MPSA18 v.s. BC550C

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A

Alienspecimen

Member
Joined 2003
Hello,

I am restoring two vintage receivers: HK330c and HK730.

I have BC550C and MPSA18 in stock. The question is: Which one would be better suited for the application and why? I intend to replace the transistors in the phono and preamp boards along with the differential pairs in the power amp stage.

Thanks in advance.

Best

P.S. I understand that neither match the "footprint" of the originals.
 
M

Monte McGuire

Member
Joined 2013
I just checked the schematic, and the first stage in the "equalizer amp" as they call it is a PNP (2SA844), so neither would be appropriate. The second stage is a 2SC1344 NPN so the BC550 would probably be ideal. Oddly enough Futurlec has both transistors in stock, so you could get originals if you'd like. I've never ordered from them, but it might work out well. Here are some links:

Q101, Q102 NPN: 2SC1344 - 2SC1344 NPN Low Frequency Transistor
Q103, Q104 PNP: 2SA844 - 2SA844 PNP General Purpose Transistor
 
Ian Finch

Ian Finch

Member
Joined 2010
Paid Member
Either type may well function to a degree in the various circuits but there are reasons why several different transistor types, including complementary pairs, were used in the amp/preamp boards. It may be possible to find suitable replacements for all of these but it may also be tricky, because you may still have problems with the range and type of circuit adjustments necessary to accommodate them. In that event, it amounts to re-designing the circuit and perhaps best left to others.

Monte's advice is makes good sense. ie. if it works and noise levels are what you'd expect for the era, don't mess with changing semis. It's asking for a whole pile of trouble by altering those transistor types with parameters selected to match the circuit or vice-versa. Also, swapping small-signal transistor types according to what you have on hand is far more likely to degrade performance than improve it.

Futurlec's parts are sometimes good, sometimes dubious because the datasheets they post are not necessarily those of the parts you buy or receive. It's a long wait and a disappointment when the type you receive doesn't match it's spec, outline etc. either.
 
A

Alienspecimen

Member
Joined 2003
Monte McGuire said:
I just checked the schematic, and the first stage in the "equalizer amp" as they call it is a PNP (2SA844), so neither would be appropriate. The second stage is a 2SC1344 NPN so the BC550 would probably be ideal. Oddly enough Futurlec has both transistors in stock, so you could get originals if you'd like. I've never ordered from them, but it might work out well. Here are some links:

Q101, Q102 NPN: 2SC1344 - 2SC1344 NPN Low Frequency Transistor
Q103, Q104 PNP: 2SA844 - 2SA844 PNP General Purpose Transistor
Click to expand...
Thank you for checking the schematic and thank you for the links.

I looked at what they offer. It seems that I do not have a choice of hfe grading. The transistors in all preamp stages require hfe > 400.

The datasheet shows that I could get ones with hfe as low as 150.
 
A

Alienspecimen

Member
Joined 2003
Ian Finch said:
Monte's advice is makes good sense. ie. if it works and noise levels are what you'd expect for the era, don't mess with changing semis. It's asking for a whole pile of trouble by altering those transistor types with parameters selected to match the circuit or vice-versa. Also, swapping small-signal transistor types according to what you have on hand is far more likely to degrade performance than improve it.
Click to expand...

Hi Ian,

Thanks for the advice.

I was considering changing the small signal transistors in the preamp stages, because of hiss. I have two HK730's and one of them is slightly "noisier" than the other one. The online reviews tell me that "music is being painted on dead-quiet canvas" (eyeroll), but neither of my receivers is "dead-quiet", so the plan was to experiment with the noisier one. Just for the record - this is the slight hiss between music passages one hears when next to the speakers.

I am not exactly sure what I should be concerned about with regards replacing active devices if the new ones have almost identical specs. What should I be looking for?

Thanks in advance.
 
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A

Alienspecimen

Member
Joined 2003
I also heard a comment that the BC550 and MPSA18 are outdated devices.

Can someone recommend more modern available equivalents?

Before choosing the BC550 and MPSA18, I spent about a month searching for replacements by looking at datasheets on Mouser and DigiKey, but eventually settled on these two, because of either of availability (as in 2N5962), or because there was no hfe grading i.e. the datasheet listed an hfe range of 150 ~ 900 (as in PN2484) and I did not know what I am going to get.

Are there any available modern low-noise/high hfe devices that exist?

Thanks in advance.
 
K

kannan_s

Member
Joined 2009
Yes - noise source could be Capacitors - and if the transistors have been chosen by the designer initially unless Preamplifier part does not work i will not replace the transistors- concentrate first on Caps
kannan
PS : even film capacitors fail ( i have seen many instances of Nakamichi Cassette recorders stop working because of old Japanese Orange colored PP caps!)
 
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