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Heathkit AR-1500 - problem with FM

S

Spint

Active Member
Picked up and AR-1500 awhile back and got everything working on it except the FM tuner.

Now- I am a bit clueless when it comes to troubleshooting FM tuner issues but seeing if anyone can tell what the issue is or where to look. I can hear stations very faintly if I turn the volume almost all the way up. Behaves the same whether FM auto or FM stereo is selected. The signal and tuning meters are moving. I made sure the pots for the FM inputs were not turned down (the ones you can access from he bottom). I realize the 3 way switch at the top of multiplex board does something with the signal meter since it seems to disable or reverse its orientation depending on where the switch is (but this has no impact on what I hear). I gave all those selector switches on the front another thorough cleaning just to be sure. The AM seems to work fine as do all the other input sources.

Another issue is that the backlight for the dial seems to be very very dim, as in can barely tell its on. All the bulbs seem to be functioning fine (all except the stereo indicator bulb which I don't think I've ever seen light up). However the signal and tuning meters are plenty bright. Just mentioning this in case its related.

Was going to go through and test every component on the FM and multiplex boards but looking for a shortcut so thought I'd post here first.
 
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Try taking the signal direct from the FM detector, bypassing the multiplex decoder. This will help indicate if the problem is in the multiplex or ahead of it. The multiplex section does have a lot of squelch circuitry and that could be killing the signal.
 
So I connected a wire from pin 2 on FM IF board to the tape monitor input (saw that trick in another thread). I definitely get good sound- not as loud as it should be but I guess that because I'm bypassing the multiplex. So I guess that narrows it down to the multiplex. The other thread I referred to said all was fine when they adjusted "ratio detector #53-14". Not sure what that is or if I should even be touching it without any knowledge. Here's the thread I'm talking about:

https://audiokarma.org/forums/index.php?threads/heathkit-ar-1500-no-sound-from-radio.745677/

By the way- I have a schematic but I don't have the manual.

Any suggestions? Thanks
 
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I don't think there is a "mute" on this unit. There is "FM stereo" and "FM auto" but both behave the same. Pressing the loudness, blend and mono selectors make no difference either.

However- I was poking around on the multiplex while it was on and all of a sudden everything came in loud and clear. Then it faded away after 5 seconds and I can't reproduce what I did. At first I thought it had something to do with the blue trimpot labels "sep adj" but now I'm not sure. I cleaned the trimpot but now that pot seems to do nothing when I rotate it. Could this trimpot be bad? Or possibly a bad electrolytic somewhere? I could easily replace all the electrolytics but hate creating unnecessary work for myself these days. I have tested all the transistors (in circuit)- no shorts. I tested resistors and while some where out of spec, nothing was blatantly off.
 
IC301 is in a socket yes? Pull it out and put it back in (to clean the pins a bit), and see if that helps. If not then similar to what you did in post #3 look for signals at IC301 pins 11 & 12.
 
Pulled IC301 cleaned and reinserted. No change.

wasn’t sure which exact pins were 11&12 so tried them all. I can figure out correct pins and recheck was just in a hurry. Got sound from a few- some clearer than others. Not sure how strong signal should be at these points.
 
The level using detector out to tape monitor should have the same level as other inputs because the circuit path is the same. Of course, the individual trim pots must be turned up.

Don't adjust the ratio detector just yet. Or, put a flag on the tuning tool so it can be returned to its original location. I would suggest that the problem is in the multiplex decoder section.

The collector of Q311 is the output of the squelch control. This should be pulling down the cathodes of D309 L and R, allowing them to pass the signal. If you can access points A and B on the multiplex board (can't tell what they are connected to on the outside) you should have audio signal direct from the actual decoder before any squelch circuits.
 
So I got it working- well kinda....

I pulled the multiplex board. Removed and cleaned IC301. Replaced the only electrolytic cap on the board (2 mfd/70v with a 2.2/100v- the one next to the ratio det trimpot). Hooked all back up. The L channel started working but R very weak- made sure the levels for both FM R and L were both fully clockwise. I do get good sound from the R channel if mono or blend button is pressed.

However whenever I switch to a different source and back to FM- nothing happens at first but within 5-10 seconds, the FM starts coming through, a little distorted at first but then it cleans up and sounds great within a few more seconds. This even happens sometimes when switching from FM auto to FM stereo. It takes a few seconds but eventually it starts coming in weakly and quickly gets stronger and sounds fine. (Except R channel is very very weak unless using mono or blend as mentioned above). All other input sources (including AM) work fine in both channels

I also noticed that the FM stereo mode only works if the stereo indicator lamp is removed- basically I can turn the FM on and off by pulling and replacing the lamp. The lamp makes no difference though it set to FM auto mode. Now I may not have the correct lamp which may be the issue- but I ordered some LED's that should fit since realize the lamp could cause issue if drawing too much current.

So progress. Any ideas what is causing the slow warmup for the FM?
 
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There is an issue with the way the decode chip interfaces to the stereo light. Let me see if I can find that thread. {edit} https://antiqueradios.com/forums/viewtopic.php?t=375988

I don't in general recommend any 'shotgun' type troubleshooting, but in this case I would suggest you find and order an NTE722 IC to replace that MC1307P. Should be available and rather cheap.
 
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Quick question- the original stereo indicator lamp is a GE #1813. I’ve read that the wrong lamp in that position can cause issues- I have this LED on hand. Is it an okay sub?

https://www.parts-express.com/5-Pac...-T3-1-4-Bayonet-LED-Lamp-for-McIntosh-070-136

If not I may need to get a suitable replacement. I do have 44’s and 47’s but don’t think they’ll work there.

As an observation- if I put a 44 or 47 there it never lights up. But if I put in the LED it stays lit no matter if tuned to a station or not. If that means anything…..
 
I think you'll be OK with the LED. May need to shunt the LED with a resistor and/or put the pull up on Q309. But let's try to get the stereo working before we worry too much about the stereo indicator.
 
Right. But for troubleshooting purposes, I was just wondering if it would cause issues if wrong lamp vs no lamp in place.
 
It doesn't look like it. There is some convoluted feedback of lamp current to the muting sections, but it looks like a bad bulb - or a bulb that does not draw enough current - would tend to unmute the system.
 
The 1813 lamp is 14 volt and the 44 and 47 are 6 volt. The 1813 only draws 100mA and the 44 and 47 draw 250mA and 150mA respectively. It's not a good idea to just swap bulbs around willy-nilly because you could burn out the source.

I agree to try a different decoder. The symptoms kind of fit the same ones I had with the MC1305 decoder I had in my AJ-29 tuner - weak channel, etc. and that's what it was in that. However, I know one guy had issues trying to use the NTE chip to sub for the 1305. An LM1305 worked well for me until I was able to locate a genuine MC1305 in, of all places, Ireland! The only difference is the LM IC didn't seem to require fine tuning of the multiplex coils as covered in the manual. Adjusting the coils made very little, if any, difference but the IC worked perfectly with great separation and fidelity.

Doug
 
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