BX765X (Techniek Radio/TV)

door Pandu Rajan , 22.06.2021, 09:47 (1032 dagen geleden)

Dear Forum Members

Few years back I had restored two of the above sets. After complete restoration I found the low frequency ( Bass) response very poor and not at all satisfactory with the tone switch in position 5. I rechecked once again every thing but the problem was same. I also compared with the bass response of the set BX798X which is exceptionally good even at very low volume. I then carried out the following changes.
1. The value of the capacitors C111 and C113 were changed from 5600pf to 22000pf.
2. R45 (3.3K) was changed to 100k and C89( 47000pf) was changed to 6800pf.
3. R57 (27K) and C93 ( 10Kpf) were kept same.

4. Even after carrying out the above modifications there was not much improvement.

5. Few days back I thought of studying the problem once again. After proper study of the schematic I applied the input directly to the variable point of the volume control R53/R54, which is directly connected to one end of R55/R56, keeping the volume at very low level because the second volume control is connected in series. There was a complete change in the Bass response.
6. Now I was sure that the problem is around the 1st volume control.
7. Then I observed that resistance R46 is directly connected across the volume control part R54. R46 is 47K and the Volume control part R54 which is 200K. The total parallel resistance comes to 40K. Also the series combination of R45 and C89 is connected across R54.
8. I disconnected R46 and now the total response is very good including the Bass. It is nearly comparable to the response of the set BX798X.
9. Now I want to know as to why the resistance R46 is used? Is it to reduce the effect of R45 and C89?

Regards
P.Rajan