, R709. Correct means within +/- 5%.
The detector diode pair (AA119's) measure good for Vf (approx 280mV at 1mA).
The AM (MW) AGC functions.
I have not measured or replaced C703, C704 or C702. Access is very difficult.
C303 was bypassed with 2.2nF with no effect.
The leakage current in C304 and C312 measures less than 1 uA at 20VDC.
The ECL80 has been exchanged (for another reason) and it did not affect performance. Automatic tuning works well.
During alignment, the set responded according to the SABA alignment instructions with no problems noted.
Any advice would be greatly appreciated, as aside from a problem with the ratiofilter I am running out of ideas.
Thank you for your attention,
The 17.5V limiting voltage seems perfect; in fact it is imperceptibly better than the 19.5V you see in the Freiburg 14.
The improved lower value of 17.5 could result from a number of causes. Some of these include R303 being 5% high, the EBF89 pentode being 5% stronger than average, and the mains voltage being high, perhaps more than 120VAC.
Since you are paying such close attention to detector operation, I recommend checking the leakage of the AA119 germanium detector diodes. It should not be more than a few uA at -20V, and should be fairly voltage independent from -1V to -20V. A great mismatch in diode leakage is also to be avoided, in particular if the S curve detector response seems skewed to one side, which could affect motor operation. But you say automatic tuning works well.
You have an excellent set, I wish I had room to own one.
Thank you for the response and advice.
There is some asymmetry in the S curve, so I will check the ratio detector diodes for leakage.
Or perhaps I will just put in a pair of 1N4148's or BAT42's I have on hand.
I take it that this radio has plenty of gain, such that there will always be plenty of signal available for detection. In this case loosing about 1V of signal in the diode drop of the two 1N4148 diodes is OK.
In some of the simpler FM radios, with only one RF and two stages of FM IF gain the signal from weak stations at the ratio detector, as measured across the electrolytic cap, can be as low as 3V, while the sound is still acceptable. In this case, the 1N4148 drop is a significant loss.
You could try common germanium diodes or shottky diodes to avoid the signal loss. I have measured plain old 1N34 diodes with less than 1uA of leakage at -20V. The original diodes may be fine, anyway.
A further inspection of the schematic shows that there are separate detectors for the automatic tuning motor drive and for audio. The automatic tuning uses a frequency discrminator for AM and FM with the 2 diodes in the EABC80. The FM audio ratio detector uses two AA119 solid state diodes, and the AM audio detector uses one tube diode in the EBF89. I expect that the AA119 are germanium diodes, but I don't see this specified in the schematic. Easy enough to measure the forward drop.
Glad to be of some help. I learned too, in particular about the separate motor/audio detector chains.
see here: AA119 and most GE Diodes ( AA111 to AA143) be on RMorg.