Quad 303 voltage/channel problems

[johndoe]

Hi all, I recently picked up a rather grubby 303 (1978) with the aim to recap and recommission, I've done a few of these this year and all have been fairly straight forward. The one I picked up has a problem I can't get my head around and I was hoping someone here might be able to please help me

The regulator board is working fine and is adjusted to 67v, I have changed the electrolytics on both amplifier boards, the left channel is working and adjusted to 33.5v, it sounds good. The right channel is causing me the problem as I can't dial in the 33.5v, its sitting around 5v, it will go up and a down a few volts either way by adjusting RV100, but is well off spec.

I've compared readings around both boards and the most significant difference I can find is 60v sitting on either side of both MR106 and MR105, which is not the same on the good board, so I guess something is quite upset. The diodes test fine.

So far Ive tested all the diodes and resistors I can and taken TR101 and TR102 out of circuit, those are fine according to the tester.

The board does pass a signal, but it's somewhat distorted, unsurprisingly!

[Mooly]

If you have +60 volts on each side of MR105 and 106 and only 5 volts on the midpoint line then the 0.3 ohm resistors must be open circuit (but there will be other damage as well such as failed output/drivers) as your measurement shows 55v across each 0.33 ohm.

The simulation shows typical voltages.

[johndoe]

Hi Mooly, thanks for taking the time to reply to me.

I've removed and checked the 0.3 resistors out of circuit and they are good (R124/R125) I checked R128/R129 while I was there,they are both good too.

I've taken a few more transistor readings and thought maybe the best way to present them was via an image



Would it be a good idea to test TR107, 103 and 105 out of circuit at this point?

Thanks.

[PJL]

It is best to measure transistor voltages on base and collector wrt emitter to help fault find.

[JonSnell]

Looks like TR106 has 11volts on its base and only 0.4 on the emitter!
That is not right at all, the junction voltage is a maximum 0.7volts for silicon devices.

[Mooly]

Originally Posted by johndoe

Hi Mooly, thanks for taking the time to reply to me.

I've removed and checked the 0.3 resistors out of circuit and they are good (R124/R125) I checked R128/R129 while I was there,they are both good too.

I've taken a few more transistor readings and thought maybe the best way to present them was via an image

Would it be a good idea to test TR107, 103 and 105 out of circuit at this point?

Thanks.

Careful voltage checks are your best bet at this point rather than removing parts.

Can we explain this?

Originally Posted by johndoe

I've compared readings around both boards and the most significant difference I can find is 60v sitting on either side of both MR106 and MR105

If you have 60 volts on each side of the diodes and only 5 volts at the midpoint then that implies 55 volts across the 0.3 ohm which is not possible. The current would be nearly 200 amps... so can we explain what is going here?

It is an anomaly that needs resolving before going further. The midpoint is measured at L1, R124, R125 junction.

If that is 5 volts then have we really got 60 volts on each end of MR105/106 ?

Lets clear that up first.

[johndoe]

Originally Posted by Mooly

If that is 5 volts then have we really got 60 volts on each end of MR105/106 ?

Lets clear that up first.

I should probably make sure I'm taking the reading correctly, I'm using pin 9 as ground when taking the measurements, is that correct?

I've checked again and both sides of MR105/106 are around 60v. R120/R121 is the same, R117 is 63v/60v and R123 is 60v either side.

The mid point is currently sitting at 7.2v, the power draw at the wall is 10 watts and nothing is getting appreciably warm to the touch (but the shed is cold! )

[Mooly]

Yes, all voltages in the power amp are referred to pins 8 and 9.

The midpoint which should be around +30 is measured at pin 5

If you have 60v on R123 then you must also have that on pin 5. If you have only 5 volts on pin 5 then something is open circuit.

So check first that pin '5' really is only at the 5 volts you mentioned in post #1.

We have to resolve that before going further.

[Trigon.]

In the marked up image in post #3, pin 5 is shown as being at 60v. Where is the quoted 5v / 7.2V 'midpoint' voltage being measured

Cheers

[johndoe]

I've started from scratch and followed the service manual instructions as I think I was reading the mid point from the wrong points

The regulator board is at 66.9v between pins 1 and 9.
The voltage across pins 5 and 9 was 60v, the lowest I now can get it to adjusting RV100 is 50v.

Hopefully I'm making more sense now.

[johndoe]

Originally Posted by Trigon.

In the marked up image in post #3, pin 5 is shown as being at 60v. Where is the quoted 5v / 7.2V 'midpoint' voltage being measured

Cheers

I think I measured pins 1 to 5

[Mooly]

Originally Posted by johndoe

Hopefully I'm making more sense now.

Maybe

Check R130, the 150k in series with the preset is OK and not gone high.

I'll look in later.

[johndoe]

You called it, I can confirm R130 is OL, thank you! I'm sorry my initial post confused things. I don't have a 150k in my bucket of bits I'll get some ordered, although I might look over my pile of scrap boards to see if I can find one looking for a new home.

Could this have caused damage elsewhere in the circuit?

[PJL]

Your voltages on the PCB image look to have problems around:
TR103 has e=66, b=60, c=66
TR105 has e=60, b=68, c=60

Although it is difficult to tell the exact point the voltages are taken from.
As said earlier, the best way to test transistors in circuit is to put your -ve probe on the emitter and measure the base and collector voltages.

[Mooly]

Originally Posted by johndoe

You called it, I can confirm R130 is OL, thank you! I'm sorry my initial post confused things. I don't have a 150k in my bucket of bits I'll get some ordered, although I might look over my pile of scrap boards to see if I can find one looking for a new home.

Could this have caused damage elsewhere in the circuit?

Excellent. The shift in midpoint would not cause any damage so you should be all good once that is replaced. You can always use two series values to get you in the ballpark such as 100k and 47 or 56k.

[johndoe]

I found two 75k's and put them in series, that did the trick. Mid point voltage is set and the right channel is now singing as well as the left channel I'll replace the bodge when a pack of 150k's turn up in the post

I'm most grateful for the assistance in locating the fault, now to invest in some filter/decoupling caps so it gets the full treatment

[Mooly]

That's good, I'm pleased to hear its all working now.