R1556A (AR88D) Help needed. - UK Vintage Radio Repair and Restoration Discussion Forum

M6SPW1974 · 24th Mar 2018, 4:08 pm

Hi people, I have here am R1556A (AR88D)which was until recently working perfectly.

A couple of days ago the audio output dropped and so does the S meter & reception.

The receiver has been modernised at some point with nearly all components replaced including the bathtubs.

Here is what happens, on the T2 output transformer (type 901666-501) there is a 2.2K resistor bridged across contacts 3 & 4 and when I increase the volume control it burns the 2.2k resistor out.

I have replaced capacitors C118 + C122 yet it still burns out the 2.2k resistor on the transformer.

Can anybody offer me any advive on what to check or is the T2 output transformer as good as dead??

Cheers in advance

Station X · 24th Mar 2018, 4:39 pm

I've had a look at the circuit and I cannot see a 2.2k resistor in that position. Looks like a mod.

I would suggest you check the continuity of the primary winding of the output transformer.

I'd also check C119, as if this goes short-circuit it will put the full HT across the output transformer primary.

M6SPW1974 · 24th Mar 2018, 4:52 pm

Hi Graham, thanks for the quick response.

Yep I also suspected the 2.2K resistor was some sort of modification as I also could not find it in the circuit diagram or parts list.

I will check out C119 as you say.

Not sure how to go about checking "continuity of the primary winding of the output transformer" as I am new to all this repair larky so you would have to explain exactly what pins on the transformer that I would need to check for continuity.

Again cheers for your help / knowledge.

Regards, Shaun.

Station X · 24th Mar 2018, 4:56 pm

Quote:

Originally Posted by M6SPW1974

Not sure how to go about checking "continuity of the primary winding of the output transformer" as I am new to all this repair larky so you would have to explain exactly what pins on the transformer that I would need to check for continuity.

Remove the 2.2k resistor and measure the resistance between tags 3 and 4.

Report back results. I don't want a "no reading" response.

M6SPW1974 · 24th Mar 2018, 5:03 pm

Just checked Graham, no C119 present on pin 3 of the 6K6G, I know the capacitor you mean ,had a few on other AR88's go before now, all that is on this one is a red wire that feeds from pin 3 on the 6K6G direct to the pin 4 of the output transformer where pin 4 is bridged to pin 3 with that 2.2k resistor.

M6SPW1974 · 24th Mar 2018, 5:06 pm

Quote:

Originally Posted by Station X

Quote:

Originally Posted by M6SPW1974

Not sure how to go about checking "continuity of the primary winding of the output transformer" as I am new to all this repair larky so you would have to explain exactly what pins on the transformer that I would need to check for continuity.

Remove the 2.2k resistor and measure the resistance between tags 3 and 4.

Report back results. I don't want a "no reading" response.

Hi Graham,no continuity on pins 3 & 4,which i presume means it is dead yes??

Luckily i still have the NOS T2 transformer that i bought off you Graham + i have another one in reserve which i know works.

Would you do away with the 2.2k resistor when i install a new T2 & reinstate a C119 on the 6K6G?

Best regards,Shaun.

Station X · 24th Mar 2018, 5:23 pm

If there's an open circuit between pins 3 and 4, then your output transformer is toast.

I may have a spare somewhere, bit it won't be cheap, as the price will reflect what I paid for it.

If you intend fitting a new transformer, you'll need to check everything first if you're to avoid destroying another one. Something as simple as the speaker being disconnected will burn out the primary.

If you search the forums you'll find details of a mode using neon lamps to protect the transformer.

EDIT. I think you've already purchased a transformer from me.

M6SPW1974 · 24th Mar 2018, 5:37 pm

Re-read my last post Graham,sorry i must of been editing it at the same time you were replying,regards.

Station X · 24th Mar 2018, 5:40 pm

That is indeed the case.

Personally I'd put the circuit back the way it was, but add a couple of neons across the primary winding between pins 3 and 4.

M6SPW1974 · 24th Mar 2018, 5:44 pm

OK Graham,so you would not bother reinstating C119?? and just put a new 2.2k across pins 3 & 4 as it was before.

Also sorry what do you mean by neons??

Best regards,Shaun.

Station X · 24th Mar 2018, 5:45 pm

The R1556A had some circuit changes around the output transformer when compared to the standard AR88D. It might be worth checking the additional 0.01uF capacitor.

