Bush SRP52 help

[tracypaper]

Hi.
Meant to put ac, any how checked with fuse out and got 18.5vac. Got a copy of the schematic thanks to ms660 and checked voltages on the transistors as I assumed these are what you would have told me to check next. All were below what schematic showed changed out the AC128 (even though tests showed it was ok) and boom deafening volume.
The sound is great but only around half volume if I go full then sound distorts as if the cartridge is of a too high an output. the voltage on the AC176 is showing 10.5 vdc where it should be 24.5 and 6v where it should be 12.00.
Should I be grateful that it's working or does it need further work?
Gaz

[ms660]

I'll let you figure that one out.

Lawrence.

[Station X]

What DC voltage do you get across C10 now? If it's low either the rectifier is faulty or something is drawing excess current and pulling down the voltage.

EDIT.

I see that you're measuring only 10.5V on VT3's collector which is the same as measuring across C10.

[Boater Sam]

To be clear, measuring the voltage out of the motor before the bridge rectifier the voltage will be AC.

After the bridge rectifier, i.e. across the 2500uf electrolytic cap C10 will be DC.

[Station X]

There's only a bit of wire or PCB track between C10 and VT3's collector, so it's not likely that excessive current draw would drop many volts in the connection.

Therefore I'd say that either C10 is electrically leaky or the rectifier is at fault. C10 has been changed for what I assume is a brand new cap connected the right way round, so that just leaves the rectifier.

[tracypaper]

Managed to find and order a couple of unused old stock rectifiers for pence, so will try them.
Thanks every one, will get back when they have arrived.
Gaz.

[Station X]

Have you tested the existing rectifier using the diode test facility on your DMM?

[Michael Maurice]

Check the value of the two resistors in the bias chain R8, R9 and RV3, replace if necessary.

[tracypaper]

Just tested bridge with DMM as per tutorial video and got 3 neg readings with neg lead touching neg term on rectifier.
Swapped leads from DMM over and now Pos lead touching neg leg on rectifier got readings of 612, 612 on the AC legs and 242 on the pos leg.
according to tut this needs changing?
Gaz.

[Station X]

I don't fully understand your results. The diode test function of a DMM measures forward volt drop in one direction and reverse resistance in the other.

Reverse resistance should be infinite, so I assume this is what you mean by neg readings?

A forward volt drop of around 0.6 Volts is fine for a silicon diode, but not 0.242V.

You should have four readings, one for each diode, but you've only given three.

Are you testing this in circuit with the fuse removed or out of circuit?

[tracypaper]

Tested in circuit with fuse present, it was late on when I did the test, will watch the tuts again and take rectifier out of circuit.

[tracypaper]

Hi.
Took the bridge out of circuit and tested as per tuts and compared results against the two tested old stock that I had ordered and it is fine.
R8 and R9 tested fine (R8 had been replaced) and RV3 appears to be the tone pot, is that the culprit?
Gaz.

[Station X]

I'm at a loss to explain what's wrong here.

You're getting 18.5 VAC on the overwind which is good.

The original rectifier and two replacements test good.

C10 has been changed and is in the circuit the correct way round.

Yet you're only seeing 10.5V "HT" instead of 24.5V "HT".

Put it back together and see what voltage you get on the overwinding with the fuse in.

[tracypaper]

Hi with the fuse out I get a constant 18.5V reading on AC, with the fuse in voltage starts at 15.5 then gradually reduces to 10.3. one wire goes direct to an AC tab on the BR and the other wire goes to the other AC tab on BR via the fuse. DC voltage goes down as well when testing voltage on the C10 pos circuit.
Gaz.

[Station X]

Here's the circuit.

Is anything getting hot?

What are the other DC test voltages looking like?

[Clydeuk]

Do you have a transformer or other supply around 20 volts that you could try to power the amplifier? Even a DC supply would do to test as the rectifier would take care of polarity. Just to eliminate the overwind. I'm wondering if there's a high resistance connection somewhere.

[tracypaper]

I have the circuit and the Listed MR is a BY122, but fitted is a BY164 could this be it? I did not try the two replacement MRs in circuit, would it be worth trying them? also the fuse looks really meaty as in it looks like its made from sheet metal as opposed to a thin wire fuse?
I will go test DC voltage at other parts of the circuit.
As a test could I bypass the fuse and wire direct to the MR?

[Station X]

Does the fuse have a rating on it? It's supposed to be 500mA or 0.5A.

So long as it's the correct value and doesn't measure high resistance there's no reason to replace it. Bypassing it is asking for trouble.

[Station X]

If you look at the specs for the two bridge rectifiers, you'll see that the BY164 has a higher peak inverse volts and forward current than the BY122, so it's a perfectly good substitute.

[ms660]

If there was a low resistance pulling the AC and DC voltages down then something would be getting hot, if nothing's getting hot then probably some series resistance getting in somewhere.

Lawrence.