low audio output from receiver

[tab a]

My Toshiba SA-3500 quit working and to further my electronics education I'm trying to fix it. It's been a while since it quit and I stuck it on the shelf, but IIRC it quit suddenly, with no unusual smells/smoke.

It powers up, and all lights seem to work but sound is just barely audible on both channels with the volume all the way up.

My reading suggested that one cause of this symptom could be a bad volume control. It's a 193F-V100KBX2. I measure about 6k ohms to 15k to 2 ohms on both channels from low to mid to high volume respectively. Doesn't seem right to me, but would this cause almost no sound??

Without a schematic to help, I thought a tone generator might be useful so I made one using a 555 per Andrew Singman's book. Starting at the speaker outputs, and working backwards, it seems to work but is not very loud. I lose the tone on the far side of both a small cap and a 10 ohm resisitor, connected in parallel to the collector of a transistor. The tone isn't very loud to begin with so I wonder if the generator just needs amplification, rather than this indicating a problem at this point in the circuit?

Any advice on troubleshooting this problem, or how to use a tone generator (google turned up almost nothing) would be greatly appreciated!

Thanks.

Thon

 

[tab a]

I've been reading everything I can find from the library, to increase my knowledge about electronics. Much of what they have though, either isn't specifically targeted to troubleshooting, or requires a schematic to be applicable. That's why I'd hoped someone might have some tips for bridging the gap between what I know, and what the books require.

Barring that, can anyone suggest other good sites to help amateurs with electronic repairs?

Thanks again.

 

[kchriste]

This is not a very good "learning" project. Fixing a piece of equipment without a schematic can be very tough. The first thing to do is try to draw a rough schematic of the left channel (The right should be identical) to better understand how to trouble shoot it. It sounds like the fault is common to both left and right channels so check the power supply rails and the speaker power on delay circuits first.

 

[tab a]

I know it's challenging to try to do this as a 'beginner' but I like to think everything I have to learn to accomplish it will very valuable practical knowledge. And, if I fail, it's not like I've ruined a receiver that's worth much.

I had thought about speaker power on delay dircuits, and had looked for a relay, which I believe accomplishes this in some amps--and causes symptoms like mine. I don't see anything that looks like a relay, so presumably there's another way to do this?

I have found a couple of shorted diodes D503 and 505, which are unmarked. I think Sam's has this receiver in the MHF series, but I only have access to the 'regular' photofacts. So if anyone has the values for those diodes (zeners?), I'd greatly appreciate it.

Thanks for your help!

 

[Nigel Goodwin]

Are you 100% sure the diodes are faulty?, have you removed them and tested out of circuit? - there are likely to be a number of diodes across low resistances so they read faulty in circuit.

 

[tab a]

Yes, I did test them after removing them, and they are shorted.

 

[Nigel Goodwin]

Yes, I did test them after removing them, and they are shorted.

In which case you need to find out exactly where they are in the circuit - if you don't have a circuit, I would suggest (as someone else did) drawing it out yourself. It's actually a VERY good learning experience!.

 

[tab a]

These two are in series between the B of one trans. and the C of another, but they are only in one channel--which allowed me to compare them with their images in the other, prior to taking them out--and I'm fairly certain they're not my underlying problem, as I let the smoke out of them, when checking voltages on transistors. (Another learning experience!)

It's easy to see the two channels symmetry on the board, and I have been comparing voltages but was guessing the problem probably wasn't there as the low sound was common to both. I'd been concentrating on a section of the board between the two channels, and had drawn it out. Just haven't figured out what it all means yet.

Thanks.

 

[Nigel Goodwin]

As it's common to both, there may be a muting circuit - this may be permanently ON. Another option is a missing supply rail to the preamp?.

Frst thing you need to do is find out if the problem is preamp or power amp.

 

[tab a]

<missing supply rail to the preamp?>

I'll try to investigate that.

<Frst thing you need to do is find out if the problem is preamp or power amp.>

It's easy to see what's preamp and what's power in a block diagram in a book. I'm not so sure when looking at a board, without a schematic. I imagine the 4 large transistors on the heat sink are power amp, but does that mean/imply most/all of the small ones are preamp? Can't be that easy, can it?

Other than FM muting, there is no mute button on this receiver.

Many thanks!

 

[Nigel Goodwin]

<missing supply rail to the preamp?>

I'll try to investigate that.

<Frst thing you need to do is find out if the problem is preamp or power amp.>

It's easy to see what's preamp and what's power in a block diagram in a book. I'm not so sure when looking at a board, without a schematic. I imagine the 4 large transistors on the heat sink are power amp, but does that mean/imply most/all of the small ones are preamp? Can't be that easy, can it?

No, those are just the output transistors - the usual place to split power and pre-amp is at the volume control.

Other than FM muting, there is no mute button on this receiver.

Not a mute 'button', an internal muting circuit that mutes the audio when you turn ON and OFF (not a speaker switching type, as you said it doesn't have speaker relays).

 

[Leftyretro]

I've been a collector and restorer of vintage, mostly 70s, hi-fi gear for many years now. The most common problem with this aged gear is dirty switch contacts and control pots. A good contact/pot cleaner can solve many problems. However every case is specific so injecting a signal source to an input connector and tracing the signal through the stages with an O-scope is one of the best methods for troubleshooting this kind of problem.

There is a great audio forum site at audiokarma.org where one can find all kinds of information and help with this kind of gear.

Good luck

Lefty

 

[tab a]

<the usual place to split power and pre-amp is at the volume control>

This is exactly the kind of info that I've never (knowingly) come across in books but that's basic to the experienced. Thanks!

I tried the contact cleaner early on, though the pot. is so buried I'm not sure how much actually got on the contacts.

Thanks for the audiokarma site.

 

[Nigel Goodwin]

<the usual place to split power and pre-amp is at the volume control>

This is exactly the kind of info that I've never (knowingly) come across in books but that's basic to the experienced. Thanks!

Have you ever heard of the 'half-spilt' method? - the volume control is normally the first point to apply it (even though it may not actually be half way).

 

[tab a]

<Have you ever heard of the 'half-spilt' method?>

THAT the troubleshooting books DO talk about.

Thanks.