Lafayette Model HA-600A VERY DEAF
 

I have another of my lock down buys on the bench today which is a Lafayette 600A. It picks a few broadcast stations quite loudly but not as loud as I would expect. Running it from a 50 ohm signal generator its take 600uV before I hear a low audio tone at 80% modulation.

I dont understand the imput path from the antemma upto the mechanical filter MF1 and hoping someone will help walk me though the circuit.

I can see the front end tuning via SW1 but was expect the signal to enter via an RF ampifier however it the manual describes FET1 as a mixer stage. There is also a dotted line shown from SW1 to near RV2 I think this is the AGC line.

What is the purpose of SW4 ?

I would normally suspect a bad tranistor in the front end possible FET1 or TR-1 or a problem with the mechanical filter. I am not sure how to fult find a mechnaical filter?

I think I need to understand the signal path first before I start faulting. So hopefully someone will be able help with the circuit operation .

Thanks Regards Chris

Re: Lafayette Model HA-600A VERY DEAF
 

The description [red text] is wrong.

FET1 and TR1 are a 'hybrid' cascode pair.

FET2 is the mixer.

FET3 is the local-oscillator.


The switched resistors in the source connection of FET1 are presumably to alter the bias/standing-current of the cascode pair depending on waveband.

Re: Lafayette Model HA-600A VERY DEAF
 

Thanks G6

Oops sorry about all the spelling errors The joy of working from my phone screen. So basically your saying the circuit discription is just wrong. The manual and the block diagram does describe FET 1 as being a mixer. But your discription makes sense to me and what I expected. What about the function of the dotted line.

Thanks Chris

Re: Lafayette Model HA-600A VERY DEAF
 

As long as the filter is not the same as KW2000 types, then it might be fine. You can prove by putting a cap across. SW4 could be an attenuator. Is it set correctly?

Re: Lafayette Model HA-600A VERY DEAF
 

Hello Hamish and thanks. So just short of the filter with small capacitor and see if the level increases. I don’t have any direct control of SW4 it must be ganged with either the band select or function switch. Probing around the FET 1 with a signal tracer I don’t get anything on SGD. but I do get a signal on the base of TR1 but nothing on the Emitter or collector. I should probably change out FET 1 as the most likely problem

Re: Lafayette Model HA-600A VERY DEAF
 

Hello Chris

The dotted line shows that RV1 and RV2 are ganged together. They form the RF gain control, with RV1 acting as an attenuator.

Paula

Re: Lafayette Model HA-600A VERY DEAF
 

If by the "dotted line" you mean the one between RV1 and RV2, this indicates that the two controls are ganged.

RV1 is an attenuator on the antenna-input; RV2 alters the DC bias fed from the AGC-amplifiier TR12 to the front-end transistors.

The control-knob is probably marked "RF Gain" or "Attenuator" on the front-panel. This variable-resistor-in-the-antenna-input may seem odd but it was a common thing in first/second-generation solid-state receivers, which handled really-strong signals rather poorly. The last thing you wanted [in the US at least] on Medium Wave/AM broadcast-band reception was lots of front-end gain when there was a broadcast station putting-out 10Kw a couple of blocks away; but equally when you were trying to listen to a weak ham-station on 28MHz you _needed_ that gain.

Even 'professional/military' solid-state receivers of the era incorporated aerial-attenuator networks as a way to address the poor dynamic-range/strong-signal-handling ability of first-generation transistor RF-amplifiers/mixers.

Re: Lafayette Model HA-600A VERY DEAF
 

Thanks G6

Great explanation. I was confused about the ganging because the line looks like it goes from the switch. But your quite right the two potentiometer are on the same shaft marked RF gain.

Re: Lafayette Model HA-600A VERY DEAF
 

You may simply have come across one of the worst circuit diagrams I've seen in long time.

David

Re: Lafayette Model HA-600A VERY DEAF
 

It’s probably me David that’s a bit clueless with the modern stuff. But to add insult to injury actual front end FET is mark FET1 on the circuit diagram but it is FET2 on the circuit board. It’s causing me a lot of head scratching. I will update my annotated schematic and upload it again tomorrow

I have removed it from the circuit board and my peak tester is reporting it as a diode although RF transistor often come up erroneously on the tester. Would a MP102 be a useful replacement or are then any easier alternatives for the SK19 to be found at RS or Farnell

Re: Lafayette Model HA-600A VERY DEAF
 

Don't do yourself down. That diagram is utter crap. It's badly drawn, wrongly labelled and thoroughly misleading. Even the coil packs - crazily removed away from the circuits they are part of, are in the wrong order.

If someone had set out to make it as confusing as possible, they couldn't have done much better.

The unknown is whether they were so clueless or whether it was deliberate obfuscation. There is the possibility that it wasn't drawn by the designer, but by someone downstream and without a scoobie.

Circuit diagrams re-worked through the eyes of a 'graphic designer' are also quite something too (this isn't one of those...).

David

Re: Lafayette Model HA-600A VERY DEAF
 

I read re the KW2000 filter that you can put either it was .1 or .01 uF across it and see if it livens up.

Having said that, they (Lafayette) might not suffer from filter problems like the KW.

Re: Lafayette Model HA-600A VERY DEAF
 

I think KW used the Kokusai filter. Did the Lafayette receiver use a real Collins/Kokusai style mechanical filter, or was it the cheaper ceramic type?

David

Re: Lafayette Model HA-600A VERY DEAF
 

That was what I wondered , in which case I am miles out.

Re: Lafayette Model HA-600A VERY DEAF
 

I think they are ceramic

Re: Lafayette Model HA-600A VERY DEAF
 

I am wrong! Chances are ceramic are ok.

Re: Lafayette Model HA-600A VERY DEAF
 

That circuit diagram strikes me as being very similar indeed to my Realistic DX-160. That certainly has ceramic filters.

You'd never call these communication receivers, but they weren't the worst sets ever marketed.

Let me know how close the 160 turns out to be.

B

Re: Lafayette Model HA-600A VERY DEAF
 

Thanks Bazz the front end drawing looks much more obviously and very similar to the 600A

Re: Lafayette Model HA-600A VERY DEAF
 

Modified Schematic

Re: Lafayette Model HA-600A VERY DEAF
 

I'm sure the incorrect block diagram is for the HA600, which didn't have the cascode front end of the HA600A.

Mine still works and is anything but deaf. There is far too much RF gain unless the dual-gang "gain" pot is turned about half-way down, at which point the wideband hash, due to overloading, disappears.

The filter is a simple ceramic type and it's inside one of the IFT cans. There are two more used as emitter decoupling in an IF stage.

As mentioned above it is really a glorified domestic receiver, and it is similar in performance to the (valve) Trio 9R59DE, which for some odd reason is quite sought after.

I bought my Lafayette new after carefully considering the choice between the two, and went for the HA600A because it could run from either 12VDC or mains. Turned out to be very useful during the early-70's power cuts, during which I could hear USA medium wave radio stations in the resulting quiet conditions.

Works OK as a basic receiver for HF broadcast AM, of course nowhere near as good as an FRG-7.

And yes, one of the first things I did when I got the set 50 years ago, was to re-draw the circuit diagram so it made sense!