Sony TA-5650 big VFET amp, 1970s

Don’t see many of these nowadays. Early 1970s Sony TA5650. One of the first amps to use VFET transistors in the output stage. Nice amplifiers and very collectable within the audiophile community. Sound very good for age. BUT!!! Now very rare, mainly due to the VFET transistors blowing up. These Toshiba devices have been out of manufacture for years and impossible to come by. Yes, look on Aliexpress and China Google and you can buy a handful, quite expensive. Of course they are crap. Open them up and you will find the device is full of cream cheese and ball bearings. The last known stock of the original genuine parts was found in a small village in Outer Asponistan, guarded by a village elder with a Yak that hadn’t been fed for a year. Now the main reason for the failure of these devices is due to a couple of components used in each channel. Sony used temperature tracking Varactor diodes to help track bias with temp. These devices had a habit of going open circuit intermittently wreaking havoc in the output stage and blowing up the parallel push pull configuration. A client had bought this amp for a high price and wanted it services, with these questionable parts changed. You don’t get many chances with this set. Muck something up and it goes in the bin. Equiv. parts are not available and, well, don’t go there. So, for those of you that may come across one, here are a couple of tips that may help you. Schematics are available, easy. Check to see if the thing runs before anything. If it does, check the power supply and the 20V setting. There is a pot to adjust if needed. You will see on the main amp board, there are two sets of these diodes, see photo attached, you will see one location highlighted with a sad face. Each of these can be replaced with two 1N4181 diodes in series. Make sure you put them in the right way round, there’s a good lad, girl, or neutral something or other. These can be heat sink compounded to the transistor below or with extended leads to the main heatsink. I will assume that you will check and or replace caps as required. Right, connect up and check there is no nasty offset on the speaker connections, after you hear the speaker relay click in. Assuming you are getting audio through, have a check of the bias. This is quite critical. The service manual explains what you should see at the test point for each channel. This setting depends on the rank revision of the VFETs used! Pain. Be careful. You have to use a very sensitive DVM, and 0V is not the chassis, use the PSU 0v. You can create oscillations etc… all of which can blow things up. The pot in each channel has not been touched for years so will be scratchy. A squirt of jollop is a good idea. A tip here is to leave it on for 5 mins and see if the heatsinks stay reasonably cool. If one end (channel left or right) gets warm quickly. Carefully check the bias. I will be shot for saying the next thing, but if it gets you out of jail without killing the amp whilst Liam Neeson is holding a gun to your head, do this. An easy test is to monitor the current drawn at the mains plug, I always do. The quiescent DC current is stated in the manual but if you cannot get a meter in line with the supply to each cannel, I have found that with no volume, about 120mA at 220V is about right. So there we have it. A quick summary of dos and don’ts. Next time, I think I will replace these VFETs with KT66s.