I have a Philips Radioplayer 179 chassis that sits bare on a shelf and acts as my workshop radio, driving an old baffle-board speaker.

Lately it has been getting harder and harder to get a clean signal out of it - lots of hash and many MW stations inaudible.

Antenna manipulations have no effect except on volume.

I have the service data sheets and have checked voltages, also done the IF alignment according to instructions on the sheet.

Screen voltages seem to be a little high on V1 and V2 (65volts, should be 50-55) but correct on other valves. Plate voltages all OK.

IF alignment achieved some increase in volume, but no improvement in selectivity or clarity of signal.

Most capacitors were replaced not too long ago, only a few ceramics in the tuning and tone circuits are original. It was working well for at least two years since then.

I've cleaned everything including the main tuning condenser (some cobwebs removed) and checked everywhere for shorts or dry joints - nothing found.

Also swapped out the 6AN7 - no help. I don't have 6BH5 or 6BD7 to swap, but voltages on the 6BD7s were spot on.

Any suggestions?

Maven

Have you considered that the "hash" might not be due to the radio but external interference?

What has changed around your house lately?

LED lighting?
Solar power?

Good points, Ian. I do have solar panels and am steadily replacing fluoro tubes and CFL units with LED. But this problem came up long after that. I've just rechecked by switching nearby LED units on and off - no effect. The location has always been electrically noisy, but other radios are not suffering the same deterioration.

Maven

Are any of the ceramic caps the Simplex brand?. I am told that they can be a issue.

Hi Maven, sounds like the whole set gain has gone down slowly.
Just a guess but I would be suspicious of the back bias resistor going high.
I think its R26? and 33 ohm.

My reason is this, the front end gain is set by the back bias voltage as a pedestal with any AGC varying on top.
If the back bias resistor goes high, the negative bias jacks up, the valves bias off, run at lower gain and the set goes "quiet".
The screen voltages being high also indicate the valves are biased off a bit too much.
I'll bet it gone high, offset the bias and lowered the gain.
I've replaced such resistors with a wire wound that will last forever.

Hope I've guessed it in one!

Cheers, Fred Lever.

I'll follow up that back-bias resistor idea. No Simplex caps that I can identify.

R26 in this set is one of a pair of 4.7Kohm resistors above the main power 40mF bypass caps C31,32 .

R24 is a 47ohm wire-wound between C32, ground, and the transformer secondary yellow tap - I suppose centre tap. The service doc says votage across R24 should be -3volts, so I guess that is the back bias.

If it has gone high, I'll get a replacement 47R wire-wound, high voltage rated.

As an untrained amateur at this, I place a high value on other people's "guesses" that are based on a lot of experience. My own "guessing" is likely to be a less reliable hypothesis. Better to test other people's informed guesses than my own less-informed ones.

Maven

Hi, I missed by thaaaaaaaat much!
Looks like your chassis has R24 as the bias resistor and a 47 ohm wire wound is much better than the original 33 ohm 1/2watt carbon I was worried about.
If you have about -3 volts measured across it that would be normal and i'm wrong.
your lack of oomph does sound like a low gain situation or a loss of signal.

My next impulse would be to clip the AGC feed to the front end to ground at the grid side of the isolating resisitor.
When you do that the signal level should jump to maximum as the bias comes off the grids and there should be 10's of volts at the AGC diode as the IF stage should then be flat out and deliver lots of audio drive and distortion.
With hi gain modern valves in the front end and a PP output stage that radio should work like ripper.
If nothing much changes then the front end gain is low for some reason or the signal is choked off by something.
My approach is to closely examine each stage and test each function for operation and efficiency, easy to say but tedious to implement especially if you are not used to doing that. That then involves scoping signal levels at input and output of stages and comparing the results to some standard like a handbook (i'm joking) or similar sets or experience.
Good luck and keep posting what happens and i'll keep chirping from the side lines.
Fred.

I would strongly advise against heavying up the back bias resistor. That is a resistor I always let sail with the wind. If there is a HV short (eg filter) better it burns than the transformer.

