Sony TAN-5550 Restoration Thread
- Thread starter MartinAudio
- Start date
HalfApt
Well-Known Member
Looking forward to what you may come up with. That's the one board I didn't update.
Crooks covering for crooks
MartinAudio
Young Collector
How nice for Ebay to promote a listing I reported.
Hi. Do you remember the source-gate, DC offset voltages of the amp with V-Fets removed?
Set it as high as possible. What rank v-fets does your unit have? For Rank 8, which is not a-typical in the 5550, you need at least 24v or higher, IIRC. I need to look at my notes. In contrast, Vgs can be start as low as 12V for Rank 3
HalfApt
Well-Known Member
So the last digit of the rank marking is the Vgs rank? Do you know if the prefix is meaningful? I have R58 (J18) and HE58 (k60).
Yes, the last digit is the rank. The number prefix indicates different generation, as in the v-fets with the 5 (53 to 58) were used for the first gen equipment.
I see sometime parts marked with 51 and 52. Those are mostly fake reprints, however there were some made by Sony but never used in any of their equipment. Likely sold as parts for DIYers (gm too high for any of the Sony designs without mods).
There was an initial series for the first produced units, primarily the TA-8650, where I found a different nomenclature that was still based on Rank 3-8, but it was limited.
Then you had other prefix numbers such as 2, 3 and 4 (IIRC) used in the second gen units: TA-N7, TA-F7 and for the dual-die parts used only on the TA-N88 for which but the last digit is also the rank.
I am thinking (and have no way to know for sure) any letters were to differentiate the batches, mfg dates, etc.
I see sometime parts marked with 51 and 52. Those are mostly fake reprints, however there were some made by Sony but never used in any of their equipment. Likely sold as parts for DIYers (gm too high for any of the Sony designs without mods).
There was an initial series for the first produced units, primarily the TA-8650, where I found a different nomenclature that was still based on Rank 3-8, but it was limited.
Then you had other prefix numbers such as 2, 3 and 4 (IIRC) used in the second gen units: TA-N7, TA-F7 and for the dual-die parts used only on the TA-N88 for which but the last digit is also the rank.
I am thinking (and have no way to know for sure) any letters were to differentiate the batches, mfg dates, etc.
I have 57 rank. The highest Source-Gate voltage on V-fet conductors (50VDC at 2SK and 80VDC at 2SJ) corresponds to the highest DC Bias voltage with V-Fets in place. I remember standard 65mV DC bias point is in the middle of POT position. May setting it to maximum overheat the V-Fets with the highest possible DC Bias?
What about DC offset at speaker outputs? Is it normal 0.3-0.8VDC without V-Fets? DC offset balance adjustment doesn't change anything without V-Fets.
Do you know what ranks can be used in TA-N7 amp?
Thanks.
You have Rank 7. Read above. You need to have at least 20 Vgs. Higher would be better.
I have some videos on this topic:
You would have to close the NF loop to test without vfets in. Have you worked on any vfet amps before? It is very easy to fry those vfets.
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Yes, 7 rank. I have 19-20Vgs. One channel can't hold bias, it drops to 15mV in 10 minutes after long time storage of the amp. Voltage after R503 drops to 22VDC. I think I need to replace old bias transistors. Do you think it's possible just to remove 3-rd connector from B board (WHT-VIO-BRN wires) in order to leave the V-Fets connected but power them off like it's described in 8650 amp service manual and test the bias of the amp?
Yes, I have N7, 8650, but I'm not a technician, just a hobbyist. I'm aware of it, that's why I asked before put them in the circuit.
Thanks for your advices.
After replacement of Q601 the bias is steady now. Increasing the bias will lower the Vgs at the V-Fets (75mV bias/18.8Vgs). Setting the bias to 40mV will keep the Vgs at 20-21V on both channels which indicates the need of replacement of degraded old bias transistors.
So, setting the dc bias adjust resistor for maximim reading, described in Sony Service Bulletin No. 62, actually means decreasing the bias setting to the minimum with the highest Vgs reading at the V-Fets in place or the maximum reading of the resistor itself 22K. The words "with the V-Fets still out of the circuit" must mean not removing the V-Fets but powering thems off, as the reading of the VOM will be completely opposite with V-Fets removed and left in place.
SM of 8650, 8550 says 19.4-21.9V is acceptable Vgs for 7 rank V-Fets presumedly at recommended DC bias setting.
I have 27-30VDC at collectors of Q501, 601 and 60-80mV at collectors of Q105-106. What are your readings? In comparison to 8650, 8550 similar schematics, collectors of Q106-107 are 0V.
Ok, I will wait for your measurements. Thanks.
Do you have DC offset fluctuations during amp operation?
No
In case you have not figured this out already, I just got done with the adjustments of a Sony 5550.
With vfets out of circuit (and with NFB bridged), I see anywhere between 13-27 Vgs OR a ~26-54v potential across R511, depending of the adjustment.
I made sure that the trim pots for the bias adjustment can adjust the bias within that range, then left them at the highest reading (pot fully CCW)
I made sure I can adjust DC offset past zero in each direction (+/-)
Fitted the vfets and made the first round adjustments. The amp is now baking over night. I left the bias at 70mV. I suspect it will drop below the recommended 65mV as the amp bakes in well, since vfets have a negative temp coefficient.
DC offset is within 2mV on either channel. Will see where is at in the morning, but I left the cover off so that is not really the right way to do it. For the first round of adjustments, it is goo enough.
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