Bush SRP30 Amplifier Problems

[Dazmond]

Hi Everyone,

I have a Bush SRP30 with amplifier problems that I can't seem to get to the bottom of, i'm still learning and although i've picked up a lot over the past year from the very kind and hugely knowledgable people on this forum, this one is proving a challenge, so I was wondering if there was anyone on here who would be kind enough to help me out.

The initial fault was no output from the amp, just a deep hum / rumbling sound and the occasional pop and crackle. When I put my iPod through it you can hear some output but the transients in the music are represented by static rather than music itself.

I've changed all the wax and hunts capacitors as I didn't see the point in going any further until these were all out, and for the sake of it I also changed the 470pf mica, oh and the filter caps too. I've also cleaned up the valve pins and checked the bases for dry joints.

After trying the amp again it's exactly the same which leaves the resistors, valves and transformers. I've checked all the resistors and they are just within tolerance, I will swap them out but at this stage I can't see these being the culprit.

I've read on the forum that those wax capacitors have a nasty habit of wrecking the output transformer and 2 of the 3 valves, (EZ80, EL84, EBC81) so to start with i've taken some voltages on each valve, i've taken them all in both DC and AC ranges where present including the rectifier in case this may be of use:

EZ80:

1) 280.2v AC
2) 2.28v AC / 103.3mv DC
3) 19.85v AC / 354.1v DC
4) 6.66v AC
5) 0.05v AC
6) 2.08v AC / 24.8mv DC
7) 280.6v AC
8) 1.660v AC / 11.2mv DC
9) 1.784v AC / 43.7mv DC

EL84:

1) 15.6mv DC
2) 14mv DC
3) 1.596v DC
4) 6.70v AC
7) 1.754v
9) 086.3v DC / 1.401v AC

EBC81:

1) 36.5v DC
2) 74.8mv DC / 0.031v AC
3) 491.8mv DC / 0.008v DC
5) 6.68v AC

Looking at the service data I bought, the first test point on the diagram from pin 3 of the EZ80 should be 312v, but as you can see from my measurements the actual is somewhat higher.

Before I jump in and start swapping valves (I don't have an EZ80 to hand so i'd have to get one) or indeed look for replacement transformers I thought i'd put this out there and see if anybody could shed any light.

Any help would be gratefully appreciated,

Many thanks,

Darren.

[Nuvistor]

Check the primary of T1, probably open circuit or the wiring around it. Unless you have made any wiring errors in the work already completed.

Frank

[Edward Huggins]

I assume the cartridge in the Garrard 4SP is working well? Edward

[Dazmond]

Hi Frank,

Thank you for that, i'll check T1 and report back with the results, definitely no errors on my part, I take a photo of everything before hand and check then check again before I power up after each replacement.

Not sure what my chances are of finding a replacement speaker transformer :S

Many thanks,

Darren.

[Boater Sam]

RS universal output transformer if it will fit.

Its probably the tranny if the HT is high.

[Nuvistor]

I suggested checking T1 as the voltage reading is only 1.7v on the EL84 anode, I presume that is correct?

Frank

[1955APREN]

HI
Your EZ 80 is working ok you do not need to replace it. The high voltage on pin3 cathode is due to the fact that the EL84 is not taking enough current. Check that the other have said that the HT voltage is reaching one tag of the primary side of the output transformer.
Regards Derrick

[Dazmond]

Hi everyone thank you for your replies,

On the output transformer I get 360v on the primary but only 1.476 on the secondary when it settles down, looks like it's seen better days?

I assume the cartridge in the Garrard 4SP is working well? Edward

No that's had it Edward i'll replace that when i've sorted the amp out

Many thanks,

Darren.

[Nuvistor]

Perhaps we have a terminology misunderstanding.
The primary winding of T1, one end connects to the Cathode of the rectifier pin 3, the other end connects to the anode of the EL84 pin 7, you should have 300vdc or there about on both sides, if not measure the resistance, without power applied to the player, between those two points and it should be about 600 ohms according to the manual.

The secondary is connected to the speakers completely isolated for DC from the primary winding.

If you know which tags on T1 you can put the meter on them to test the winding, if OK then the check the wiring.

Frank

[Dazmond]

Hi Frank,

Yes sorry my mistake, on the red wire connected to pin 3 of the rectifier I get 360v, on the blue wire connected to pin 7 of the EL84 I get 1.476v, if I measure across the 2 I get infinite resistance.

Many thanks,

Darren.

[Nuvistor]

Well unless there is a wiring error the primary winding is O/C, possibly been overrun by a faulty EL84 or faulty grid bias (C2, R10).
Examine T1 carefully, it may just be a break in the winding where it connects to the tag, if so it may be easy to repair.
Failing that there is the RS transformer suggested by Sam, or a rewind.

Frank

[Dazmond]

Thank you Frank, i'll have a close look at it tomorrow and let you know what I find, i've just found an output transformer from an old Murphy A851G somebody gave me, not sure if I could use it as a replacement?

I've replaced C2 but i'll swap out R10 too before I do anything else just to be safe.

Many thanks,

Darren.

[Edward Huggins]

I think that Murphy player used a single PCL83 valve. I'm not quite sure if the anode load impedance on it's OPT might not quite match that needed for the EL84? Edward

[Dazmond]

Hi Folks,

I've had a look at the transformer, no breaks in the winding that I can physically see so it looks like being a new one, i've had a look on RS for a universal output transformer but i'm presented with a lot of results, could anyone recommend one specifically?

Many thanks,

Darren.

[peter_sol]

That murphy one will be fine
Just try it.

[Dazmond]

The Murphy has 2 x ECL86's, does that help at all with compatibility?

I thought I'd add some photos if this helps at all.

Many thanks,
Darren.

[peter_sol]

Ok
Sorry I have given you wrong Info. That transformer is the same as the Bush SRP31, which is push pull and not suitable for your player.

[Dazmond]

Ok Peter no problem, thank you for your help all the same.

Many thanks,

Darren.

[Nuvistor]

This is the only RS transformer I can find that may be suitable.
http://uk.rs-online.com/web/p/audio-...rmers/2106475/

It can be matched to various input/output impeadance which look suitable for the player.

However there is a big caveat. The max primary current is 40ma, you will need to confirm the anode current is less than this or alter the HT/ Bias on the EL84 to acheive it. The circuit gives a cathode current of approx 43 ma, that includes the screen current of about 4ma, that's very close to the RS transformer limit. It should not be a major problem to reduce the Ia but it will reduce the output a little, probably not noticeable.
The other perhaps less difficult problem is will it physically fit.

Long time since I did this so anyone else any thoughts.

just found this, know nothing about them though.
http://www.ampmaker.com/store/5W-sin...ansformer.html

Frank

[Edward Huggins]

That's a nice beefy tranny. Just to say that an ECL86 OPT might do as the pentode section has similar characterstics to an EL84. It was deliberately designed that way to be a 1/2 ECC83 and EL84 in one envelope. Edward