BRT400 gone low gain

[John Mann]

Hello all.

Any ideas on this one?
My BRT400 was running nicely, I went and left it for about an hour, when I got back it had gone very quiet although signals just about detectable.

I found that the gain of each RF stage was about 2 (it should be 7). The anode voltages were 145V, they should be 100V higher according to the manual and the anode resistors for both stages were getting hot.

Now since both RF stages are affected I looked for what they have in common which is AVC and RF gain. However I can't find anything obviously wrong. The AVC line is at zero volts and the RF gain pot is smoothly adjusting the (low) gain. Voltage on the RF gain pot is about 14V at minimum, decreasing to 0V at max.

Any thoughts?

Thanks, John.

[Station X]

Low HT voltage along with hot anode resistors is a sure sign of excess anode current.

Measure the control grid voltages of AGC controlled stages and check that they change as the RF gain control is adjusted and stations are tuned in. This is best done with a DMM having a high impedance input.

[G6Tanuki]

Is there a decoupling-capacitor common to the HT (or screen-grid) feeds to the RF-stages?

if so, check it hasn't decided to mutate into a low-value resistor.

[turretslug]

ISTR there's an anti-hunting capacitor between the amplified AGC line and the 115 (ish) V screen-grid rail. One of various candidates for checking...

[John Mann]

Thanks for all replies. I'll check all the suggestions and report back.

Graham, access to the valve bases in this set is not easy. However on one of the valves the grid appears to be at 0V. Gain is so low there is no chance of the AVC coming into action. The RF gain is a resistance in series with the cathode so does not affect the grid.

John.

[Station X]

The RF gain is a resistance in series with the cathode so does not affect the grid.

Yes it does. In a circuit using auto bias (ie a resistor in the cathode lead) the absolute value of the grid bias voltage should be equal to the cathode voltage, neglecting the effect of agc.

Even though it's usual to measure the cathode voltage wrt to chassis, it's the grid to cathode voltage that counts.

[Skywave]

I've had a look at the cct. diag.: its layout does not make for easy analysis. However, it seems that the two R.F. stages have separate decoupling arrangements for their respective anode supplies of HT.

The first RF amp., V1, has R7 in series with R10, the common connection of those two being joined to C23, which goes to 0v. / chassis.
The second RF amp., V2, has a similar arrangement, the components being R17, R20 and C42 respectively.
R7 & R17 = 4700 Ω; R10 & R20 = 1000 Ω; C23 & C42 = 0.05 µF

Although the voltages are low on both anodes - which implies a fault common to V1 and V2 - there is the possibility that C23 and C42 are each defective, which would explain the low anode voltages and the hot resistors.

Although I am aware that it is not easy to get to some components in this receiver, if you can gain access to them without too much difficulty, I'd change them 'on spec.' That eliminates two likely candidates. Moreover, if they are not defective, on account of their age, it would be a good idea to replace them anyway.

Plus:

Originally Posted by Station X

Even though it's usual to measure the cathode voltage w.r.t. to chassis, it's the grid to cathode voltage that counts.

Exactly: another line of investigation to pursue.

So it would be a good idea to measure the grid-cathode voltage (with a DMM with a high input resistance) at V1 and V2. If the grid-cathode voltages are substantially + ve, that's the cause of the problem.
Hence, the Qs. arising from those measurements are:
1. Why is that voltage "substantially + ve"?
2. Where is the source of that voltage?

Al. / Oct. 31, '13. //

[Biggles]

If it's any help I had an internal inter-winding s/c on one of the variable bandwidth IFT's which caused exactly the symptoms you are describing. The fault manifested itself as a lot of heavy crackling sounds before it went down totally. When I found it was one the IF transformers I gave up on the set. I still have it. They are not easy to work on as the valve bases are very tightly populated. I can't remember which IFT it was unfortunately.
Alan.

[Skywave]

That's interesting, Alan: although I can appreciate that you're not too happy about it!
Anyway, on the basis of what you've stated, would I be correct in deducing that the consequence of that fault was to make the AGC line substantially +ve, thus biassing the two R.F. amp. valves hard on?

