Fountain record player, distorted left channel

[PWH]

Fountain is a NZ brand.

I can't find a diagram, although the board is marked Fountain 164, so I am in the dark. Its definitely the amp which is faulty, I have swapped L+R speakers, and cartridge. I have changed the caps on the left channel (top in the picture.) All passive parts seem to test OK cold. The only significant difference is the two diodes. The LHS one has 17V on the orange side and 16.6 v on the blue. The working channel has 14.5V on the orange side and 14V on the blue.
As best I can tell, the Output Transistor is a AY21 (obviously one of a pair.) I think the 'F' is a brand?
Any clues on how to fix the distortion would be appreciated.

[Kentode]

Those two preset trimmers are for the half rail voltage and quiescent current.

Measure the good side and compare with the bad side. You might get away with an adjustment, but if the tracks are corroded then you'll have to replace them.

Hope this helps.

[Herald1360]

The amp looks like a straightforward complementary output job of a watt or so.

Can't find AY21 data and it's unusual (here) to see TO5/39 devices used. Are they germanium?

AC153/176 would be a common pair if so, but cases are TO1.

Midpoint voltage and more interesting quiescent current (measure across emitter resistors) would be useful to know......

[dazzlevision]

The big "F" on that TO5 transistor is probably Fairchild.

[PWH]

Originally Posted by Kentode

Those two preset trimmers are for the half rail voltage and quiescent current.

Measure the good side and compare with the bad side.


Hope this helps.

Than you both.

WRT the photo, Top left trimmer .26V, 0, 0 Top right 1.62V,0,0 (Distorted)
Bottom left .28V, 0, 0 Bottom right, 1.60V, 0, 0. all the wipers are to ground so common.


I don't know which are the emitter resistors to measure.

[Kentode]

Hmm, those voltages appear to be within 0.02 of each other. Try gently pressing on the wipers with an insulated probe.

Please can you post two more pictures, one of the top as in your first post and one of the underside by turning the PCB from top to bottom .

The polarity of the electrolytic capacitors seems ok, you might have a dry joint somewhere.

['LIVEWIRE?']

The emitter resistors will be two low-value, maybe 2.2 or 3.3 ohms each connected between the emitters of the two output transistors. At the junction of the two is an electrolytic capacitor, the other end of which goes to the loudspeaker. With respect to the common rail, which may be positive in this case, about half the supply voltage should be measured at the junction of the two resistors, whilst the voltage measured across each emitter resistor will indicate the quiescent current being drawn by the amplifier. A meter (analog or DMM) on a low DC voltage range will be needed for the latter measurement, which should only be a fraction of a volt.

[Kentode]

Here's a circuit diagram and instructions from a mono amplifier, these might help you trace the components on your PCB.

The numbers in the black boxes are the voltages.

['LIVEWIRE?']

Looking again at the photos, the 470uf Electrolytic in the lower amplifier (as viewed from the photo) appears to be connected with its Positive terminal at the speaker end, whereas it should be wired with its negative terminal to the speaker. If I'm right, this could be the cause of the distortion. (I've re-typed this post because Ken was posting a circuit diagram whilst I was typing, and I expected an amplifier using germanium transistors to be positive earth, whereas this one is negative earth! The amp. whose circuit Ken posted, though, is not the one in the pictures, but I would still carefully check the polarity of the two speaker coupling capacitors, since one is almost certainly the wrong way round. In a positive earth amp. the positive end of the cap. in normally connected to the speaker, which is returned to the negative rail, whereas the reverse is true in a negative ground circuit. Re-reading the OP's post, it would appear that he has inadvertently fitted the 470uf speaker coupling cap in the LH channel the wrong way round.

[Kentode]

Ooh, well spotted 'LIVEWIRE?', l asked for the PCB side photo so I could trace the left hand side of the circuit.



My apologies for not posting a sample circuit of the correct polarity.

I was distracted by the voltages...

['LIVEWIRE?']

I hope I haven't confused the OP with my comments. To set the record straight, the Fountain amplifier is almost certainly Positive earth. In such circuits the speaker is nrmally returned to the positive rail. If this is the case here, then the speaker coupling cap. should be wired with it's Negative terminal to the junction of the two emitter resistors, and it's Positive terminal to the speaker wire. If the cap in the LH channel has been run reverse polarity for any length of time, I'd be inclined to fit a new one. In the immortal words of Lance-Corporal Jones 'They don't like it up 'em!'

[Kentode]

Agreed. Here's hoping the OP took lots of ' before' pictures.

[vidjoman]

The photo of the board looks to me like 2 identical layouts side by side. All the electrolytics are connected the same way in both halves. Some have obviously been changed as there are different makes visible but the polarities are the same.
A photo of the underside would confirm if it's just 2 mono circuits side by side.

['LIVEWIRE?']

With all due respect, vidjoman, the upper (light Blue) 470uf (speaker coupling) capacitor is connected with it's negative terminal to the speaker wire, whereas the lower (black) 470uf cap. is connected with it's positive terminal to the speaker wire. As I previously noted, if, as most of these circuits are/were designed, it has a positive earth (or positive common rail if you prefer) and the speaker is returned to the positive line, the upper (LH Channel )speaker coupling cap. is connected the wrong way round. The negative marker on the light blue cap. clearly points to the edge of the board.

[Station X]

I don't get that.

If you chopped the PCB in half along the horizontal axis you'd be left with two identical amplifiers? They're not mirror images.

[Kentode]

We'd better wait until the OP posts the PCB picture I've asked for.

[ms660]

Originally Posted by Station X

I don't get that.

If you chopped the PCB in half along the horizontal axis you'd be left with two identical amplifiers? They're not mirror images.

Me neither, I see two identical amplifiers with all the electrolytics fitted the correct way round in the top amplifier if the orientation of the electrolytics in the bottom amplifier are as per original.

Lawrence.

[PWH]

Originally Posted by Station X

If you chopped the PCB in half along the horizontal axis you'd be left with two identical amplifiers? They're not mirror images.

Correct. You could chop the board horizontally and have two 'monoblocks'

Attached is a picture of 'before'. You can see the PSU cap with a fuse for each amp. Also I have swapped the two driver transistors with the small heatsinks with no change. I'll take a photo of the underside and report back. As you can see the heatsinks can move and make contact with parts. Could that have caused damage?


Thank you all for your interest and help.

[Techman]

I would remove the two output transistors in the bad channel and check the junctions in all directions with an ohm meter - think of the transistor junctions as two diodes.

[PWH]

Attached is the photo of the underside. I swapped the output transistors over. No difference but at least that shows they are all good.