Identified a fault in a hitachi HA-610 with Q703R, which tested good out of circuit but in situ the base voltage was off, which was in turn causing later stages to have incorrect DC voltages too.
The original transistor is 2SA836.
For troubleshooting, I swapped it for a BC558 because I have plenty on hand, and it corrected the fault.
I have enough BC558 (good vintage parts, most of them) that I could replace Q704 and Q703 on both sides with matched pairs, however I am not sure whether this is a suitable replacement, or if I should use a different type to maintain the original performance of the amplifier.
Any advice?
The original transistor is 2SA836.
For troubleshooting, I swapped it for a BC558 because I have plenty on hand, and it corrected the fault.
I have enough BC558 (good vintage parts, most of them) that I could replace Q704 and Q703 on both sides with matched pairs, however I am not sure whether this is a suitable replacement, or if I should use a different type to maintain the original performance of the amplifier.
Any advice?
Q703 forms part of a Norton Mirror and should be fine if the CE voltage does not exceed 30volts, which it is likely to. A BC556 is a better choice.
It will make no difference to the performance of the amplifier as it is not part of the gain stage.
No need to replace any others due to 'just in case' reasons.
It will make no difference to the performance of the amplifier as it is not part of the gain stage.
No need to replace any others due to 'just in case' reasons.
I think I have some BC556/557 maybe even a 559 (low noise) or two.
So they don't need to match? I wasn't sure what this topology was, at first I thought LTP but then I can see it's not quite. Thank you for naming it, I will read up.
So they don't need to match? I wasn't sure what this topology was, at first I thought LTP but then I can see it's not quite. Thank you for naming it, I will read up.