Marshall TSL60 OT Primary resistance

[jstbrowsin]

Hi there,

If possible could one of you kind gents who owns (or has worked on) one of these amps please give me the output transformers primary resistance reading. The one I have here is a Dagnall D2504 and I need to get it rewound but the rewinding company needs the PR spec from me before they can do anything. The reading I currently have is 40Ω on each side (centre tap to plate) due to it blowing in the weekend it just died and is outputting about a 10th of the volume now.

I would buy a new MM one but shipping to NZ from states would kill the good deal we have on the dollar here and besides I've had my fun with this amp its time to move on to something different

Can anyone help me please?

TIA

Trevor C

[pdf64]

This hammond is probably equivilant
http://www.hammondmfg.com/pdf/EDB1750P.pdf
There are NZ and Aussie distributors, so check they aren't cheaper than a rewind.

[g1]

Those primary readings sound fairly normal. Why do you suspect the OT and how have you verified it is bad?

[jstbrowsin]

Those primary readings sound fairly normal. Why do you suspect the OT and how have you verified it is bad?

I have tried the following : Checked my speaker cab I then traced the problem to the power section by means of signal though return of FX loop so I swapped out power tubes, swapped PI tube with no change in volume from either, other than that I have just taken usual voltage readings with DMM and its all normal. Tubes aren't red-plating though which made me think the same way as you, but because the tubes are good, voltages are normal and I have really low output I suspected the OT.

Is there any other tests I can make? I have a scope here.

Trevor C

[jstbrowsin]

Just a quick note that the readings I took from the center tap of the OT to the plates of the power tubes were purely resistance readings as I don't have an impedance meter here, sorry I guess I should have mentioned that before.

[pdf64]

What's the plate to plate resistance?
However, even if resistances check out (ie 80 ohms), there can still be bad insulation that gets punched through by large enough signal voltages.
Are there 1 ohm cathode resistors?
If, when signal is applied the current changes plenty but the plate voltage doesn't, it indicates that the tube is driving into a very low impedance, eg shorted turns.

[jstbrowsin]

[QUOTE=pdf64;229565]What's the plate to plate resistance?

Do I measure that with amp on or off?
I tested with a scope and got clean signal all the way to the output of power tubes so I'm sure those results reflect a damaged OT yes?

"However, even if resistances check out (ie 80 ohms), there can still be bad insulation that gets punched through by large enough signal voltages."

That's what I have been told by my tech friend who is out of town atm so can't help me for about a month


"Are there 1 ohm cathode resistors?"

Well there was originally but the tech changed the bias circuit to smooth out the bias drift problem this amp previously had. Now it has a {10Ω 5W resistor in place of the original 1Ω and a 2200µF cap in place of the 1n4007 diode} bias circuit. But the tubes aren't red-plating so I don't see how that would be an issue, unless of course I'm missing something here???

[pdf64]

The plate to plate OT primary resistance is measured with amp off and isolated, caps discharged.
It should be the sum of the 2 plate to centre tap resistances, ie 80 ohms in this case, but if there's a 'solid' short, it may be less than 80 ohms.
If that cathode bypass cap is removed, should be able to monitor Vac at the cathode. Indicates that signal current must be flowing. If that current isn't resulting in a large waveform at the plate, then the plate load is likely to be very low value, eg a short.
If you have a neon bulb, could try the Geofex shorted winding test http://www.geofex.com/FX_images/xform_test.gif

[jstbrowsin]

The plate to plate is 80Ω just as you expected it to be I was really hoping it to be different .
The cathode bypass cap do you mean the 2200µF cap that I previously mentioned? Does this mean I have to power it up again to measure the Vac? Or just input a sine wave and measure with scope? Sorry if that is a noobish question to you but again I'm not a tech.

Thanks for you continued help guys, oh I just noticed the thanks button cool.

Trevor C

[jmaf]

The plate to plate OT primary resistance is measured with amp off and isolated, caps discharged.
It should be the sum of the 2 plate to centre tap resistances, ie 80 ohms in this case, but if there's a 'solid' short, it may be less than 80 ohms.

If you have one turn shorted for example, the DCR will vary almost nothing at all, but the inductance will suffer and the entire transformer may continue to melt and short due to the heat and arcing of that one shorted turn.

