Roberts STA-100 ii
This is a New Zealand made amplifier from the 1970's? from the time when imported HiFi gear was restricted and very expensive.
New Zealand designers stepped up to the mark and produced some really fantastic products; this is one of them, very, very good build quality, fibreglass circuit boards and top quality components as well as really well made case work ...and styling!
I'm not a designer, not sure if this design is "cutting edge" for the time but these sound superb (this one needs some repair work) but I had another and the sound was excellent and so this one will be a keeper when fixed.
The output devices are the "humble" and common 2N3055/MJ2955 complimentary pair. You may have read some disparaging comments about them, however their specs more than adequate for audio amplifiers (quite a few NAD amplifier models used them in designs from years later and of course had outstanding reviews). As a New Zealand made "boutique" brand reviews are hard to come by and this model particularly is very hard to find info on (always interested in any info out there please?) if there had been reviews this would have had glowing recommendations I'm sure.
This one has a failed regulator circuit and needs a zener diode and two (complimentary) transistors MJ243/253 - and more checking to see why they failed?
More to come on the outcome of the repair....I'll amend post when it's done. As of August 2021.
Other work? Electrolytic capacitors are good quality and measured ones so far are fine. There are some Tantalum capacitors that I may change as these can fail (and contemporary advice is mixed but leaning heavily to "change always")
I will of course clean all switches and potentiometers and the faceplate!
It's beautifully made and I want to keep as original as possible.
One of the best features of this model is the RCA connectors (as well as DIN) the DIN connectors were very common on early NZ made audio even for speaker connections and that is an issue with modern speaker cable recommendations.
Another feature not common on such an early amplifier is that the input switching including the Phono amplifier has internal remote switch to keep cable length to a minimum and reduce hum and other interference.
The pre-amp and power amp can be separated (to, particularly, back in the day probably add an equalizer- they were very popular) and also allowing mix match of other power or pre-amps. .
Other Roberts models:
I know of the STA 36, STA 50 and STA 60 (including versions 1,2 &3? of some) and an RE-05. The RE-05 ( I think that was the model?) had a particularly difficult Integrated circuit that seemed to fail often - not sure if there is an easy fix or replacement for that; from memory it was a pre-amplifiier (14 pin DIL) the one I saw (a friend had 8 of the amps had all got the same problem, failed IC, some that had got hot and been physically destroyed) N.B. The STA 100 does not have that IC nor any others, its all discrete.
New Zealand designers stepped up to the mark and produced some really fantastic products; this is one of them, very, very good build quality, fibreglass circuit boards and top quality components as well as really well made case work ...and styling!
I'm not a designer, not sure if this design is "cutting edge" for the time but these sound superb (this one needs some repair work) but I had another and the sound was excellent and so this one will be a keeper when fixed.
The output devices are the "humble" and common 2N3055/MJ2955 complimentary pair. You may have read some disparaging comments about them, however their specs more than adequate for audio amplifiers (quite a few NAD amplifier models used them in designs from years later and of course had outstanding reviews). As a New Zealand made "boutique" brand reviews are hard to come by and this model particularly is very hard to find info on (always interested in any info out there please?) if there had been reviews this would have had glowing recommendations I'm sure.
This one has a failed regulator circuit and needs a zener diode and two (complimentary) transistors MJ243/253 - and more checking to see why they failed?
More to come on the outcome of the repair....I'll amend post when it's done. As of August 2021.
Other work? Electrolytic capacitors are good quality and measured ones so far are fine. There are some Tantalum capacitors that I may change as these can fail (and contemporary advice is mixed but leaning heavily to "change always")
I will of course clean all switches and potentiometers and the faceplate!
It's beautifully made and I want to keep as original as possible.
One of the best features of this model is the RCA connectors (as well as DIN) the DIN connectors were very common on early NZ made audio even for speaker connections and that is an issue with modern speaker cable recommendations.
Another feature not common on such an early amplifier is that the input switching including the Phono amplifier has internal remote switch to keep cable length to a minimum and reduce hum and other interference.
The pre-amp and power amp can be separated (to, particularly, back in the day probably add an equalizer- they were very popular) and also allowing mix match of other power or pre-amps. .
Other Roberts models:
I know of the STA 36, STA 50 and STA 60 (including versions 1,2 &3? of some) and an RE-05. The RE-05 ( I think that was the model?) had a particularly difficult Integrated circuit that seemed to fail often - not sure if there is an easy fix or replacement for that; from memory it was a pre-amplifiier (14 pin DIL) the one I saw (a friend had 8 of the amps had all got the same problem, failed IC, some that had got hot and been physically destroyed) N.B. The STA 100 does not have that IC nor any others, its all discrete.