LED's are no guarantee of an RFI free zone. I have send two LED lights back to place of purchase for generating RFI across the spectrum. I also have an NBN box that works off a tower. These apparently have no AM filters & kill signal. They should never have been let into the country. It has a devastating effect on signal.

Short out the Antenna with a 0.01mfd cap & tell us if there is still hash. A sizzling hash is often produced by a Mica cap when they fail. Ones on HV DC are the common culprits. If its not affected by volume control, its in the audio.


Marc

Well, I followed up every resistor in the set. R24 in fact turned out to be spot on its 47ohm, but about a dozen others were significantly off, and have been replaced. Result: Voltages read correctly, including bias, and I'm getting much better sound, but still a degree of distortion.

I have done an alignment as best I could understand the service doc - but what is the difference between "peaking" the various IF and Antenna trimmers and just getting max volume?

Since my attempted alignment, all the BC stations are at the right points on the dial, but reception is very uneven across the band. I "guess" I have not aligned correctly.

As to noise - my workshop and the outdoor single-wire antenna are less than ten meters from my 3-phase solar inverter. But cheap little radios in the same location don't pick up any significant distortion. Just this 7-valve Philips 179. Could internal caps in the IF coil cans have failed? This happened on my older Philips 123.

Anyone recommend a good tutorial on alignment procedures? And testing for source of RFI?

Maven

The difference is the wire antenna and the ferrite rod antenna in the "cheap little radios"..

I have the same issue at my place. Wire antenna is unusable for most of the band, ferrite rod antenna much better.

Except I haven't yet found the source of the noise. You have! The solar inverter!

Re the alignment, IFs caps will be OK if you can find a peak with each adjustment. It's very rare for these to be bad.

IF slugs may peak in two places. Use the OUTER one.

Use a very weak signal when peaking the IFs etc.by ear because otherwise the AGC will mask your adjustment.

Re the uneven reception across the band, sounds like you have introduced a tracking error in the RF alignment. You should normally peak the RF trimmers on a weak signal at around 1200 kHz and the slugs likewise at around 600 kHz. Use the weakest signal you can find.

There is normally a marker on the dial for setting the pointer. Make sure this is mechanically correct.

The danger if you don't have an accurate signal generator is the IF might get tuned to other than the recommended frequency - usually 455kHz or whatever the manual says. If this is wrong you will never get the RF section to track properly.

Ian T. said

QUOTE: Wire antenna is unusable for most of the band, ferrite rod antenna much better.

Perhaps before re-doing the alignment, I should replace the long wire with a ferrite rod, or a loop.
This set came from a radiogram - I believe the original cabinet would have included a loop antenna.

Jaycar has both on offer:

CAT.NO: LF1020
"Small ferrite rod complete with pre-wound standard broadcast band coil. Ideally suitable to our 60-160pf mini tuning capacitor "

CAT.NO: LT3001
"a loop antenna that can be easily left on top of your stereo cabinet of stored out of sight. It has built-in stand and fly leads to fit the antenna spring terminals on most tuners.

Frequency range: 520 - 1660kHz
Dimensions: 135(W) x 120(H)mm"

Any suggestions which would be best, and any tricks to matching a ferrite rod to the set?

Maven

I would be suspicious at trying to fit a loop antenna as normally the aerial coil is on the ferrite loop. You may be better with the old wire loop type as used on the really early radio's. as the ferrite may not track properly.

You will get variations in signal strength across the band due to atmospherics & distance. Where there is an adjustable padder it is set first and the aerial trimmer last. Aerial trimmer here is done only with the "long wire attached" nothing else and on the weakest station that can be found around 1500 kHz. Standard length for along wire was around 25 feet. I have space; one of mine here (not being used) is 70m.

There is a new kid on the block for killing AM signal & some FM and is sent to you to run NBN. This wondrous piece of non compliant ..... Will wipe out an AM signal for 15 feet on a strong station: Some much for regulations.

Fax & switch mode PSU can get into a 175kHz IF & computers are also good at RFI, as are Compact Fluro lights and some LED lights with Switch mode PSU's.