Channel out on old Technics

[EdByrnes]

This old Technics (Panasonic SA-5170) has been running forever. Yesterday it lost power. It was the main 2Amp fuse. I replaced that and now when ever it is fired up, it blows the 3Amp speaker protection fuse on the left channel.

Any ideas on what it might be? There was a lilttle electrical smell but no components look discolored. I used a vacuum to get a ton of dust out of it. Thanks.

The best,
_________________
Ed Byrnes

[RiffMaster]

Blowing speaker protection fuse suggests failure of the power amplifier output transistors. If a DVM is available, measure the dc offset on the speaker terminals with speakers disconnected. The voltage should be 0v +- 0.3v or thereabouts. Anthing greater than this is a fault.

As to the cause. If the output voltage offset measured previously is close to 0v, should then check for short circuits in the speaker and wiring. If <>0v then check power stage for shorted transistors etc. Not sure of the topology employed for your amplifier is discrete or integrated circuit, to be more detailed, but if discrete there may be an underlying reason for failure such dry joints, component etc.

[EdByrnes]

Thanks RiffMaster, you were right! It's been quite awhile since I tried to trace circuits and then it was on a hobby level. The speakers are okay. I located the power transistors, on the etched side of the board, under two of the our power transistors, the board looked a bit burned. Leaving the components on the board I checked the resistance of the white 2W resistors (.22 ohm K) in front of each power transistor.

All were at zero ohms except the right most, it was at 1.3. I am thinking that maybe failure of this resistor caused the two right most power transistors to fry?

Readings between E-B-C-B were all zero while the left two were at .001-,003. I think I can be sure about trying to replace the bad power transistors, if they are available. I'll replace the resistor too. Is there more I should check?

When I replace the power transistors, I plan to scrape the white stuff off the heat sink and try to find new adhesive.
Thanks very much for the help.

The best,
_________________
Ed Byrnes

[RiffMaster]

Hi Ed,

I had a similar situation with a '70's Pioneer amp using similar NEC transistors. Over here in the UK, spares are virtually non-existant and those that are were prohibitively expensive. In this instance substitution by a modern type was very successful.

Failure of either power transistor will often result in damaged bias resistors (the two white 2W resistors (.22 ohm K) used here). Given the failure mode, it would be wise to check or even replace the three adjacent small signal transistors since the circuit is dc coupled and may have been stressed. Also check resistors for value and electrolytics for leakage.

It appears that TR60(7?) may be part of the bias circuit, setting quiescent current and compensating for temperature rise due to close proximity with the heatsink.

White material used under the transistors is special heatsink thermal compound to improve heat conduction away from the transistor junction to the heatsink. Also should there be a mica insulating washer, care is needed to avoid fracturing. Not sure if the transistor mounting tabs are internally connected to the collector but often are!

Suggest using a variac, if one is available, to power up for the first time after a re-build. That way if something is amiss it wont result in instant destruction of the new power transistors. Its always a good idea to monitor the quiescent current while slowly increasing the supply voltage and so would expect something between 10 to 50mA when operating at zero volume with speakers disconnected. If using a DVM this current may be measured as a small voltage (mV) across one of the 0.22 ohm resistors.

Hope the above helps.

Regards Richard

[EdByrnes]

Yikes! Now it's getting complicated! I found the Power Transistors and ordered them through

http://www.acme-sales.net/acmecart/html/main/portal.html

at 8 USD each and that included shipping. So .22 ohm K is .22 ohms, I thought it might have been .22 times 1000, 220 ohms, thanks I needed to know that...now only to find them!

I have a DVM but no transistor checker. I built a PNP checker in high school which beeped if the transistor was good, but I have misplaced it sometime in the 38 years since.

I do have a variac and will use it when powering on the first time. I have an old analog amp meter so I can monitor at zero volume, if the meter goes down to the mA range.

I think I will try replacing those Power Transistors and .22 ohm resistors and hope that is all I need to replace. I have some old computer carcases and plan to install a small fan to cool the heat sink, if I get the receiver working long enough to need it.

Thanks again RiffMaster, have a great one and let me know if you think of anything else I should consider.

The best,
~Ed
_________________
Ed Byrnes

[Guest]

The 'K' as part of the resistor specification represents tolerance. I think from memory it is +-10% and certainly in this configuration the exact value is not critical. As a check simply measuring a good one will confirm if in the right order.

Just a thought, as an alternative you can measure the quiescent current by connecting the meter in series with one of the 0.22 ohms. Its just a matter of convenience.

Richard

[EdByrnes]

Thanks again Richard,

Thanks for both teaching me that the K indicates tolerance and to simply (not so simple apparently for me) check the resistance on one of the good resisters to determine resistance. I am waiting on the Power Transistors and I must locate that resistor. And of course, it was just a week or two ago that I threw some "save for parts" receivers in the garbage.

Oh yeah, just for fun...I found this picture of the SA-5170 on the web.



Mine has lost its case.

The best,

_________________
Ed Byrnes

[hayee]

i have an old technics SA 5170 amplifier.the output power transistors have failed and not available in the local market.the manufacturer was NEC plz suggest any alternate transistors

[hayee]

i have an old technics SA 5170 amplifier.the output power transistors have failed and not available in the local market.the manufacturer was NEC plz suggest any alternate transistors.is there any site where i could see the circuit diagram of this amplifier

[hayee]

i have an old technics SA 5170 amplifier.the output power transistors have failed and not available in the local market.the manufacturer was NEC plz suggest any alternate transistors.is there any site where i could see the circuit diagram of this amplifier