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Transistor Leakage

M

mcdmgb

AK Subscriber
Subscriber
I have been playing with a Sansui R-30 receiver in a first attempt at DIY repair. The DC offset is approximately 35 volts in each channel, its obviously pretty sick. I have been using a recently acquired Sencore TF 166 transistor checker and have been checking the transistors in circuit. The gain of most if not all of the transistors has checked out ok, but when I check leakage, the numbers all over the place. The lowest readings is 20 microamps, with the highest being 100+ microamps. Is the 20 microamp amount of leakage considered okay and is the transistor is acceptable? At what point is the amount of leakage unacceptable, and the transistor replaced?

Also, if anyone has a manual for a Sencore TF 166 I would be interested in purchasing a copy.

Mike
 
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Hi,
If you measured the DC at speaker terminal is 35V for R30, that means one power transistor is in full conduction mode!
I fear it can be an indication that the one of the power transistor is blown.
Suggest that you look for replacement.
 
Yes I understand that at least one of the power transistors is fried. However, the thrust of my question has to do with interpeting the results of the transistor checker, and what is considered to be an acceptable level of leakage.

Mike
 
I see. Normally measure leakage current for transistors require the devices to be out of circuit.
Found the link for you: go to www. tpub.com then do a google search under " transistor Leakage "
It says:
Small Si 0.1ua
Large Si 1-50ua
I have a R50 and the speaker DC offset is 35 left 25mv right channels.
MC
 
Leakage will be directly proportional to the voltage applied. I'd think the testers likely don't apply much voltage, so I'd be leary of any leakage over 50µA.

Seems a manual for the tester ought to give at least a basic guide...
 
I tested all the transistors in circuit with my Sencore TF 166. Of the 26 transistors, 24 of them tested bad, either by poor gain or having leakage greater than 50 microamps. The only good ones are a couple of pre-amp transistors. Is this rate of failure common or otherwise to be expected in older equipment?

Mike
 
Out of my a dozen, or so, vintage receivers/ amps that I have and restored, only 3 gave me transistors problems:
1. The HK 430, which has a cracking noise transistors at the input of power amplifier. This transistor is well known being troublesome. Got it replaced and wholething sings again.
2. The Pioneer SA600 integrated amp. First stage of tone control. This caused slider noise when adjusting the volume control ( at its input ). Again this Hitachi transistor is welknown for its aged poor performance. I believe this is a genuine case of leaking.
3. Getting poor quality treble out of the Marantz 2250B, both channel. Renewed all capacitors but no improverment. Suspect leakage transistors at power amp but no time to troubleshoot yet.

The rest, including a Sansui R50 do not ( seemed to ) have transistor problem, Electrolytic capacitors are a common cause of leakage souce. Replace them will definitely improve the performance ( tuner ) and sound quality.



No sure about in circuit measurements because the measurements could be due to bias voltages or leaky capacitors.
 
As far as I know, the sencore transistor checker is a standard one that can't perform in-circuit measurements. No wonder so many of them appear to be malfunctioning.
I'd check one by one, starting with the output transistor pair after desoldering them. Certainly a boring task
 
I'm still looking for the manual for this tester. It says on the case that it is an in circuit/out of circuit checker, however that's not to say that it can perform both the gain test and leakage test in circuit. If one assumes that the leakage test results are erroronious, then about half of the trasistors fail on the basis of the gain test. Based soley on leakage, most if not all of the failed transistors are in the amp section.

Mike
 
I just finished scanning the TF166 Service Manual since I am in no position to copy it. However, the files run about 1,500 kilobytes per scan so loading them here is out of the question, but if you PM me your address I'll burn the lot onto a CD and you can print them out or just view them on your PC. There are 27 files, each one showing two pages as the manual is 5.5 x 8.5 inches. Oh yeah, it does in-circuit testing.

Rob
 
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