Lafayette LR-3500 questions

[papagiorgio]

Hi, I just pulled a Lafayette LR-3500 from someone's trash and fired it up. At first there was no sound, but that seemed to resolve itself after a few power cycles. I think the protection relay was stuck from lack of use and worked itself free. The receiver now seems to be mostly working (I am surprised at how good the AM radio sounds, using the stock antenna), but the unit still has a few issues. The most significant is that there is no sound (well actually a tiny bit of sound when volume is set really high) when the tone controls are in play. Sound is fine when switch is in "flat" mode, but no sound in "Tone" mode. Both L&R channels are lost, which is interesting because the tone systems seem to be independent for each channel. I was therefore suspecting a power-supply problem since that could affect both channels. I measured the voltages on the 7xx board - some of the readings I got were:

- Pin 3 ("+B"): -40v measured; schematic says -38.1v

- Pin 4 ("-B"): +39.9v measured; schematic says +38.2v

- Pin 5 ("+35v"): +18.6v measured; schematic say +31.8v

- Pin 6 ("+13v"): 11.5v measured; schematic says +13v

The reading for the "+35v" line seems troublesome at only +18.6v. I started probing around Q701, which seems to be most-directly in control of this supply line. The capacitors checked out ok with my ESR, and the resistors where in range of their markings. Having said that, R702 is marked on the schematic I have (which is difficult to read) as 22 ohms, whereas what is installed is 270 ohms (see picture). There may have been some rework done in the area, but the transistors look original. I am wondering if there was any update to the schematic which changed R702 to be 270 ohms, or if this is a mistake that I should correct. I am reluctant simply to swap out the resistor as I don't want to blow the transistor. Can anyone tell me the value of R702 as installed in your LR-3500?

Update: I tested the Tone/Flat switch and it appears to be working normally.

Second Update: I am now suspecting C521. It is hard to get to, but is showing some brown leakage at the bottom...

I have a few other questions
- does anyone have a good-quality schematic scan, user manual, and/or service manual for the LR-3500? It looks like hifiengine has some manuals, but of course I can't register anymore to download the files. I have some poor-quality split-page schematic scans that I got from radiomuseum.org. I see this question has been asked several times before, but I haven't seen an answer...
- does anyone have other theories as to why I lose all sound in "tone" mode? What common points of failure could there be that would impact both channels? The High Filter and Loudness functions are working. The problem occurs with all inputs and all speaker combinations (headphones, mains, etc.).
- on a different note, several of the lamps are out, including the one for the signal meter, and also at least one backlighting the frequency display. I don't see any part numbers for these lamps on the schematic - does anyone have part numbers for these lamps? I would like to avoid having to take the front apart just to get the numbers.



Thanks for any help.
-Paul

 

[peteCWA]

Hi, I just pulled a Lafayette LR-3500 from someone's trash and fired it up. At first there was no sound, but that seemed to resolve itself after a few power cycles. I think the protection relay was stuck from lack of use and worked itself free. The receiver now seems to be mostly working (I am surprised at how good the AM radio sounds, using the stock antenna), but the unit still has a few issues. The most significant is that there is no sound (well actually a tiny bit of sound when volume is set really high) when the tone controls are in play. Sound is fine when switch is in "flat" mode, but no sound in "Tone" mode. Both L&R channels are lost, which is interesting because the tone systems seem to be independent for each channel. I was therefore suspecting a power-supply problem since that could affect both channels. I measured the voltages on the 7xx board - some of the readings I got were:

- Pin 3 ("+B"): -40v measured; schematic says -38.1v

- Pin 4 ("-B"): +39.9v measured; schematic says +38.2v

- Pin 5 ("+35v"): +18.6v measured; schematic say +31.8v

- Pin 6 ("+13v"): 11.5v measured; schematic says +13v

The reading for the "+35v" line seems troublesome at only +18.6v. I started probing around Q701, which seems to be most-directly in control of this supply line. The capacitors checked out ok with my ESR, and the resistors where in range of their markings. Having said that, R702 is marked on the schematic I have (which is difficult to read) as 22 ohms, whereas what is installed is 270 ohms (see picture). There may have been some rework done in the area, but the transistors look original. I am wondering if there was any update to the schematic which changed R702 to be 270 ohms, or if this is a mistake that I should correct. I am reluctant simply to swap out the resistor as I don't want to blow the transistor. Can anyone tell me the value of R702 as installed in your LR-3500?

