[TechAssist] Re: Sony KV27TW75 Blowing Q601 and Q602
- From: "Leonard Caillouet" <lcaillo@xxxxxxxxxxxx>
- To: <techassist@xxxxxxxxxxxxx>
- Date: Sun, 24 Apr 2005 17:25:55 -0400
Peter,
While I have never done so, looking at the specs for the 4834 and 4664, it
seems that the former should be a good sub for the latter.
I assume the HOT did not blow.
In all of the Sony sets with the switching pair, you need to check the
timing of the circuit carefully. The duty cycle of the transistors should
be 50%, within a couple of percent. The things that cause the transistors
to fail in short order are usually timing related. Several things to
consider are the small timing caps, the matching of the gains of the
transistors, the gain of the transistors, and the transformer. I always
crank the supply up slowly and look at the switching frequency. If it is
very low(I'll have to check the training manual on Mon at the shop to get
the frequencies for the LN1 that you should see at various ac voltages), the
transformer may be bad. If the duty cycle is not matched then one of the
caps is likely bad or the transistors are not matched. Be sure to use
higher rank matched transistors. I always buy at least 4 at a time and
match them for gain and junction bias. You can mark them up enough to cover
the cost of the extra transistors and still be cheaper than the Sony
supplied parts and get a better match.
As for VDRs that people say to change, they will usually cause problems
immediately, but they are a lot less of a problem than others will imply.
Not an issue in this set, if I recall correctly there are none.
Of course, always check any electrolytics in the base circuits of the HOT
and look at the collector waveform.
Leonard Caillouet
Electronics World
1261 NW 76 Blvd
Gainesville, FL 32606
352-332-5608
FAX 352-332-5668
----- Original Message -----
From: "Peter oliver" <oliver_peter1@xxxxxxxxxxx>
To: <techassist@xxxxxxxxxxxxx>
Sent: Sunday, April 24, 2005 12:27 AM
Subject: [TechAssist] Sony KV27TW75 Blowing Q601 and Q602
Sony
KV27TW75
Chassis LN-1 (very simmilar to the AA-1 in the KV27TS27....)
MFD 1992
Set came in with blown HOT, SW mode transistors and fuse.
Connections on HDT were ok but wax had melted out of it. Replaced then HDT
with one from a AA-2 chassis (same #) Pulled flyback (1-439-498-12) it rang
31 times one I had on the shelf (a leftover that didn't fix another set rang
46 times) so I swaped it out with the one on the shel. used 2SD1880 (from
B&D). Used 2SC4834 M instead of 2SC4664 B for Q601 and Q602. Set ran long
enough to show a picture for a minitue then blew Q601 & Q602 and the fuse.
Found a service bultien in the filing cabinet to change R647 to 1Ω. But
is still blowing Q601 &Q602 about a miniute after the picture appears (the
2nd time used so left over pulls from a power supply C4106 to test) Ideas..
Caps etc...
Peter Oliver
Tucson Radio TV
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