Al. / Nov. 01, '13 //

[Biggles]

To be honest Skywave it was a few years ago and I can't remember how it affected the valves but I wouldn't be surprised if it put a pretty heavy positive into the grid circuits somewhere. With a normal IF transformer construction the primary and secondary are well separated but I think they are different in the BRT. When I have time!? I might have another go at it but the chances of getting a replacement IFT are slim. The problem is it's a multi tapped one to vary the selectivity if I remember correctly. It crackled violently and was on and off for quite a while before it eventually gave up the ghost, which was when I finally managed to pinpoint the fault.
Alan.

[turretslug]

That's a worrying possibility- and not quick or easy surgery, even with a replacement IFT. Being optimistic, the broken-down winding might be an overlaid coupling winding, switched in for "wide" bandwidth- perhaps this could be cut away, at least leaving the normal and narrower switched bandwidth options. This also relies on the underlying winding not having come to harm with the original breakdown...

BRT400 IFTs might be rare and difficult to come by but it hasn't escaped my notice that 455kHz IFTs from a well known, commonplace, large and heavy WW2 US communications receiver have a sporadic auction site presence (often from the barely-veiled breaking of a set). Whilst this is regrettable, it might at least offer a lifeline to a set like this- they'll need adapting to fit and won't give all the selectivity options (and probably slightly different ones, anyway) but better than nothing

[Skywave]

Biggles; turretslug: all very interesting what you say about faults with non-standard IFTs - and which might be relevant to the fault that John Mann has reported. However, John did say that he would "report back" (post #5), so I suggest that we now wait to see what John comes back with before we develop this thread further.

Al.

[John Mann]

Apologies for not being able to reply earlier, I have been away since Friday.

The problem appears to be fixed and was embarrassingly simple, although it leaves a few questions in its wake. I was becoming increasingly puzzled that I couldn't find anything wrong with the RF stages. I therefore began to look at other parts of the circuit. On V5, the first IF, the screen was at rail voltage, 150V. A finger test revealed the can of V5 was cold. Luckily I had a spare W81 and with that in place the set came back to life.

So what about those RF anode voltages? Well the manual says that gain measurements should be made at 1MHz, so that's what it was tuned to when I checked the voltages, although no particular settings were specified here. On the MW and LW ranges an extra 10K anode resistance is switched in series with the common 1K. Selecting one of the HF ranges produced the expected voltage. However, it is these extra resistors which to my mind were getting hot (with that clean and glistening appearance of melted wax) and I can't explain that.

It must also be the case that my gain measurements were wrong, although I did check them several times. Clearly the scope was loading the circuits so maybe I should have checked input and output at the same time using both scope channels.

The two capacitors that Skywave suggests for replacement are easy to get at so I may do that, certainly there have been a few failures of various Dublier capacitors over the last year or so.

John.

[turretslug]

That's something of a relief! I believe that the W81 valves used as RF/IF amps have a relatively high anode current under typical biasing, so those 10k resistors might be dissipating getting around a watt (assuming around 250V HT means about 100V across the resistors with 145V anode measurements), dependent on AGC, of course. Even with sizeable resistors, that'll feel rather warm and enough to give the wax film a sheen.

There are a fair sprinkling of "liability" capacitors in this set- the anti-hunting one mentioned (between AGC and IF1 screen grid) and V9 and V13 grid coupling caps (as a result of the audio filtering/shaping circuitry, V9 has no less than 3 of these). Unfortunately, there were at least 3 substantially different circuits (early/late loctal/miniature valve) so quoting designations is difficult.

[Skywave]

Originally Posted by John Mann

It must also be the case that my gain measurements were wrong, although I did check them several times. Clearly the scope was loading the circuits so maybe I should have checked input and output at the same time using both scope channels.

Presumably you were using a X10 'scope probe: typically about 15 pF capacitance at its tip: that's quite enough to throw the tuned ccts. off tune. When you stated that you had measured the gain of the RF stages, the thought did cross my mind: "I wonder how he did that?", but I didn't enquire. Measuring the gain of RF amps. - and obtaining reliable figures - can be quite a tricky task.
Anyway, you've fixed it now - that's the main issue - and learnt a few useful things during the journey.

Al.

[Radio Wrangler]

X1/X10 switchable probes load even more, even with the switch in x10.

David

[Skywave]

"Load even more" than . . . what, exactly?

Al.

[Herald1360]

A x10 dedicated probe, presumably.