UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Microvitec Cub 1431 Power but no picture.

Lucien Nunes · 26th Mar 2018, 12:42 am

I used to see hundreds of these. The TDA4600-based power supply in the Series 4 MS4F was unreliable and they later returned to their previous discrete design. Check for HT (115V) at D10 cathode. If absent, provided there is no short on the HT rail the PSU is probably blown up. With R18 and R227 lifted at one end, lack of HT is conclusive diagnosis.

As with many SMPSUs, it is necessary to change all faulty parts at once, otherwise powering back up when only some parts have been changed, can destroy the replacements. When changing the TDA4600 and TR1, check everything nearby in the non-isolated area. I used to change about 12 components as a matter of course such as those in the base drive path of TR1, without bothering to test, and they usually worked first time.

IIRC, another common problem that could kill both line and frame was a solid short on C306, the reservoir cap for the 24V flyback supply to the frame output. In this case there will be some volts on the HT but no EHT. The TDA3651 was also a fairly common casualty.

As mentioned above there is not usually any need to discharge the EHT, so long as you are not disconnecting either the EHT lead or the tube dag ground. But note that unlike most other Cub chassis, this model uses a diode-split line output transformer that will remain charged for weeks, rather than discharging in seconds through the focus pot.

26th Mar 2018, 12:42 am	#7
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: Microvitec Cub 1431 Power but no picture. I used to see hundreds of these. The TDA4600-based power supply in the Series 4 MS4F was unreliable and they later returned to their previous discrete design. Check for HT (115V) at D10 cathode. If absent, provided there is no short on the HT rail the PSU is probably blown up. With R18 and R227 lifted at one end, lack of HT is conclusive diagnosis. As with many SMPSUs, it is necessary to change all faulty parts at once, otherwise powering back up when only some parts have been changed, can destroy the replacements. When changing the TDA4600 and TR1, check everything nearby in the non-isolated area. I used to change about 12 components as a matter of course such as those in the base drive path of TR1, without bothering to test, and they usually worked first time. IIRC, another common problem that could kill both line and frame was a solid short on C306, the reservoir cap for the 24V flyback supply to the frame output. In this case there will be some volts on the HT but no EHT. The TDA3651 was also a fairly common casualty. As mentioned above there is not usually any need to discharge the EHT, so long as you are not disconnecting either the EHT lead or the tube dag ground. But note that unlike most other Cub chassis, this model uses a diode-split line output transformer that will remain charged for weeks, rather than discharging in seconds through the focus pot.