Update: I tested the Tone/Flat switch and it appears to be working normally.

Second Update: I am now suspecting C521. It is hard to get to, but is showing some brown leakage at the bottom...

You said: "I tested the Tone/Flat switch and it appears to be working normally" What does that mean; Are you getting sound and tone adjustment when switch is in the "Tone" position?
R702 was never changed. It should be 22 ohms. Note, when checking the Pin 5 voltage above, check it on both sides of the 270 ohm resistor.
Also check the voltages on Q701, the 2SC1096. Emitter should be roughly 38 volts, Base should be roughly 29 volts, Collector should be roughly 31 volts. Pin 5 voltage supplies the FET's in left and right channel base and treble tone circuits on board EPA-0217A. The Pin 5 voltage also supplies voltage to boards EPA-0219A and EPA-0229A.

Lafayette created a separate operating manual and a service manual for the LR-3500.
There are 14 lamps in this receiver. Different styles but I believe they are all 8 volts.
The included parts list only includes Lafayette part numbers which would be zero helpful today.

 

[papagiorgio]

First let me thank you for taking the time to respond - your help is greatly appreciated.

My comment about the Tone/Flat switch was simply that I used an ohmmeter to test out the switch itself, and it checked out ok. I still get no sound in "Tone" position, while in "Flat" I do.

The voltage readings for Q701 are:
- Emitter: 31.8v
- Base: 32.3v
- Collector: 38.9v
This roughly matches your numbers, except that the Emitter and Collector voltages appear to be swapped (see my comment about the schematic below).

Readings for R702:
- Q701 side: 31.8
- Pin 5 side: 21.4v (this is up slightly from earlier readings, possibly due to the replacement of C521, but I cannot be certain).

Replacing R702 with 22 ohms (actually slightly less - 21.5 ohms - since I put it in parallel with the 270 ohms resistor) yields:
R702 (21.5 ohms)
- Q701 side: 30.3v
- Pin 5 side: 31.4v (very close to +31.8v from schematic )
The voltage readings for Q701 are not significantly changes.

So it looks like the problem with the Pin 5 supply is the incorrect R702. Unfortunately testing with the 21.5 ohms did not resolve my Tone problem.

I have to say I am a bit confused by what the schematic shows (see pictures) and what is actually present. Unless I am reading it wrong - a definite possibility - the schematic is showing the collector (line without arrowhead) attached to R702 and the emitter (line with arrowhead) connected to R704. On the board this is reversed. Am I reading the schematic wrong? Q701 is marked "NEC C1096", same markings as Q5. The opposite orientation of Q5 and Q701 on the schematic may also suggest a schematic error.

Also not sure about the empty thru-holes parallel to Q701...

 

[papagiorgio]

OK so I think I've made progress. I had suspected C522 but couldn't get good access to the capacitor to test it with my ESR. When I finally did, it tested low ESR (maybe too low). I then touched R536 (right after shutdown) and it was hot, suggesting C522 was shorting to ground. Continuity test with the ohmmeter confirmed that C522 was shorted. Not having a good replacement on-hand I simply pulled C522, powered back on (volume way down and only using headphone output), and the tone controls were functional. The system was noisy, but no doubt due to the missing cap and also partial disassembly of covers etc that I had done. So next step is to replace C522.

FWIW I also looked at the bulbs - the signal meter, and I assume tuning meter, have Stanley 8v 0.25A fuse-type lamps. The dial has Stanley 8v 0.25A auto-type lamp. I have some pictures below.

Thanks peteCWA for your help. I'll report back if/when I have everything put back together and working again.

 

[peteCWA]

Congrats on finding the shorted capacitor. Last week when I was looking at the schematic, I circled C522 on my schematic but got tied up with work related stuff and didn't get back to the forum.


8 volt fuse type lamps are available from Parts Express, Amazon, Ebay, and probably a few other places.
I checked my stock for those other lamps and don't seem to have any more.