Toshiba sb420 amp fault

[Jime174]

Hi all.

I'm having a problem with this amp and this seemed like a good place to seek help.

The right channel had no output in stereo mode, but when switched to mono it works. The balance pot will pan between the left and right as expected in mono too.

I'm using a turntable and this occurs on phono 1 and 2 inputs. It also happens on speaker 1 and 2 outputs.

I've plugged the turntable into the tuner input and although much quieter as would be expected, there is output from the left channel and still none from the right. Switching to mono again gives output on both channels (presumably duplicating the right channel on both speakers.

I've not cleaned the switches and pots yet but they all seem to operate smoothly with no crackling.

I've had the cover off and looking at the PCB, I can't see any exploded capacitors or other obvious fault. There's another PCB under this which appears to have tone and source controls connected to it but I've not looked at that yet.

I have a multimeter and basic soldering kit but that's about it.

Can anyone offer any advice as to where to start in identifying where the fault lies? If be very grateful.

Ps. It has links in the back where I can de couple the pre and main amp circuits if that would help.

[Edward Huggins]

Suspect there may be a RH Channel fault on your cartridge. If not check its pin connections at the cartridge or the lead to the RH phono plug.

[SiriusHardware]

Hi (Jim?) welcome to the forum. As a new user your first few posts will be delayed by moderation, but that will wear off after a while.

The attached block diagram is taken from the service manual which is available to download free from several sites including Elektrotanya. The block diagram shows that the stereo / mono switch (S4) is fairly early on in the unit (right after the massive array of switches which direct the chosen source to the input of the preamp / tone section) so to judge from the sensible tests you have already carried out, your problem is something which is prior to S4 but still common to the right hand channel for all sources.

For example if one channel of the MONITOR switch S2, or the wiring to it, has gone open circuit that would result in the loss of all input on that channel no matter what the source.

If you haven't already done so it isn't a bad idea to 'exercise' (robustly operate) all of the switches to see if that makes the problem go away for at least some of the time, starting with the 'monitor' switch as that can affect all sources.

[SiriusHardware]

Suspect there may be a RH Channel fault on your cartridge. If not check its pin connections at the cartridge or the lead to the RH phono plug

Unlike Edward, I've only just realised you were using the same source for all tests on different inputs. Do you have a source with line-level output which you can use to check the TAPE and AUX inputs with (tuner, cassette deck, CD or DVD player?).

If not, just swap the L+R audio connections from the turntable over and see if the fault moves to the other channel.

[Jime174]

Thank you both for the replies.

I should have said that I reversed the phono plugs and the fault remained so I'm reasonably confident that the turntable is ok.

I'll get some contact cleaner and work the switches as suggested.

[Jime174]

And yes you are correct. It's jim ��

[SiriusHardware]

Originally Posted by Jime174

I should have said that I reversed the phono plugs and the fault remained so I'm reasonably confident that the turntable is ok.

It seemed likely that you would have done that, given that you had done other common sense checks. It does help us to know everything that you have already tried so we don't keep suggesting things which you have already thought of.

Although switch cleaning is not a bad thing to do anyway, if your fault is due to a tarnished / dirty switch contact that will usually become obvious if you just operate each switch back and forwards through its whole range while listening to see if that has any effect on the problem. Once you identify a likely dirty switch then you can usually get away with cleaning just that one switch. It's often one which has rarely ever been moved, like the 'Monitor' switch which will have spent 99% of its working life in the 'Source' position.

The fault could still be down to a physical problem like a cracked solder joint on a switch pole or wire. Also block diagrams can be deceptively simple, and may not show individual components which are in the signal path and could have failed, so if working the switches produces no obvious change come back to us and we will trace the 'missing' signal more logically.

[Jime174]

Thank you. I need to set aside some time to look more thoroughly. Hopefully it is something simple like dirty contacts. The faulty channel seems completely dead ie. No hum or any other noise when the volume is turned up.

I did also get continuity on the 4 tone arm pins where they connect to the head shell and the relevent point on the RCA plugs so just another check that I forgot to mention.

I'll come back to you when I've had a chance to investigate further if that's ok?

Hopefully I can fix it as its a nice sounding set up. (Toshiba turntable SB255) even through cheap eltax bookshelves.

Thank you again

[SiriusHardware]

No rush -

Couple more suggestions for you, as and when you are ready -

If you connect a stereo source (ideally line level) to the Tape 1 PLAY inputs and select TAPE 1 on the MONITOR switch, do you get audio out on both channels?

If you connect a stereo source to the Tape 2 PLAY inputs and select TAPE 2 on the monitor switch, do you get audio out on both channels?

[Jime174]

Thanks. I'll report back

[Jime174]

Update.

You were right. The source selector was dirty and causing the problem.

Sounds great now.

Thank you for your help. I appreciate it.

Can I just ask a question about the TT stylus please?

The cartridge is a dual dms210.

1. I'm assuming it's a budget cartridge. It's it a capable one?

2. I need a new stylus. Dn211 I believe. Is it worth getting one for this cartridge and if so, where is the best value supplier?

Thank you

[SiriusHardware]

Hi Jim, sorry I didn't see this until now, this is such a fast moving section of the forum that you can easily miss an update to a thread if you blink. I'm glad the fault on the amp proved to be so straightforward.

My knowledge of turntables, cartridges and styli is however next to zero but luckily there are people on this forum who know every last thing about that subject - time for me to step aside and hand over to those who know best.

[Edward Huggins]

Don't rush to buy a stylus, examine yours under a x12 glass. If you decide to buy one there are number of reputable suppliers - The Stylus Lady, Musonic, Mantra Audio.....

[Jime174]

Thanks both.

The stylus seems to be beyond salvage as when I removed it to clean it ones of the little wings that attached to the rubber? Coupling that attaches the metal with the sapphire on to the plastic part fell off. I assumed it's this that translates the stylus movement to the cartridge for one of the channels.