Bush AC91.

[Geordie McBoyne]

I've gathered up enough parts to reconstruct a Bush AC91, the three waveband version of the Bush DAC90. The chassis is a bit rough to say the least but it can be restored to working order. A salvaged cabinet has been found and from an eBay seller an excellent reproduction back. To complete the job two more control knobs must be found.                                  This is the chassis with a mains transformer so there isn't any of the heat issues the AC/DC versions suffer from.
In some respects the chassis resembles that in the model SUG3. Doesn't have the bi-focal negative feedback.

Geordie McBoyne.

[bobhowe]

Hi David Glad you won the Bush AC91 chassis are you going to do a blanket change of capacitors or a basic check up before a HKS plug in ? kind regards Bob

[Ed_Dinning]

Hi David, I think this set uses the auto transformer on which my design is based; so a potential live chassis.

I may have some knobs, I'll look on my return



Ed

[bobhowe]

Hi Ed you are correct it has an Auto transformer but if I am correct it has not got the separate tapping of 6.3V for the dial lamps like your Auto transformer . Hi David are I am correct in saying about the Auto transformer does not have a separate tapping for the dial lamps ? kind regards Bob

[Geordie McBoyne]

Hi Bob,
the attachment shows the differences between the AC only model AC91 and the universal version, the DAC91. The tuning scale lamp in the AC91 is 6.3volts 0.3amp and in the DAC91 it is 3.5volts 0.3amp.
The small console models SUG91 and DUG31 are electrically similar.

Geordie McBoyne.

[Ed_Dinning]

Yes I see , scale lamps 6.3v and stabilised by tap on autotrans

Ed

[Geordie McBoyne]

Last night the broken CCH35 valve was replaced, the wiring around the mains transformer checked. Surprisingly, the single pole on-off switch is OK, contact resistance 0.3ohms.
Connect up to the variac, supply the chassis with 100volts. After several minutes a loud hum coming from the 'speaker, 'scope connected to the reservoir capacitor. All too evident the smoothing capacitor is useless. Original is 16 + 32mfd, found a good 32 + 32mfd capacitor which should be OK to use in this receiver. The Mullard Maintenance Manual tells us that the CY31 rectifier's HT reservoir capacitor can be 32mfd when used with a 150ohm series anode resistor, which it is.
The attachment shows the waveform across the reservoir capacitor.

Geordie McBoyne.

[bobhowe]

Hi David it seams that the smoothing capacitor on the DAC90 were useless as well but the DAC90A usually reformed ok different make and type ? . What is the next step after replacing the smoothing capacitor ? change the grid coupling capacitor an see how it performs or a capacitors blanket change ? kind regards Bob

[Geordie McBoyne]

The HT smoothing capacitor was replaced today so there is no hum coming from the loudspeaker. Full mains voltage now supplied to the receiver. The audio amplifier works OK, loud hum when the top cap of the EBC33 is touched. Quite a lot of background crackling and noise, gets worse when the chassis is tapped. No clicks from the loudspeaker when the wave-change switch is operated. Measure the cathode volts on the of the EF39 IF amplifier - zero volts. The valve is faulty. Replaced with a CV1053, Now the cathode voltage on pin 8 is 90volts! Bad earthing connection due the valve fixing screw having lost it's head. Replaced the 6BA screw. Cathode volts is now 2V. An aerial is connected and on the medium waveband weak reception of one station can be heard. Greatest Hits on 261 metres. Transmitter is only three miles from my location. One of the tuning cores in a IF transformer is broken. It's possible more than one tuning core could be broken. It's a very common fault in these receivers. Also, the DAC90, SUG3 etc have the same problem. An easy fix assuming the offending core isn't too badly damaged.

Geordie McBoyne.

[bobhowe]

Hi David glad you have given Bush AC91 chassis a second chance as if you had not it would have been in landfill . This will become a good outcome. with your patience kind regards Bob