First radio (DAC90A)

[Silicon]

Are the resistor values taken from the service manual or are they measured with a multimeter?

Using your information I calculate the cathode current of the UL41 to be 57.3mA.
That is higher than it should be. It could damage the output transformer.
The service manual gives a cathode current of 31.5mA (i.e. 27.5mA +4mA).

The value of R17 is important. It sets the bias for the valve.

Can you measure the voltage on the control grid (pin 6) relative to the cathode (pin 3).

[davek0974]

Yes, R17 is a new item, checked before fitting.

I will get the grid voltage tonight.

What was the formula for working out that current, I got it wrong

Thanks

[davek0974]

Originally Posted by davek0974

What was the formula for working out that current, I got it wrong

Oops, must have made a calculator boo-boo
I see the current is indeed 8.6v/150 ohm = 57.3mA
This was also backed up by an online bias calculator which showed 8.1W dissipation.

That is indeed hot, presumably the only way to create that is to overdrive the grid or have a partially shorted OPT??

Am i right in believing that the OPT is only good for around 40mA max?

Will check the grid with reference to the cathode tonight and also measure the OPT value.

[paulsherwin]

It is normally caused by a leak inside the output valve or a leaky grid coupling capacitor. However, double check the value of the cathode resistor. It never does any harm to increase these a bit to reduce power dissipation at the cost of slightly reduced maximum volume, but you should sort out the underlying fault condition first.

You are right that this fault will be stressing the output transformer. It may (partially) explain the low HT.

[davek0974]

I have just received a NOS CV1977/UL41, I can try that tonight, the radio is currently fitted with a UL46 which I had in the loft and should be a direct replacement for the UL41, I guess it too could be leaky.

[Herald1360]

Originally Posted by davek0974

That is indeed hot, presumably the only way to create that is to overdrive the grid or have a partially shorted OPT??

Am i right in believing that the OPT is only good for around 40mA max?

Yes and no.....

Grid overdrive can happen from a leaky coupling capacitor, internal leakage between pins from deposits (presumably from the cathode) on the glass, or low value cathode resistor, resulting in excessive anode current.

Excessive anode current, though it may cause OPT primary problems, can't itself be caused by OPT issues. The effective internal resistance of a pentode is so high that its anode current is largely independent of any change in anode voltage either from lower anode circuit resistance or signal voltage. It's effectively a (grid) voltage controlled current source. A completely shorted OPT primary will have almost no effect on anode current, though anode dissipation will increase slightly due to the slight increase in anode voltage from the lack of dc drop on the primary.

[davek0974]

Thanks for that, I have much to learn

[davek0974]

I must have measured the wrong cathode resistor yesterday, or just completely goofed up

The value for the UL41 grid with bias with ref to the cathode is -4.8v which seems good, the drop across the cathode resistor is 4.64v over 150 ohms so 31mA which seems good too.

Is there any more I should be looking at with reference to the voltage list I made earlier??

[davek0974]

Having got that part sorted, I have a NOS UBC41 on its way just in case that was making it a bit deaf.

Failing that I think it's just about done, is there any thing else I should look at before stuffing back in its nice shiny cabinet?

[Nickthedentist]

It's worth checking that the inner/rear suspension on the speaker is OK as they tend to pull away from the speaker chassis if the set's been stored in a damp environment.

[HamishBoxer]

Mica valves bases can some times go "leaky".

[davek0974]

Originally Posted by Nickthedentist

It's worth checking that the inner/rear suspension on the speaker is OK as they tend to pull away from the speaker chassis if the set's been stored in a damp environment.

Already fixed that one

A cocktail stick and some glue sorted that out.

[davek0974]

Originally Posted by HamishBoxer

Mica valves bases can some times go "leaky".

My ones are black so I'm guessing not mica?

[davek0974]

I'm getting some very low figures for the early stages, is it an indicator of an issue or just ignore it?

R8, V2&3 cathode resistor shows a drop of 0.263v across 330r which gives 0.0008mA?

R2, V1 cathode resistor shows a drop of 0.62v across 220r which gives 0.0028mA?

These seem very low?

Ignore or investigate?

[G6Tanuki]

What kind of meter are you using to measure this? Modern digital meters use very-low voltages/currents and can get impressively confused when trying to measure a resistor-paralleled-by-a-capacitor.

[davek0974]

The only one I have is a Fluke DMM, I am bidding on an Avo8 on eBay but doubtful if I'll win it for sensible cash.

[DangerMan]

Did you include the mica capacitors in the list of "wax" capacitors that you changed?

If so, it may be that the set needs the front end to be realigned.

Looking at the voltages you've listed, I think you have a typo... pin2 of UCH42 and UF41 should both be 98V according to the Trader sheet (they are both fed from the same supply rail) and so it's no surprise your measurements show them as virtually the same. Definitely not a fault.

Pete

[davek0974]

The mica caps were not included in the pile of bits I got, so they are the originals.

The voltages were the same (89/90v) but v2 cathode is shared with v3 cathode so the readings would be different no?

The currents seem very low, is that ok?

Is an alignment difficult? It seems ok in the trader sheet, I have a signal generator that covers 10hz to 1mhz but is not an RF signal gen, is that adequate?
Also have no specific tools for the IF coils, are any needed?
Are there any tips or hints for aligning it?

[G6Tanuki]

The wax-encapsulated-mica capacitors are generally rather reliable - I'd look at other things before replacing them.

Leaky paxolin valve-sockets can be an issue though.

[davek0974]

I guess replacement is the way to check the sockets?