Sansui 555a - resistors replacement

[pete_mac]

And even going back to what pete_mac wrote; I read that it may be well to replace the carbon resistors with metal film resistors in the preamplifier section. What do you think?

Based up the feedback of others, it's worth trying if the only cost if parts (ie. you're supply the labour free of charge)

The feedback of a few other AKers is good. I note that Sansui used high-end Riken resistors in select locations in their late model high end amps. Whilst metal films are not boutique, they should be quieter than carbon films.

 

[harobike]

Thank you, so I try with metal film resistors Could you just tell me which part of the diagram are the most sensitive? I marked in red the part where I think I should replace the resistors, but may be I wrong?

 

[smurfer77]

As others have said, replacing the resistors, when they are not out of spec or damaged, is of questionable value. I sometimes do it just for the heck of it but it is considered being a bit extreme by some. If you are going that far to (potentially/arguably) extract the maximum performance from the device, might as well replace the lot instead of focusing on certain sections of the circuit IMO since it's also considered going overboard

 

[smurfer77]

The crucial thing about that T0-1 case transistor is that it has an isolated can, there is no contact with any of the 3 legs, which allows it to be in metal to metal contact with the heatsink(s). Because they are so hard to source now, that's what makes the T0-92 substitutes attractive because of the naturally isolated package - albeit plastic.
There are few transistors of other case styles with isolated metal cans, I believe some kinds of T0-39 case transistors are suitable.

As hyperion, skippy and others have noted in various useful posts you have to be careful with the bias transistors that the substitutes don't have collectors connected a conducting case, then connected to chassis for thermal management.

When 2SC281 is used for other positions in amps I tend to use KSC1815 for lower power (<0.4 W) and gain applications, and instead use the ZTX693B for higher power (<1 W) or gain (hFE is 500 for this guy) applications. I have used ZTX693B for bias transistors though and monitored temperature without any thermal management and it was not getting too hot on the 555A or similar amps.

Hyperion has mentioned the possibility of some TO-39 package transistors and I wanted give a specific option that i can vouch for, the 2N2102 (and similar). Very similar specs to KSC1815 but higher power handling (similar or slightly better than ZTX694B in this respect, but with the cool looking metal case package for when you really need that for aesthetics/matching). Have used it a number of times for power handling or aesthetic reasons.

 

[zastin17]

Just a note - don't handle the 2SC281 metal-can transistors unless you really have to - the leads can snap off at the base of the body fairly easily and then you are proper-fcked because it is hard to replace them these days.....just saying......I learned from experience with my AU-999.

Did the same on my Au-666 tore the dang leads off both 2SC281, yes, i'm that kind of special!! haha.

Soldering them worked for a few moments, before it just falls off as the leads that are left are just too stubby.

I am using recommended KSC1815 a replacements in this thread.

I have replaced all the main driver transistors on the Driver boards 2SC708, and 2SA537 with KSC2690, and KSA1220AYS. And I will replace the UFO transistors on the Filterblocks with them aswell. As 1x is wide open. The other UFOs are fine, so I just leave them for now.

I also replaced the original blown Outputs 2SD188/2SA627 with NTE88, NTE87 Compliments.

 

[slimecity]

I have replaced all the main driver transistors on the Driver boards 2SC708, and 2SA537 with KSC2690, and KSA1220AYS. And I will replace the UFO transistors on the Filterblocks with them aswell. As 1x is wide open. The other UFOs are fine, so I just leave them for now.

I also replaced the original blown Outputs 2SD188/2SA627 with NTE88, NTE87 Compliments.

Each to their own - however a couple of thoughts on this:

I would always replace all the old UFO trannies, they may test OK out of circuit, however under operation in-circuit you may get issues. Given their age adn relatively primitive construction techniques at the time they were made, I always shotgun these when I see them, and dont look back. New trannies will give you more predictable operation and for the cost per item, the effort is low.

You will find plenty of posts on AK about NTE parts. NTE buy up parts in bulk and relabel these to give them their own designation. A "NTE88" part could be a wide range of original parts with widely differing specs (and potentially also quality). So you dont know what you will get, and each part could vary in specs from the last one. You are more likely to find more standard replacements for these outputs. I havent research your case, but Im think probably OnSemi MJ15020, 21 (etc.