Sherwood AX-7030R amplifier. - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 8th Nov 2019, 6:10 pm

I'm repairing a Sherwood AX-7030R amplifier for a client. It's dead, no standby light and all the fuses are OK.
Removed lid and did a few initial tests, check the driver and output transistors and for anything obvious. Found a 2SC2168 driver transistor with a short circuit between emitter and base. Out with it and test it again, it's OK! Check the PCB, no short circuit. So the transistor has been refitted, again no S/C.
Anyway, it turns out the standby transformer has an open circuit primary.
The secondary voltage will have to be determined. The 330uF reservoir capacitor is a 35 volt component so I'd guess it's a 25 volt transformer but I'd better make sure before a new one is fitted.
The third attachment shows that the standby transformer doesn't look too good.

Anyone familiar with this amplifier?

DFWB.

paulsherwin · 8th Nov 2019, 7:25 pm

If you are confident that you correctly measured a fault in that transistor, then I would replace it anyway. It may have developed an intermittent fault, and suitable replacements only cost pennies.

turretslug · 8th Nov 2019, 7:27 pm

Hopefully a reasonable assumption- the small blue relay in the third picture appears to be marked "12VDC", presumably coil voltage and presumably driven more-or-less directly from the standby supply by the small plastic transistor, though the small, slightly cooked and stood-off adjacent resistor may be dropping some voltage. I'd do a bit of track-following for guidance here,

Good luck, Colin

sortedradio · 8th Nov 2019, 9:28 pm

The service manual is available foc from https://elektrotanya.com/sherwood_ax...wnload.html#dl

Michael Maurice · 8th Nov 2019, 10:48 pm

Standby transformer should be approx 8 - 12V, no more, I'd go for 10V.

Circuit attached.

FERNSEH · 9th Nov 2019, 2:41 pm

"Standby transformer should be approx 8 - 12V, no more, I'd go for 10V."

Hi Michael,
many thanks for the information. I've found this unused 8 volt transformer which I'll try out later today.

DFWB.

FERNSEH · 9th Nov 2019, 11:59 pm

That's the amplifier fixed. The "8volt" transformer is delivering slightly under 10 volts to the bridge rectifiers. There is 10.5 volts across the reservoir capacitor C502.
That short circuit transistor was in fact caused by the collector tab of the 2SC2168 coming into contact with the safety resistor R1711, see second attachment.

DFWB.

paulsherwin · 10th Nov 2019, 1:13 am

OK, fair enough.

8th Nov 2019, 6:10 pm	#1
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Sherwood AX-7030R amplifier. I'm repairing a Sherwood AX-7030R amplifier for a client. It's dead, no standby light and all the fuses are OK. Removed lid and did a few initial tests, check the driver and output transistors and for anything obvious. Found a 2SC2168 driver transistor with a short circuit between emitter and base. Out with it and test it again, it's OK! Check the PCB, no short circuit. So the transistor has been refitted, again no S/C. Anyway, it turns out the standby transformer has an open circuit primary. The secondary voltage will have to be determined. The 330uF reservoir capacitor is a 35 volt component so I'd guess it's a 25 volt transformer but I'd better make sure before a new one is fitted. The third attachment shows that the standby transformer doesn't look too good. Anyone familiar with this amplifier? DFWB. Attached Thumbnails

8th Nov 2019, 7:25 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Sherwood AX-7030R amplifier. If you are confident that you correctly measured a fault in that transistor, then I would replace it anyway. It may have developed an intermittent fault, and suitable replacements only cost pennies.

8th Nov 2019, 7:27 pm	#3
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,400	Re: Sherwood AX-7030R amplifier. Hopefully a reasonable assumption- the small blue relay in the third picture appears to be marked "12VDC", presumably coil voltage and presumably driven more-or-less directly from the standby supply by the small plastic transistor, though the small, slightly cooked and stood-off adjacent resistor may be dropping some voltage. I'd do a bit of track-following for guidance here, Good luck, Colin

8th Nov 2019, 9:28 pm	#4
sortedradio Heptode Join Date: Jan 2008 Location: Sheffield, South Yorkshire, UK. Posts: 656	Re: Sherwood AX-7030R amplifier. The service manual is available foc from https://elektrotanya.com/sherwood_ax...wnload.html#dl __________________ Martin BVWS member GQRP Club

8th Nov 2019, 10:48 pm	#5
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Sherwood AX-7030R amplifier. Standby transformer should be approx 8 - 12V, no more, I'd go for 10V. Circuit attached. Attached Thumbnails __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

9th Nov 2019, 2:41 pm	#6
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Sherwood AX-7030R amplifier. "Standby transformer should be approx 8 - 12V, no more, I'd go for 10V." Hi Michael, many thanks for the information. I've found this unused 8 volt transformer which I'll try out later today. DFWB. Attached Thumbnails

9th Nov 2019, 11:59 pm	#7
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Sherwood AX-7030R amplifier. That's the amplifier fixed. The "8volt" transformer is delivering slightly under 10 volts to the bridge rectifiers. There is 10.5 volts across the reservoir capacitor C502. That short circuit transistor was in fact caused by the collector tab of the 2SC2168 coming into contact with the safety resistor R1711, see second attachment. DFWB. Attached Thumbnails

10th Nov 2019, 1:13 am	#8
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Sherwood AX-7030R amplifier. OK, fair enough.