Hi all,
Another thread on this Vfet amp – this one was known to be deceased from when bought from E-bay. However, through a well-trodden tale of woe, I have a set of V-fets for it, just a case of figuring out what caused a problem in the first place,
First of all – service manual, I have refurbed a TA5650 previously and you can download plenty of V-fet information readily. You may have to have several goes before you find a really clear copy, but you are looking for the service manual and any bulletin you can find, either for the TA4650 or the TA5650.
Second of all it arrived! Since the seller gave no useful information on what or why it wasn’t working I figured check Vfets – heck I MIGHT be lucky
Yeah, I wasn’t 🙄
Not 100% sure that they, the V-Fets hadn’t been disturbed, for all I know at this point the amp was bought to salvage a set of transistors, as otherwise the internals are ( I’m 99% sure) completely un-touched and a reminder that more people smoked in the 80’s. Nothing burned and no obvious failures like a bulged cap or burnt resistor.
I'm intending to write this up as I go for information. This tale might not have a happy ending. But I have a dead 5650 and a dead 4650 to go at as well as what seems to be a set of V-fets
All the gear and no idea
Step 1 then: where does this fit on the Sony development cycle? These amps had at least one major update it seems, to try and reduce their propensity for self immolation. This example will need the fitted 1.5k, ¼ watt resistors R313, R314, R363 and R364 exchanging for 3.3k on the Power amp (F) board. DC bias was probably set at 75mv too but that’s irrelevant as it’ll be re-set later.
OK Party time at RS components, or your preferred parts peddler. £$£$£ add .gif of throwing cash...
I’ll take each board in turn at this stage, but my rules of engagement are:
Cap values – same, I believe the V-fet amps have to wake up in a certain ‘order’ from what I’ve read so I’m sticking with Sony’s values. Plus it’s nice to know that the character of the amp will be preserved. The only real gripe is that the power supply caps on the TA-4650 are 4700uF whereas on the substantially similar TA5640 they are a whopping 10,000uf. I had planned to split the difference with 6800uF but ultimately I bottled it – your first explicit invitation for “Comments Please”
Cap voltages – a variable; I figure that capacitor technology must have improved since this was built and I’m not trying to save every penny so if it fits and is a bigger voltage I’m ok with that – to a point eg a 10v cap might become a 25v cap of the same value.
Cap Types
Frankly, 0.47uf is not a value I associate with electrolytic capacitors and even 3.3uF is a push so these are going to be switched to polyester types. Same for C351 and C301 (10uF/16V) as they sit I the audio path. Equally, where available I prefer polymer types over electrolytic. That is MY preference your science may vary. Don't grumble, whatever, 40 year old etc...
Resistors
Just normal ¼ watt types nothing fancy, nothing fancy coming out nothing fancy going in. But they are checked if it says 3.3k it better measure that before soldering.
Final disclaimer, any specific components referenced are my choice, and probably based on the flimsiest of logic..
Sooooooo…..
Board 1 – power amp board (AKA the F board)
Pre-power on aims:
FIRST replace the so-called ‘death diodes’ VD1221 with 2x 1N4148 there are three on the Power amp board D414, D352 and D302. The latter are thermally mated to the transistors next to them, I’ve read both, that it’s worth doing, and it’s not worth doing. Sony did it, and it would have been another step in manufacturing; ergo, why not?
As a side note I’ve not yet come across a bust VD1221, maybe if they got this far they’re ok? I'm four amps in at this point
All Electrolytics will be replaced. Come on, this thing is more than 40 years old. Let’s just assume a component with a rated hours life time is past its best! (As I am slightly older than the amp, let’s also give thanks that this logic doesn’t extend any further, Oh hang on a minute…)
Not many capacitors really:
TA-4650 - F board
Qty old Value voltage New Value Voltage RS
C417 1 0.47uF 50v 0.47uF 63v 108-2429
C301. C351 2 10 uF 16v 4.7 uF 63v 171-9241
C306, C356 2 10 uF 16v 10 uF 25v 714-9622
C307, C357 2 10 uF 16v 10 uF 25v 714-9622
C303. C353 2 100 uF 10v 100 uF 16v 839-2341
C305, C355 2 220 uF 6.3v 220 uF 25v 839-2180
C418 1 10 uF 25v 10 uF 35v 839-2316
C420. C421 2 100 uF 50v 100 uF 100v 571-650
Cap count: 14
Note especially C301 and C351, are the electrolytics in the signal path!! So changing to ‘not that’ makes me feel better. The 4.7uf value doubles (?) the L.F. cut-off point but it seems that Sony buried it in the dirt to begin with, sub 1.5 Hz still feels OK, but we haven’t listened yet 😀
At this point there are no guarantees that this will actually work, come power up, but since these mods will happen anyway my logic is to simply remove variables before that point. Less likely a problem will be a bad cap if it’s a new cap!
Bias resistors RT301 and RT351 won’t have moved in years and again there’s [STILL] that smell of tobacco so I picture a thin film of Marlboro in the resistive layer – and goodbye. To be replaced with your favourite VR no doubt, but in this case I chose Bourns 2.2K multi turn trimmer 3296Y-1-222LF or RS no 785-9767.
Interestingly measured resistances on the old pots were close to mid-range (bagged and tagged for later examination if needed)
R423 which forms part of what appears to be a filter network has got hot at some point, at least enough to burn the markings. On removal it measured 1.4k rather than the 1k value it’s supposed to be. So I’ve swapped out for a ½ w 1k. Hopefully that was the right thing to do… Worryingly it’s a resistor that will see some pain if transistors fail on the input side
Pretty happy with the amp (F) board at this point, just a little nearer powering up.
To be continued....