See this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=50017

M6SPW1974 · 24th Mar 2018, 6:28 pm

I presume you mean the circuit diagram where it shows a capacitor inline with the high impedance headphone socket?

24th Mar 2018, 4:08 pm	#1
M6SPW1974 Pentode Join Date: Nov 2017 Location: Millom, South Cumberland, UK. Posts: 114	R1556A (AR88D) Help needed. Hi people, I have here am R1556A (AR88D)which was until recently working perfectly. A couple of days ago the audio output dropped and so does the S meter & reception. The receiver has been modernised at some point with nearly all components replaced including the bathtubs. Here is what happens, on the T2 output transformer (type 901666-501) there is a 2.2K resistor bridged across contacts 3 & 4 and when I increase the volume control it burns the 2.2k resistor out. I have replaced capacitors C118 + C122 yet it still burns out the 2.2k resistor on the transformer. Can anybody offer me any advive on what to check or is the T2 output transformer as good as dead?? Cheers in advance Attached Thumbnails

24th Mar 2018, 4:39 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: R1556A (AR88D) Help needed. I've had a look at the circuit and I cannot see a 2.2k resistor in that position. Looks like a mod. I would suggest you check the continuity of the primary winding of the output transformer. I'd also check C119, as if this goes short-circuit it will put the full HT across the output transformer primary. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

24th Mar 2018, 4:52 pm	#3
M6SPW1974 Pentode Join Date: Nov 2017 Location: Millom, South Cumberland, UK. Posts: 114	Re: R1556A (AR88D) Help needed. Hi Graham, thanks for the quick response. Yep I also suspected the 2.2K resistor was some sort of modification as I also could not find it in the circuit diagram or parts list. I will check out C119 as you say. Not sure how to go about checking "continuity of the primary winding of the output transformer" as I am new to all this repair larky so you would have to explain exactly what pins on the transformer that I would need to check for continuity. Again cheers for your help / knowledge. Regards, Shaun.

24th Mar 2018, 5:03 pm	#5
M6SPW1974 Pentode Join Date: Nov 2017 Location: Millom, South Cumberland, UK. Posts: 114	Re: R1556A (AR88D) Help needed. Just checked Graham, no C119 present on pin 3 of the 6K6G, I know the capacitor you mean ,had a few on other AR88's go before now, all that is on this one is a red wire that feeds from pin 3 on the 6K6G direct to the pin 4 of the output transformer where pin 4 is bridged to pin 3 with that 2.2k resistor.

24th Mar 2018, 5:23 pm	#7
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: R1556A (AR88D) Help needed. If there's an open circuit between pins 3 and 4, then your output transformer is toast. I may have a spare somewhere, bit it won't be cheap, as the price will reflect what I paid for it. If you intend fitting a new transformer, you'll need to check everything first if you're to avoid destroying another one. Something as simple as the speaker being disconnected will burn out the primary. If you search the forums you'll find details of a mode using neon lamps to protect the transformer. EDIT. I think you've already purchased a transformer from me. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

24th Mar 2018, 5:37 pm	#8
M6SPW1974 Pentode Join Date: Nov 2017 Location: Millom, South Cumberland, UK. Posts: 114	Re: R1556A (AR88D) Help needed. Re-read my last post Graham,sorry i must of been editing it at the same time you were replying,regards.

24th Mar 2018, 5:40 pm	#9
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: R1556A (AR88D) Help needed. That is indeed the case. Personally I'd put the circuit back the way it was, but add a couple of neons across the primary winding between pins 3 and 4. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

24th Mar 2018, 5:44 pm	#10
M6SPW1974 Pentode Join Date: Nov 2017 Location: Millom, South Cumberland, UK. Posts: 114	Re: R1556A (AR88D) Help needed. OK Graham,so you would not bother reinstating C119?? and just put a new 2.2k across pins 3 & 4 as it was before. Also sorry what do you mean by neons?? Best regards,Shaun.

24th Mar 2018, 5:45 pm	#11
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: R1556A (AR88D) Help needed. The R1556A had some circuit changes around the output transformer when compared to the standard AR88D. It might be worth checking the additional 0.01uF capacitor. See this thread:- https://www.vintage-radio.net/forum/...ad.php?t=50017 __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

24th Mar 2018, 6:28 pm	#12
M6SPW1974 Pentode Join Date: Nov 2017 Location: Millom, South Cumberland, UK. Posts: 114	Re: R1556A (AR88D) Help needed. I presume you mean the circuit diagram where it shows a capacitor inline with the high impedance headphone socket?