Grundig 3028

[LyntonP]

Bought this radio from a fellow forum member a couple weeks ago. All original untouched (EL84 changed in 1964).
Replaced all paper and small electrolytic caps.
Connected via variac and lamp limiter and slowly increased voltage whilst monitoring HT.
HT reached 100 volts and lamp limiter started to light.
Changed C65 and 66 And fitted s/s bridge rectifier.
Repeated test with same result. Some very faint crackling in speaker when tuning but no stations.
Left to soak at 100v for a few minutes then some flashing inside EABC80. Nothing major but switched off straight away.
Tried again, no flashing but lamp limiter still starting to illuminate at 100 Volts DC.
Checked all my installation and it is ok.
Circuit diagram I have is different to the way that the EABC80 is wired with pins 1 and 2 reversed, but since they are both diodes I don’t suppose it makes any difference.
So I must have a short somewhere, but where to begin?
Lynton

[Techman]

Sorry Lynton, but that's completely the wrong way to go about things. Never in my life or in my wildest dreams would I think about changing a load of components before I'd initially powered up the set to ascertain the state of play - and I qualified as a radio & TV engineer back in the 70s. You could have introduced any number of man made faults. You'll now need to do some serious fault finding and hope you haven't made any mistakes with your mass component changing.

[LyntonP]

Originally Posted by Techman

Sorry Lynton, but that's completely the wrong way to go about things. Never in my life or in my wildest dreams would I think about changing a load of components before I'd initially powered up the set to ascertain the state of play - and I qualified as a radio & TV engineer back in the 70s. You could have introduced any number of man made faults. You'll now need to do some serious fault finding and hope you haven't made any mistakes with your mass component changing.

Opinion noted and acknowledged. As a relative novice to radio restoration I understand that there are different schools of thought.
I have been told “Don’t power the set up until you have replaced the old capacitors” which I feel is a sensible approach.
This is by no means the first set I have worked on and have always had good results using this method even with sets much older than this.
I have been extremely careful to change the components using the correct values and double or triple checking the installation, using before and after photos just to be sure.
Unfortunately I don’t have years of experience to draw on and finding someone experienced who I could sit alongside to gain more knowledge is impossible.
Let us hope that with a little encouragement from other members of the forum I will have a successful outcome.
Regards
Lynton

[stevehertz]

Indeed Lynton, agreed on all points. Absolutely nothing wrong with performing a blanket change of known, time and time again proven, troublesome components, pre-testing of valves, cleaning switches, etc etc before even switching on a set that one is restoring. In fact it makes good sense as not only does it remove dozens of faults in one fell swoop, it also avoids potential damage to transformers and other components that may be fatally stressed by switching on an old set. The issue of changing components without making a mistake is an obvious one, and it's up to the restorer to perform such component changes one at a time, and frankly, that's not difficult to do.

[cathoderay57]

Hi Lynton, regardless of different approaches to diagnostics there's likely no serious harm done. If the EABC80 has an internal short it would have manifested itself in any case. I'm not a fan of either lamp limiters or variacs and never use either. They make any voltage measurements you take within the circuit misleading and pointless. I recommend that with power off, and being sure the HT caps have discharged, you make a resistance check across the HT line. Checking the value you measure against the circuit should show if you have a short somewhere. Obviously you will get a low readng initially as the meter charges the HT caps. If the reading seems reasonable then remove the EABC80 since it appears suspect (or fit a replacement from another set if you have one), then power up with full mains and without the limiter, and measure valve voltages compared to those in the service sheet. I have never known an EABC80 to flash over (EL84s yes) but they aren't the most reliable of valves. The voltage on the EABC80 diodes is low so the flash over would presumably have been the triode which should have given a crack at the speaker. Cheers, Jerry

[simpsons]

Hi Lynton

You will see from the schematic that when the PU button is depressed that the IF and AM/FM Front End is disconnected from the HT supply as is the EM34. Only the audio amplifier EABC80 and Output, EL84, are left in the circuit. Switch 3i 3k refers.

Press PU, remove the EL84 and EABC80 and see what is what then replace them one at a time etc; etc.

This will at least narrow down the fault, well hopefully!!

Chris

[LyntonP]

I followed Jerry’s advice. Measuring across the HT line gave a reading of 2.5K ohms. What would be a normal reading?
Switched on directly to mains and R15 1K (connects HT to pin 8 of EF89 and IF can T2) got very hot and gave off a wisp of smoke.
Switched off immediately and investigated connections around EF89. It is a very busy area compounded by the fact that I had to use two capacitors for both C41 and 42 to achieve correct values. Couldn’t really see any shorts but I tidied the area as best I could resoldering all the connections.
Tried again and the radio worked on LW, MW, and SW but not on FM. All modes tested with an external aerial. So there must have been a short somewhere? R15 still reads good at 1K.
Enough volume to rattle the sides of my shed! Tone controls work well but I’m sure that the wave change switches need further cleaning. Not really sure how to achieve this since they are almost impossible to reach.
After about 15 min HT voltage settled at 250 volts (initially 290). Maybe a little low? I have a German circuit diagram which indicates 270 to 280v
Tried cleaning pins and socket of ECC85 but had no effect. Wiggling the valve has no effect apart from a slight “rustling” in the speaker.
Need to check valve voltages but too cold in the shed to continue so back in the house for a cup of tea and a warm.
RE using the tape input to check the audio section- I have seen a chap on Utube do this where he injects an audio signal. Good suggestion which I have to try but volume when the radio came to life was really good.
Thank you gentlemen for your comments which are very helpful.
More work to do and I’m sure more questions to follow.
Photos show a few before and after progress.
Cheers
Lynton