If you have a decent precision LCR meter, then use that, you shouldn't see more than a few % difference in inductance between the two halves.

What I also used to do to check OT primaries:
- Touch the power tube plates with a multimeter in the voltage scale, plates on both sides of the primary should cause the same pop out of the speaker. (This is done on a live amp, so if you're not 100% sure please don't try.)
- If you unsolder the transformer leads, you can touch them quickly and repeatedly to a lower DC voltage, like a battery, and see if the speaker pops the same on both plates.
- If you unsolder the leads, you can apply a low AC voltage to the primary and see if both halves produce the same voltage on the secondary. You can do the same on the secondary and read the primaries.

Hope these ideas help.

[Enzo]

There is an impedance selector switch, is there not? Those get dirty or fail. Try this: Power off, unplug the speaker, plug a cord into the speaker jack, and on the far end of that cord, measure resistance from tip to sleeve. You should see the almost zero ohms of the transformer winding. If you see substantially more, then either the winding is open or the wiring or the switch.

And...
Looks like the output transformer secondaries plug onto posts W3,4,5,6. W6 appears to be the common, and W4 the other end. Pull both those wires off the posts - no I don;t know which colors they are - and use clip wires to connect them directly to a speaker. Same low volume? Or back to loud?


And...
Most any OT will work in this one's place, at least well enough to tell us if the transformer is the problem. I have an OT for Fender Bassman head I use for this in all situations. Impedance be damned, it doesn't matter for this test. You would pull the primary wires off the existing OT, and clip them tothe promary of the sustitute OT. COnnect the secondary of the substitute directly to a speaker, don't worry about NFB or anything. If that su8b transformer works, when the original would not, then the original is probably bad. Assuming we have ruled out the impedance switch etc.

[jstbrowsin]

There is an impedance selector switch, is there not? Those get dirty or fail. Try this: Power off, unplug the speaker, plug a cord into the speaker jack, and on the far end of that cord, measure resistance from tip to sleeve. You should see the almost zero ohms of the transformer winding. If you see substantially more, then either the winding is open or the wiring or the switch.

And...
Looks like the output transformer secondaries plug onto posts W3,4,5,6. W6 appears to be the common, and W4 the other end. Pull both those wires off the posts - no I don;t know which colors they are - and use clip wires to connect them directly to a speaker. Same low volume? Or back to loud?


And...
Most any OT will work in this one's place, at least well enough to tell us if the transformer is the problem. I have an OT for Fender Bassman head I use for this in all situations. Impedance be damned, it doesn't matter for this test. You would pull the primary wires off the existing OT, and clip them tothe promary of the sustitute OT. COnnect the secondary of the substitute directly to a speaker, don't worry about NFB or anything. If that su8b transformer works, when the original would not, then the original is probably bad. Assuming we have ruled out the impedance switch etc.

Ah my Enzo good to talk to you again. Its pretty late here so I can't try anything til the morning but I thought I'd share my OT scope readings with all of you here.

The first is the input wave, 2nd the wave on both sides of the primary and finally the wave that again is the same on all the secondary pins. Does this mean that my OT is in good condition? I certainly hope so. I'm gigging on friday and amp I borrowed sounds like crap




Cheers
Trevor C.

[jstbrowsin]

There is an impedance selector switch, is there not? Those get dirty or fail. Try this: Power off, unplug the speaker, plug a cord into the speaker jack, and on the far end of that cord, measure resistance from tip to sleeve. You should see the almost zero ohms of the transformer winding. If you see substantially more, then either the winding is open or the wiring or the switch.

Well I thought before I go to bed I'd test the speaker jack resistance . . . it's approx 0.9Ω on both sides of the switch 16/8.

ok off to bed for me now

Trevor C

[pdf64]

That 3rd waveform (from the secondary) looks to have high frequency oscillation superimposed on it. Was another scope probe connected somewhere else while that pic was being taken? That may cause oscillation not present otherwise.

[jstbrowsin]

That 3rd waveform (from the secondary) looks to have high frequency oscillation superimposed on it. Was another scope probe connected somewhere else while that pic was being taken? That may cause oscillation not present otherwise.

Hi ,

No the signal gen earth was directly hooked to the probe earth and I only have a single ch scope. Does that mean I have a problem again?