First radio (DAC90A)

[DangerMan]

Originally Posted by davek0974

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

The voltages are normally measured with respect to the chassis, so they should be the same, which your readings show they are. All these voltages change slightly with the AGC action anyway, and the shared cathode voltage is only about a volt, so this is all ok.

I wasn't suggesting you should change the mica's. just that doing so would alter the RF alignment possibly enough to lose sensitivity.

[Sideband]

Originally Posted by davek0974

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?

Then leave the alignment well alone! Not only do you not have a suitable signal generator (you need an RF generator) but attempting to adjust I.F transformers with the wrong tools means almost instant destruction of the dust cores. It's doubtful that the alignment will be very far out anyway unless someone has already had a twiddle.

To answer the basic question...no alignment is not difficult but it requires a) an RF signal generator, b) the correct alignment tools (you can make good one's from PLASTIC knitting needles), c) the alignment instructions and d) knowledge of how to use the RF generator.

[davek0974]

Originally Posted by DangerMan

I wasn't suggesting you should change the mica's. just that doing so would alter the RF alignment possibly enough to lose sensitivity.

Great, thanks, I'll leave them as they are.

[davek0974]

Originally Posted by Sideband

To answer the basic question...no alignment is not difficult but it requires a) an RF signal generator, b) the correct alignment tools (you can make good one's from PLASTIC knitting needles), c) the alignment instructions and d) knowledge of how to use the RF generator.

Of course, I'm not one to aimlessly twiddle settings, especially in a working set. I knew I did not have the correct tools which is why I mentioned it. None of the IF or trimmer settings have been got at - they all have the original sealing paint on them.

I'll leave alignment practice for another set and another day, keep an eye out for an RF sig gen too.

[davek0974]

I have the new UBC41 to try tonight, hopefully it will clean things up a little

When is a set declared fit for use?

This one is working, the af stage voltages are now reading good but it does have some of rf/if stage voltages and currents, mostly low, would that matter?

T-cut brought the cabinet up to a lovely shine

[HamishBoxer]

Yes certainly the UL41 base.Sorry have I got confused here thinking you had a hum on sound?

[davek0974]

Originally Posted by HamishBoxer

Yes certainly the UL41 base.Sorry have I got confused here thinking you had a hum on sound?

I seem to have hum between stations, as I can only get about three stations, that's a lot of hum

You can hear some mild hum between speech etc but I have no idea what a "good" DAC90A sounds like really, too used to the modern stuff where it is silent between speech.

[paulsherwin]

You should have better sensitivity than that. Double check the frame aerial connections and the wavechange switching arrangements. I assume you've already cleaned the wavechange switch contacts, but do them again

[davek0974]

Thanks, I'll re-check the aerial connections and aerial continuity tonight, I'll also clean and check the switch again.

The high end of my MW scale is pretty bogged up with noise from a SMPS, not sure if its the printer or HDD psu, certainly chucking out some noise, I might try shutting that lot down for a test.

[paulsherwin]

Yes, shut down all potental sources of interference before drawing any conclusions.

[wireful3]

Hi Dave, My DAC90A certainly receives more than three stations so the sensitivity seems low. The internal loop is quite directional so rotating the set might make a difference.

You mention low voltage measurements, if they are only low by 10% I wouldn't expect this to have much effect so the problem must be elsewhere. Paul's suggestion is a good starting point.

[davek0974]

These were the values i got with a Fluke DMM, (tech sheet values in brackets)

UCH42
Pin 2 89v (98)
Pin 3 54v (47)
Pin 5 60v (47)
Pin 7 0.67v (0.8)

UF41
Pin 2 90v (74)
Pin 5 60v (47)

UBC41
Pin 2 59v (74)
Pin 7 0.33v (1.3)

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?

The UBC41 values were with the old valve, I will try the new one tonight.

[Herald1360]

Originally Posted by davek0974

R8, V2&3 cathode resistor shows a drop of 0.263v across 330r which gives 0.0008mA? No, that's 0.8mA or 0.0008A
R2, V1 cathode resistor shows a drop of 0.62v across 220r which gives 0.0028mA? No, that's 2.8mA or 0.0028A


The UBC41 values were with the old valve, I will try the new one tonight.

HTH

[davek0974]

D'oh!

[davek0974]

Well, the UBC41 made a massive difference, many more stations now on MW, easier to tune too. It has also reduced the interference I was getting from the SMPS in my PC stuff a lot. Can anyone explain why that valve is so vital?

The LW is still dead, some noise and a whistle at one end and a gurgling sound at the other end very dead in the middle, no hum though

The noise I have on MW is more like a sloshing sound than a hum now, a shhhhhhh sort of noise.

Don't know if that's normal or not.

Can anyone explain why that valve is so vital

All the valves are vital (they wouldn't be there otherwise!), the ageing of some just make the set a bit worse, others stop it entirely.

[davek0974]

Yes I figured that

It just seemed odd that this one was singled out many posts ago as being worthy of changing. I think this is the one with the detector and AGC diodes in it, would a failing detector do it?

['LIVEWIRE?']

A failing detector diode would indeed mean that only the stronger signals would be received,, probably 5 Live, Talksport & Absolute R. in your area. What puzzles me is why some of the voltage readings you get with your Fluke DMM are lower than the values in the Service Sheet, given that a DMM has a very high impedance, usually 10Megohms, and thus imposes very little load on a circuit, whereas the readings on the Service Sheet would no doubt have been taken with an AVO 7, which is only 1000 OPV, or, at best, an AVO 8 (20000 OPV).

[paulsherwin]

The HT is down though, which will suppress all the voltages derived from it, all things being equal.

I'm surprised that a new UBC41 has made such a difference. It is basically an AF amplifier triode, though it's true that it also contains the detector and AGC diodes. I suggested it might be responsible for the hum, but the other effects are unexpected.

[davek0974]

Those three stations were indeed the only ones I could get before!

I have won a very nice Avo8 and should have it monday/Tuesday I will do another complete voltage check with that just for comparison.

I don't mind the HT being down but only if it's not being dragged down by a failing component. I would like to use this set in the office so want it to be fairly stable and reliable as possible.

What sort of current should be seen through the main HT dropper, I think it's R13, 10k?

Apart from valve bases and the mica caps, there is little left to change really.

I'll check the rating on the dial lamps too, I hear they can be wrong as well.