Hacker RV14 front end problems

[bristle66]

Many thanks guys.  I'll order some of the Vishays now.  R37 and R38 are OK.  Any thoughts on the replacement R39?

[ppppenguin]

For making the set work at all the value of R39 isn't critical. If I didn't have a spare to hand I would be tempted to stick a 100W old fashioned lightbulb in its place. If it lights up more than dimly and stays that way then something is very wrong. You can also put a meter on current range in series to see the actual current taken.

Doing it as a lash-up with croc clip leads is fine, but keep your hands away from it. Set it all up, turn on the set, see what happens and turn off again before touching anything.

[Mike Watterson]

And it can't be said too often, the biggest issue isn't electrolytics, resistor values can be way out, but what will destroy valves, audio transformer and mains transformer is capacitors coupling anodes to grids. They go leaky and it's only evident with a high voltage tester, not a regular capacitor meter or DMM.

The leaky caps from g2 pins will lower gain, but do no damage.

The paper dielectric caps had a 10 to 15 year life. They can be in waxed card, tar or even plastic.

One way to see is if your DMM is 10M or 20M on volts, is to power up the radio with no valves in it. Connect -ve of DMM to any cathode or 0V line.

Without valves and powered on (the dead HT resistor can be temporarily be replaced by a short):
Check g1 on all sockets. Should be zero (won't be with valves in, be negative or zero compared to cathode.
Check g2 on all sockets. Should be HT volts (won't be with valves in, be less then).

If any g1 is not zero the series capacitor needs replaced. Exact value not critical, can be x2.

If any g2 is not at HT, the capacitor to 0V needs replaced. Exact value not critical, can be even x4.

Rating should be HT +50V (minimum) and polyester types are fine.

[bristle66]

OK.  Update.  "Those" capacitors have been replaced with Vishays.  R39 is now a 200 ohm wirewound rated at 6 watts - which is all I had to hand.  I've replaced the Gram input with a normal phono socket and installed a SPDT switch in place of the old tape output.  Which means I can now switch between line input and radio without having to faff around with long reach plugs.  Result - a fully functioning radio with sounds rather good to my untutored ears.  Not as God made it of course but then I'm not a purist.

The only outstanding matter - touch wood - is the HT.  With the new wirewound in place, it measures 275v, which Mark has suggested is a bit high.  For the avoidance of doubt, this is the voltage measured after R39.  The voltage drop across R39 is 15v.

[Mike Watterson]

Likely 470 Ohms is OK with silicon rectifier. Not too critical. I'd use a 330 or 560 if that's all I had.

Power rating at least Vsquared/R + 25% margin, so even 2.5 W (3W better) might do fine. Likely the leaky caps on grids meant massive currents earlier.

[ppppenguin]

Valves are pretty tolerant. I doubt that 275V HT will cause any trouble. As Mike says, 470R would be better but 200R will work.

[Mark Hennessy]

I always leave R39 at 120 ohms.

If the HT is too high after a silicon rectifier swap, that's because the person who did it neglected to install a series resistor, or used a value that was too low. Looking at my notes, I've fitted anything between 33 ohms and 100 ohms. I've done loads over the years - mostly for a good friend who was selling them - but it's been around a decade since the last one, so without digging out my notes, I can't remember all the details.

The advantage of adding the series resistor is 2-fold: it saves changing the value of R39 and it avoids stressing the first of the capacitors (C36). It is possible to include the resistor inside the original rectifier (Westinghouse FC 124), as the attachment shows. The resistor is in contact with the aluminium body, so gets a reasonable cooling effect from that.

Part of the trouble is knowing what the HT actually should be. It's not quoted in the manual. They do state the anode voltages at 250V (and 240V for the screens). But the RV20 manual quotes 270V (and 275V for the screens). The latter values make more sense to me - or at least, the fact the anode is lower than the screen seems logical. But otherwise, these sets are practically identical, and both have the same audio output specifications. So, perhaps the HT should be 275V after all? But I've tended to go a bit lower, and still hit the output power spec.

If you don't want to pick apart the rectifier swap then I guess it's OK to vary R39. Check the voltage rating of C36 isn't being exceeded, especially when the set is first switched on. I normally replace C35/C36 with parts rated at 450V - it's easy to cut open the original part and fit replacements inside (I'm not big on re-stuffing, but I make an effort with these).

[Mike Watterson]

I always leave R39 at 120 ohms.

If the HT is too high after a silicon rectifier swap, that's because the person who did it neglected to install a series resistor, or used a value that was too low. Looking at my notes, I've fitted anything between 33 ohms and 100 ohms. I've done loads over the years - mostly for a good friend who was selling them - but it's been around a decade since the last one, so without digging out my notes, I can't remember all the details.

The advantage of adding the series resistor is 2-fold: it saves changing the value of R39 and it avoids stressing the first of the capacitors (C36).

Totally agree that a separate resistor between rectifier and 1st cap is the proper thing. I didn't look at schematic and didn't realise R39 wasn't before any caps.

In any case the HT caps voltage rating ought to be good enough for the situation were the heater supply is disconnected or output valve dead/missing or O/P transformer O/C. Some makers skimped on voltage rating to save costs, assuming always normal operating conditions.

[bristle66]

Many thanks, guys. C35/36 are rated at 500V. But, just to be on the safe side, I'll fit a series resistor as recommended by Mark. I'll experiment with values to get the HT closer to 250V.

I'm indebted to all of you for your guidance and your patience. I'm sure hand holding must be a pain but, believe me, it's necessary and greatly appreciated. Thanks again