[stevehertz]

A bit of side advice, the switching on these sets is quite complex, and the actual switch contacts are prone to bad 'blackening' aka corrosion and high resistance or even open circuit. So don't be afraid to give the switch banks plenty of sprays with switch cleaner and operate all the switches to effect a clean. Repeat if necessary. There should be no crackling if you 'tickle' the switches, or very little.

[cathoderay57]

Hi Lynton, nice work, result. I would have said a cold resistance measurement of 2.5k across the HT line was too low; if you think about it you would be drawing about 100mA before the valves had even warmed up. It looks like you found the short (R42+R15=2.3k so it would appear the bottom of R15 was shorting to chassis) so no harm done now you've cleared the fault. I agree with Stevehertz, do give the switchbank a healthy dose of switch cleaner. Assuming you're using an aerosol spray with a thin tube attached to the nozzle you have to try to point the spray to where you think the contacts are. It's fiddly but doable. If there are still no sigs on FM after cleaning then it's time for more voltage checks. Measure both the anode voltages of the ECC85; service data says they should be 124v at pin 1 and 139v at pin 6. If low/no volts on one or the other anode check R4 and R7 both ends. If no volts either end then the switch contact S6 has either failed or is still dirty. If the volts are significantly higher at the anodes then the ECC85 has probably lost emission and might require replacing. First, clean the ECC85 valve pins and sockets since if the cathode pins are not grounding it can cause the same symptom. ECC85s run hot so oxidation of pins and sockets is common. There are numerous other causes of loss of FM including faults at the EABC80 because different diodes are used for AM and FM. Are all 3 heaters lighting up in the EABC80? It occurs to me that the internal sparks you mentioned at the start could possibly have been one of the diode heaters failing. Cheers, Jerry

[Gabe001]

I had a dodgy eabc80 in the past with an internal short which knocked out FM only. Voltage readings will help. If it flashed over it's certainly suspect

[robreddog]

hi, i repaired 3 of these radios in lockdown and two of them needed ecc85's replacing.
Rob.

[Laurence Smith]

Hi,
I also restored one of these sets for a friend last year, and the ECC85 required replacement in order for it to work on FM.

[LyntonP]

Hi Jerry
Ok voltage readings as follows:-
FM switch 6b 238 volts. 6c 231 volts.
R4 6.8K HT side 231 volts, Valve side 214 volts.
R7 15K Ht Side 231 volts, valve side 193 volts.
ECC85 Pin 1 212 volts.
ECC85 Pin 6 189 volts.
EABC80 I can see 3 very faint heaters glowing.
I have sprayed and operated all switches several times and they are much quieter (electrically) during operation.
So my uneducated guess is that the FM valve is kaput. The comments seem to support this guess.
Cheers
Lynton

[cathoderay57]

Yes, I'd agree. The ECC85 is drawing very low current indeed. You should be able to source a replacement without too much expense. If I were you I would avoid untested valves - there are a lot of duds in circulation. Well done, best of luck! Cheers, Jerry

[LyntonP]

Originally Posted by robreddog

hi, i repaired 3 of these radios in lockdown and two of them needed ecc85's replacing.
Rob.

Out of interest where did you obtain the valves from?

[robreddog]

post in the wanted section or ebay, watford valves,
they are quite common.
Rob

[LyntonP]

Originally Posted by robreddog

post in the wanted section or ebay, watford valves,
they are quite common.
Rob

Thanks Rob

Lynton

[LyntonP]

New ECC85 arrived and fitted. FM now working ok.
Voltage readings now
HT 225 volts
ECC85 pin 1 130 volts, pin 6 116 volts.
Magic eye has now faded completely.
I did check the resistors on the back of the valve and both very close to1 Megohm.
I don’t have a VVM only an AVO so cannot check AGC voltage accurately.
Started to restore the cabinet. Unsure of original gloss coat but it was as hard as nails!
Lynton

[cathoderay57]

Hi Lynton, nice work. Those Grundigs sound very good and usually FM sensitivity is excellent. Cheers, Jerry

[stevehertz]

You will probably need to stain the wood itself as well as maybe using a 'finish' that has a tint. As everyone knows on here (I keep banging on about it!) my favourite is Wilko Quick Dry varnish that comes in many varieties and colours. It is so easy to apply and sand and dries to a lovely self levelling finish.