The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

Go back and read my last post, I asked for voltages at some points, this is to determine where the fault may be.

Check the voltage at R70, it should be at least 8V, if it is not then there is a problem with the main voltage regulator circuit, D22 TR23.

If the voltage is OK then it could be the 5V regulator circuit, Z10.

Check the voltage between pin 11 and pin 21 of the PLL, to make sure that it has a supply, it sounds like a DC supply issue at this point. I would also check R124 for 5V from the main 5V supply. It interests me that the meter bulb blew at the same time, I suggest that you check your DC power supply ...

Check to see if there is any voltage in the CAL mode, and the S/RF mode, it could be bad wire on the S/RF/CAL/SWR switch. The S/RF and CAL functions get the feeds from different places, I would be surprised if they have both failed. It may also be a good idea to use the earth track of the main PCB f...

It sounds like you have shorted out the MPU, you will have to find another one to replace it with.

In the worst case I would be interested in buying the radio from you for parts.

You need to find out if it is an electrical or mechanical issue, so measure the voltage that is feeding the meter when you TX, if there is voltage but no movement then the meter is stuck. Usually undoing BOTH the tiny screws 1/2 a turn or so will free it, sometimes you need to go further than that, ...

What test gear do you have available to you ?

Remove the top cover, and touch the IC that is just behind the two vertical PCBs on the main PCB, marked as IC101 (it an NJM2902N), if it feels hot when the radio is on then replace it.

It was probably on the back of the pot originally.

One side of the pot goes to R415 (3.3K), which then connects to 0V, the other side goes to the RX only 8VDC supply, as found at the NB switch, and the feed side of the resistor that then connects to the RF gain pot (R417) (there are two, one goes to 0V, the other to the RX only DC). Once you have th...

Remove that clarifier mod to return the clarifier to original, and re-do the loop alignment and see if that gives better results, if you have replaced C72 as well and the VCO output is still noisy, then the PLL may be at fault. The reason I want to get the VCO and loop problem sorted first, is that ...

You will have to use tantalums, as they are critical parts, they form a special filter circuit to smooth out the pulses and steps which are generated by the PLL to control the VCO, using electrolytic caps probably won't work. You may also want to check C72, that is supposed to be a tantalum as well,...

Now check the freq TP10 and see if it is stable when you are injecting each of the following 1V, 3V, 6V. it does not drift with the fixed voltages OK, in that case check for a stable 10.240MHz on pin 7 and pin 8 of the PLL, if it is unstable replace C64, C65, C66, C67 and re-check. If that still fa...

OK, we shall try and fix the loop freq problem first, and go from there. Because it is a LOOP all parts are incorrect, the frequency drift could be the cause of the voltage shift, or the voltage drifting could be the cause of the freq drift, an easy way to find out if you disconnect the PD output fr...

the audio amp i replaced the original came out of a used/parts radio, for what its worth The odd voltage on the audio amp may mean the used one you put in could be faulty as well, if you have a new one try putting that in and re-checking the voltages, if they are about the same and it still has no ...

I am a little confused, it is 20KHz off frequency ? if the loop is running in the 40-50MHz range then something is not right if you are getting the TX being only 20KHz off. Is the loop freq the same (or very close) on TX/RX ? What loop freq do you get on CH1 at TP1, what freq do you get at TP10 and ...

All those voltage check out OK... TR34 and TR35 are the AM modulators, TR34 is a 2SC945, TR35 is a 2SC1419 they are at the rear right corner next to the audio amp. I am just trying to figure out what could be causing your radios faults, so far everything has checked out, apart from the slightly high...

There is usually (but not always) a variable VR on the power supply PCB.

I think I had some notes here for adjusting the voltage when the VR is not present, I will have to have a look.

What is the PCB number on the power supply board, there are different versions.

I kind of forgot about this one.. Have you tried doing an alignment on it, and did you notice anything that seems incorrect during the alignment ? It is likely to just be a capacitor (or a bunch of them) somewhere that are killing the DC supplies to certain parts of the circuit, the voltages you gav...

Glad to have helped. Have a good one.

Thanks for the info.

There are a few possible causes, I would first check VR2 to make sure it is not cracked or just incorrectly set (VR2 is the SSB signal meter), if that checks out, then check R61 , if that is OK then replace Q15. If the problem is only one LSB or USB (it works on the other sideband) then it could jus...

The TX alignment issue is probably due to the original clarifier mod, check for correct component values for the resistors that are connected by VR5. On the TX side R187 should be 2.7K, R186 should be 100 Ohm, there should also be diodes in D75 and D52. On the RX side R175 should be 3.3K, R174 shoul...

