"Gral" - "StereosoniC"  model TS-135 amplifier

 

General:

Around 1960 issued solid-state stereo amplifier for USA and European markets made in Japan.
This is a "Stereosonic" rebranded amp as there were obviously copyright issues with the brand-name "Gral" in some regions.

 

front ^

I have removed the "Stereosonic" sign on the front situated above the pilot-lamp, as I suspected there was another brand-name under it and it was.
I replaced the "Stereosonic" sign as you can see here on the photo, to the middle on the front plate, here it sits nicely and respects the history of the amp.
Under the "Stereosonic" sign there was the original brand name "Gral" as you can see above. This particular amp was initially bought in the UK.

 

Owners manual:

Not available

 

interior ^
 

Special futures:

This amplifier comes with cinch input sockets.
Light and small footprint amp. Metal case.
Well build and good parts used.
This amp employs all germanium transistors.
The pcb's of the amp are sitting vertically in the amp.

 

Specifications:

Power output: 10 watts RMS per channel into 8 Ohms (stereo)

Frequency response: 20Hz to 20kHz

Total harmonic distortion: 0.5% at RMS output

Damping factor: >15

Input sensitivity: 2.5mV (tape head), 80mV (phono ceramic), 300mV (line)

Signal to noise ratio: better than 50dB (tapehead), 70dB (line)

Speaker load impedance: 8 to 16 Ohms (minimum)

Semiconductors: 18 x transistors, 6 x diodes

Solid aluminum front plate with stamped lettering

Weight: 8lbs

Year: 1960-1964

 

Bottom view ^

Germanium power transistors (AD149 Philips (Uce 40V version, repaired) ^
Original power transistors are: 2SB471(B) Hitachi Japan.

 

Schematic:

Not yet available.

Power rail voltage is -44 Volt assymetric, it should be -40 V, but the power transformer has only one 220V~ connection:
consequently due to the detrimental power change to 230V in Europe the voltage of the power rail is few Volts too high.
Power supply + is connected to ground.
OTL and DTL class B push-pull power amp section.
Early direct coupled power amp section.
Complementary driver stage, semi complementary power stage.
Remarkable small power transformer that works great: the power transformer can handle this 230 Voltage.
All Hitachi germanium transistors inside.

 

Modifications:

I replaced some diodes, capacitors and resistors in the power supply and power rail.
Also I exchanged the speaker output screws for sockets as they were of bad quality.
The power transistors 2SB471B were broken, so I replaced them with AD149K,
in one amp the drivers 2SB77-2SD77 were broken too, replaced them too for AC127-AC128.
You may also use AD166 (Siemens) or AL102, AD142 (Ates) as 40V power transistors.
The pre-amp sections all have 2SB75 transistors used.

The amps frequency response was not quite flat due to aging transistors,
so I removed a couple of caps in the tone amp to get a nice flat response up to 20 kHz.
The pcb's are placed vertically in the amp and one pcb was quite some curved by heat in the amp.
To get the pcb right again I took a thick solid copper wire and soldered it to both pcb's keeping the distance
and ensuring a good 'ground' to both, replacing a thin stranded wire.
The tape-head amp can get quite noisy and has a tendency to oscilation when not terminated correctly.
This amp has + to ground. The power-supply diodes were 1A 100V types by Hitachi, these were replaced by 10A 500V general purpose diodes.
The already "star" grounding topology should be extended by 2 wires (green) at least to complete the grounding as it should be.
Also the power line - leads (red) should be extended by 1 wire to get the power rail right.
There was no spark suppression cap on the mains switch, causing serious sparking when the amp was switched off. I inserted a X2 capacitor.

 

Pcb's & back view, speakers are fused with 2A at the back panel. ^

 

Sound:

The sound is very good and powerful and distortionless for such a small and early amplifier.
The Tone-pots do not meet modern standards. The Loudness & Balance switches work great.

 

The nice and well crafted solid aluminium front plate after cleaning ^

 

The restored amp looks good after repainting the hood, now it needs some polishing for a nice shine ^.

 

Below more photos of the restore of this GRAL integrated amplifier by Theo de Veth.

 

Main amplifier board copper side ^

Replaced Siemens versus original Hitachi power transistors ^

Power supply pcb ^

Hitachi power supply diode on the copper-side, the other 3 are on the component-side

 

Replaced capacitors, some in the signal-path replaced by foil caps. ^

 

Front panel detail ^

Back panel detail ^

Original setup of the L / R amp pcb before the restore by Theo ^

Presented to you by ON9MMK on:

Thanks Theo de Veth for the excellent photo's.

The vintage website.

© Hans Hilberink - PE1MMK - ON9MMK ® Theo de Veth, 2003/2017/2024 - Last update: 11-08-2024.