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FerrographF307 Amplifier Upgrade Instructions |
Ferrograph published the following instructions for upgrading the output stage of the F307 amplifier when the 39285 output transistors became unavailable.
The scans and text were provided by Richard Newman.
Although the instructions refer to type 16020 transistors, Ferrograph sometimes supplied 2N3055H with the kit. These transistors are suitable equivalents to the 16020, and are still readily available.
Remember to ensure that the replacement transistors are attached to the heatsinks with electrically insulating washers and bushes, and thermally conductive grease.
In addition to the changes described here, the kit included
replacement DC power supply fuses. Fuses FS701 and FS702 should be
increased in rating from 0.75A to 1.0A.
Power devices type 39285 fitted to F307 amplifiers prior to Ser. No. A2000 are no longer available, and have been replaced with type 16020 power transistor, currently used on "Mark 2" amplifiers.
Where one or more of the 39285 transistors have failed it will be necessary to replace all the power devices on both channels and as these are physically larger, also to redrill the heat sinks to accommodate them.
When fitting new power transistors, the printed board circuitry must be modified. This involves breaking the printed circuit between emitter VT503 and junction R500/R502. A 22 ohm 1/2W 20% resistor is then connected between emitter VT503 and collector VT502 (mounted on the printed circuit side of the board).
After modifying and assembling the power amplifier, the d.c. quiescent current must be measured and adjusted to between 25 and 30 mA by value selection of resistor R518, which is effectively shunted across R506. For the LH channel this resistor is mounted on a tagboard just forward of the mains transformer screening plate and wired to R506 mounted on the power amplifier printed circuit board itself. On the RH channel R518 is physically mounted above R506.
On some F307 amplifiers R518 may be a 500 ohm carbon potentiometer fitted to the heat sink.
Under no circumstances should the amplifier be operated without R518 fitted or with this component having such a value that the quiescent current is well in excess of this range. In practice it will be found that the current increases as the value of R518 is raised. If R518 is a fixed resistor it will be advisable to temporarily replace this with a potentiometer e.g. 500 ohms or 1000 ohms (initially set to zero ohms) for the purposes of adjusting the quiescent current and to increase its value to obtain 25-30mA. The amplifier should then be operated for approximately 30 minutes when the quiescent current should not rise above 45mA. If, however, this figure is exceeded, the potentiometer resistance should be adjusted to reduce the current to below this limit. The potentiometer can then be removed, its value measured and replaced with a fixed resistor of approximately equal resistance.
Please note that in the event of a driver or output transistor
becoming defective, all four devices (VT500, 501, 502 & 503) should be
removed and tested before replacing any component, since a fault in,
for example, an output transistor may be caused by a defective driver
device, or vice versa, which may not immediately be apparent.