Toshiba Receiver SA-220L loud pop when switching on

[reactolite]

Just bought this receiver for a fiver from the Heart Foundation (good cause) and when switching on I get a very loud pop but when I switch off the music dies away slowly, 2 seconds or so. Looking at the schematic I see there is a mylar cap .047 200v across the main switch could this be the culprit or is this cap merely to prevent the switch from arcing and excessive contact wear?

Love to solve this problem as the FM sounds wonderfull - except for very slight background hum in both speakers but not affected by volume adjustment.

Your thoughts & suggestions would be much appreciated. Interesting to note there is no earth cable. Please keep it simple as I am not very technical!

[peter_sol]

The pop is most likely normal for that piece of equipment.
It will not damage the speakers.

['LIVEWIRE?']

That 0.047uf is, as you suggest, a suppressor capacitor, designed to reduce mains borne interference, and to protect the switch contacts. One thing puzzles me. If it's a UK?European market ser - i.e. one designed for 220/240vac operation, the capacitor ought to be rated at 300v or more. Either remove it, or replace it with a class x capacitor
These are designed for the purpose and are rated at 300vac or more. The value isn't critical. A 0.01uf or 0.022uf will be adequate. Why that 200v capacitor didn't go shortcircuit or blow apart when fed with 240v I don't know!!

[reactolite]

Originally Posted by peter_sol

It will not damage the speakers.

Sorry I didn't describe the 'pop' very well, it's not a quiet & innocuous noise but a great loud electrical thumping noise which is alarming and I do not think this is normal even for a 30 year plus receiver.

Originally Posted by 'LIVEWIRE?'

Why that 200v capacitor didn't go shortcircuit or blow apart when fed with 240v I don't know!!

I'm glad you mention this as even I wondered why there was a 200v cap! But the voltage selector at the back has a range of between 220v and 240V so it is designed for the European market.

[Focus Diode]

Originally Posted by reactolite

it's not a quiet & innocuous noise but a great loud electrical thumping noise which is alarming....

I agree this is not normal. Years ago I had a Fidelity stereo record player which did this from new, the loud splutter when switching on was most alarming, even though it was expected. I'm sure an identical model I saw didn't do this....

I never got round to looking at the problem. I'd guess they should be some form of muting circuit to reduce the initial audible surge of power which may not be working in your receiver, possibly from new.

I'd be interested to hear what others think, perhaps someone else has a similar receiver to yourself who can confirm if this is "normal" or not and of course offer suggestions to help solve the problem.

Brian

[Gillian]

Hi everyone,I think it is probably a DC settling fault
I would be checking or replacing C607 ,C608 33uf 25v capacitors in the right and left channels of the power amplifier respectively.
also C604,C603 also in the power amp .
It may be just one channel making the noise ,but it may be alarming so you can't work out which one is making it.
Also output coupling electrolytic capacitors C609,C610 may be worth checking for value or leakage.
Please observe carefully the polarity of the capacitors when replacing or refitting,as they don't like being reversed and heat up and smoke ,as discussed elsewhere in this forum.The hum is most likely the smoothing capacitors in the power supply as well .
They are all electrochemical devices and age and as the amp is circa 1975 that is a lot of years.
Gill

[reactolite]

Thanks Jill, I have located the four caps you mention but as for the smoothing capacitor(s) is it C803?

[sp10mk11]

Hello I had a Toshiba SA-320L from the same range, the power switch was part of the speaker selector, this had no muting circuit and would produce a gentle pop when switched on, so I made a speaker muting relay circuit and this solved the problem of the switch on pop.

I have had various pieces of equipment that have thumped etc when switched on.

Good luck with this

Gary

[reactolite]

Thanks Gary, that's interesting but unfortunatley your solution would be way above my skill set also the thud I get is not a gentle pop as you describe.

[Gillian]

Hi Indeed it is C803 and that should hopefully be the source of your hum,
Gill

[paulsherwin]

Lots of equipment of this era did produce a pronounced switch on thump as part of normal operation. It's not possible to say if yours is abnormal without listening to it.

[nigelr2000]

Seconded my old 70's Amstrad amp gave a ker thump at switch on and a squeak if it was on phono when it was turned off !

