Vintage Toshiba 330 power amp problem, possible repair?

[Mooly]

Its the only way to do it

[Mooly]

I was thinking some more on this...

If these two transistors, the 2SA841's, are a recognised issue (and I don't know if they really are or not) and if you are really keen to try and get on with this then you could always get a couple of transistors from Maplin who have shops in many towns. Either a pair of BC556's or a pair of 2N5401's are suitable.

If you did do that then come back here before switching on and we'll confirm absolutely 100% you are fitting them the right way round as the pin outs are different.

[peacocksuit]

Mooly I already ordered 2SA970's and they have arrived along with the multi meter. They where only cheap though the trans so if need be i can order different ones as well.

Il probably have a look at it new years day when im off work as im out all day tomorrow and in work Monday. Its taken months to get this far so now that its all ready to go there is no point rushing it. The system is older than I am so I few more days wont upset it too much .

Cheers.

Originally Posted by Mooly

I was thinking some more on this...

If these two transistors, the 2SA841's, are a recognised issue (and I don't know if they really are or not) and if you are really keen to try and get on with this then you could always get a couple of transistors from Maplin who have shops in many towns. Either a pair of BC556's or a pair of 2N5401's are suitable.

If you did do that then come back here before switching on and we'll confirm absolutely 100% you are fitting them the right way round as the pin outs are different.

[Mooly]

Well don't rush with it

The 2SA970 seems to have the same pin-outs as the 2SA841 (as far as my data sheets show). When you fit the transistors make absolutely sure you measure the DC offset BEFORE connecting speakers. That is to say, the DC voltage at the amplifier speaker terminals MUST be zero volts.

If you have ANY doubts over the transistor pin outs then ASK and we'll identify them using your meter first.

[peacocksuit]

Having already got the repalcement transistors Im going to trya nd fit them today and see if that solves the problem first. Should be fairly straight forwayd (I hope!). Iv just looked the ones they have sent me and printed on them is:

TCSA790 GR

I ordered:

2SA970

Are these the same thing?

[Mooly]

They should be but I would never take anything on trust...

Two choices... fit 'em and see what gives or at least do a basic test on them. Did you get a meter ?

If you fit them read the post above regarding NOT connecting speakers until you have confirmed there is NO dc voltage across the speaker terminals.

Good luck... ask if your not sure

[peacocksuit]

Stuck them in anyway and it works like a charm! Thanks for all the help!

Im really pleased with myself!

Now just a knackered volume pot to look at. Il make a new post and describe this fault in more detail.

Thanks again everyone

[Mooly]

That's great to hear, well done. It seems its a known issue by the sound of it.

[peacocksuit]

I am resurcting this thread rather than starting a new one as this post is about the same amp. The amp is still working fine and sounding great but has developed a rather worrying foible. When I switch it on and off it makes my plasma tv, which is connected to the same double gang power socket (and therefor circuit) go off for a split second. Its as though it causes a sudden lack of power.

Is this a sign of something a miss with the bigger capacitors not storing enough power or something?

Another thing is that when i switched it off it would take a few seconds for the sound t gradually fade away. Now it goes off almost immediately.

[Mooly]

No problem... easy one first.

The amp carrying on for a while and fading out, and now carrying for less time indicates a few possibilities. Most, or quite likely... the bias of the output stage has drifted a little and its drawing a wee bit more current. This setting is something that you should check as per the manual. More current means it pulls the charge out or the reservoir caps quicker. Another possibility you MUST check. That the DC offset is still zero volts DC on both channels. Thats just means measuring the DC voltage across the speaker with no signal. If there is an offset then that too pulls more current. And least likely... the caps are failing.

TV going off when you switch the amp on and off. I would guess the switch in the amp is arcing and upsetting the PSU in the TV. Try the amp on a different socket. Check all the connections in the plug for security. Consider adding a supressor to the switch. The fact it does it when you switch off as well as on suggests arcing rather than a dip in supply voltage caused by a surge.