Of course, no CH9.. but yes, check the path and source of the voltage feeding pin 11, if you disconnect it from the ch selector and test the ch selector side for voltage it should say whether the selector is at fault, or if there is a short circuit somewhere (it should be feeding 0V), the PLL pin wi...

There is an error with the N codes, the ch1 code should be 1001111, not 1101111, check for problems with the connections for pin 11, try checking the diode matrix that is used for the CH9 funtion, or lifting the wire from the ch selector for that pin.

I think you checked IC4 instead of IC5, IC4 is the audio amp, IC5 is the MB3756

It should be an 18V Zener diode.

Also make sure that any diodes or resistors on the clarifier circuit have been restored and are not bridged or anything, often they are, which could put too much strain the the MB3756 if the clarifier is turned fully in one direction.

D60 is mounted between pin 2 of the MB3756 and 0V, it is on the track side of the PCB, it was a factory addition, to help protect the device, not all radios have it in place, if not add it.

You may want to re-check the clarifier mod, in case it is shorting the TX and RX lines together, it sounds like the MB3756 may have failed.

Make sure you connect your negative test lead to one of the metal cans on the PCB, and not the chassis, as the chassis is not a true 0V.

There is a capacitor near the audio amp that can short out when it is old, it kills the audio, check C107.

Also what voltages are you getting on IC5 ?

does SSB TX work at all ? is there an actual carrier getting out on AM ?

All of these units, (including the HR2510) have problems when they get older, what you need to start out with is to replace ALL of the electrolytic capacitors on the MPU PCB, there is only one which you do not replace, the large one that is next to the MPU, as it is a special memory backup capacitor...

Crystal sets are not my area, but I do know that there are two versions of the 135, I have a diagram for one of them, and that version (which may or may not be like your one) does not give any easy way to add more channels, yes there is one crystal that can be switched but it will only work on RX, n...

Because it is a crystal radio instead of a PLL based one it complicates things a fair amount. I suspect that it MAY be possible to instal a crystal mixer PCB, to shift one of the frequency banks to another range, to give higher channels, but it is not something I have done on a crystal radio, so I c...

If you have a scope there, it may be an idea to check pin 2 and pin 4 of the MN3101, it generates the clock frequency for the MN3008, check to see if the clock frequency drops off on those two pins when the echo fails, if it stays OK, then the fault is probably with the MN3008

I would say to check all the resistors and capacitors, if they are OK try replacing the LM324, it is a common part, and will rule that out, failing that it could be one of the echo IC's themselves.

INteresting, and pretty clever. I do wonder though if the resistance and tuning changes with temperature, as it warms up it may increase its resistance as the molecules expand, perhaps if you build it you could check the SWR at room temp, and again when it is hotter after it has been used and see if...

Glad you got it sorted.

it looks like the VCO signal is missing, check for the same frequencies at Q402 (it is just a buffer/amplifier), check all three legs, one is 0V, so wont have anything, the other two should have the frequencies on it I mentioned.

If there is nothing, then check Q403 and the alignment of L401.

Check the voltages across the IC's, specifically pin 1 and 3 of MN3101 and see if it drops off when the echo does.

Also check pin 3 and ground of the LM324, in case of a problem with the 100K resistors.

Does the TX LED light up and stay lit up the whole time, even when the echo fails?

OK, if the VCO and PLL are functioning, you should see 16.270 MHz on CH1 in RX, and 13.4825MHz in TX on PLL pin 23. If the signal is missing, then I would suggest you start by checking power supplies, the radio uses Zener diodes, it is possible one has failed, or a capacitor is shorting dragging dow...

Personally I put a 1K series resistor on the middle leg of TR24, and change R124 and R126 to 4.7K, that increases your mod level on all modes with no ill effects.

I am pretty sure that alignment info is on the main radiomods site too, my memory is failing me, but I expect it is there.

If you think about it you will understand that you only need to slide by about 6K down, as the upwards slide from the channel below it would cover the gap, giving full coverage.

Glad to hear it

Please post the diagram for the mic.

I would definitely keep what is done to a minimum, the more slide range you give, the more it will drift, AM would be OK, but SSB will really suffer for it. As far as a super diode goes, I have no idea, someone once used to market them as such, but as far a an actual part number goes, I do not know,...

I would start by looking to see if the PLL and VCO are functioning correctly, I would definitely be looking for things common to both RX and TX.

Can you please post a copy of the diagram here, I do not have that one.

Do you have the exact circuit diagram for the radio, I do not have a very good one for the KEPC-332, it is only just readable in places.

I would say to check (in order) R41, TR42 and the 220 Ohm 2W resistor, it looks like it is either R193 or R183 (I cant make it out), VR10, C174, R228, R194, C173.

OK, but why did you remove the mod info you posted ?

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