[julie_m]

It's normal for transistor amplifiers to make a loud "pop" at switch-on, as the power smoothing and loudspeaker coupling capacitors charge up. If the capacitors are faulty (and they can and do die of old age), then they could be taking longer than normal to charge. The music dies away slowly, if the amplifier is switched off with a source still connected, as the power smoothing capacitor discharges.

Changing these capacitors might not get rid of the thump altogether, but should reduce background hum (smoothing) and might improve the bass response and reduce distortion on loud passages (speaker coupling). You can fafely fit a larger value capacitor in the smoothing position; a new capacitor of the same rating probably will be physically smaller than the one it is replacing. just fit the largest capacitance value you can get at the same voltage rating. An additional ceramic capacitor of 100 nF. - 470 nF. and the same or better voltage, wired as near as possible to the output transistors on each channel will help to prevent high-frequency instability.

[PE9ZZ_JO22KI]

A loud thud isn't normal. It should be really soft. This is an amp with single supply. The output caps (C609 and C610) charge at powerup. If the voltage at the hot side rises too fast you get this thud. Check C804. This is the powerup delay. If it's dried out you get your loud thud. A replacement should also cure your 100 Hz hum.

Tjerk, 9ZZ

[reactolite]

Thanks, this sounds a likely solution. It's not the switch as I have tried powering on from the wall socket with receiver switch in the on position, same thud. Also, if this is any clue, using the balance knob I get a thud from both L & R channels so not related to just one cap of a dual cap circuit? Bear in mind I'm no techi!
Another clue is that the unit was standing for some time switched off and when I plugged in my head phones I got a whacking great crack from some discharge. Looking at the schematic I see C609 & C610 are connected to Phones Out.

[Gillian]

By Jove,Yes C804!!.It may be that the others may need checking as well in my defence.
Gill

[PE9ZZ_JO22KI]

Tjerk, 9ZZ

[reactolite]

Well I have taken the plunge and have ordered all the (power supply side) caps including all the ones suggested by several members. So as soon as they arrive I will fit them (one at a time then test to discover if the fault is cured and there is any improvement?) A reminder I am no techi!!. Will post again once I am able to get going.
PS thanks everyone for all your input.

[reactolite]

Ok so following on from yesterdays post. Today received my order for new caps also desoldering braid 3mm (worked well this) and received a complementary quantity of solder (very nice). Changed C804 which was suspect No1 - no improvement. Continued changing and testing as I went along and got to cap C606 and thought there was an improvement regarding headphones in and out whilst unit on, less noticeable crackling. Continued doing this until I had replaced 14 caps in all:
C804, C609, C610, C603 C604, C607, C608, C806, C805, C509, C510, C803, C601, C602. They all look ok to me but unfortunately I have no way of testing them.

Results are: still a hefty wallop on switch on
Still a slight hum which is stable in volume (both channels) no matter what control is changed; balance, treble, volume etc.
But at least the headphones can be plugged in and out whilst unit is on without any harsh crackling.

Could the hum be normal, perhaps I'm being over critical. If not could it be some grounding problem, there is no earth wire.

One point to mention, replacing caps C607 & C608 the schematic calls for 33uf 25v and the closest I could order was 47uf 25v but the caps I removed were actually 220uf 16v is this common to find different values on components when removed from circuit board?[/B]

Yesterday I had sold a small item on ebay and pocketed just over £15, my order came to £12.41 so happliy this paid for the above. I did enjoy doing this project. I've learnt how to desolder and replace capacitors (the correct way round!) and for once I've not made something worse by tinkering with it!!

Are there any more suggestions - although I'm not sure I want to spend much mor on this project!

[PE9ZZ_JO22KI]

Did you actually measure anything or did you just go on a wanton replacement spree? I still suspect the circuit around C804 (with its associated hum), so if replacing it doesn't solve it there still must be something wrong. Maybe a grounding issue. Take your scope and see what the voltage across C805 (emitter TR801) does if you switch on. It should rise in a few seconds, not be there immediately. Then switch to AC and increase the gain. There should be no ripple at all. The collector of TR801 will have ripple of a few hundred millivolts.

As for C607/8: these are bootstrap capacitors. Their value is not very critical. Larger values give better low frequency response.

Oh, and do your speakers a favor and use a dummy load of 10 ohm/10W.

Tjerk, 9ZZ