Dynatron SRX25 Radiogram with Garrard 40B, Sound Issue

[Kanyehobbitkind]

Originally Posted by Edward Huggins

To help us to help you better, when you use the word "Table" do you really mean the Turntable or the complete Dynatron Unit?
Also you say "Phono Speakers", do you mean speakers that have a Phono plug at the end of the connecting cable rather than a DIN plug?

I mean the complete Dynatron unit when I mention the table and the Phono plug is the end of the speaker cable as I don't have DIN speakers which can connect to the plug without an adapter.

Thank you
Kane

[Kanyehobbitkind]

Originally Posted by Michael Maurice

First things first. Use a meter with one probe connected to chassis and with the other measure the voltage on the four output transistors at the back. This is a positive earth chassis so readings will be negative. two transistors should read -30V approx the other two should read -15V

Take those readings and report back.

Starting from the power swith

First - -181.1 Millivolt
Second - -30.6 Volts
Third - -163.3 Millivolts
Fourth - -30.7 Volts

I did try to convert from Millivolts to volts but it's just showing as not even 1 volt coming from them

Thank you
Kane

[Station X]

You're missing the 15V supply then.

Check the voltage on the fuses as described earlier.

[Kanyehobbitkind]

Originally Posted by Station X

Yes. The voltage check is to check that voltage is actually present at the fuses. This will show whether or not amp's power supply is working correctly.

Don't concern yourself with sourcing new transistors at this stage. They may not be needed. Under no circumstances change components because you "think" they may be faulty.

The left fuse was showing at -30.6 whilst the right was at -30.7 ( I held the black wire (Negative) and the red wire (Positive) to the top of the of the fuses)

Thank you
Kane

[Station X]

Can you clarify that please. Was one lead connected to the chassis? If you're using a DMM it doesn't matter which.

You should be able to take four readings in all, two fuses with two ends.

[Station X]

In anticipation of the direction this thread is likely to take, I've attached an extract from the service manual.

There are plenty of voltages to be checked here.

To avoid red herrings check that with your speakers connected to the adaptors, but not the the amp, you measure a low resistance (not a short-circuit) across the DIN pins of the adaptor

[Michael Maurice]

OK so I know what is going on. There are two diodes which are NKT279, they look like transistors, they develop tin whiskers.

Look at the picture which I've copied from your picture.

You will see two blocks of small heatsinks (arrowed)

Remove the pillar and then remove only the top two transistors.

Remove them from their heatsink (the copper part)

Wrap each transistor in one thin layer of Sellotape then refit the heatsink and refit both to the other heatsink and secure with the pillar.

Repeat for the other block.

If the picture hasn't come out well, I'll take more later and post them here.

[Kanyehobbitkind]

Originally Posted by Station X

Can you clarify that please. Was one lead connected to the chassis? If you're using a DMM it doesn't matter which.

You should be able to take four readings in all, two fuses with two ends.

Sorry it was meant to say the black wire was connected to the chassis whilst the red was at the top of the fuses, was I meant to try both sides?

Thank you
Kane

[Kanyehobbitkind]

Originally Posted by Station X

In anticipation of the direction this thread is likely to take, I've attached an extract from the service manual.

There are plenty of voltages to be checked here.

To avoid red herrings check that with your speakers connected to the adaptors, but not the the amp, you measure a low resistance (not a short-circuit) across the DIN pins of the adaptor

I'm sorry but I'm a little confused now with regards to the speakers, I'm meant to connect the speakers to the 2 DIN to phono adaptors then not connect them to the Dynatron table unit?

Also thank you for the extract I will look through them all ASAP

Thank you
Kane

[Kanyehobbitkind]

Originally Posted by Michael Maurice

Wrap each transistor in one thin layer of Sellotape then refit the heatsink and refit both to the other heatsink and secure with the pillar.

Repeat for the other block.

When you say repeat for the other block does this mean on the other side?

Thank you
Kane

[Michael Maurice]

No same side, you do the same for both sets of transistors

[Station X]

I'll leave this one to Michael who obviously knows these amps well.

[Kanyehobbitkind]

Originally Posted by Michael Maurice

OK so I know what is going on. There are two diodes which are NKT279, they look like transistors, they develop tin whiskers.

Look at the picture which I've copied from your picture.

You will see two blocks of small heatsinks (arrowed)

Remove the pillar and then remove only the top two transistors.

Remove them from their heatsink (the copper part)

Wrap each transistor in one thin layer of Sellotape then refit the heatsink and refit both to the other heatsink and secure with the pillar.

Repeat for the other block.

If the picture hasn't come out well, I'll take more later and post them here.

When you say "Wrap each transistor in one thin layer of Sellotape then refit the heatsink and refit both to the other heatsink and secure with the pillar."

Do you mean just switch them around once I've sellotaped them, then the same with the other two?
Sorry I'm about to do it that's all and got confused by the wording.


Thank you
Kane

[Techman]

No. Do exactly as described. There's no need to disconnect the legs, but be very careful not to break them off. Open up the heatsink very slightly and carefully remove each top transistor and wrap the tape as described and carefully re-insert them in the copper heatsink. It sounds like a bodge but will probably last for another fifty years - done it quite a few times on these.

[paulsherwin]

To explain further, these parts are prone to growing very fine threads of metal ('tin whiskers') between the case and the functional bits inside. In this design, the cases are clipped inside metal heatsinks, which are connected to earth. Fitting a layer of sellotape around the case stops it being earthed through the heatsink, and the tin whiskers then have no effect.

[Kanyehobbitkind]

Originally Posted by Techman

No. Do exactly as described. There's no need to disconnect the legs, but be very careful not to break them off. Open up the heatsink very slightly and carefully remove each top transistor and wrap the tape as described and carefully re-insert them in the copper heatsink. It sounds like a bodge but will probably last for another fifty years - done it quite a few times on these.

It's the "refit both to the other heatsink" that is confusing me, it's just put them back exactly where they came from?

Thank you
Kane

[paulsherwin]

Yes.

[Kanyehobbitkind]

Attached is the photo of them both once I'd sellotaped them, is that alright?
Now I just try and test the system to see if the speakers work?

Thank you
Kane

[paulsherwin]

That looks about right. Put them back in the heatsinks though.

If you measure the voltages around the output transistors again, they should have changed.

[Kanyehobbitkind]

Originally Posted by paulsherwin

To explain further, these parts are prone to growing very fine threads of metal ('tin whiskers') between the case and the functional bits inside. In this design, the cases are clipped inside metal heatsinks, which are connected to earth. Fitting a layer of sellotape around the case stops the it being earthed through the heatsink, and the tin whiskers then have no effect.

Right I did that but still no sound at all, tried headphones too but nothing attached is a picture of the headphone socket going to the speaker output from the inside, not sure why but it may shed some light on it.

I honestly wouldn't even mind if it was white noise coming out.

Thank you
Kane