Hacker RV14 front end problems

[bristle66]

I'm presently working on a Hacker Mayflower 1.  I've replaced all of the old electrolytics and a good number of off spec resistors.  The selenium rectifier was replaced with diodes before it came to me.  The valves all test good.  So it should be good to go.  Trouble is - it ain't.  The audio stage is fine.  Feed Spotify in through the gram input and it sings like a bird.  No hum or crackles.  The radio is a different story.  The EM84 opens and closes but volume is faint and there is considerable hum.  Advice please.

[Mark Hennessy]

Just to say that this question has also been asked on UKVRRR and the Hacker Radio Group. Mentioning this to avoid the repetition that can result from cross-posted questions.

I think it might be useful to state what test equipment you have access to so we know what tests to suggest. Also, what is your experience with these sorts of things?

But if the EM84 is moving when you tune across the band, that suggests that quite a lot of it is working, and it might just be an issue between the detector and the audio stage.

This might be an obvious question, but have you eliminated the gram socket? When you insert a vintage long-reach phono plug, you break the connection between the detector and the following amplifier. These connections are notorious for tarnishing, and not making the correct contact when the plug has been removed - this could easily be responsible for the symptoms you describe. Connecting an oscilloscope or a bench amp to the tape output would be a good test, or simply getting a phono to phono lead and connecting the tape output to the gram input. Ideally, using long-reach plugs, as mentioned.

If the rectifier has been converted to silicon, there should be a series resistor to limit the HT. What is the HT at the moment?

[bristle66]

Many thanks Mark.

My test equipment - if it can be called that - is restricted to a multimeter, a capacitance meter and a (very basic) valve tester. My experience is likewise limited. In in the last 12 months, I've tackled an Hacker RV20, an RP37A and a couple of Bush DAC 90as - all of which were relatively easy to sort. But essentially, I'm an amateur who knows how to use a soldering iron and very little else.

Turning to your questions. I've tried connecting the speaker output to the gram input as suggested but it makes no difference. The magic eye moves but there is no sound. No hum either.

The HT supply is 278 volts. The series resistor is 120 ohms.

[Mark Hennessy]

Are you sure you mean the speaker output? I said tape output.

If you look at the diagram*, you'll see that joining these two will bypass the potentially troublesome switch in the gram input socket.

* By "diagram", you'll have to look to the RV20 schematic to see this, as the RV14 schematic doesn't show it. The socket was added later in the RV14's production run - they mention it in the manual, but didn't re-draw the schematic.

If you don't have a phono-to-phono lead with the requisite long-reach plug tips, a crocodile lead can be used on the inside of the chassis (with all safety precautions taken, of course). But if you've managed to get an audio signal in, I guess that means you've got at least one suitable plug.

The HT is a tad high, but shouldn't cause any problems in the short term. Let's not worry about that until we've got the rest of it working.

It's a shame you don't have an oscilloscope or a test amplifier. The amplifier in the radio will do the job as a test amplifier for now, but it's really nice to have a known-good amplifier/speaker on the bench. I made a simple one decades ago which I use all the time for radio work. In the short term, a set of PC speakers will do the job, and can be found at car boot sales and charity shops for not much money if you don't already have something suitable. Likewise, oscilloscopes can be found for sensible money, but there is quite a learning curve with them.

Earlier you said "volume is faint and there is considerable hum", which I took to mean you could just about hear radio stations, but they were swamped by the hum. Now you're saying "No hum either". Was that during the test? Where the speaker output was connected to the gram input? In which case, the speaker might not have been connected to the amplifier, depending on how you made the connection... As soon as you unplug the speaker connection from the rear panel, you've - well - disconnected the speaker

(Forgive me for stating the obvious, but you say you're a novice, and until I've got a better idea of what exactly that means, I've got to err on the side of caution)

[bristle66]

You are of course correct.  It's tape output not speaker output.

Anyway, I can report progress - of a sort.  Your observations about the gram input prompted me to replace it with one George Christofi was kind enough to give me some time back.  Result - the radio immediately fired up.  The old input was clearly defective.  Withdrawing the long reach phono plug did not restore the radio signal and what I assumed to be a very faint signal was possibly crosstalk.  Whatever, we now have a fully functioning radio.  Well, almost.

After half an hour of Radio Three, the 120 ohm series resistor decided to give up the ghost.  I replaced it with a 10 watt 180 ohm wirewound I had to hand.  It failed within minutes.  I have no more suitable wirewounds available and will have to send off for some.  But we appear to to have exchanged a radio problem for a power supply one. 

Before I order anything, it would be useful to have your thoughts on (a) what could be causing the resistors to fail and (b) what value I should be going for, given that the HT is a little high.

[Mark Hennessy]

The gram input socket will probably respond to switch cleaner, so don't throw it away just yet.

I assume you're referring to R39? Always give the component designation to avoid confusion

When you measured the HT, which side of R39 did you measure it on? The rectifier side, or the output transformer side?

[bristle66]

Apologies - I'm still learning the language. Yes , it's R39.

The HT was measured on the rectifier side.

[Mark Hennessy]

No worries!

The next thing is to measure the other side - the side connected to the output transformer and the rest of the radio. That's the side that I'd call "HT". Others are more experienced with valve radios than I am, so they'll tell me if I'm wrong.

For your wire-wound resistors to fail, there must be quite a large voltage across them. That will be proved by measuring the voltage between the centre tap of the output transformer and chassis (fairly easily done from above the chassis) - it will be quite a bit less than 278V. If it's 240 volts, for example, that means that you would have 38V across R39. You could just put your voltmeter across R39 and measure that directly - saves the mental arithmetic, but does involve taking live measurements on an upside-down chassis.

When you know the voltage across R39, you can work out the power it is being asked to handle (V-squared over R). So for my fictitious 38V, across 120 ohms that's 12 watts. The original resistor is probably only rated for 2 or 3 watts, I'd guess...

Assuming you've changed C35, almost the only thing that can take enough current to damage R39 will be the output valves.

I'm not saying they're faulty. In practice, it's more likely that they are conducting too much current because of something else. That would usually be C31 and C32.

Re-reading your first post, I see you've changed the electrolytics. That suggests you haven't changed C31 and C32. If true, then do that before using the radio again. There is a risk of consequential damage until these have been done.

BTW, these "grid coupler" caps are another known trouble-spot on valve gear (not just the Mayflower). They almost always leak (electrically). This has the effect of making the output valve(s) conduct more than they should. They are so notorious that many people refer to them as "that capacitor".

With those replaced, you're probably sorted. Fingers crossed! They are 50nF, but 47nF is perfectly fine. Watch the voltage rating - they need to be rated for HT at least, so I'd go for a minimum of 300V, or 400V to be sure.

As a suggestion, these would do the job: https://uk.farnell.com/vishay/mkt1813347...dp/1166874

They are rated for 630V DC, and aren't too expensive. It's worth buying a handful.

[Mark Hennessy]

Just thought I'd drop a copy of the schematic here, for anyone following. Just the audio stages...

[ppppenguin]

Please heed Mark's warning about the grid coupler capacitors. The "consequential damage" can be serious: the output valves and output transformer.

If you haven't already done so, please also check R37 and R38. These are the cathode resistors for the output valves. If they have gone low (below about 200 ohms) that will also cause excess current to be taken by the output valves.